Red Hat JBoss Web Server 2.1
Hibernate Core Reference Guide
An Introductory Guide for Hibernate with Red Hat JBoss Web Server 2.1.0
Last Updated: 2017-10-18
Red Hat JBoss Web Server 2.1 Hibernate Core Reference Guide
An Introductory Guide for Hibernate with Red Hat JBoss Web Server 2.1.0
Misha Husnain Ali
Red Hat Engineering Content Services
mhusnain@redhat.com
Tom Wells
twells@redhat.com
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Abstract
Using Hibernate with Red Hat JBoss Web Server
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Table of Contents
CHAPTER 1. HIBERNATE
1.1. ABOUT HIBERNATE CORE
1.2. JAVA PERSISTENCE API (JPA)
1.2.1. About JPA
1.2.2. Hibernate EntityManager
1.2.3. Configuration
1.2.3.1. Hibernate Configuration Properties
1.2.3.2. Hibernate JDBC and Connection Properties
1.2.3.3. Hibernate Cache Properties
1.2.3.4. Hibernate Transaction Properties
1.2.3.5. Miscellaneous Hibernate Properties
1.2.3.6. Hibernate SQL Dialects
1.3. CONNECTION POOLING
1.3.1. About Connection Pooling
1.3.2. C3P0 Connection Pool
1.3.3. Use JNDI to Obtain a Connection
1.3.4. Other Connection Specific Configurations
1.4. HIBERNATE ANNOTATIONS
1.4.1. Hibernate Annotations
1.5. ENVERS
1.5.1. About Hibernate Envers
1.5.2. About Auditing Persistent Classes
1.5.3. Auditing Strategies
1.5.3.1. About Auditing Strategies
1.5.4. Getting Started with Entity Auditing
1.5.4.1. Add Auditing Support to a JPA Entity
1.5.5. Configuration
1.5.5.1. Configure Envers Parameters
1.5.5.2. Enable or Disable Auditing at Runtime
1.5.5.3. Configure Conditional Auditing
1.5.5.4. Envers Configuration Properties
1.5.6. Queries
1.5.6.1. Retrieve Auditing Information
CHAPTER 2. USING HIBERNATE
2.1. INTRODUCTION TO USING HIBERNATE
2.2. HIBERNATE TUTORIAL
2.3. YOUR FIRST HIBERNATE APPLICATION
2.3.1. About the Hibernate Application Tutorial
2.3.2. Set Up Your Hibernate Application
2.3.3. Create the First Class
2.3.4. The Mapping File
2.3.5. Hibernate Configuration
2.3.6. Building with Maven
2.3.7. Startup and Helpers
2.3.8. Loading and Storing Objects
2.4. MAPPING ASSOCIATIONS
2.4.1. About Mapping Associations
2.4.2. Mapping the Person Class
2.4.3. A Unidirectional Set-based Association
2.4.4. Working the Association
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2.4.5. Collection of Values
2.4.6. Bi-directional Associations
2.4.7. Working Bi-directional Links
2.5. THE EVENTMANAGER WEB APPLICATION
2.5.1. About the EventManager
2.5.2. Writing the Basic Servlet
2.5.3. Processing and Rendering
2.5.4. Deploying and Testing
2.5.5. Summary
CHAPTER 3. HIBERNATE ARCHITECTURE
3.1. OVERVIEW
3.2. INSTANCE STATES
3.3. JMX INTEGRATION
3.4. JCA SUPPORT
3.5. CONTEXTUAL SESSIONS
CHAPTER 4. PERSISTENT CLASSES
4.1. ABOUT PERSISTENT CLASSES
4.2. POJO EXAMPLE
4.2.1. A Simple POJO Example
4.2.2. Implement a No-argument Constructor
4.2.3. Provide an Optional Identifier Property
4.2.4. Prefer Optional Non-final Classes
4.2.5. Declare Optional Accessors and Mutators for Persistent Fields
4.3. ADDITIONAL INFORMATION
4.3.1. Implementing Inheritance
4.3.2. Implementing equals() and hashCode()
4.3.3. Dynamic Models
4.3.4. Tuplizers
4.3.5. EntityNameResolvers
CHAPTER 5. BASIC O/R MAPPING
5.1. MAPPING DECLARATION
5.1.1. About Mapping Declaration
5.1.2. Doctype
5.1.3. EntityResolver
5.1.4. Hibernate-mapping
5.1.5. Class
5.1.6. id
5.1.7. Generator
5.1.8. Hi/Lo Algorithm
5.1.9. UUID Algorithm
5.1.10. Identity Columns and Sequences
5.1.11. Assigned Identifiers
5.1.12. Primary Keys Assigned by Triggers
5.1.13. Enhanced Identifier Generators
5.1.14. Identifier Generator Optimization
5.1.15. composite-id
5.1.16. Discriminator
5.1.17. Version (optional)
5.1.18. Timestamp (optional)
5.1.19. Property
5.1.20. Many-to-one
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5.1.21. One-to-one
5.1.22. Natural-id
5.1.23. Component and Dynamic-component
5.1.24. Properties
5.1.25. Subclass
5.1.26. Joined-subclass
5.1.27. Union-subclass
5.1.28. Join
5.1.29. Key
5.1.30. Column and Formula Elements
5.1.31. Import
5.1.32. Any
5.2. HIBERNATE TYPES
5.2.1. Entities and Values
5.2.2. Basic Value Types
5.2.3. Custom Value Types
5.3. ADDITIONAL INFORMATION
5.3.1. Mapping a Class Multiple Times
5.3.2. SQL Quoted Identifiers
5.4. METADATA ALTERNATIVES
5.4.1. About Metadata Alternatives
5.4.2. Using XDoclet Markup
5.4.3. Using JDK 5.0 Annotations
5.4.4. Generated Properties
5.4.5. Auxiliary Database Objects
CHAPTER 6. COLLECTION MAPPING
6.1. PERSISTENT COLLECTIONS
6.2. COLLECTION MAPPINGS
6.2.1. About Collection Mappings
6.2.2. Collection Foreign Keys
6.2.3. Collection Elements
6.2.4. Indexed Collections
6.2.5. Collections of Values and Many-to-many Associations
6.2.6. One-to-many Associations
6.3. ADVANCED COLLECTION MAPPINGS
6.3.1. Sorted Collections
6.3.2. Bidirectional Associations
6.3.3. Bidirectional Associations with Indexed Collections
6.3.4. Ternary Associations
6.3.5. Using an idbag
6.4. COLLECTION EXAMPLE
6.4.1. Collection Examples
CHAPTER 7. ASSOCIATION MAPPINGS
7.1. ABOUT ASSOCIATION MAPPINGS
7.2. UNIDIRECTIONAL ASSOCIATIONS
7.2.1. Unidirectional Many-to-one
7.2.2. Unidirectional One-to-one
7.2.3. Unidirectional One-to-many
7.3. UNIDIRECTIONAL ASSOCIATIONS WITH JOIN TABLES
7.3.1. Unidirectional One-to-many with Join Tables
7.3.2. Unidirectional Many-to-one with Join Tables
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7.3.3. Unidirectional One-to-one with Join Tables
7.3.4. Unidirectional Many-to-many with Join Tables
7.4. BIDIRECTIONAL ASSOCIATIONS
7.4.1. Bidirectional One-to-many and Many-to-one
7.4.2. Bidirectional One-to-one
7.5. BIDIRECTIONAL ASSOCIATIONS WITH JOIN TABLES
7.5.1. Bidirectional One-to-many and Many-to-one with Join Tables
7.5.2. Bidirectional One to one with Join Tables
7.5.3. Bidirectional Many-to-many with Join Tables
7.6. OTHER ASSOCIATION MAPPINGS
7.6.1. Complex Association Mappings
CHAPTER 8. COMPONENT MAPPING
8.1. ABOUT COMPONENT MAPPING
8.2. DEPENDENT OBJECTS
8.3. COLLECTIONS OF DEPENDENT OBJECTS
8.4. COMPONENTS AS MAP INDICES
8.5. COMPONENTS AS COMPOSITE IDENTIFIERS
8.6. DYNAMIC COMPONENTS
CHAPTER 9. INHERITANCE MAPPING
9.1. INHERITANCE MAPPING STRATEGIES
9.1.1. About Inheritance Mapping Strategies
9.1.2. Table Per Class Hierarchy
9.1.3. Table Per Subclass
9.1.4. Table Per Subclass: Using a Discriminator
9.1.5. Mixing Table Per Class Hierarchy with Table Per Subclass
9.1.6. Table Per Concrete Class
9.1.7. Table Per Concrete Class Using Implicit Polymorphism
9.1.8. Mixing Implicit Polymorphism With Other Inheritance Mappings
9.2. LIMITATIONS
9.2.1. Inheritance Mapping Limitations
CHAPTER 10. WORKING WITH OBJECTS
10.1. ABOUT WORKING WITH OBJECTS
10.2. HIBERNATE OBJECT STATES
10.3. MAKING OBJECTS PERSISTENT
10.4. LOADING AN OBJECT
10.5. QUERYING
10.5.1. About Querying
10.5.2. Executing Queries
10.5.3. Iterating Results
10.5.4. Queries that Return Tuples
10.5.5. Scalar Results
10.5.6. Bind Parameters
10.5.7. Pagination
10.5.8. Scrollable Iteration
10.5.9. Externalizing Named Queries
10.5.10. Filtering Collections
10.5.11. Criteria Queries
10.5.12. Queries in Native SQL
10.6. MODIFYING OBJECTS
10.6.1. Modifying Persistent Objects
10.6.2. Modifying Detached Objects
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10.7. OTHER OBJECT OPERATIONS
10.7.1. Automatic State Detection
10.7.2. Deleting Persistent Objects
10.7.3. Replicating an Object Between Two Datastores
10.7.4. Flushing the Session
10.7.5. Transitive Persistence
10.7.6. Using Metadata
CHAPTER 11. TRANSACTIONS AND CONCURRENCY
11.1. ABOUT TRANSACTIONS AND CONCURRENCY
11.2. SESSION AND TRANSACTION SCOPES
11.2.1. About Session and Transaction Scopes
11.2.2. Unit of Work
11.2.3. Long Conversations
11.2.4. Considering Object Identity
11.2.5. Common Issues
11.3. DATABASE TRANSACTION DEMARCATION
11.3.1. About Database Transaction Demarcation
11.3.2. Non-managed Environment
11.3.3. Using JTA
11.3.4. Exception Handling
11.3.5. Transaction Timeout
11.4. OPTIMISTIC CONCURRENCY CONTROL
11.4.1. About Optimistic Concurrency Control
11.4.2. Application Version Checking
11.4.3. Extended Session and Automatic Versioning
11.4.4. Detached Objects and Automatic Versioning
11.4.5. Customizing Automatic Versioning
11.5. PESSIMISTIC LOCKING
11.5.1. About Pessimistic Locking
11.5.2. Connection Release Modes
CHAPTER 12. INTERCEPTORS AND EVENTS
12.1. INTERCEPTORS
12.2. EVENT SYSTEM
12.3. HIBERNATE DECLARATIVE SECURITY
CHAPTER 13. BATCH PROCESSING
13.1. ABOUT BATCH PROCESSING
13.2. BATCH INSERTS
13.3. BATCH UPDATES
13.4. THE STATELESSSESSION INTERFACE
13.5. DML-STYLE OPERATIONS
CHAPTER 14. THE HIBERNATE QUERY LANGUAGE (HQL)
14.1. ABOUT THE HIBERNATE QUERY LANGUAGE
14.2. CASE SENSITIVITY
14.3. THE FROM CLAUSE
14.4. ASSOCIATIONS AND JOINS
14.5. FORMS OF JOIN SYNTAX
14.6. REFERRING TO IDENTIFIER PROPERTY
14.7. THE SELECT CLAUSE
14.8. AGGREGATE FUNCTIONS
14.9. POLYMORPHIC QUERIES
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14.10. THE WHERE CLAUSE
14.11. EXPRESSIONS
14.12. THE ORDER BY CLAUSE
14.13. THE GROUP BY CLAUSE
14.14. SUBQUERIES
14.15. HQL EXAMPLES
14.16. BULK UPDATE AND DELETE
14.17. HQL TIPS
14.18. COMPONENTS
14.19. ROW VALUE CONSTRUCTOR SYNTAX
CHAPTER 15. CRITERIA QUERIES
15.1. CREATING A CRITERIA INSTANCE
15.2. NARROWING THE RESULT SET
15.3. ORDERING THE RESULTS
15.4. ASSOCIATIONS
15.5. DYNAMIC ASSOCIATION FETCHING
15.6. EXAMPLE QUERIES
15.7. PROJECTIONS, AGGREGATION AND GROUPING
15.8. DETACHED QUERIES AND SUBQUERIES
15.9. QUERIES BY NATURAL IDENTIFIER
CHAPTER 16. NATIVE SQL
16.1. ABOUT NATIVE SQL
16.2. USING SQLQUERIES
16.2.1. Using a SQLQuery
16.2.2. Scalar Queries
16.2.3. Entity Queries
16.2.4. Handling Associations and Collections
16.2.5. Returning Multiple Entities
16.2.6. Returning Non-managed Entities
16.2.7. Handling Inheritance
16.2.8. Parameters
16.3. NAMED SQL QUERIES
16.3.1. About Named SQL Queries
16.3.2. Using return-property to Explicitly Specify Column/Alias Names
16.3.3. Using Stored Procedures for Querying
16.3.4. Rules/limitations for Using Stored Procedures
16.4. ADDITIONAL SQL FUNCTIONS
16.4.1. Custom SQL for Create, Update and Delete
16.4.2. Custom SQL for Loading
CHAPTER 17. FILTERING DATA
17.1. ABOUT FILTERING DATA
17.2. ABOUT HIBERNATE FILTERS
17.3. USING HIBERNATE FILTERS
17.4. HIBERNATE FILTERS EXAMPLE
CHAPTER 18. XML MAPPING
18.1. WORKING WITH XML DATA
18.1.1. About Working with XML Data
18.1.2. Specifying XML and Class Mapping Together
18.1.3. Specifying Only an XML Mapping
18.2. XML MAPPING METADATA
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18.2.1. About XML Mapping Metadata
18.2.2. Manipulating XML Data
CHAPTER 19. IMPROVING PERFORMANCE
19.1. FETCHING STRATEGIES
19.1.1. About Fetching Strategies
19.1.2. Working with Lazy Associations
19.1.3. Tuning Fetch Strategies
19.1.4. Single-ended Association Proxies
19.1.5. Initializing Collections and Proxies
19.1.6. Using Batch Fetching
19.1.7. Using Subselect Fetching
19.1.8. Using Lazy Property Fetching
19.2. UNDERSTANDING COLLECTION PERFORMANCE
19.2.1. Taxonomy
19.2.2. Lists, Maps, idbags and Sets
19.2.3. Bags and Lists as Inverse Collections
19.2.4. One Shot Delete
19.3. MONITORING PERFORMANCE
19.3.1. About Monitoring Performance
19.3.2. Monitoring a SessionFactory
19.3.3. Performance Metrics
CHAPTER 20. TOOLSET GUIDE
20.1. ABOUT THE TOOLSET GUIDE
20.2. AUTOMATIC SCHEMA GENERATION
20.2.1. About Automatic Schema Generation
20.2.2. Customizing the Schema
20.2.3. Running the Tool
20.2.4. Database Properties
20.2.5. Using Ant
20.2.6. Incremental Schema Updates
20.2.7. Using Ant for Incremental Schema Updates
20.2.8. Schema Validation
20.2.9. Using Ant for Schema Validation
CHAPTER 21. A PARENT/CHILD EXAMPLE
21.1. ABOUT THE PARENT/CHILD EXAMPLE
21.2. ABOUT COLLECTIONS
21.3. BIDIRECTIONAL ONE-TO-MANY EXAMPLE
21.4. CASCADING LIFE CYCLE
21.5. CASCADES AND UNSAVED-VALUE
21.6. CONCLUSION
CHAPTER 22. WEBLOG APPLICATION EXAMPLE
22.1. PERSISTENT CLASSES
22.2. HIBERNATE MAPPINGS
22.3. HIBERNATE CODE
CHAPTER 23. VARIOUS MAPPINGS EXAMPLE
23.1. EMPLOYER/EMPLOYEE
23.2. AUTHOR/WORK
23.3. CUSTOMER/ORDER/PRODUCT
23.4. MISCELLANEOUS EXAMPLE MAPPINGS
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23.4.1. About the Miscellaneous Example Mappings
23.4.2. Typed One-to-one Association
23.4.3. Composite Key Example
23.4.4. Many-to-many with Shared Composite Key Attribute
23.4.5. Content Based Discrimination
23.4.6. Associations on Alternate Keys
CHAPTER 24. BEST PRACTICES
24.1. HIBERNATE BEST PRACTICES
CHAPTER 25. DATABASE PORTABILITY CONSIDERATIONS
25.1. PORTABILITY BASICS
25.2. DIALECT
25.3. DIALECT RESOLUTION
25.4. IDENTIFIER GENERATION
APPENDIX A. REVISION HISTORY
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CHAPTER 1. HIBERNATE
1.1. ABOUT HIBERNATE CORE
Hibernate Core is an object/relational mapping library. It provides the framework for mapping Java
classes to database tables, allowing applications to avoid direct interaction with the database.
Report a bug
1.2. JAVA PERSISTENCE API (JPA)
1.2.1. About JPA
The Java Persistence API (JPA) is the standard for using persistence in Java projects. Java EE 6
applications use the Java Persistence 2.0 specification, documented here:
http://www.jcp.org/en/jsr/detail?id=317.
Hibernate EntityManager implements the programming interfaces and life-cycle rules defined by the
specification. It provides the JBoss Enterprise Web Server with a complete Java Persistence solution.
Report a bug
1.2.2. Hibernate EntityManager
Hibernate EntityManager implements the programming interfaces and life-cycle rules defined by the JPA
2.0 specification. It provides the JBoss Enterprise Web Server with a complete Java Persistence
solution.
Report a bug
1.2.3. Configuration
1.2.3.1. Hibernate Configuration Properties
Table 1.1. Properties
Property Name Description
hibernate.dialect The classname of a Hibernate
org.hibernate.dialect.Dialect. Allows
Hibernate to generate SQL optimized for a particular
relational database.
In most cases Hibernate will be able to choose the
correct org.hibernate.dialect.Dialect
implementation, based on the JDBC metadata
returned by the JDBC driver.
hibernate.show_sql Boolean. Writes all SQL statements to console. This
is an alternative to setting the log category
org.hibernate.SQL to debug.
Hibernate Core Reference Guide
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hibernate.format_sql Boolean. Pretty print the SQL in the log and console.
hibernate.default_schema Qualify unqualified table names with the given
schema/tablespace in generated SQL.
hibernate.default_catalog Qualifies unqualified table names with the given
catalog in generated SQL.
hibernate.session_factory_name The org.hibernate.SessionFactory will be
automatically bound to this name in JNDI after it has
been created. For example,
jndi/composite/name.
hibernate.max_fetch_depth Sets a maximum "depth" for the outer join fetch tree
for single-ended associations (one-to-one, many-to-
one). A 0 disables default outer join fetching. The
recommended value is between 0 and 3.
hibernate.default_batch_fetch_size Sets a default size for Hibernate batch fetching of
associations. The recommended values are 4, 8,
and 16.
hibernate.default_entity_mode Sets a default mode for entity representation for all
sessions opened from this SessionFactory.
Values include: dynamic-map, dom4j, pojo.
hibernate.order_updates Boolean. Forces Hibernate to order SQL updates by
the primary key value of the items being updated.
This will result in fewer transaction deadlocks in
highly concurrent systems.
hibernate.generate_statistics Boolean. If enabled, Hibernate will collect statistics
useful for performance tuning.
hibernate.use_identifier_rollback Boolean. If enabled, generated identifier properties
will be reset to default values when objects are
deleted.
hibernate.use_sql_comments Boolean. If turned on, Hibernate will generate
comments inside the SQL, for easier debugging.
Default value is false.
hibernate.id.new_generator_mappings Boolean. This property is relevant when using
@GeneratedValue. It indicates whether or not the
new IdentifierGenerator implementations
are used for
javax.persistence.GenerationType.AUT
O,
javax.persistence.GenerationType.TAB
LE and
javax.persistence.GenerationType.SEQ
UENCE. Default value is true.
Property Name Description
CHAPTER 1. HIBERNATE
11
IMPORTANT
For hibernate.id.new_generator_mappings, new applications should keep the
default value of true. Existing applications that used Hibernate 3.3.x may need to change
it to false to continue using a sequence object or table based generator, and maintain
backward compatibility.
Report a bug
1.2.3.2. Hibernate JDBC and Connection Properties
Table 1.2. Properties
Property Name Description
hibernate.jdbc.fetch_size A non-zero value that determines the JDBC fetch
size (calls Statement.setFetchSize()).
hibernate.jdbc.batch_size A non-zero value enables use of JDBC2 batch
updates by Hibernate. The recommended values are
between 5 and 30.
hibernate.jdbc.batch_versioned_data Boolean. Set this property to true if the JDBC driver
returns correct row counts from executeBatch().
Hibernate will then use batched DML for
automatically versioned data. Default value is to
false.
hibernate.jdbc.factory_class Select a custom
org.hibernate.jdbc.Batcher. Most
applications will not need this configuration property.
hibernate.jdbc.use_scrollable_resultset Boolean. Enables use of JDBC2 scrollable resultsets
by Hibernate. This property is only necessary when
using user-supplied JDBC connections. Hibernate
uses connection metadata otherwise.
hibernate.jdbc.use_streams_for_binary Boolean. This is a system-level property. Use
streams when writing/reading binary or
serializable types to/from JDBC.
hibernate.jdbc.use_get_generated_keys Boolean. Enables use of JDBC3
PreparedStatement.getGeneratedKeys()
to retrieve natively generated keys after insert.
Requires JDBC3+ driver and JRE1.4+. Set to false if
JDBC driver has problems with the Hibernate
identifier generators. By default, it tries to determine
the driver capabilities using connection metadata.
hibernate.connection.provider_class The classname of a custom
org.hibernate.connection.ConnectionP
rovider which provides JDBC connections to
Hibernate.
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hibernate.connection.isolation Sets the JDBC transaction isolation level. Check
java.sql.Connection for meaningful values,
but note that most databases do not support all
isolation levels and some define additional, non-
standard isolations. Standard values are 1, 2, 4,
8.
hibernate.connection.autocommit Boolean. This property is not recommended for use.
Enables autocommit for JDBC pooled connections.
hibernate.connection.release_mode Specifies when Hibernate should release JDBC
connections. By default, a JDBC connection is held
until the session is explicitly closed or disconnected.
The default value auto will choose
after_statement for the JTA and CMT
transaction strategies, and after_transaction
for the JDBC transaction strategy.
Available values are auto (default) | on_close |
after_transaction | after_statement.
This setting only affects Sessions returned from
SessionFactory.openSession. For
Sessions obtained through
SessionFactory.getCurrentSession, the
CurrentSessionContext implementation
configured for use controls the connection release
mode for those Sessions.
hibernate.connection.<propertyName> Pass the JDBC property <propertyName> to
DriverManager.getConnection().
hibernate.jndi.<propertyName> Pass the property <propertyName> to the JNDI
InitialContextFactory.
Property Name Description
Report a bug
1.2.3.3. Hibernate Cache Properties
Table 1.3. Properties
Property Name Description
hibernate.cache.region.factory_class The classname of a custom CacheProvider.
hibernate.cache.use_minimal_puts Boolean. Optimizes second-level cache operation to
minimize writes, at the cost of more frequent reads.
This setting is most useful for clustered caches and,
in Hibernate3, is enabled by default for clustered
cache implementations.
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13
hibernate.cache.use_query_cache Boolean. Enables the query cache. Individual queries
still have to be set cacheable.
hibernate.cache.use_second_level_cac
he
Boolean. Used to completely disable the second level
cache, which is enabled by default for classes that
specify a <cache> mapping.
hibernate.cache.query_cache_factory The classname of a custom QueryCache interface.
The default value is the built-in
StandardQueryCache.
hibernate.cache.region_prefix A prefix to use for second-level cache region names.
hibernate.cache.use_structured_entri
es
Boolean. Forces Hibernate to store data in the
second-level cache in a more human-friendly format.
hibernate.cache.default_cache_concur
rency_strategy
Setting used to give the name of the default
org.hibernate.annotations.CacheConcu
rrencyStrategy to use when either
@Cacheable or @Cache is used.
@Cache(strategy="..") is used to override
this default.
Property Name Description
Report a bug
1.2.3.4. Hibernate Transaction Properties
Table 1.4. Properties
Property Name Description
hibernate.transaction.factory_class The classname of a TransactionFactory to
use with Hibernate Transaction API. Defaults to
JDBCTransactionFactory).
jta.UserTransaction A JNDI name used by
JTATransactionFactory to obtain the JTA
UserTransaction from the application server.
hibernate.transaction.manager_lookup
_class
The classname of a
TransactionManagerLookup. It is required
when JVM-level caching is enabled or when using
hilo generator in a JTA environment.
hibernate.transaction.flush_before_c
ompletion
Boolean. If enabled, the session will be automatically
flushed during the before completion phase of the
transaction. Built-in and automatic session context
management is preferred.
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hibernate.transaction.auto_close_ses
sion
Boolean. If enabled, the session will be automatically
closed during the after completion phase of the
transaction. Built-in and automatic session context
management is preferred.
Property Name Description
Report a bug
1.2.3.5. Miscellaneous Hibernate Properties
Table 1.5. Properties
Property Name Description
hibernate.current_session_context_cl
ass
Supply a custom strategy for the scoping of the
"current" Session. Values include jta | thread |
managed | custom.Class.
hibernate.query.factory_class Chooses the HQL parser implementation:
org.hibernate.hql.internal.ast.ASTQu
eryTranslatorFactory or
org.hibernate.hql.internal.classic.C
lassicQueryTranslatorFactory.
hibernate.query.substitutions Used to map from tokens in Hibernate queries to
SQL tokens (tokens might be function or literal
names). For example,
hqlLiteral=SQL_LITERAL,
hqlFunction=SQLFUNC.
hibernate.hbm2ddl.auto Automatically validates or exports schema DDL to
the database when the SessionFactory is
created. With create-drop, the database schema
will be dropped when the SessionFactory is
closed explicitly. Property value options are
validate | update | create | create-drop
hibernate.hbm2ddl.import_files Comma-separated names of the optional files
containing SQL DML statements executed during the
SessionFactory creation. This is useful for
testing or demonstrating. For example, by adding
INSERT statements, the database can be populated
with a minimal set of data when it is deployed. An
example value is /humans.sql,/dogs.sql.
File order matters, as the statements of a given file
are executed before the statements of the following
files. These statements are only executed if the
schema is created (i.e. if
hibernate.hbm2ddl.auto is set to create or
create-drop).
CHAPTER 1. HIBERNATE
15
hibernate.hbm2ddl.import_files_sql_e
xtractor
The classname of a custom
ImportSqlCommandExtractor. Defaults to the
built-in SingleLineSqlCommandExtractor.
This is useful for implementing a dedicated parser
that extracts a single SQL statement from each
import file. Hibernate also provides
MultipleLinesSqlCommandExtractor,
which supports instructions/comments and quoted
strings spread over multiple lines (mandatory
semicolon at the end of each statement).
hibernate.bytecode.use_reflection_op
timizer
Boolean. This is a system-level property, which
cannot be set in the hibernate.cfg.xml file.
Enables the use of bytecode manipulation instead of
runtime reflection. Reflection can sometimes be
useful when troubleshooting. Hibernate always
requires either CGLIB or javassist even if the
optimizer is turned off.
hibernate.bytecode.provider Both javassist or cglib can be used as byte
manipulation engines. The default is javassist.
Property value is either javassist or cglib
Property Name Description
Report a bug
1.2.3.6. Hibernate SQL Dialects
IMPORTANT
The hibernate.dialect property should be set to the correct
org.hibernate.dialect.Dialect subclass for the application database. If a dialect
is specified, Hibernate will use sensible defaults for some of the other properties. This
means that they do not have to be specified manually.
Table 1.6. SQL Dialects (hibernate.dialect)
RDBMS Dialect
DB2 org.hibernate.dialect.DB2Dialect
DB2 AS/400 org.hibernate.dialect.DB2400Dialect
DB2 OS390 org.hibernate.dialect.DB2390Dialect
Firebird org.hibernate.dialect.FirebirdDialec
t
Hibernate Core Reference Guide
16
FrontBase org.hibernate.dialect.FrontbaseDiale
ct
H2 Database org.hibernate.dialect.H2Dialect
HypersonicSQL org.hibernate.dialect.HSQLDialect
Informix org.hibernate.dialect.InformixDialec
t
Ingres org.hibernate.dialect.IngresDialect
Interbase org.hibernate.dialect.InterbaseDiale
ct
Mckoi SQL org.hibernate.dialect.MckoiDialect
Microsoft SQL Server 2000 org.hibernate.dialect.SQLServerDiale
ct
Microsoft SQL Server 2005 org.hibernate.dialect.SQLServer2005D
ialect
Microsoft SQL Server 2008 org.hibernate.dialect.SQLServer2008D
ialect
Microsoft SQL Server 2012 org.hibernate.dialect.SQLServer2008D
ialect
MySQL5 org.hibernate.dialect.MySQL5Dialect
MySQL5 with InnoDB org.hibernate.dialect.MySQL5InnoDBDi
alect
MySQL with MyISAM org.hibernate.dialect.MySQLMyISAMDia
lect
Oracle (any version) org.hibernate.dialect.OracleDialect
Oracle 9i org.hibernate.dialect.Oracle9iDialec
t
Oracle 10g org.hibernate.dialect.Oracle10gDiale
ct
RDBMS Dialect
CHAPTER 1. HIBERNATE
17
Oracle 11g org.hibernate.dialect.Oracle10gDiale
ct
Pointbase org.hibernate.dialect.PointbaseDiale
ct
PostgreSQL org.hibernate.dialect.PostgreSQLDial
ect
PostgreSQL 9.2 org.hibernate.dialect.PostgreSQL82Di
alect
Postgres Plus Advanced Server org.hibernate.dialect.PostgresPlusDi
alect
Progress org.hibernate.dialect.ProgressDialec
t
SAP DB org.hibernate.dialect.SAPDBDialect
Sybase org.hibernate.dialect.SybaseASE15Dia
lect
Sybase 15.7 org.hibernate.dialect.SybaseASE157Di
alect
Sybase Anywhere org.hibernate.dialect.SybaseAnywhere
Dialect
RDBMS Dialect
Report a bug
1.3. CONNECTION POOLING
1.3.1. About Connection Pooling
Hibernate includes a rudimentary internal connection pooling algorithm for development and testing
purposes. Use a third party pool for improved performance and stability.
To set a third party connection pool, replace the hibernate.connection.pool_size property with
the appropriate settings for your selected connection pool. Setting a new value automatically disabled
Hibernate's internal connection pool.
Report a bug
1.3.2. C3P0 Connection Pool
C3P0 is an open source JDBC connection pool that is distributed with Hibernate. C3P0 is available in the
lib/ directory. Hibernate uses
Hibernate Core Reference Guide
18
org.hibernate.service.jdbc.connections.internal.C3P0ConnectionProvider for
connection pooling if the hibernate.c3p0 connection properties are set.
The following table contains hibernate.c3p0 connection properties to set to use the c3p0 connection
pool in Hibernate:
Table 1.7. Configuration Properties for the C3p0 Connection Pool
Property Details
hibernate.c3p0.min_size The minimum number of concurrent connections per
connection identity.
hibernate.c3p0.max_size The maximum number of concurrent connections per
connection identity.
hibernate.c3p0.timeout The period an idle connection is allowed to remain in
the connection pool before it is closed and removed
from the pool.
hibernate.c3p0.max_statements The maximum size of the statement cache.
Report a bug
1.3.3. Use JNDI to Obtain a Connection
To use Hibernate inside an application server, configure Hibernate to obtain connections from an
application server (javax.sql.Datasource) that is registered in JNDI. Set more than one of the
following properties to configure JNDI to obtain a connection:
hibernate.connection.datasource (mandatory)
hibernate.jndi.url
hibernate.jndi.class
hibernate.connection.username
hibernate.connection.password
Report a bug
1.3.4. Other Connection Specific Configurations
Pass custom connection properties by appending the name of the new property to
hibernate.connection. For example, to specify a charSet property, specify a new charSet
connection property names hibernate.connection.charSet.
Pass custom plugin strategies to obtain JDBC connections by implementing the
org.hibernate.service.jdbc.connections.spi.ConnectionProvider and by specifying a
custom implementation with the hibernate.connection.provider_class property.
Report a bug
CHAPTER 1. HIBERNATE
19
1.4. HIBERNATE ANNOTATIONS
1.4.1. Hibernate Annotations
Table 1.8. Hibernate Defined Annotations
Annotation Description
AccessType Property Access type.
Any Defines a ToOne association pointing to several
entity types. Matching the according entity type is
done through a metadata discriminator column. This
kind of mapping should be only marginal.
AnyMetaDef Defines @Any and @manyToAny metadata.
AnyMedaDefs Defines @Any and @ManyToAny set of metadata.
Can be defined at the entity level or the package
level.
BatchSize Batch size for SQL loading.
Cache Add caching strategy to a root entity or a collection.
Cascade Apply a cascade strategy on an association.
Check Arbitrary SQL check constraints which can be defined
at the class, property or collection level.
Columns Support an array of columns. Useful for component
user type mappings.
ColumnTransformer Custom SQL expression used to read the value from
and write a value to a column. Use for direct object
loading/saving as well as queries. The write
expression must contain exactly one '?' placeholder
for the value.
ColumnTransformers Plural annotation for @ColumnTransformer. Useful
when more than one column is using this behavior.
DiscriminatorFormula Discriminator formula to be placed at the root entity.
DiscriminatorOptions Optional annotation to express Hibernate specific
discriminator properties.
Entity Extends Entity with Hibernate features.
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20
Fetch Defines the fetching strategy used for the given
association.
FetchProfile Defines the fetching strategy profile.
FetchProfiles Plural annotation for @FetchProfile.
Filter Adds filters to an entity or a target entity of a
collection.
FilterDef Filter definition.
FilterDefs Array of filter definitions.
FilterJoinTable Adds filters to a join table collection.
FilterJoinTables Adds multiple @FilterJoinTable to a collection.
Filters Adds multiple @Filters.
Formula To be used as a replacement for @Column in most
places. The formula has to be a valid SQL fragment.
Generated This annotated property is generated by the
database.
GenericGenerator Generator annotation describing any kind of
Hibernate generator in a detyped manner.
GenericGenerators Array of generic generator definitions.
Immutable Mark an Entity or a Collection as immutable. No
annotation means the element is mutable.
An immutable entity may not be updated by the
application. Updates to an immutable entity will be
ignored, but no exception is thrown.
@Immutable placed on a collection makes the
collection immutable, meaning additions and
deletions to and from the collection are not allowed. A
HibernateException is thrown in this case.
Index Defines a database index.
JoinFormula To be used as a replacement for @JoinColumn in
most places. The formula has to be a valid SQL
fragment.
Annotation Description
CHAPTER 1. HIBERNATE
21
LazyCollection Defines the lazy status of a collection.
LazyToOne Defines the lazy status of a ToOne association (i.e.
OneToOne or ManyToOne).
Loader Overwrites Hibernate default FIND method.
ManyToAny Defines a ToMany association pointing to different
entity types. Matching the according entity type is
done through a metadata discriminator column. This
kind of mapping should be only marginal.
MapKeyType Defines the type of key of a persistent map.
MetaValue Represents a discriminator value associated to a
given entity type.
NamedNativeQueries Extends NamedNativeQueries to hold Hibernate
NamedNativeQuery objects.
NamedNativeQuery Extends NamedNativeQuery with Hibernate features.
NamedQueries Extends NamedQueries to hold Hibernate
NamedQuery objects.
NamedQuery Extends NamedQuery with Hibernate features.
NaturalId Specifies that a property is part of the natural id of the
entity.
NotFound Action to do when an element is not found on an
association.
OnDelete Strategy to use on collections, arrays and on joined
subclasses delete. OnDelete of secondary tables is
currently not supported.
OptimisticLock Whether or not a change of the annotated property
will trigger an entity version increment. If the
annotation is not present, the property is involved in
the optimistic lock strategy (default).
OptimisticLocking Used to define the style of optimistic locking to be
applied to an entity. In a hierarchy, only valid on the
root entity.
Annotation Description
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22
OrderBy Order a collection using SQL ordering (not HQL
ordering).
ParamDef A parameter definition.
Parameter Key/value pattern.
Parent Reference the property as a pointer back to the
owner (generally the owning entity).
Persister Specify a custom persister.
Polymorphism Used to define the type of polymorphism Hibernate
will apply to entity hierarchies.
Proxy Lazy and proxy configuration of a particular class.
RowId Support for ROWID mapping feature of Hibernate.
Sort Collection sort (Java level sorting).
Source Optional annotation in conjunction with Version and
timestamp version properties. The annotation value
decides where the timestamp is generated.
SQLDelete Overwrites the Hibernate default DELETE method.
SQLDeleteAll Overwrites the Hibernate default DELETE ALL
method.
SQLInsert Overwrites the Hibernate default INSERT INTO
method.
SQLUpdate Overwrites the Hibernate default UPDATE method.
Subselect Maps an immutable and read-only entity to a given
SQL subselect expression.
Synchronize Ensures that auto-flush happens correctly and that
queries against the derived entity do not return stale
data. Mostly used with Subselect.
Table Complementary information to a table either primary
or secondary.
Tables Plural annotation of Table.
Annotation Description
CHAPTER 1. HIBERNATE
23
Target Defines an explicit target, avoiding reflection and
generics resolving.
Tuplizer Defines a tuplizer for an entity or a component.
Tuplizers Defines a set of tuplizers for an entity or a
component.
Type Hibernate Type.
TypeDef Hibernate Type definition.
TypeDefs Hibernate Type definition array.
Where Where clause to add to the element Entity or target
entity of a collection. The clause is written in SQL.
WhereJoinTable Where clause to add to the collection join table. The
clause is written in SQL.
Annotation Description
NOTE
The annotation "Entity" is deprecated and scheduled for removal in future releases.
Report a bug
1.5. ENVERS
1.5.1. About Hibernate Envers
Hibernate Envers is an auditing and versioning system, providing the JBoss Enterprise Web Server with
a means to track historical changes to persistent classes. Audit tables are created for entities annotated
with @Audited, which store the history of changes made to the entity. The data can then be retrieved
and queried.
Envers allows developers to:
audit all mappings defined by the JPA specification,
audit all hibernate mappings that extend the JPA specification,
audit entities mapped by or using the native Hibernate API
log data for each revision using a revision entity, and
query historical data.
Report a bug
Hibernate Core Reference Guide
24
1.5.2. About Auditing Persistent Classes
Auditing of persistent classes is done in the JBoss Enterprise Web Server through Hibernate Envers and
the @Audited annotation. When the annotation is applied to a class, a table is created, which stores the
revision history of the entity.
Each time a change is made to the class, an entry is added to the audit table. The entry contains the
changes to the class, and is given a revision number. This means that changes can be rolled back, or
previous revisions can be viewed.
Report a bug
1.5.3. Auditing Strategies
1.5.3.1. About Auditing Strategies
Auditing strategies define how audit information is persisted, queried and stored. There are currently two
audit strategies available with Hibernate Envers:
Default Audit Strategy
This strategy persists the audit data together with a start revision. For each row that is inserted,
updated or deleted in an audited table, one or more rows are inserted in the audit tables, along with
the start revision of its validity.
Rows in the audit tables are never updated after insertion. Queries of audit information use
subqueries to select the applicable rows in the audit tables, which are slow and difficult to index.
Validity Audit Strategy
This strategy stores the start revision, as well as the end revision of the audit information. For each
row that is inserted, updated or deleted in an audited table, one or more rows are inserted in the audit
tables, along with the start revision of its validity.
At the same time, the end revision field of the previous audit rows (if available) is set to this revision.
Queries on the audit information can then use between start and end revision, instead of subqueries.
This means that persisting audit information is a little slower because of the extra updates, but
retrieving audit information is a lot faster.
This can also be improved by adding extra indexes.
Report a bug
1.5.4. Getting Started with Entity Auditing
1.5.4.1. Add Auditing Support to a JPA Entity
Task Summary
This topic covers adding auditing support for a JPA entity.
Procedure 1.1. Add Auditing Support to a JPA Entity
1. Configure the available auditing parameters to suit the deployment (refer to Section 1.5.5.1,
“Configure Envers Parameters”)
CHAPTER 1. HIBERNATE
25
2. Open the JPA entity to be audited.
3. Import the org.hibernate.envers.Audited interface.
4. Apply the @Audited annotation to each field or property to be audited, or apply it once to the
whole class.
Example 1.1. Audit Two Fields
Example 1.2. Audit an entire Class
import org.hibernate.envers.Audited;
import javax.persistence.Entity;
import javax.persistence.Id;
import javax.persistence.GeneratedValue;
import javax.persistence.Column;
@Entity
public class Person {
@Id
@GeneratedValue
private int id;
@Audited
private String name;
private String surname;
@ManyToOne
@Audited
private Address address;
// add getters, setters, constructors, equals and hashCode
here
}
import org.hibernate.envers.Audited;
import javax.persistence.Entity;
import javax.persistence.Id;
import javax.persistence.GeneratedValue;
import javax.persistence.Column;
@Entity
@Audited
public class Person {
@Id
@GeneratedValue
private int id;
private String name;
private String surname;
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26
Result
The JPA entity has been configured for auditing. A table called Entity_AUD will be created to store
the historical changes.
Report a bug
1.5.5. Configuration
1.5.5.1. Configure Envers Parameters
Task Summary
This topic covers configuring the available Envers parameters.
Procedure 1.2. Configure Envers Parameters
1. Open the persistence.xml file for the application.
2. Add, remove or configure Envers properties as required. For a list of available properties, refer
to Section 1.5.5.4, “Envers Configuration Properties”.
Example 1.3. Example Envers Parameters
@ManyToOne
private Address address;
// add getters, setters, constructors, equals and hashCode
here
}
<persistence-unit ...>
<provider>org.hibernate.ejb.HibernatePersistence</provider>
<class>...</class>
<properties>
<property name="hibernate.dialect" ... />
<!-- other hibernate properties -->
<property name="hibernate.ejb.event.post-insert"
value="org.hibernate.ejb.event.EJB3PostInsertEventListener,org.hibernate
.envers.event.AuditEventListener" />
<property name="hibernate.ejb.event.post-update"
value="org.hibernate.ejb.event.EJB3PostUpdateEventListener,org.hibernate
.envers.event.AuditEventListener" />
<property name="hibernate.ejb.event.post-delete"
value="org.hibernate.ejb.event.EJB3PostDeleteEventListener,org.hibernate
.envers.event.AuditEventListener" />
<property name="hibernate.ejb.event.pre-collection-update"
value="org.hibernate.envers.event.AuditEventListener" />
CHAPTER 1. HIBERNATE
27
Result
Auditing has been configured for all JPA entities in the application.
Report a bug
1.5.5.2. Enable or Disable Auditing at Runtime
Summary
This task covers the configuration steps required to enable/disable entity version auditing at runtime.
Procedure 1.3. Enable/Disable Auditing
1. Subclass the AuditEventListener class.
2. Override the following methods that are called on Hibernate events:
onPostInsert
onPostUpdate
onPostDelete
onPreUpdateCollection
onPreRemoveCollection
onPostRecreateCollection
3. Specify the subclass as the listener for the events.
4. Determine if the change should be audited.
5. Pass the call to the superclass if the change should be audited.
Report a bug
1.5.5.3. Configure Conditional Auditing
Summary
<property name="hibernate.ejb.event.pre-collection-remove"
value="org.hibernate.envers.event.AuditEventListener" />
<property name="hibernate.ejb.event.post-collection-recreate"
value="org.hibernate.envers.event.AuditEventListener" />
<property name="org.hibernate.envers.versionsTableSuffix" value="_V"
/>
<property name="org.hibernate.envers.revisionFieldName"
value="ver_rev" />
<!-- other envers properties -->
</properties>
</persistence-unit>
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Hibernate Envers persists audit data in reaction to various Hibernate events, using a series of event
listeners. These listeners are registered automatically if the Envers jar is in the class path. This task
covers the steps required to implement conditional auditing, by overriding some of the Envers event
listeners.
Procedure 1.4. Implement Conditional Auditing
1. Set the hibernate.listeners.envers.autoRegister Hibernate property to false in the
persistence.xml file.
2. Subclass each event listener to be overridden. Place the conditional auditing logic in the
subclass, and call the super method if auditing should be performed.
3. Create a custom implementation of org.hibernate.integrator.spi.Integrator, similar
to org.hibernate.envers.event.EnversIntegrator. Use the event listener subclasses
created in step two, rather than the default classes.
4. Add a META-INF/services/org.hibernate.integrator.spi.Integrator file to the
jar. This file should contain the fully qualified name of the class implementing the interface.
Result
Conditional auditing has been configured, overriding the default Envers event listeners.
Report a bug
1.5.5.4. Envers Configuration Properties
Table 1.9. Entity Data Versioning Configuration Parameters
Property Name Default Value Description
org.hibernate.envers.audit_table_
prefix
A string that is prepended to the
name of an audited entity, to
create the name of the entity that
will hold the audit information.
org.hibernate.envers.audit_table_
suffix
_AUD A string that is appended to the
name of an audited entity to
create the name of the entity that
will hold the audit information. For
example, if an entity with a table
name of Person is audited,
Envers will generate a table called
Person_AUD to store the
historical data.
org.hibernate.envers.revision_field
_name
REV The name of the field in the audit
entity that holds the revision
number.
org.hibernate.envers.revision_type
_field_name
REVTYPE The name of the field in the audit
entity that holds the type of
revision. The current types of
revisions possible are: add, mod
and del.
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29
org.hibernate.envers.revision_on_
collection_change
true This property determines if a
revision should be generated if a
relation field that is not owned
changes. This can either be a
collection in a one-to-many
relation, or the field using the
mappedBy attribute in a one-to-
one relation.
org.hibernate.envers.do_not_audit
_optimistic_locking_field
true When true, properties used for
optimistic locking (annotated with
@Version) will automatically be
excluded from auditing.
org.hibernate.envers.store_data_a
t_delete
false This property defines whether or
not entity data should be stored in
the revision when the entity is
deleted, instead of only the ID,
with all other properties marked
as null. This is not usually
necessary, as the data is present
in the last-but-one revision.
Sometimes, however, it is easier
and more efficient to access it in
the last revision. However, this
means the data the entity
contained before deletion is
stored twice.
org.hibernate.envers.default_sche
ma
null (same as normal tables) The default schema name used
for audit tables. Can be
overridden using the
@AuditTable(schema="...
") annotation. If not present, the
schema will be the same as the
schema of the normal tables.
org.hibernate.envers.default_catal
og
null (same as normal tables) The default catalog name that
should be used for audit tables.
Can be overridden using the
@AuditTable(catalog="..
.") annotation. If not present,
the catalog will be the same as
the catalog of the normal tables.
org.hibernate.envers.audit_strateg
y
org.hibernate.envers.strategy.Def
aultAuditStrategy
This property defines the audit
strategy that should be used when
persisting audit data. By default,
only the revision where an entity
was modified is stored.
Alternatively,
org.hibernate.envers.st
rategy.ValidityAuditStr
ategy stores both the start
revision and the end revision.
Together, these define when an
audit row was valid.
Property Name Default Value Description
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30
org.hibernate.envers.audit_strateg
y_validity_end_rev_field_name
REVEND The column name that will hold
the end revision number in audit
entities. This property is only valid
if the validity audit strategy is
used.
org.hibernate.envers.audit_strateg
y_validity_store_revend_timestam
p
false This property defines whether the
timestamp of the end revision,
where the data was last valid,
should be stored in addition to the
end revision itself. This is useful to
be able to purge old audit records
out of a relational database by
using table partitioning.
Partitioning requires a column that
exists within the table. This
property is only evaluated if the
ValidityAuditStrategy is
used.
org.hibernate.envers.audit_strateg
y_validity_revend_timestamp_field
_name
REVEND_TSTMP Column name of the timestamp of
the end revision at which point the
data was still valid. Only used if
the
ValidityAuditStrategy is
used, and
org.hibernate.envers.au
dit_strategy_validity_s
tore_revend_timestamp
evaluates to true.
Property Name Default Value Description
Report a bug
1.5.6. Queries
1.5.6.1. Retrieve Auditing Information
Summary
Hibernate Envers provides the functionality to retrieve audit information through queries. This topic
provides examples of those queries.
NOTE
Queries on the audited data will be, in many cases, much slower than corresponding
queries on live data, as they involve correlated subselects.
Example 1.4. Querying for Entities of a Class at a Given Revision
The entry point for this type of query is:
AuditQuery query = getAuditReader()
CHAPTER 1. HIBERNATE
31
Constraints can then be specified, using the AuditEntity factory class. The query below only
selects entities where the name property is equal to John:
The queries below only select entities that are related to a given entity:
The results can then be ordered, limited, and have aggregations and projections (except grouping)
set. The example below is a full query.
Example 1.5. Query Revisions where Entities of a Given Class Changed
The entry point for this type of query is:
Constraints can be added to this query in the same way as the previous example. There are
additional possibilities for this query:
AuditEntity.revisionNumber()
Specify constraints, projections and order on the revision number in which the audited entity was
modified.
AuditEntity.revisionProperty(propertyName)
Specify constraints, projections and order on a property of the revision entity, corresponding to the
revision in which the audited entity was modified.
AuditEntity.revisionType()
Provides accesses to the type of the revision (ADD, MOD, DEL).
The query results can then be adjusted as necessary. The query below selects the smallest revision
number at which the entity of the MyEntity class, with the entityId ID has changed, after revision
number 42:
.createQuery()
.forEntitiesAtRevision(MyEntity.class, revisionNumber);
query.add(AuditEntity.property("name").eq("John"));
query.add(AuditEntity.property("address").eq(relatedEntityInstance));
// or
query.add(AuditEntity.relatedId("address").eq(relatedEntityId));
List personsAtAddress = getAuditReader().createQuery()
.forEntitiesAtRevision(Person.class, 12)
.addOrder(AuditEntity.property("surname").desc())
.add(AuditEntity.relatedId("address").eq(addressId))
.setFirstResult(4)
.setMaxResults(2)
.getResultList();
AuditQuery query = getAuditReader().createQuery()
.forRevisionsOfEntity(MyEntity.class, false, true);
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Queries for revisions can also minimize/maximize a property. The query below selects the revision at
which the value of the actualDate for a given entity was larger than a given value, but as small as
possible:
The minimize() and maximize() methods return a criteria, to which constraints can be added,
which must be met by the entities with the maximized/minimized properties.
There are two boolean parameters passed when creating the query.
selectEntitiesOnly
This parameter is only valid when an explicit projection is not set.
If true, the result of the query will be a list of entities that changed at revisions satisfying the
specified constraints.
If false, the result will be a list of three element arrays. The first element will be the changed entity
instance. The second will be an entity containing revision data. If no custom entity is used, this will
be an instance of DefaultRevisionEntity. The third element array will be the type of the
revision (ADD, MOD, DEL).
selectDeletedEntities
This parameter specified if revisions in which the entity was deleted should be included in the
results. If true, the entities will have the revision type DEL, and all fields, except id, will have the
value null.
Example 1.6. Query Revisions of an Entity that Modified a Given Property
The query below will return all revisions of MyEntity with a given id, where the actualDate
property has been changed.
Number revision = (Number) getAuditReader().createQuery()
.forRevisionsOfEntity(MyEntity.class, false, true)
.setProjection(AuditEntity.revisionNumber().min())
.add(AuditEntity.id().eq(entityId))
.add(AuditEntity.revisionNumber().gt(42))
.getSingleResult();
Number revision = (Number) getAuditReader().createQuery()
.forRevisionsOfEntity(MyEntity.class, false, true)
// We are only interested in the first revision
.setProjection(AuditEntity.revisionNumber().min())
.add(AuditEntity.property("actualDate").minimize()
.add(AuditEntity.property("actualDate").ge(givenDate))
.add(AuditEntity.id().eq(givenEntityId)))
.getSingleResult();
AuditQuery query = getAuditReader().createQuery()
.forRevisionsOfEntity(MyEntity.class, false, true)
.add(AuditEntity.id().eq(id));
.add(AuditEntity.property("actualDate").hasChanged())
CHAPTER 1. HIBERNATE
33
The hasChanged condition can be combined with additional criteria. The query below will return a
horizontal slice for MyEntity at the time the revisionNumber was generated. It will be limited to the
revisions that modified prop1, but not prop2.
The result set will also contain revisions with numbers lower than the revisionNumber. This means
that this query cannot be read as "Return all MyEntities changed in revisionNumber with prop1
modified and prop2 untouched."
The query below shows how this result can be returned, using the
forEntitiesModifiedAtRevision query:
Example 1.7. Query Entities Modified in a Given Revision
The example below shows the basic query for entities modified in a given revision. It allows entity
names and corresponding Java classes changed in a specified revision to be retrieved:
There are a number of other queries that are also accessible from
org.hibernate.envers.CrossTypeRevisionChangesReader:
List<Object> findEntities(Number)
Returns snapshots of all audited entities changed (added, updated and removed) in a given
revision.Executes n+1 SQL queries, where n is a number of different entity classes modified
within the specified revision.
List<Object> findEntities(Number, RevisionType)
Returns snapshots of all audited entities changed (added, updated or removed) in a given revision
filtered by modification type. Executes n+1 SQL queries, where n is a number of different entity
classes modified within specified revision.
Map<RevisionType, List<Object>> findEntitiesGroupByRevisionType(Number)
Returns a map containing lists of entity snapshots grouped by modification operation (e.g.
addition, update and removal). Executes 3n+1 SQL queries, where n is a number of different
entity classes modified within specified revision.
Report a bug
AuditQuery query = getAuditReader().createQuery()
.forEntitiesAtRevision(MyEntity.class, revisionNumber)
.add(AuditEntity.property("prop1").hasChanged())
.add(AuditEntity.property("prop2").hasNotChanged());
AuditQuery query = getAuditReader().createQuery()
.forEntitiesModifiedAtRevision(MyEntity.class, revisionNumber)
.add(AuditEntity.property("prop1").hasChanged())
.add(AuditEntity.property("prop2").hasNotChanged());
Set<Pair<String, Class>> modifiedEntityTypes = getAuditReader()
.getCrossTypeRevisionChangesReader().findEntityTypes(revisionNumber);
Hibernate Core Reference Guide
34
CHAPTER 2. USING HIBERNATE
2.1. INTRODUCTION TO USING HIBERNATE
Working with object-oriented software and a relational database can be cumbersome and time
consuming in today's enterprise environments. Hibernate is an object/relational mapping tool for Java
environments. The term object/relational mapping (ORM) refers to the technique of mapping a data
representation from an object model to a relational data model with a SQL-based schema.
Hibernate not only takes care of the mapping from Java classes to database tables (and from Java data
types to SQL data types), but also provides data query and retrieval facilities. It can also significantly
reduce development time otherwise spent with manual data handling in SQL and JDBC.
Hibernate's goal is to relieve the developer from 95 percent of common data persistence related
programming tasks. Hibernate may not be the best solution for data-centric applications that only use
stored-procedures to implement the business logic in the database, it is most useful with object-oriented
domain models and business logic in the Java-based middle-tier. However, Hibernate can certainly help
you to remove or encapsulate vendor-specific SQL code and will help with the common task of result set
translation from a tabular representation to a graph of objects.
If you are new to Hibernate and Object/Relational Mapping or even Java, please follow these steps:
Read the Hibernate Tutorial for a tutorial with step-by-step instructions. The source code for the
tutorial is included in the distribution in the doc/reference/tutorial/ directory.
Read the Architecture chapter to understand the environments where Hibernate can be used.
View the eg/ directory in the Hibernate distribution. It contains a simple standalone application.
Copy your JDBC driver to the lib/ directory and edit etc/hibernate.properties,
specifying the correct values for your database. From a command prompt in the distribution
directory, type ant eg (using Ant), or under Windows, type build eg.
Use this reference documentation as your primary source of information. Consider reading Java
Persistence with Hibernate (http://www.manning.com/bauer2) if you need more help with
application design, or if you prefer a step-by-step tutorial. Also visit
http://caveatemptor.hibernate.org and download the example application for Java Persistence
with Hibernate.
FAQs are answered on the Hibernate website.
Third party demos, examples, and tutorials are linked on the Hibernate website.
The Community Area on the Hibernate website is a good resource for design patterns and
various integration solutions (Tomcat, JBoss Enterprise Application Platform, Struts, EJB, etc.).
If you have questions, use the user forum linked on the Hibernate website. We also provide a JIRA issue
tracking system for bug reports and feature requests. If you are interested in the development of
Hibernate, join the developer mailing list. If you are interested in translating this documentation into your
language, contact us on the developer mailing list.
Commercial development support, production support, and training for Hibernate is available through
JBoss Inc. (see http://www.hibernate.org/SupportTraining/). Hibernate is a Professional Open Source
project and a critical component of the JBoss Enterprise Middleware System (JEMS) suite of products.
Report a bug
CHAPTER 2. USING HIBERNATE
35
2.2. HIBERNATE TUTORIAL
Intended for new users, this chapter provides an step-by-step introduction to Hibernate, starting with a
simple application using an in-memory database. The tutorial is based on an earlier tutorial developed by
Michael Gloegl. All code is contained in the tutorials/web directory of the project source.
IMPORTANT
This tutorial expects the user have knowledge of both Java and SQL. If you have a limited
knowledge of JAVA or SQL, it is advised that you start with a good introduction to that
technology prior to attempting to learn Hibernate.
NOTE
The distribution contains another example application under the tutorial/eg project
source directory.
Report a bug
2.3. YOUR FIRST HIBERNATE APPLICATION
2.3.1. About the Hibernate Application Tutorial
For this example, we will set up a small database application that can store events we want to attend and
information about the host(s) of these events.
NOTE
Although you can use whatever database you feel comfortable using, we will use
HSQLDB (an in-memory, Java database) to avoid describing installation/setup of any
particular database servers.
Report a bug
2.3.2. Set Up Your Hibernate Application
The first thing we need to do is to set up the development environment. We will be using the "standard
layout" advocated by alot of build tools such as Maven. Maven, in particular, has a good resource
describing this layout. As this tutorial is to be a web application, we will be creating and making use of
src/main/java, src/main/resources and src/main/webapp directories.
We will be using Maven in this tutorial, taking advantage of its transitive dependency management
capabilities as well as the ability of many IDEs to automatically set up a project for us based on the
maven descriptor.
<project xmlns="http://maven.apache.org/POM/4.0.0"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://
maven.apache.org/xsd/maven-4.0.0.xsd">
<modelVersion>4.0.0</modelVersion>
<groupId>org.hibernate.tutorials</groupId>
<artifactId>hibernate-tutorial</artifactId>
Hibernate Core Reference Guide
36
<version>1.0.0-SNAPSHOT</version>
<packaging>war</packaging>
<name>First Hibernate Tutorial</name>
<build>
<!--we dont want the version to be part of the generated war file name-->
<finalName>${artifactId}</finalName>
<!--we dont want to use the jars maven provided, we want to use JBoss'
ones-->
<plugins>
<plugin>
<artifactId>maven-war-plugin</artifactId>
<configuration>
<packagingExcludes>WEB-INF/lib/*.jar</packagingExcludes>
</configuration>
</plugin>
</plugins>
</build>
<dependencies>
<dependency>
<groupId>org.hibernate</groupId>
<artifactId>hibernate-core</artifactId>
<version>3.3.2.GA_CP03</version>
<!-- please check the release notes for the correct version you're using -
->
</dependency>
<!-- Because this is a web app, we also have a dependency on the servlet
api. -->
<dependency>
<groupId>javax.servlet</groupId>
<artifactId>servlet-api</artifactId>
<version>2.5</version>
</dependency>
<!-- Hibernate uses slf4j for logging, for our purposes here use the simple
backend -->
<dependency>
<groupId>org.slf4j</groupId>
<artifactId>slf4j-simple</artifactId>
<version>1.5.8</version>
</dependency>
<!-- Hibernate gives you a choice of bytecode providers between cglib and
javassist -->
<dependency>
<groupId>javassist</groupId>
<artifactId>javassist</artifactId>
<version>3.12.0.GA</version>
</dependency>
</dependencies>
</project>
CHAPTER 2. USING HIBERNATE
37
NOTE
It is not a requirement to use Maven. If you wish to use another technology to build this
tutoial (such as Ant), the layout will remain the same. The only change is that you will
need to manually account for all the needed dependencies. If you use Ivy to provide
transitive dependency management you would still use the dependencies mentioned
below. Otherwise, you will need to find all the dependencies, both explicit and transitive,
and add them to the projects classpath. If working from the Hibernate distribution bundle,
this would mean hibernate3.jar, all artifacts in the lib/required directory and all
files from either the lib/bytecode/cglib or lib/bytecode/javassist directory;
additionally you will need both the servlet-api jar and one of the slf4j logging backends.
Save this file as pom.xml in the project root directory.
Report a bug
2.3.3. Create the First Class
Next, we create a class that represents the event we want to store in the database; it is a simple
JavaBean class with some properties:
package org.hibernate.tutorial.domain;
import java.util.Date;
public class Event {
private Long id;
private String title;
private Date date;
public Event() {}
public Long getId() {
return id;
}
private void setId(Long id) {
this.id = id;
}
public Date getDate() {
return date;
}
public void setDate(Date date) {
this.date = date;
}
public String getTitle() {
return title;
}
public void setTitle(String title) {
Hibernate Core Reference Guide
38
This class uses standard JavaBean naming conventions for property getter and setter methods, as well
as private visibility for the fields. Although this is the recommended design, it is not required. Hibernate
can also access fields directly, the benefit of accessor methods is robustness for refactoring.
The id property holds a unique identifier value for a particular event. All persistent entity classes (there
are less important dependent classes as well) will need such an identifier property if we want to use the
full feature set of Hibernate. In fact, most applications, especially web applications, need to distinguish
objects by identifier, so you should consider this a feature rather than a limitation. However, we usually
do not manipulate the identity of an object, hence the setter method should be private. Only Hibernate
will assign identifiers when an object is saved. Hibernate can access public, private, and protected
accessor methods, as well as public, private and protected fields directly. The choice is up to you and
you can match it to fit your application design.
The no-argument constructor is a requirement for all persistent classes; Hibernate has to create objects
for you, using Java Reflection. The constructor can be private, however package or public visibility is
required for runtime proxy generation and efficient data retrieval without bytecode instrumentation.
Save this file to the src/main/java/org/hibernate/tutorial/domain directory.
Report a bug
2.3.4. The Mapping File
Hibernate needs to know how to load and store objects of the persistent class. This is where the
Hibernate mapping file comes into play. The mapping file tells Hibernate what table in the database it
has to access, and what columns in that table it should use.
The basic structure of a mapping file looks like this:
Hibernate DTD is sophisticated. You can use it for auto-completion of XML mapping elements and
attributes in your editor or IDE. Opening up the DTD file in your text editor is the easiest way to get an
overview of all elements and attributes, and to view the defaults, as well as some comments. Hibernate
will not load the DTD file from the web, but first look it up from the classpath of the application. The DTD
file is included in hibernate-core.jar (it is also included in the hibernate3.jar, if using the
distribution bundle).
IMPORTANT
We will omit the DTD declaration in future examples to shorten the code. It is, of course,
not optional.
this.title = title;
}
}
<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
"-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping package="org.hibernate.tutorial.domain">
[...]
</hibernate-mapping>
CHAPTER 2. USING HIBERNATE
39
Between the two hibernate-mapping tags, include a class element. All persistent entity classes
(again, there might be dependent classes later on, which are not first-class entities) need a mapping to a
table in the SQL database:
So far we have told Hibernate how to persist and load object of class Event to the table EVENTS. Each
instance is now represented by a row in that table. Now we can continue by mapping the unique
identifier property to the tables primary key. As we do not want to care about handling this identifier, we
configure Hibernate's identifier generation strategy for a surrogate primary key column:
The id element is the declaration of the identifier property. The name="id" mapping attribute declares
the name of the JavaBean property and tells Hibernate to use the getId() and setId() methods to
access the property. The column attribute tells Hibernate which column of the EVENTS table holds the
primary key value.
The nested generator element specifies the identifier generation strategy (aka how are identifier
values generated?). In this case we choose native, which offers a level of portability depending on the
configured database dialect. Hibernate supports database generated, globally unique, as well as
application assigned, identifiers. Identifier value generation is also one of Hibernate's many extension
points and you can plugin in your own strategy.
NOTE
native is no longer considered the best strategy in terms of portability.
Lastly, we need to tell Hibernate about the remaining entity class properties. By default, no properties of
the class are considered persistent:
<hibernate-mapping package="org.hibernate.tutorial.domain">
<class name="Event" table="EVENTS">
</class>
</hibernate-mapping>
<hibernate-mapping package="org.hibernate.tutorial.domain">
<class name="Event" table="EVENTS">
<id name="id" column="EVENT_ID">
<generator class="native"/>
</id>
</class>
</hibernate-mapping>
<hibernate-mapping package="org.hibernate.tutorial.domain">
<class name="Event" table="EVENTS">
<id name="id" column="EVENT_ID">
<generator class="native"/>
</id>
<property name="date" type="timestamp" column="EVENT_DATE"/>
<property name="title"/>
Hibernate Core Reference Guide
40
Similar to the id element, the name attribute of the property element tells Hibernate which getter and
setter methods to use. In this case, Hibernate will search for getDate(), setDate(), getTitle()
and setTitle() methods.
NOTE
Why does the date property mapping include the column attribute, but the title does
not? Without the column attribute, Hibernate by default uses the property name as the
column name. This works for title, however, date is a reserved keyword in most
databases so you will need to map it to a different name.
The title mapping also lacks a type attribute. The types declared and used in the mapping files are
not Java data types; they are not SQL database types either. These types are called Hibernate mapping
types, converters which can translate from Java to SQL data types and vice versa. Again, Hibernate will
try to determine the correct conversion and mapping type itself if the type attribute is not present in the
mapping. In some cases this automatic detection using Reflection on the Java class might not have the
default you expect or need. This is the case with the date property. Hibernate cannot know if the
property, which is of java.util.Date, should map to a SQL date, timestamp, or time column. Full
date and time information is preserved by mapping the property with a timestamp converter.
NOTE
Hibernate makes this mapping type determination using reflection when the mapping files
are processed. This can take time and resources, so if startup performance is important
you should consider explicitly defining the type to use.
Save this mapping file as
src/main/resources/org/hibernate/tutorial/domain/Event.hbm.xml.
Report a bug
2.3.5. Hibernate Configuration
At this point, you should have the persistent class and its mapping file in place. It is now time to configure
Hibernate. First let's set up HSQLDB to run in "server mode"
NOTE
We do this do that the data remains between runs.
We will utilize the Maven exec plugin to launch the HSQLDB server by running: mvn exec:java -
Dexec.mainClass="org.hsqldb.Server" -Dexec.args="-database.0
file:target/data/tutorial" You will see it start up and bind to a TCP/IP socket; this is where our
application will connect later. If you want to start with a fresh database during this tutorial, shutdown
HSQLDB, delete all files in the target/data directory, and start HSQLDB again.
Hibernate will be connecting to the database on behalf of your application, so it needs to know how to
obtain connections. For this tutorial we will be using a standalone connection pool (as opposed to a
</class>
</hibernate-mapping>
CHAPTER 2. USING HIBERNATE
41
javax.sql.DataSource). Hibernate comes with support for two third-party open source JDBC
connection pools: c3p0 and proxool. However, we will be using the Hibernate built-in connection pool for
this tutorial.
WARNING
The built-in Hibernate connection pool is not intended for production use.
For Hibernate's configuration, we can use a simple hibernate.properties file, a more sophisticated
hibernate.cfg.xml file, or even complete programmatic setup. Most users prefer the XML
configuration file:
<?xml version='1.0' encoding='utf-8'?>
<!DOCTYPE hibernate-configuration PUBLIC
"-//Hibernate/Hibernate Configuration DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-configuration-
3.0.dtd">
<hibernate-configuration>
<session-factory>
<!-- Database connection settings -->
<property
name="connection.driver_class">org.hsqldb.jdbcDriver</property>
<property
name="connection.url">jdbc:hsqldb:hsql://localhost</property>
<property name="connection.username">sa</property>
<property name="connection.password"></property>
<!-- JDBC connection pool (use the built-in) -->
<property name="connection.pool_size">1</property>
<!-- SQL dialect -->
<property
name="dialect">org.hibernate.dialect.HSQLDialect</property>
<!-- Enable Hibernate's automatic session context management -->
<property name="current_session_context_class">thread</property>
<!-- Disable the second-level cache -->
<property
name="cache.provider_class">org.hibernate.cache.NoCacheProvider</property>
<!-- Echo all executed SQL to stdout -->
<property name="show_sql">true</property>
<!-- Drop and re-create the database schema on startup -->
<property name="hbm2ddl.auto">update</property>
Hibernate Core Reference Guide
42
NOTE
Notice that this configuration file specifies a different DTD
You configure Hibernate's SessionFactory. SessionFactory is a global factory responsible for a
particular database. If you have several databases, for easier startup you should use several
<session-factory> configurations in several configuration files.
The first four property elements contain the necessary configuration for the JDBC connection. The
dialect property element specifies the particular SQL variant Hibernate generates.
NOTE
Hibernate is able to correctly determine which dialect to use in most cases.
Hibernate's automatic session management for persistence contexts is particularly useful in this context.
The hbm2ddl.auto option turns on automatic generation of database schemas directly into the
database. This can also be turned off by removing the configuration option, or redirected to a file with the
help of the SchemaExport Ant task. Finally, add the mapping file(s) for persistent classes to the
configuration.
Save this file as hibernate.cfg.xml into the src/main/resources directory.
Report a bug
2.3.6. Building with Maven
We will now build the tutorial with Maven. You will need to have Maven installed; it is available from the
Maven download page. Maven will read the /pom.xml file we created earlier and know how to perform
some basic project tasks. First, lets run the compile goal to make sure we can compile everything so
far:
<mapping resource="org/hibernate/tutorial/domain/Event.hbm.xml"/>
</session-factory>
</hibernate-configuration>
[hibernateTutorial]$ mvn compile
[INFO] Scanning for projects...
[INFO] ---------------------------------------------------------------
---------
[INFO] Building First Hibernate Tutorial
[INFO] task-segment: [compile]
[INFO] ---------------------------------------------------------------
---------
[INFO] [resources:resources]
[INFO] Using default encoding to copy filtered resources.
[INFO] [compiler:compile]
[INFO] Compiling 1 source file to
/home/steve/projects/sandbox/hibernateTutorial/target/classes
[INFO] ---------------------------------------------------------------
---------
CHAPTER 2. USING HIBERNATE
43
Report a bug
2.3.7. Startup and Helpers
It is time to load and store some Event objects, but first you have to complete the setup with some
infrastructure code. You have to startup Hibernate by building a global
org.hibernate.SessionFactory object and storing it somewhere for easy access in application
code. A org.hibernate.SessionFactory is used to obtain org.hibernate.Session instances.
A org.hibernate.Session represents a single-threaded unit of work. The
org.hibernate.SessionFactory is a thread-safe global object that is instantiated once.
We will create a HibernateUtil helper class that takes care of startup and makes accessing the
org.hibernate.SessionFactory more convenient.
Save this code as src/main/java/org/hibernate/tutorial/util/HibernateUtil.java
This class not only produces the global org.hibernate.SessionFactory reference in its static
[INFO] BUILD SUCCESSFUL
[INFO] ---------------------------------------------------------------
---------
[INFO] Total time: 2 seconds
[INFO] Finished at: Tue Jun 09 12:25:25 CDT 2009
[INFO] Final Memory: 5M/547M
[INFO] ---------------------------------------------------------------
---------
package org.hibernate.tutorial.util;
import org.hibernate.SessionFactory;
import org.hibernate.cfg.Configuration;
public class HibernateUtil {
private static final SessionFactory sessionFactory =
buildSessionFactory();
private static SessionFactory buildSessionFactory() {
try {
// Create the SessionFactory from hibernate.cfg.xml
return new Configuration().configure().buildSessionFactory();
}
catch (Throwable ex) {
// Make sure you log the exception, as it might be swallowed
System.err.println("Initial SessionFactory creation failed." +
ex);
throw new ExceptionInInitializerError(ex);
}
}
public static SessionFactory getSessionFactory() {
return sessionFactory;
}
}
Hibernate Core Reference Guide
44
initializer; it also hides the fact that it uses a static singleton. We might just as well have looked up the
org.hibernate.SessionFactory reference from JNDI in an application server or any other location
for that matter.
If you give the org.hibernate.SessionFactory a name in your configuration, Hibernate will try to
bind it to JNDI under that name after it has been built. Another, better option is to use a JMX deployment
and let the JMX-capable container instantiate and bind a HibernateService to JNDI. Such advanced
options are discussed later.
You now need to configure a logging system. Hibernate uses commons logging and provides two
choices: Log4j and JDK 1.4 logging. Most developers prefer Log4j: copy log4j.properties from the
Hibernate distribution in the etc/ directory to your src directory, next to hibernate.cfg.xml. If you
prefer to have more verbose output than that provided in the example configuration, you can change the
settings. By default, only the Hibernate startup message is shown on stdout.
The tutorial infrastructure is complete and you are now ready to do some real work with Hibernate.
Report a bug
2.3.8. Loading and Storing Objects
We are now ready to start doing some real worjk with Hibernate. Let's start by writing an EventManager
class with a main() method:
package org.hibernate.tutorial;
import org.hibernate.Session;
import java.util.*;
import org.hibernate.tutorial.domain.Event;
import org.hibernate.tutorial.util.HibernateUtil;
public class EventManager {
public static void main(String[] args) {
EventManager mgr = new EventManager();
if (args[0].equals("store")) {
mgr.createAndStoreEvent("My Event", new Date());
}
HibernateUtil.getSessionFactory().close();
}
private void createAndStoreEvent(String title, Date theDate) {
Session session =
HibernateUtil.getSessionFactory().getCurrentSession();
session.beginTransaction();
Event theEvent = new Event();
theEvent.setTitle(title);
theEvent.setDate(theDate);
session.save(theEvent);
CHAPTER 2. USING HIBERNATE
45
In createAndStoreEvent() we created a new Event object and handed it over to Hibernate. At that
point, Hibernate takes care of the SQL and executes an INSERT on the database.
A org.hibernate.Session is designed to represent a single unit of work (a single atmoic piece of work to
be performed). For now we will keep things simple and assume a one-to-one granularity between a
Hibernate org.hibernate.Session and a database transaction. To shield our code from the actual
underlying transaction system we use the Hibernate org.hibernate.Transaction API. In this
particular case we are using JDBC-based transactional semantics, but it could also run with JTA.
What does sessionFactory.getCurrentSession() do? First, you can call it as many times and
anywhere you like once you get hold of your org.hibernate.SessionFactory. The
getCurrentSession() method always returns the "current" unit of work. Remember that we switched
the configuration option for this mechanism to "thread" in our
src/main/resources/hibernate.cfg.xml? Due to that setting, the context of a current unit of
work is bound to the current Java thread that executes the application.
IMPORTANT
Hibernate offers three methods of current session tracking. The "thread" based method is
not intended for production use; it is merely useful for prototyping and tutorials such as
this one. Current session tracking is discussed in more detail later on.
A org.hibernate.Session begins when the first call to getCurrentSession() is made for the current
thread. It is then bound by Hibernate to the current thread. When the transaction ends, either through
commit or rollback, Hibernate automatically unbinds the org.hibernate.Session from the thread and
closes it for you. If you call getCurrentSession() again, you get a new org.hibernate.Session and
can start a new unit of work.
Related to the unit of work scope, should the Hibernate org.hibernate.Session be used to execute one or
several database operations? The above example uses one org.hibernate.Session for one operation.
However this is pure coincidence; the example is just not complex enough to show any other approach.
The scope of a Hibernate org.hibernate.Session is flexible but you should never design your application
to use a new Hibernate org.hibernate.Session for every database operation. Even though it is used in
the following examples, consider session-per-operation an anti-pattern. A real web application is shown
later in the tutorial which will help illustrate this.
See the "Transactions and Concurrency" Chapter for more information about transaction handling and
demarcation. The previous example also skipped any error handling and rollback.
To run this, we will make use of the Maven exec plugin to call our class with the necessary classpath
setup: mvn exec:java -Dexec.mainClass="org.hibernate.tutorial.EventManager" -
Dexec.args="store"
NOTE
You may need to perform mvn compile first.
You should see Hibernate starting up and, depending on your configuration, lots of log output. Towards
the end, the following line will be displayed:
session.getTransaction().commit();
}
}
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46
This is the INSERT executed by Hibernate.
To list stored events an option is added to the main method:
A new listEvents() method is also added:
Here, we are using a Hibernate Query Language (HQL) query to load all existing Event objects from the
database. Hibernate will generate the appropriate SQL, send it to the database and populate Event
objects with the data. You can create more complex queries with HQL. See the Hibernate Query
Language chapter for further information.
Now we can call our new functionality, again using the Maven exec plugin: mvn exec:java -
Dexec.mainClass="org.hibernate.tutorial.EventManager" -Dexec.args="list"
Report a bug
2.4. MAPPING ASSOCIATIONS
2.4.1. About Mapping Associations
So far we have mapped a single persistent entity class to a table in isolation. Let's expand on that a bit
and add some class associations. We will add people to the application and store a list of events in which
they participate.
Report a bug
2.4.2. Mapping the Person Class
[java] Hibernate: insert into EVENTS (EVENT_DATE, title, EVENT_ID) values
(?, ?, ?)
if (args[0].equals("store")) {
mgr.createAndStoreEvent("My Event", new Date());
}
else if (args[0].equals("list")) {
List events = mgr.listEvents();
for (int i = 0; i < events.size(); i++) {
Event theEvent = (Event) events.get(i);
System.out.println(
"Event: " + theEvent.getTitle() + " Time: " +
theEvent.getDate()
);
}
}
private List listEvents() {
Session session =
HibernateUtil.getSessionFactory().getCurrentSession();
session.beginTransaction();
List result = session.createQuery("from Event").list();
session.getTransaction().commit();
return result;
}
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The first cut of the Person class looks like this:
Save this to a file named src/main/java/org/hibernate/tutorial/domain/Person.java
Next, create the new mapping file as
src/main/resources/org/hibernate/tutorial/domain/Person.hbm.xml
Finally, add the new mapping to Hibernate's configuration immediately after the existing mapping for
Event.hbm.xml:
Create an association between these two entities. Persons can participate in events, and events have
participants. The design questions you have to deal with are: directionality, multiplicity, and collection
behavior.
Report a bug
2.4.3. A Unidirectional Set-based Association
By adding a collection of events to the Person class, you can easily navigate to the events for a
particular person, without executing an explicit query - by calling Person#getEvents. Multi-valued
associations are represented in Hibernate by one of the Java Collection Framework contracts; here we
choose a java.util.Set because the collection will not contain duplicate elements and the ordering is
not relevant to our examples:
package org.hibernate.tutorial.domain;
public class Person {
private Long id;
private int age;
private String firstname;
private String lastname;
public Person() {}
// Accessor methods for all properties, private setter for 'id'
}
<hibernate-mapping package="org.hibernate.tutorial.domain">
<class name="Person" table="PERSON">
<id name="id" column="PERSON_ID">
<generator class="native"/>
</id>
<property name="age"/>
<property name="firstname"/>
<property name="lastname"/>
</class>
</hibernate-mapping>
<mapping resource="events/Event.hbm.xml"/>
<mapping resource="events/Person.hbm.xml"/>
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Before mapping this association, let's consider the other side. We could just keep this unidirectional or
create another collection on the Event, if we wanted to be able to navigate it from both directions. This
is not necessary, from a functional perspective. You can always execute an explicit query to retrieve the
participants for a particular event. This is a design choice left to you, but what is clear from this
discussion is the multiplicity of the association: "many" valued on both sides is called a many-to-many
association. Hence, we use Hibernate's many-to-many mapping:
Hibernate supports a broad range of collection mappings, a set being most common. For a many-to-
many association, or n:m entity relationship, an association table is required. Each row in this table
represents a link between a person and an event. The table name is decalred using the table attribute
of the set element. The identifier column name in the association, for the person side, is defined with
the key element, the column name for the event's side with the column attribute of the many-to-many.
You also have to tell Hibernate the class of the objects in your collection (the class on the other side of
the collection of references).
The database schema for this mapping is therefore:
public class Person {
private Set events = new HashSet();
public Set getEvents() {
return events;
}
public void setEvents(Set events) {
this.events = events;
}
}
<class name="Person" table="PERSON">
<id name="id" column="PERSON_ID">
<generator class="native"/>
</id>
<property name="age"/>
<property name="firstname"/>
<property name="lastname"/>
<set name="events" table="PERSON_EVENT">
<key column="PERSON_ID"/>
<many-to-many column="EVENT_ID" class="Event"/>
</set>
</class>
_____________ __________________
| | | | _____________
| EVENTS | | PERSON_EVENT | | |
|_____________| |__________________| | PERSON |
| | | | |_____________|
| *EVENT_ID | <--> | *EVENT_ID | | |
| EVENT_DATE | | *PERSON_ID | <--> | *PERSON_ID |
| TITLE | |__________________| | AGE |
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49
Report a bug
2.4.4. Working the Association
Now we will bring some people and events together in a new method in EventManager:
After loading a Person and an Event, simply modify the collection using the normal collection methods.
There is no explicit call to update() or save(); Hibernate automatically detects that the collection has
been modified and needs to be updated. This is called automatic dirty checking. You can also try it by
modifying the name or the date property of any of your objects. As long as they are in persistent state,
that is, bound to a particular Hibernate org.hibernate.Session, Hibernate monitors any changes
and executes SQL in a write-behind fashion. The process of synchronizing the memory state with the
database, usually only at the end of a unit of work, is called flushing. In our code, the unit of work ends
with a commit, or rollback, of the database transaction.
You can load person and event in different units of work. Or you can modify an object outside of a
org.hibernate.Session, when it is not in persistent state (if it was persistent before, this state is
called detached). You can even modify a collection when it is detached:
|_____________| | FIRSTNAME |
| LASTNAME |
|_____________|
private void addPersonToEvent(Long personId, Long eventId) {
Session session =
HibernateUtil.getSessionFactory().getCurrentSession();
session.beginTransaction();
Person aPerson = (Person) session.load(Person.class, personId);
Event anEvent = (Event) session.load(Event.class, eventId);
aPerson.getEvents().add(anEvent);
session.getTransaction().commit();
}
private void addPersonToEvent(Long personId, Long eventId) {
Session session =
HibernateUtil.getSessionFactory().getCurrentSession();
session.beginTransaction();
Person aPerson = (Person) session
.createQuery("select p from Person p left join fetch
p.events where p.id = :pid")
.setParameter("pid", personId)
.uniqueResult(); // Eager fetch the collection so we can
use it detached
Event anEvent = (Event) session.load(Event.class, eventId);
session.getTransaction().commit();
// End of first unit of work
aPerson.getEvents().add(anEvent); // aPerson (and its collection)
is detached
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The call to update makes a detached object persistent again by binding it to a new unit of work, so any
modifications you made to it while detached can be saved to the database. This includes any
modifications (additions/deletions) you made to a collection of that entity object.
This is not much use in our example, but it is an important concept you can incorporate into your own
application. Complete this exercise by adding a new action to the main method of the EventManager
and call it from the command line. If you need the identifiers of a person and an event - the save()
method returns it (you might have to modify some of the previous methods to return that identifier):
This is an example of an association between two equally important classes : two entities. As mentioned
earlier, there are other classes and types in a typical model, usually "less important". Some you have
already seen, like an int or a java.lang.String. We call these classes value types, and their
instances depend on a particular entity. Instances of these types do not have their own identity, nor are
they shared between entities. Two persons do not reference the same firstname object, even if they
have the same first name. Value types cannot only be found in the JDK , but you can also write
dependent classes yourself such as an Address or MonetaryAmount class. In fact, in a Hibernate
application all JDK classes are considered value types.
You can also design a collection of value types. This is conceptually different from a collection of
references to other entities, but looks almost the same in Java.
Report a bug
2.4.5. Collection of Values
Let's add a collection of email addresses to the Person entity. This will be represented as a
java.util.Set of java.lang.String instances:
// Begin second unit of work
Session session2 =
HibernateUtil.getSessionFactory().getCurrentSession();
session2.beginTransaction();
session2.update(aPerson); // Reattachment of aPerson
session2.getTransaction().commit();
}
else if (args[0].equals("addpersontoevent")) {
Long eventId = mgr.createAndStoreEvent("My Event", new
Date());
Long personId = mgr.createAndStorePerson("Foo", "Bar");
mgr.addPersonToEvent(personId, eventId);
System.out.println("Added person " + personId + " to event " +
eventId);
}
private Set emailAddresses = new HashSet();
public Set getEmailAddresses() {
return emailAddresses;
}
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The mapping of this Set is as follows:
The difference compared with the earlier mapping is the use of the element part which tells Hibernate
that the collection does not contain references to another entity, but is rather a collection whose elements
are values types, here specifically of type string. The lowercase name tells you it is a Hibernate
mapping type/converter. Again the table attribute of the set element determines the table name for the
collection. The key element defines the foreign-key column name in the collection table. The column
attribute in the element element defines the column name where the email address values will actually
be stored.
Here is the updated schema:
You can see that the primary key of the collection table is in fact a composite key that uses both
columns. This also implies that there cannot be duplicate email addresses per person, which is exactly
the semantics we need for a set in Java.
You can now try to add elements to this collection, just like we did before by linking persons and events.
It is the same code in Java:
public void setEmailAddresses(Set emailAddresses) {
this.emailAddresses = emailAddresses;
}
<set name="emailAddresses" table="PERSON_EMAIL_ADDR">
<key column="PERSON_ID"/>
<element type="string" column="EMAIL_ADDR"/>
</set>
_____________ __________________
| | | | _____________
| EVENTS | | PERSON_EVENT | | |
___________________
|_____________| |__________________| | PERSON | |
|
| | | | |_____________| |
PERSON_EMAIL_ADDR |
| *EVENT_ID | <--> | *EVENT_ID | | |
|___________________|
| EVENT_DATE | | *PERSON_ID | <--> | *PERSON_ID | <--> |
*PERSON_ID |
| TITLE | |__________________| | AGE | |
*EMAIL_ADDR |
|_____________| | FIRSTNAME |
|___________________|
| LASTNAME |
|_____________|
private void addEmailToPerson(Long personId, String emailAddress) {
Session session =
HibernateUtil.getSessionFactory().getCurrentSession();
session.beginTransaction();
Person aPerson = (Person) session.load(Person.class, personId);
// adding to the emailAddress collection might trigger a lazy load
of the collection
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This time we did not use a fetch query to initialize the collection. Monitor the SQL log and try to optimize
this with an eager fetch.
Report a bug
2.4.6. Bi-directional Associations
Next you will map a bi-directional association. You will make the association between person and event
work from both sides in Java. The database schema does not change, so you will still have many-to-
many multiplicity.
NOTE
A relational database is more flexible than a network programming language, in that it
does not need a navigation direction; data can be viewed and retrieved in any possible
way.
First, add a collection of participants to the Event class:
Now map this side of the association in Event.hbm.xml.
These are normal set mappings in both mapping documents. Notice that the column names in key and
many-to-many swap in both mapping documents. The most important addition here is the
inverse="true" attribute in the set element of the Event's collection mapping.
What this means is that Hibernate should take the other side, the Person class, when it needs to find
out information about the link between the two. This will be a lot easier to understand once you see how
the bi-directional link between our two entities is created.
Report a bug
2.4.7. Working Bi-directional Links
aPerson.getEmailAddresses().add(emailAddress);
session.getTransaction().commit();
}
private Set participants = new HashSet();
public Set getParticipants() {
return participants;
}
public void setParticipants(Set participants) {
this.participants = participants;
}
<set name="participants" table="PERSON_EVENT" inverse="true">
<key column="EVENT_ID"/>
<many-to-many column="PERSON_ID"
class="org.hibernate.tutorial.domain.Person"/>
</set>
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53
First, keep in mind that Hibernate does not affect normal Java semantics. How did we create a link
between a Person and an Event in the unidirectional example? You add an instance of Event to the
collection of event references, of an instance of Person. If you want to make this link bi-directional, you
have to do the same on the other side by adding a Person reference to the collection in an Event. This
process of "setting the link on both sides" is absolutely necessary with bi-directional links.
Many developers program defensively and create link management methods to correctly set both sides
(for example, in Person):
The get and set methods for the collection are now protected. This allows classes in the same package
and subclasses to still access the methods, but prevents everybody else from altering the collections
directly. Repeat the steps for the collection on the other side.
What about the inverse mapping attribute? For you, and for Java, a bi-directional link is simply a
matter of setting the references on both sides correctly. Hibernate, however, does not have enough
information to correctly arrange SQL INSERT and UPDATE statements (to avoid constraint violations).
Making one side of the association inverse tells Hibernate to consider it a mirror of the other side. That
is all that is necessary for Hibernate to resolve any issues that arise when transforming a directional
navigation model to a SQL database schema. The rules are straightforward: all bi-directional
associations need one side as inverse. In a one-to-many association it has to be the many-side, and in
many-to-many association you can select either side.
Report a bug
2.5. THE EVENTMANAGER WEB APPLICATION
2.5.1. About the EventManager
A Hibernate web application uses Session and Transaction almost like a standalone application.
However, some common patterns are useful. You can now write an EventManagerServlet. This
servlet can list all events stored in the database, and it provides an HTML form to enter new events.
Report a bug
2.5.2. Writing the Basic Servlet
protected Set getEvents() {
return events;
}
protected void setEvents(Set events) {
this.events = events;
}
public void addToEvent(Event event) {
this.getEvents().add(event);
event.getParticipants().add(this);
}
public void removeFromEvent(Event event) {
this.getEvents().remove(event);
event.getParticipants().remove(this);
}
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First we need create our basic processing servlet. Since our servlet only handles HTTP GET requests,
we will only implement the doGet() method:
Save this servlet as
src/main/java/org/hibernate/tutorial/web/EventManagerServlet.java
The pattern applied here is called session-per-request. When a request hits the servlet, a new Hibernate
Session is opened through the first call to getCurrentSession() on the SessionFactory. A
database transaction is then started. All data access occurs inside a transaction irrespective of whether
the data is read or written. Do not use the auto-commit mode in applications.
Do not use a new Hibernate Session for every database operation. Use one Hibernate Session that is
scoped to the whole request. Use getCurrentSession(), so that it is automatically bound to the
current Java thread.
package org.hibernate.tutorial.web;
// Imports
public class EventManagerServlet extends HttpServlet {
protected void doGet(
HttpServletRequest request,
HttpServletResponse response) throws ServletException,
IOException {
SimpleDateFormat dateFormatter = new SimpleDateFormat(
"dd.MM.yyyy" );
try {
// Begin unit of work
HibernateUtil.getSessionFactory().getCurrentSession().beginTransaction();
// Process request and render page...
// End unit of work
HibernateUtil.getSessionFactory().getCurrentSession().getTransaction().com
mit();
}
catch (Exception ex) {
HibernateUtil.getSessionFactory().getCurrentSession().getTransaction().rol
lback();
if ( ServletException.class.isInstance( ex ) ) {
throw ( ServletException ) ex;
}
else {
throw new ServletException( ex );
}
}
}
}
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55
Next, the possible actions of the request are processed and the response HTML is rendered. We will get
to that part soon.
Finally, the unit of work ends when processing and rendering are complete. If any problems occurred
during processing or rendering, an exception will be thrown and the database transaction rolled back.
This completes the session-per-request pattern. Instead of the transaction demarcation code in
every servlet, you could also write a servlet filter. See the Hibernate website and Wiki for more
information about this pattern called Open Session in View. You will need it as soon as you consider
rendering your view in JSP, not in a servlet.
Report a bug
2.5.3. Processing and Rendering
Now you can implement the processing of the request and the rendering of the page.
This coding style, with a mix of Java and HTML, would not scale in a more complex application—keep in
mind that we are only illustrating basic Hibernate concepts in this tutorial. The code prints an HTML
header and a footer. Inside this page, an HTML form for event entry and a list of all events in the
database are printed. The first method is trivial and only outputs HTML:
// Write HTML header
PrintWriter out = response.getWriter();
out.println("<html><head><title>Event Manager</title></head>
<body>");
// Handle actions
if ( "store".equals(request.getParameter("action")) ) {
String eventTitle = request.getParameter("eventTitle");
String eventDate = request.getParameter("eventDate");
if ( "".equals(eventTitle) || "".equals(eventDate) ) {
out.println("<b><i>Please enter event title and date.</i>
</b>");
}
else {
createAndStoreEvent(eventTitle,
dateFormatter.parse(eventDate));
out.println("<b><i>Added event.</i></b>");
}
}
// Print page
printEventForm(out);
listEvents(out, dateFormatter);
// Write HTML footer
out.println("</body></html>");
out.flush();
out.close();
private void printEventForm(PrintWriter out) {
out.println("<h2>Add new event:</h2>");
out.println("<form>");
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The listEvents() method uses the Hibernate Session bound to the current thread to execute a
query:
Finally, the store action is dispatched to the createAndStoreEvent() method, which also uses the
Session of the current thread:
The servlet is now complete. A request to the servlet will be processed in a single Session and
Transaction. As earlier in the standalone application, Hibernate can automatically bind these objects
to the current thread of execution. This gives you the freedom to layer your code and access the
SessionFactory in any way you like. Usually you would use a more sophisticated design and move
the data access code into data access objects (the DAO pattern). See the Hibernate Wiki for more
examples.
Report a bug
out.println("Title: <input name='eventTitle' length='50'/><br/>");
out.println("Date (e.g. 24.12.2009): <input name='eventDate'
length='10'/><br/>");
out.println("<input type='submit' name='action' value='store'/>");
out.println("</form>");
}
private void listEvents(PrintWriter out, SimpleDateFormat
dateFormatter) {
List result = HibernateUtil.getSessionFactory()
.getCurrentSession().createCriteria(Event.class).list();
if (result.size() > 0) {
out.println("<h2>Events in database:</h2>");
out.println("<table border='1'>");
out.println("<tr>");
out.println("<th>Event title</th>");
out.println("<th>Event date</th>");
out.println("</tr>");
Iterator it = result.iterator();
while (it.hasNext()) {
Event event = (Event) it.next();
out.println("<tr>");
out.println("<td>" + event.getTitle() + "</td>");
out.println("<td>" +
dateFormatter.format(event.getDate()) + "</td>");
out.println("</tr>");
}
out.println("</table>");
}
}
protected void createAndStoreEvent(String title, Date theDate) {
Event theEvent = new Event();
theEvent.setTitle(title);
theEvent.setDate(theDate);
HibernateUtil.getSessionFactory()
.getCurrentSession().save(theEvent);
}
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57
2.5.4. Deploying and Testing
To deploy this application for testing we must create a Web ARchive (WAR). First we must define the
WAR descriptor as src/main/webapp/WEB-INF/web.xml
To build and deploy call mvn package in your project directory and copy the hibernate-
tutorial.war file into your $JBOSS_HOME/server/$CONFIG/deploy directory.
Once deployed and JBoss is running, access the application at
http://localhost:8080/hibernate-tutorial/eventmanager. Watch the server log (in
$JBOSS_HOME/server/$CONFIG/log/server.log) to see Hibernate initialize when the first request
hits your servlet (the static initializer in HibernateUtil is called) and to get the detailed output if any
exceptions occurs.
Report a bug
2.5.5. Summary
This tutorial covered the basics of writing a simple standalone Hibernate application and a small web
application. More tutorials are available from the Hibernate website.
Report a bug
<?xml version="1.0" encoding="UTF-8"?>
<web-app version="2.4"
xmlns="http://java.sun.com/xml/ns/j2ee"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee
http://java.sun.com/xml/ns/j2ee/web-app_2_4.xsd">
<servlet>
<servlet-name>Event Manager</servlet-name>
<servlet-
class>org.hibernate.tutorial.web.EventManagerServlet</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>Event Manager</servlet-name>
<url-pattern>/eventmanager</url-pattern>
</servlet-mapping>
</web-app>
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CHAPTER 3. HIBERNATE ARCHITECTURE
3.1. OVERVIEW
The diagram below provides a high-level view of the Hibernate architecture:
We do not have the scope in this document to provide a more detailed view of all the runtime
architectures available; Hibernate is flexible and supports several different approaches. We will,
however, show the two extremes: "minimal" architecture and "comprehensive" architecture.
This next diagram illustrates how Hibernate utilizes database and configuration data to provide
persistence services, and persistent objects, to the application.
The "minimal" architecture has the application provide its own JDBC connections and manage its own
transactions. This approach uses a minimal subset of Hibernate's APIs:
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59
The "comprehensive" architecture abstracts the application away from the underlying JDBC/JTA APIs
and allows Hibernate to manage the details.
Here are some definitions of the objects depicted in the diagrams:
SessionFactory (org.hibernate.SessionFactory)
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SessionFactory (org.hibernate.SessionFactory)
A threadsafe, immutable cache of compiled mappings for a single database. A factory for Session
and a client of ConnectionProvider, SessionFactory can hold an optional (second-level)
cache of data that is reusable between transactions at a process, or cluster, level.
Session (org.hibernate.Session)
A single-threaded, short-lived object representing a conversation between the application and the
persistent store. It wraps a JDBC connection and is a factory for Transaction. Session holds a
mandatory first-level cache of persistent objects that are used when navigating the object graph or
looking up objects by identifier.
Persistent objects and collections
Short-lived, single threaded objects containing persistent state and business function. These can be
ordinary JavaBeans/POJOs. They are associated with exactly one Session. Once the Session is
closed, they will be detached and free to use in any application layer (for example, directly as data
transfer objects to and from presentation).
Transient and detached objects and collections
Instances of persistent classes that are not currently associated with a Session. They may have
been instantiated by the application and not yet persisted, or they may have been instantiated by a
closed Session.
Transaction (org.hibernate.Transaction)
(Optional) A single-threaded, short-lived object used by the application to specify atomic units of work.
It abstracts the application from the underlying JDBC, JTA or CORBA transaction. A Session might
span several Transactions in some cases. However, transaction demarcation, either using the
underlying API or Transaction, is never optional.
ConnectionProvider (org.hibernate.connection.ConnectionProvider)
(Optional) A factory for, and pool of, JDBC connections. It abstracts the application from underlying
Datasource or DriverManager. It is not exposed to application, but it can be extended and/or
implemented by the developer.
TransactionFactory (org.hibernate.TransactionFactory)
(Optional) A factory for Transaction instances. It is not exposed to the application, but it can be
extended and/or implemented by the developer.
Extension Interfaces
Hibernate offers a range of optional extension interfaces you can implement to customize the
behavior of your persistence layer. See the API documentation for details.
Given a "minimal" architecture, the application bypasses the Transaction/TransactionFactory
and/or ConnectionProvider APIs to communicate with JTA or JDBC directly.
Report a bug
3.2. INSTANCE STATES
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61
An instance of a persistent class can be in one of three different states. These states are defined in
relation to a persistence context. The Hibernate Session object is the persistence context. The three
different states are as follows:
transient
The instance is not associated with any persistence context. It has no persistent identity or primary
key value.
persistent
The instance is currently associated with a persistence context. It has a persistent identity (primary
key value) and can have a corresponding row in the database. For a particular persistence context,
Hibernate guarantees that persistent identity is equivalent to Java identity in relation to the in-memory
location of the object.
detached
The instance was once associated with a persistence context, but that context was closed, or the
instance was serialized to another process. It has a persistent identity and can have a corresponding
row in the database. For detached instances, Hibernate does not guarantee the relationship between
persistent identity and Java identity.
Report a bug
3.3. JMX INTEGRATION
JMX is the J2EE standard for the management of Java components. Hibernate can be managed via a
JMX standard service. AN MBean implementation is provided in the distribution:
org.hibernate.jmx.HibernateService.
For an example of how to deploy Hibernate as a JMX service on the JBoss Enterprise Application
Platform, please see the JBoss User Guide. JBoss Enterprise Application Platform also provides these
benefits if you deploy using JMX:
Session Management: the Hibernate Session's life cycle can be automatically bound to the
scope of a JTA transaction. This means that you no longer have to manually open and close the
Session; this becomes the job of a JBoss EJB interceptor. You also do not have to worry about
transaction demarcation in your code (if you would like to write a portable persistence layer use
the optional Hibernate Transaction API for this). You call the HibernateContext to access
a Session.
HAR deployment: the Hibernate JMX service is deployed using a JBoss service deployment
descriptor in an EAR and/or SAR file, as it supports all the usual configuration options of a
Hibernate SessionFactory. However, you still need to name all your mapping files in the
deployment descriptor. If you use the optional HAR deployment, JBoss will automatically detect
all mapping files in your HAR file.
Consult the JBoss Enterprise Application Platform user guide for more information about these options.
Report a bug
3.4. JCA SUPPORT
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Hibernate can also be configured as a JCA connector. Please see the website for more information.
Please note, however, that at this stage Hibernate JCA support is under development.
Report a bug
3.5. CONTEXTUAL SESSIONS
Most applications using Hibernate need some form of "contextual" session, where a given session is in
effect throughout the scope of a given context. However, across applications the definition of what
constitutes a context is typically different; different contexts define different scopes to the notion of
current. Applications using Hibernate prior to version 3.0 tended to utilize either home-grown
ThreadLocal-based contextual sessions, helper classes such as HibernateUtil, or utilized third-
party frameworks, such as Spring or Pico, which provided proxy/interception-based contextual sessions.
Starting with version 3.0.1, Hibernate added the SessionFactory.getCurrentSession() method.
Initially, this assumed usage of JTA transactions, where the JTA transaction defined both the scope and
context of a current session. Given the maturity of the numerous stand-alone JTA
TransactionManager implementations, most, if not all, applications should be using JTA transaction
management, whether or not they are deployed into a J2EE container. Based on that, the JTA-based
contextual sessions are all you need to use.
However, as of version 3.1, the processing behind SessionFactory.getCurrentSession() is now
pluggable. To that end, a new extension interface,
org.hibernate.context.CurrentSessionContext, and a new configuration parameter,
hibernate.current_session_context_class, have been added to allow pluggability of the scope
and context of defining current sessions.
See the Javadocs for the org.hibernate.context.CurrentSessionContext interface for a
detailed discussion of its contract. It defines a single method, currentSession(), by which the
implementation is responsible for tracking the current contextual session. Out-of-the-box, Hibernate
comes with three implementations of this interface:
org.hibernate.context.JTASessionContext: current sessions are tracked and scoped
by a JTA transaction. The processing here is exactly the same as in the older JTA-only
approach. See the Javadocs for details.
org.hibernate.context.ThreadLocalSessionContext:current sessions are tracked by
thread of execution. See the Javadocs for details.
org.hibernate.context.ManagedSessionContext: current sessions are tracked by
thread of execution. However, you are responsible to bind and unbind a Session instance with
static methods on this class: it does not open, flush, or close a Session.
The first two implementations provide a "one session - one database transaction" programming model.
This is also also known and used as session-per-request. The beginning and end of a Hibernate session
is defined by the duration of a database transaction. If you use programmatic transaction demarcation in
plain JSE without JTA, you are advised to use the Hibernate Transaction API to hide the underlying
transaction system from your code. If you use JTA, you can utilize the JTA interfaces to demarcate
transactions. If you execute in an EJB container that supports CMT, transaction boundaries are defined
declaratively and you do not need any transaction or session demarcation operations in your code. Refer
to the Transactions and Concurrency Chapter for more information and code examples.
The hibernate.current_session_context_class configuration parameter defines which
org.hibernate.context.CurrentSessionContext implementation should be used. For
backwards compatibility, if this configuration parameter is not set but a
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63
org.hibernate.transaction.TransactionManagerLookup is configured, Hibernate will use the
org.hibernate.context.JTASessionContext. Typically, the value of this parameter would just
name the implementation class to use. For the three out-of-the-box implementations, however, there are
three corresponding short names: "jta", "thread", and "managed".
Report a bug
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CHAPTER 4. PERSISTENT CLASSES
4.1. ABOUT PERSISTENT CLASSES
Persistent classes are classes in an application that implement the entities of the business problem (e.g.
Customer and Order in an E-commerce application). Not all instances of a persistent class are
considered to be in the persistent state. For example, an instance can instead be transient or detached.
Hibernate works best if these classes follow some simple rules, also known as the Plain Old Java Object
(POJO) programming model. However, none of these rules are hard requirements. Indeed, Hibernate3
assumes very little about the nature of your persistent objects. You can express a domain model in other
ways (using trees of Map instances, for example).
Report a bug
4.2. POJO EXAMPLE
4.2.1. A Simple POJO Example
Most Java applications require a persistent class representing felines. For example:
package eg;
import java.util.Set;
import java.util.Date;
public class Cat {
private Long id; // identifier
private Date birthdate;
private Color color;
private char sex;
private float weight;
private int litterId;
private Cat mother;
private Set kittens = new HashSet();
private void setId(Long id) {
this.id=id;
}
public Long getId() {
return id;
}
void setBirthdate(Date date) {
birthdate = date;
}
public Date getBirthdate() {
return birthdate;
}
void setWeight(float weight) {
this.weight = weight;
}
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65
The four main rules of persistent classes are explored in more detail in the following sections.
Report a bug
4.2.2. Implement a No-argument Constructor
Cat has a no-argument constructor. All persistent classes must have a default constructor (which can be
non-public) so that Hibernate can instantiate them using Constructor.newInstance(). It is
recommended that you have a default constructor with at least package visibility for runtime proxy
generation in Hibernate.
public float getWeight() {
return weight;
}
public Color getColor() {
return color;
}
void setColor(Color color) {
this.color = color;
}
void setSex(char sex) {
this.sex=sex;
}
public char getSex() {
return sex;
}
void setLitterId(int id) {
this.litterId = id;
}
public int getLitterId() {
return litterId;
}
void setMother(Cat mother) {
this.mother = mother;
}
public Cat getMother() {
return mother;
}
void setKittens(Set kittens) {
this.kittens = kittens;
}
public Set getKittens() {
return kittens;
}
// addKitten not needed by Hibernate
public void addKitten(Cat kitten) {
kitten.setMother(this);
kitten.setLitterId( kittens.size() );
kittens.add(kitten);
}
}
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Report a bug
4.2.3. Provide an Optional Identifier Property
Cat has a property called id. This property maps to the primary key column of a database table. The
property might have been called anything, and its type might have been any primitive type, any primitive
"wrapper" type, java.lang.String or java.util.Date. If your legacy database table has
composite keys, you can use a user-defined class with properties of these types (see the section on
composite identifiers later in the chapter.)
The identifier property is strictly optional. You can leave them off and let Hibernate keep track of object
identifiers internally. We do not recommend this, however.
In fact, some functionality is available only to classes that declare an identifier property:
Transitive reattachment for detached objects (cascade update or cascade merge).
Session.saveOrUpdate()
Session.merge()
We recommend that you declare consistently-named identifier properties on persistent classes and that
you use a nullable (i.e., non-primitive) type.
Report a bug
4.2.4. Prefer Optional Non-final Classes
A central feature of Hibernate, proxies, depends upon the persistent class being either non-final, or the
implementation of an interface that declares all public methods.
You can persist final classes that do not implement an interface with Hibernate. You will not, however,
be able to use proxies for lazy association fetching which will ultimately limit your options for
performance tuning.
You should also avoid declaring public final methods on the non-final classes. If you want to use a
class with a public final method, you must explicitly disable proxying by setting lazy="false".
Report a bug
4.2.5. Declare Optional Accessors and Mutators for Persistent Fields
Cat declares accessor methods for all its persistent fields. Many other ORM tools directly persist
instance variables. It is better to provide an indirection between the relational schema and internal data
structures of the class. By default, Hibernate persists JavaBeans style properties and recognizes method
names of the form getFoo, isFoo and setFoo. If required, you can switch to direct field access for
particular properties.
Properties need not be declared public - Hibernate can persist a property with a default, protected or
private get / set pair.
Report a bug
4.3. ADDITIONAL INFORMATION
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67
4.3.1. Implementing Inheritance
A subclass must also observe the first and second rules. It inherits its identifier property from the
superclass, Cat. For example:
Report a bug
4.3.2. Implementing equals() and hashCode()
You have to override the equals() and hashCode() methods if you:
intend to put instances of persistent classes in a Set (the recommended way to represent many-
valued associations); and
intend to use reattachment of detached instances
Hibernate guarantees equivalence of persistent identity (database row) and Java identity only inside a
particular session scope. When you mix instances retrieved in different sessions, you must implement
equals() and hashCode() if you wish to have meaningful semantics for Sets.
The most obvious way is to implement equals()/hashCode() by comparing the identifier value of both
objects. If the value is the same, both must be the same database row, because they are equal. If both
are added to a Set, you will only have one element in the Set). Unfortunately, you cannot use that
approach with generated identifiers. Hibernate will only assign identifier values to objects that are
persistent; a newly created instance will not have any identifier value. Furthermore, if an instance is
unsaved and currently in a Set, saving it will assign an identifier value to the object. If equals() and
hashCode() are based on the identifier value, the hash code would change, breaking the contract of the
Set. See the Hibernate website for a full discussion of this problem. This is not a Hibernate issue, but
normal Java semantics of object identity and equality.
It is recommended that you implement equals() and hashCode() using Business key equality.
Business key equality means that the equals() method compares only the properties that form the
business key. It is a key that would identify our instance in the real world (a natural candidate key):
package eg;
public class DomesticCat extends Cat {
private String name;
public String getName() {
return name;
}
protected void setName(String name) {
this.name=name;
}
}
public class Cat {
...
public boolean equals(Object other) {
if (this == other) return true;
if ( !(other instanceof Cat) ) return false;
final Cat cat = (Cat) other;
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A business key does not have to be as solid as a database primary key candidate. Immutable or unique
properties are usually good candidates for a business key.
Report a bug
4.3.3. Dynamic Models
NOTE
The following features are currently considered experimental and may change in the near
future.
Persistent entities do not necessarily have to be represented as POJO classes or as JavaBean objects
at runtime. Hibernate also supports dynamic models (using Maps of Maps at runtime) and the
representation of entities as DOM4J trees. With this approach, you do not write persistent classes, only
mapping files.
By default, Hibernate works in normal POJO mode. You can set a default entity representation mode for
a particular SessionFactory using the default_entity_mode configuration option.
The following examples demonstrate the representation using Maps. First, in the mapping file an
entity-name has to be declared instead of, or in addition to, a class name:
if ( !cat.getLitterId().equals( getLitterId() ) ) return false;
if ( !cat.getMother().equals( getMother() ) ) return false;
return true;
}
public int hashCode() {
int result;
result = getMother().hashCode();
result = 29 * result + getLitterId();
return result;
}
}
<hibernate-mapping>
<class entity-name="Customer">
<id name="id"
type="long"
column="ID">
<generator class="sequence"/>
</id>
<property name="name"
column="NAME"
type="string"/>
<property name="address"
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Even though associations are declared using target class names, the target type of associations can also
be a dynamic entity instead of a POJO.
After setting the default entity mode to dynamic-map for the SessionFactory, you can, at runtime,
work with Maps of Maps:
One of the main advantages of dynamic mapping is quick turnaround time for prototyping, without the
need for entity class implementation. However, you lose compile-time type checking and will likely deal
with many exceptions at runtime. As a result of the Hibernate mapping, the database schema can easily
be normalized and sound, allowing to add a proper domain model implementation on top later on.
Entity representation modes can also be set on a per Session basis:
column="ADDRESS"
type="string"/>
<many-to-one name="organization"
column="ORGANIZATION_ID"
class="Organization"/>
<bag name="orders"
inverse="true"
lazy="false"
cascade="all">
<key column="CUSTOMER_ID"/>
<one-to-many class="Order"/>
</bag>
</class>
</hibernate-mapping>
Session s = openSession();
Transaction tx = s.beginTransaction();
// Create a customer
Map david = new HashMap();
david.put("name", "David");
// Create an organization
Map foobar = new HashMap();
foobar.put("name", "Foobar Inc.");
// Link both
david.put("organization", foobar);
// Save both
s.save("Customer", david);
s.save("Organization", foobar);
tx.commit();
s.close();
Session dynamicSession = pojoSession.getSession(EntityMode.MAP);
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Please note that the call to getSession() using an EntityMode is on the Session API, not the
SessionFactory. That way, the new Session shares the underlying JDBC connection, transaction,
and other context information. This means you do not have to call flush() and close() on the
secondary Session, and also leave the transaction and connection handling to the primary unit of work.
Report a bug
4.3.4. Tuplizers
org.hibernate.tuple.Tuplizer, and its sub-interfaces, are responsible for managing a particular
representation of a piece of data given that representation's org.hibernate.EntityMode. If a given
piece of data is thought of as a data structure, then a tuplizer is the thing that knows how to create such a
data structure and how to extract values from and inject values into such a data structure. For example,
for the POJO entity mode, the corresponding tuplizer knows how create the POJO through its
constructor. It also knows how to access the POJO properties using the defined property accessors.
There are two high-level types of Tuplizers, represented by the
org.hibernate.tuple.entity.EntityTuplizer and
org.hibernate.tuple.component.ComponentTuplizer interfaces. EntityTuplizers are
responsible for managing the above mentioned contracts in regards to entities, while
ComponentTuplizers do the same for components.
Users can also plug in their own tuplizers. Perhaps you require that a java.util.Map implementation
other than java.util.HashMap be used while in the dynamic-map entity-mode. Or perhaps you need
to define a different proxy generation strategy than the one used by default. Both would be achieved by
defining a custom tuplizer implementation. Tuplizer definitions are attached to the entity or component
mapping they are meant to manage. Going back to the example of our customer entity:
// Create a customer
Map david = new HashMap();
david.put("name", "David");
dynamicSession.save("Customer", david);
...
dynamicSession.flush();
dynamicSession.close();
...
// Continue on pojoSession
<hibernate-mapping>
<class entity-name="Customer">
<!--
Override the dynamic-map entity-mode
tuplizer for the customer entity
-->
<tuplizer entity-mode="dynamic-map"
class="CustomMapTuplizerImpl"/>
<id name="id" type="long" column="ID">
<generator class="sequence"/>
</id>
<!-- other properties -->
...
</class>
</hibernate-mapping>
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71
Report a bug
4.3.5. EntityNameResolvers
The org.hibernate.EntityNameResolver interface is a contract for resolving the entity name of a
given entity instance. The interface defines a single method resolveEntityName which is passed the
entity instance and is expected to return the appropriate entity name (null is allowed and would indicate
that the resolver does not know how to resolve the entity name of the given entity instance). Generally
speaking, an org.hibernate.EntityNameResolver is going to be most useful in the case of
dynamic models. One example might be using proxied interfaces as your domain model. The hibernate
test suite has an example of this exact style of usage under the
org.hibernate.test.dynamicentity.tuplizer2. Here is some of the code from that package for illustration.
public class CustomMapTuplizerImpl
extends org.hibernate.tuple.entity.DynamicMapEntityTuplizer {
// override the buildInstantiator() method to plug in our custom
map...
protected final Instantiator buildInstantiator(
org.hibernate.mapping.PersistentClass mappingInfo) {
return new CustomMapInstantiator( mappingInfo );
}
private static final class CustomMapInstantiator
extends org.hibernate.tuple.DynamicMapInstantitor {
// override the generateMap() method to return our custom map...
protected final Map generateMap() {
return new CustomMap();
}
}
}
/**
* A very trivial JDK Proxy InvocationHandler implementation where we
proxy an interface as
* the domain model and simply store persistent state in an internal Map.
This is an extremely
* trivial example meant only for illustration.
*/
public final class DataProxyHandler implements InvocationHandler {
private String entityName;
private HashMap data = new HashMap();
public DataProxyHandler(String entityName, Serializable id) {
this.entityName = entityName;
data.put( "Id", id );
}
public Object invoke(Object proxy, Method method, Object[] args)
throws Throwable {
String methodName = method.getName();
if ( methodName.startsWith( "set" ) ) {
String propertyName = methodName.substring( 3 );
data.put( propertyName, args[0] );
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}
else if ( methodName.startsWith( "get" ) ) {
String propertyName = methodName.substring( 3 );
return data.get( propertyName );
}
else if ( "toString".equals( methodName ) ) {
return entityName + "#" + data.get( "Id" );
}
else if ( "hashCode".equals( methodName ) ) {
return new Integer( this.hashCode() );
}
return null;
}
public String getEntityName() {
return entityName;
}
public HashMap getData() {
return data;
}
}
/**
*
*/
public class ProxyHelper {
public static String extractEntityName(Object object) {
// Our custom java.lang.reflect.Proxy instances actually bundle
// their appropriate entity name, so we simply extract it from
there
// if this represents one of our proxies; otherwise, we return
null
if ( Proxy.isProxyClass( object.getClass() ) ) {
InvocationHandler handler = Proxy.getInvocationHandler( object
);
if ( DataProxyHandler.class.isAssignableFrom(
handler.getClass() ) ) {
DataProxyHandler myHandler = ( DataProxyHandler ) handler;
return myHandler.getEntityName();
}
}
return null;
}
// various other utility methods ....
}
/**
* The EntityNameResolver implementation.
* IMPL NOTE : An EntityNameResolver really defines a strategy for how
entity names should be
* resolved. Since this particular impl can handle resolution for all of
our entities we want to
* take advantage of the fact that SessionFactoryImpl keeps these in a Set
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In order to register an org.hibernate.EntityNameResolver users must either:
1. Implement a custom tupilizer, implementing the getEntityNameResolvers method.
2. Register it with the org.hibernate.impl.SessionFactoryImpl (which is the
implementation class for org.hibernate.SessionFactory) using the
registerEntityNameResolver method.
so that we only ever
* have one instance registered. Why? Well, when it comes time to
resolve an entity name,
* Hibernate must iterate over all the registered resolvers. So keeping
that number down
* helps that process be as speedy as possible. Hence the equals and
hashCode impls
*/
public class MyEntityNameResolver implements EntityNameResolver {
public static final MyEntityNameResolver INSTANCE = new
MyEntityNameResolver();
public String resolveEntityName(Object entity) {
return ProxyHelper.extractEntityName( entity );
}
public boolean equals(Object obj) {
return getClass().equals( obj.getClass() );
}
public int hashCode() {
return getClass().hashCode();
}
}
public class MyEntityTuplizer extends PojoEntityTuplizer {
public MyEntityTuplizer(EntityMetamodel entityMetamodel,
PersistentClass mappedEntity) {
super( entityMetamodel, mappedEntity );
}
public EntityNameResolver[] getEntityNameResolvers() {
return new EntityNameResolver[] { MyEntityNameResolver.INSTANCE };
}
public String determineConcreteSubclassEntityName(Object
entityInstance, SessionFactoryImplementor factory) {
String entityName = ProxyHelper.extractEntityName( entityInstance
);
if ( entityName == null ) {
entityName = super.determineConcreteSubclassEntityName(
entityInstance, factory );
}
return entityName;
}
...
}
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CHAPTER 5. BASIC O/R MAPPING
5.1. MAPPING DECLARATION
5.1.1. About Mapping Declaration
Object/relational mappings are usually defined in an XML document. The mapping document is designed
to be readable and hand-editable. The mapping language is Java-centric, meaning that mappings are
constructed around persistent class declarations and not table declarations.
Please note that even though many Hibernate users choose to write the XML by hand, a number of tools
exist to generate the mapping document. These include XDoclet, Middlegen and AndroMDA.
Here is an example mapping:
<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
"-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping package="eg">
<class name="Cat"
table="cats"
discriminator-value="C">
<id name="id">
<generator class="native"/>
</id>
<discriminator column="subclass"
type="character"/>
<property name="weight"/>
<property name="birthdate"
type="date"
not-null="true"
update="false"/>
<property name="color"
type="eg.types.ColorUserType"
not-null="true"
update="false"/>
<property name="sex"
not-null="true"
update="false"/>
<property name="litterId"
column="litterId"
update="false"/>
<many-to-one name="mother"
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We will now discuss the content of the mapping document. We will only describe, however, the document
elements and attributes that are used by Hibernate at runtime. The mapping document also contains
some extra optional attributes and elements that affect the database schemas exported by the schema
export tool (for example, the not-null attribute).
Report a bug
5.1.2. Doctype
All XML mappings should declare the doctype shown. The actual DTD can be found at the URL above,
in the directory hibernate-x.x.x/src/org/hibernate, or in hibernate3.jar. Hibernate will
always look for the DTD in its classpath first. If you experience lookups of the DTD using an Internet
connection, check the DTD declaration against the contents of your classpath.
Report a bug
5.1.3. EntityResolver
Hibernate will first attempt to resolve DTDs in its classpath. It does this is by registering a custom
org.xml.sax.EntityResolver implementation with the SAXReader it uses to read in the xml files.
This custom EntityResolver recognizes two different systemId namespaces:
a hibernate namespace is recognized whenever the resolver encounters a systemId starting
with http://hibernate.sourceforge.net/. The resolver attempts to resolve these
entities via the classloader which loaded the Hibernate classes.
a user namespace is recognized whenever the resolver encounters a systemId using a
classpath:// URL protocol. The resolver will attempt to resolve these entities via (1) the
current thread context classloader and (2) the classloader which loaded the Hibernate classes.
column="mother_id"
update="false"/>
<set name="kittens"
inverse="true"
order-by="litter_id">
<key column="mother_id"/>
<one-to-many class="Cat"/>
</set>
<subclass name="DomesticCat"
discriminator-value="D">
<property name="name"
type="string"/>
</subclass>
</class>
<class name="Dog">
<!-- mapping for Dog could go here -->
</class>
</hibernate-mapping>
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77
The following is an example of utilizing user namespacing:
Where types.xml is a resource in the your.domain package and contains a custom mapping type.
Report a bug
5.1.4. Hibernate-mapping
This element has several optional attributes. The schema and catalog attributes specify that tables
referred to in this mapping belong to the named schema and/or catalog. If they are specified, tablenames
will be qualified by the given schema and catalog names. If they are missing, tablenames will be
unqualified. The default-cascade attribute specifies what cascade style should be assumed for
properties and collections that do not specify a cascade attribute. By default, the auto-import
attribute allows you to use unqualified class names in the query language.
schema (optional): the name of a database schema.
catalog (optional): the name of a database catalog.
default-cascade (optional - defaults to none): a default cascade style.
default-access (optional - defaults to property): the strategy Hibernate should use for accessing
all properties. It can be a custom implementation of PropertyAccessor.
default-lazy (optional - defaults to true): the default value for unspecified lazy attributes of class
and collection mappings.
auto-import (optional - defaults to true): specifies whether we can use unqualified class names of
classes in this mapping in the query language.
<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
"-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd" [
<!ENTITY types SYSTEM "classpath://your/domain/types.xml">
]>
<hibernate-mapping package="your.domain">
<class name="MyEntity">
<id name="id" type="my-custom-id-type">
...
</id>
<class>
&types;
</hibernate-mapping>
<hibernate-mapping
schema="schemaName"
catalog="catalogName"
default-cascade="cascade_style"
default-access="field|property|ClassName"
default-lazy="true|false"
auto-import="true|false"
package="package.name"
/>
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package (optional): specifies a package prefix to use for unqualified class names in the mapping
document.
If you have two persistent classes with the same unqualified name, you should set auto-
import="false". An exception will result if you attempt to assign two classes to the same "imported"
name.
The hibernate-mapping element allows you to nest several persistent <class> mappings, as shown
above. It is, however, good practice (and expected by some tools) to map only a single persistent class,
or a single class hierarchy, in one mapping file and name it after the persistent superclass. For example,
Cat.hbm.xml, Dog.hbm.xml, or if using inheritance, Animal.hbm.xml.
Report a bug
5.1.5. Class
You can declare a persistent class using the class element. For example:
name (optional): the fully qualified Java class name of the persistent class or interface. If this attribute is
missing, it is assumed that the mapping is for a non-POJO entity.
table (optional - defaults to the unqualified class name): the name of its database table.
discriminator-value (optional - defaults to the class name): a value that distinguishes individual
subclasses that is used for polymorphic behavior. Acceptable values include null and not null.
mutable (optional - defaults to true): specifies that instances of the class are (not) mutable.
schema (optional): overrides the schema name specified by the root <hibernate-mapping> element.
<class
name="ClassName"
table="tableName"
discriminator-value="discriminator_value"
mutable="true|false"
schema="owner"
catalog="catalog"
proxy="ProxyInterface"
dynamic-update="true|false"
dynamic-insert="true|false"
select-before-update="true|false"
polymorphism="implicit|explicit"
where="arbitrary sql where condition"
persister="PersisterClass"
batch-size="N"
optimistic-lock="none|version|dirty|all"
lazy="true|false"
entity-name="EntityName"
check="arbitrary sql check condition"
rowid="rowid"
subselect="SQL expression"
abstract="true|false"
node="element-name"
/>
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catalog (optional): overrides the catalog name specified by the root <hibernate-mapping> element.
proxy (optional): specifies an interface to use for lazy initializing proxies. You can specify the name of
the class itself.
dynamic-update (optional - defaults to false): specifies that UPDATE SQL should be generated at
runtime and can contain only those columns whose values have changed.
dynamic-insert (optional - defaults to false): specifies that INSERT SQL should be generated at
runtime and contain only the columns whose values are not null.
select-before-update (optional - defaults to false): specifies that Hibernate should never perform
an SQL UPDATE unless it is certain that an object is actually modified. Only when a transient object has
been associated with a new session using update(), will Hibernate perform an extra SQL SELECT to
determine if an UPDATE is actually required.
polymorphism (optional - defaults to implicit): determines whether implicit or explicit query
polymorphism is used.
where (optional): specifies an arbitrary SQL WHERE condition to be used when retrieving objects of this
class.
persister (optional): specifies a custom ClassPersister.
batch-size (optional - defaults to 1): specifies a "batch size" for fetching instances of this class by
identifier.
optimistic-lock (optional - defaults to version): determines the optimistic locking strategy.
lazy (optional): lazy fetching can be disabled by setting lazy="false".
entity-name (optional - defaults to the class name): Hibernate3 allows a class to be mapped multiple
times, potentially to different tables. It also allows entity mappings that are represented by Maps or XML
at the Java level. In these cases, you should provide an explicit arbitrary name for the entity.
check (optional): an SQL expression used to generate a multi-row check constraint for automatic
schema generation.
rowid (optional): Hibernate can use ROWIDs on databases. On Oracle, for example, Hibernate can use
the rowid extra column for fast updates once this option has been set to rowid. A ROWID is an
implementation detail and represents the physical location of a stored tuple.
subselect (optional): maps an immutable and read-only entity to a database subselect. This is useful if
you want to have a view instead of a base table. See below for more information.
abstract (optional): is used to mark abstract superclasses in <union-subclass> hierarchies.
It is acceptable for the named persistent class to be an interface. You can declare implementing classes
of that interface using the <subclass> element. You can persist any static inner class. Specify the class
name using the standard form i.e. e.g.Foo$Bar.
Immutable classes, mutable="false", cannot be updated or deleted by the application. This allows
Hibernate to make some minor performance optimizations.
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The optional proxy attribute enables lazy initialization of persistent instances of the class. Hibernate will
initially return CGLIB proxies that implement the named interface. The persistent object will load when a
method of the proxy is invoked. See "Initializing collections and proxies" below.
Implicit polymorphism means that instances of the class will be returned by a query that names any
superclass or implemented interface or class, and that instances of any subclass of the class will be
returned by a query that names the class itself. Explicit polymorphism means that class instances will be
returned only by queries that explicitly name that class. Queries that name the class will return only
instances of subclasses mapped inside this <class> declaration as a <subclass> or <joined-
subclass>. For most purposes, the default polymorphism="implicit" is appropriate. Explicit
polymorphism is useful when two different classes are mapped to the same table This allows a
"lightweight" class that contains a subset of the table columns.
The persister attribute lets you customize the persistence strategy used for the class. You can, for
example, specify your own subclass of org.hibernate.persister.EntityPersister, or you can
even provide a completely new implementation of the interface
org.hibernate.persister.ClassPersister that implements, for example, persistence via stored
procedure calls, serialization to flat files or LDAP. See org.hibernate.test.CustomPersister for
a simple example of "persistence" to a Hashtable.
The dynamic-update and dynamic-insert settings are not inherited by subclasses, so they can
also be specified on the <subclass> or <joined-subclass> elements. Although these settings can
increase performance in some cases, they can actually decrease performance in others.
Use of select-before-update will usually decrease performance. It is useful to prevent a database
update trigger being called unnecessarily if you reattach a graph of detached instances to a Session.
If you enable dynamic-update, you will have a choice of optimistic locking strategies:
version: check the version/timestamp columns
all: check all columns
dirty: check the changed columns, allowing some concurrent updates
none: do not use optimistic locking
It is strongly recommended that you use version/timestamp columns for optimistic locking with Hibernate.
This strategy optimizes performance and correctly handles modifications made to detached instances
(i.e. when Session.merge() is used).
There is no difference between a view and a base table for a Hibernate mapping. This is transparent at
the database level, although some DBMS do not support views properly, especially with updates.
Sometimes you want to use a view, but you cannot create one in the database (i.e. with a legacy
schema). In this case, you can map an immutable and read-only entity to a given SQL subselect
expression:
<class name="Summary">
<subselect>
select item.name, max(bid.amount), count(*)
from item
join bid on bid.item_id = item.id
group by item.name
</subselect>
<synchronize table="item"/>
<synchronize table="bid"/>
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Declare the tables to synchronize this entity with, ensuring that auto-flush happens correctly and that
queries against the derived entity do not return stale data. The <subselect> is available both as an
attribute and a nested mapping element.
Report a bug
5.1.6. id
Mapped classes must declare the primary key column of the database table. Most classes will also have
a JavaBeans-style property holding the unique identifier of an instance. The <id> element defines the
mapping from that property to the primary key column.
name (optional): the name of the identifier property.
type (optional): a name that indicates the Hibernate type.
column (optional - defaults to the property name): the name of the primary key column.
unsaved-value (optional - defaults to a "sensible" value): an identifier property value that indicates an
instance is newly instantiated (unsaved), distinguishing it from detached instances that were saved or
loaded in a previous session.
access (optional - defaults to property): the strategy Hibernate should use for accessing the property
value.
If the name attribute is missing, it is assumed that the class has no identifier property.
The unsaved-value attribute is almost never needed in Hibernate3.
There is an alternative <composite-id> declaration that allows access to legacy data with composite
keys. Its use is strongly discouraged for anything else.
Report a bug
5.1.7. Generator
The optional <generator> child element names a Java class used to generate unique identifiers for
instances of the persistent class. If any parameters are required to configure or initialize the generator
instance, they are passed using the <param> element.
<id name="name"/>
...
</class>
<id
name="propertyName"
type="typename"
column="column_name"
unsaved-value="null|any|none|undefined|id_value"
access="field|property|ClassName">
node="element-name|@attribute-name|element/@attribute|."
<generator class="generatorClass"/>
</id>
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All generators implement the interface org.hibernate.id.IdentifierGenerator. This is a very
simple interface. Some applications can choose to provide their own specialized implementations,
however, Hibernate provides a range of built-in implementations. The shortcut names for the built-in
generators are as follows:
increment
generates identifiers of type long, short or int that are unique only when no other process is
inserting data into the same table. Do not use in a cluster.
identity
supports identity columns in DB2, MySQL, MS SQL Server, Sybase and HypersonicSQL. The
returned identifier is of type long, short or int.
sequence
uses a sequence in DB2, PostgreSQL, Oracle, SAP DB, McKoi or a generator in Interbase. The
returned identifier is of type long, short or int
hilo
uses a hi/lo algorithm to efficiently generate identifiers of type long, short or int, given a table and
column (by default hibernate_unique_key and next_hi respectively) as a source of hi values.
The hi/lo algorithm generates identifiers that are unique only for a particular database.
seqhilo
uses a hi/lo algorithm to efficiently generate identifiers of type long, short or int, given a named
database sequence.
uuid
uses a 128-bit UUID algorithm to generate identifiers of type string that are unique within a network
(the IP address is used). The UUID is encoded as a string of 32 hexadecimal digits in length.
guid
uses a database-generated GUID string on MS SQL Server and MySQL.
native
selects identity, sequence or hilo depending upon the capabilities of the underlying database.
assigned
lets the application assign an identifier to the object before save() is called. This is the default
strategy if no <generator> element is specified.
select
<id name="id" type="long" column="cat_id">
<generator class="org.hibernate.id.TableHiLoGenerator">
<param name="table">uid_table</param>
<param name="column">next_hi_value_column</param>
</generator>
</id>
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retrieves a primary key, assigned by a database trigger, by selecting the row by some unique key and
retrieving the primary key value.
foreign
uses the identifier of another associated object. It is usually used in conjunction with a <one-to-
one> primary key association.
sequence-identity
a specialized sequence generation strategy that utilizes a database sequence for the actual value
generation, but combines this with JDBC3 getGeneratedKeys to return the generated identifier value
as part of the insert statement execution. This strategy is only supported on Oracle 10g drivers
targeted for JDK 1.4. Comments on these insert statements are disabled due to a bug in the Oracle
drivers.
Report a bug
5.1.8. Hi/Lo Algorithm
The hilo and seqhilo generators provide two alternate implementations of the hi/lo algorithm. The first
implementation requires a "special" database table to hold the next available "hi" value. Where
supported, the second uses an Oracle-style sequence.
Unfortunately, you cannot use hilo when supplying your own Connection to Hibernate. When
Hibernate uses an application server datasource to obtain connections enlisted with JTA, you must
configure the hibernate.transaction.manager_lookup_class.
Report a bug
5.1.9. UUID Algorithm
The UUID contains: IP address, startup time of the JVM that is accurate to a quarter second, system
time and a counter value that is unique within the JVM. It is not possible to obtain a MAC address or
memory address from Java code, so this is the best option without using JNI.
Report a bug
<id name="id" type="long" column="cat_id">
<generator class="hilo">
<param name="table">hi_value</param>
<param name="column">next_value</param>
<param name="max_lo">100</param>
</generator>
</id>
<id name="id" type="long" column="cat_id">
<generator class="seqhilo">
<param name="sequence">hi_value</param>
<param name="max_lo">100</param>
</generator>
</id>
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5.1.10. Identity Columns and Sequences
For databases that support identity columns (DB2, MySQL, Sybase, MS SQL), you can use identity
key generation. For databases that support sequences (DB2, Oracle, PostgreSQL, Interbase, McKoi,
SAP DB) you can use sequence style key generation. Both of these strategies require two SQL queries
to insert a new object. For example:
For cross-platform development, the native strategy will, depending on the capabilities of the
underlying database, choose from the identity, sequence and hilo strategies.
Report a bug
5.1.11. Assigned Identifiers
If you want the application to assign identifiers, as opposed to having Hibernate generate them, you can
use the assigned generator. This special generator uses the identifier value already assigned to the
object's identifier property. The generator is used when the primary key is a natural key instead of a
surrogate key. This is the default behavior if you do not specify a <generator> element.
The assigned generator makes Hibernate use unsaved-value="undefined". This forces
Hibernate to go to the database to determine if an instance is transient or detached, unless there is a
version or timestamp property, or you define Interceptor.isUnsaved().
Report a bug
5.1.12. Primary Keys Assigned by Triggers
Hibernate does not generate DDL with triggers. It is for legacy schemas only.
In the above example, there is a unique valued property named socialSecurityNumber. It is defined
by the class, as a natural key and a surrogate key named person_id, whose value is generated by a
trigger.
Report a bug
5.1.13. Enhanced Identifier Generators
<id name="id" type="long" column="person_id">
<generator class="sequence">
<param name="sequence">person_id_sequence</param>
</generator>
</id>
<id name="id" type="long" column="person_id" unsaved-value="0">
<generator class="identity"/>
</id>
<id name="id" type="long" column="person_id">
<generator class="select">
<param name="key">socialSecurityNumber</param>
</generator>
</id>
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Starting with release 3.2.3, there are 2 new generators which represent a re-thinking of 2 different
aspects of identifier generation. The first aspect is database portability; the second is optimization
Optimization means that you do not have to query the database for every request for a new identifier
value. These two new generators are intended to take the place of some of the named generators
described above, starting in 3.3.x. However, they are included in the current releases and can be
referenced by FQN.
The first of these new generators is org.hibernate.id.enhanced.SequenceStyleGenerator
which is intended, firstly, as a replacement for the sequence generator and, secondly, as a better
portability generator than native. This is because native generally chooses between identity and
sequence which have largely different semantics that can cause subtle issues in applications eyeing
portability. org.hibernate.id.enhanced.SequenceStyleGenerator, however, achieves
portability in a different manner. It chooses between a table or a sequence in the database to store its
incrementing values, depending on the capabilities of the dialect being used. The difference between this
and native is that table-based and sequence-based storage have the same exact semantic. In fact,
sequences are exactly what Hibernate tries to emulate with its table-based generators. This generator
has a number of configuration parameters:
sequence_name (optional, defaults to hibernate_sequence): the name of the sequence or
table to be used.
initial_value (optional, defaults to 1): the initial value to be retrieved from the
sequence/table. In sequence creation terms, this is analogous to the clause typically named
"STARTS WITH".
increment_size (optional - defaults to 1): the value by which subsequent calls to the
sequence/table should differ. In sequence creation terms, this is analogous to the clause
typically named "INCREMENT BY".
force_table_use (optional - defaults to false): should we force the use of a table as the
backing structure even though the dialect might support sequence?
value_column (optional - defaults to next_val): only relevant for table structures, it is the
name of the column on the table which is used to hold the value.
optimizer (optional - defaults to none).
The second of these new generators is org.hibernate.id.enhanced.TableGenerator, which is
intended, firstly, as a replacement for the table generator, even though it actually functions much more
like org.hibernate.id.MultipleHiLoPerTableGenerator, and secondly, as a re-implementation
of org.hibernate.id.MultipleHiLoPerTableGenerator that utilizes the notion of pluggable
optimizers. Essentially this generator defines a table capable of holding a number of different increment
values simultaneously by using multiple distinctly keyed rows. This generator has a number of
configuration parameters:
table_name (optional - defaults to hibernate_sequences): the name of the table to be
used.
value_column_name (optional - defaults to next_val): the name of the column on the table
that is used to hold the value.
segment_column_name (optional - defaults to sequence_name): the name of the column on
the table that is used to hold the "segment key". This is the value which identifies which
increment value to use.
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segment_value (optional - defaults to default): The "segment key" value for the segment
from which we want to pull increment values for this generator.
segment_value_length (optional - defaults to 255): Used for schema generation; the column
size to create this segment key column.
initial_value (optional - defaults to 1): The initial value to be retrieved from the table.
increment_size (optional - defaults to 1): The value by which subsequent calls to the table
should differ.
optimizer (optional - defaults to ): Refer to Section 5.1.14, “Identifier Generator Optimization”
for further information.
Report a bug
5.1.14. Identifier Generator Optimization
For identifier generators that store values in the database, it is inefficient for them to hit the database on
each and every call to generate a new identifier value. Instead, you can group a bunch of them in
memory and only hit the database when you have exhausted your in-memory value group. This is the
role of the pluggable optimizers. Currently only the two enhanced generators support this operation.
none (generally this is the default if no optimizer was specified): this will not perform any
optimizations and hit the database for each and every request.
hilo: applies a hi/lo algorithm around the database retrieved values. The values from the
database for this optimizer are expected to be sequential. The values retrieved from the
database structure for this optimizer indicates the "group number". The increment_size is
multiplied by that value in memory to define a group "hi value".
pooled: as with the case of hilo, this optimizer attempts to minimize the number of hits to the
database. Here, however, we simply store the starting value for the "next group" into the
database structure rather than a sequential value in combination with an in-memory grouping
algorithm. Here, increment_size refers to the values coming from the database.
Report a bug
5.1.15. composite-id
<composite-id
name="propertyName"
class="ClassName"
mapped="true|false"
access="field|property|ClassName">
node="element-name|."
<key-property name="propertyName" type="typename"
column="column_name"/>
<key-many-to-one name="propertyName class="ClassName"
column="column_name"/>
......
</composite-id>
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A table with a composite key can be mapped with multiple properties of the class as identifier properties.
The <composite-id> element accepts <key-property> property mappings and <key-many-to-
one> mappings as child elements.
The persistent class must override equals() and hashCode() to implement composite identifier
equality. It must also implement Serializable.
Unfortunately, this approach means that a persistent object is its own identifier. There is no convenient
"handle" other than the object itself. You must instantiate an instance of the persistent class itself and
populate its identifier properties before you can load() the persistent state associated with a composite
key. We call this approach an embedded composite identifier, and discourage it for serious applications.
A second approach is what we call a mapped composite identifier, where the identifier properties named
inside the <composite-id> element are duplicated on both the persistent class and a separate
identifier class.
In this example, both the composite identifier class, MedicareId, and the entity class itself have
properties named medicareNumber and dependent. The identifier class must override equals() and
hashCode() and implement Serializable. The main disadvantage of this approach is code
duplication.
The following attributes are used to specify a mapped composite identifier:
mapped (optional - defaults to false): indicates that a mapped composite identifier is used, and
that the contained property mappings refer to both the entity class and the composite identifier
class.
class (optional - but required for a mapped composite identifier): the class used as a composite
identifier.
We will describe a third, even more convenient approach, where the composite identifier is implemented
as a component class. The attributes described below apply only to this alternative approach:
name (optional - required for this approach): a property of component type that holds the
composite identifier. Please see chapter 9 for more information.
access (optional - defaults to property): the strategy Hibernate uses for accessing the
property value.
class (optional - defaults to the property type determined by reflection): the component class
used as a composite identifier. Please see the next section for more information.
The third approach, an identifier component, is recommended for almost all applications.
Report a bug
<composite-id>
<key-property name="medicareNumber"/>
<key-property name="dependent"/>
</composite-id>
<composite-id class="MedicareId" mapped="true">
<key-property name="medicareNumber"/>
<key-property name="dependent"/>
</composite-id>
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5.1.16. Discriminator
The <discriminator> element is required for polymorphic persistence using the table-per-class-
hierarchy mapping strategy. It declares a discriminator column of the table. The discriminator column
contains marker values that tell the persistence layer what subclass to instantiate for a particular row. A
restricted set of types can be used: string, character, integer, byte, short, boolean, yes_no,
true_false.
column (optional - defaults to class): the name of the discriminator column.
type (optional - defaults to string): a name that indicates the Hibernate type
force (optional - defaults to false): "forces" Hibernate to specify the allowed discriminator values,
even when retrieving all instances of the root class.
insert (optional - defaults to true): set this to false if your discriminator column is also part of a
mapped composite identifier. It tells Hibernate not to include the column in SQL INSERTs.
formula (optional): an arbitrary SQL expression that is executed when a type has to be evaluated. It
allows content-based discrimination.
Actual values of the discriminator column are specified by the discriminator-value attribute of the
<class> and <subclass> elements.
The force attribute is only useful if the table contains rows with "extra" discriminator values that are not
mapped to a persistent class. This will not usually be the case.
The formula attribute allows you to declare an arbitrary SQL expression that will be used to evaluate
the type of a row. For example:
Report a bug
5.1.17. Version (optional)
The <version> element is optional and indicates that the table contains versioned data. This is
particularly useful if you plan to use long transactions. See below for more information:
<discriminator
column="discriminator_column"
type="discriminator_type"
force="true|false"
insert="true|false"
formula="arbitrary sql expression"
/>
<discriminator
formula="case when CLASS_TYPE in ('a', 'b', 'c') then 0 else 1 end"
type="integer"/>
<version
column="version_column"
name="propertyName"
type="typename"
access="field|property|ClassName"
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column (optional - defaults to the property name): the name of the column holding the version number.
name: the name of a property of the persistent class.
type (optional - defaults to integer): the type of the version number.
access (optional - defaults to property): the strategy Hibernate uses to access the property value.
unsaved-value (optional - defaults to undefined): a version property value that indicates that an
instance is newly instantiated (unsaved), distinguishing it from detached instances that were saved or
loaded in a previous session. Undefined specifies that the identifier property value should be used.
generated (optional - defaults to never): specifies that this version property value is generated by the
database. See the discussion of generated properties for more information.
insert (optional - defaults to true): specifies whether the version column should be included in SQL
insert statements. It can be set to false if the database column is defined with a default value of 0.
Version numbers can be of Hibernate type long, integer, short, timestamp or calendar.
A version or timestamp property should never be null for a detached instance. Hibernate will detect any
instance with a null version or timestamp as transient, irrespective of what other unsaved-value
strategies are specified. Declaring a nullable version or timestamp property is an easy way to avoid
problems with transitive reattachment in Hibernate. It is especially useful for people using assigned
identifiers or composite keys.
Report a bug
5.1.18. Timestamp (optional)
The optional <timestamp> element indicates that the table contains timestamped data. This provides
an alternative to versioning. Timestamps are a less safe implementation of optimistic locking. However,
sometimes the application might use the timestamps in other ways.
column (optional - defaults to the property name): the name of a column holding the timestamp.
name: the name of a JavaBeans style property of Java type Date or Timestamp of the persistent class.
access (optional - defaults to property): the strategy Hibernate uses for accessing the property value.
unsaved-value="null|negative|undefined"
generated="never|always"
insert="true|false"
node="element-name|@attribute-name|element/@attribute|."
/>
<timestamp
column="timestamp_column"
name="propertyName"
access="field|property|ClassName"
unsaved-value="null|undefined"
source="vm|db"
generated="never|always"
node="element-name|@attribute-name|element/@attribute|."
/>
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unsaved-value (optional - defaults to null): a version property value that indicates that an instance is
newly instantiated (unsaved), distinguishing it from detached instances that were saved or loaded in a
previous session. Undefined specifies that the identifier property value should be used.
source (optional - defaults to vm): Where should Hibernate retrieve the timestamp value from? From the
database, or from the current JVM? Database-based timestamps incur an overhead because Hibernate
must hit the database in order to determine the "next value". It is safer to use in clustered environments.
Not all Dialects are known to support the retrieval of the database's current timestamp. Others may
also be unsafe for usage in locking due to lack of precision (Oracle 8, for example).
generated (optional - defaults to never): specifies that this timestamp property value is actually
generated by the database.
NOTE
<Timestamp> is equivalent to <version type="timestamp">. And <timestamp
source="db"> is equivalent to <version type="dbtimestamp">
Report a bug
5.1.19. Property
The <property> element declares a persistent JavaBean style property of the class.
name: the name of the property, with an initial lowercase letter.
column (optional - defaults to the property name): the name of the mapped database table column. This
can also be specified by nested <column> element(s).
type (optional): a name that indicates the Hibernate type.
update, insert (optional - defaults to true): specifies that the mapped columns should be included
in SQL UPDATE and/or INSERT statements. Setting both to false allows a pure "derived" property
<property
name="propertyName"
column="column_name"
type="typename"
update="true|false"
insert="true|false"
formula="arbitrary SQL expression"
access="field|property|ClassName"
lazy="true|false"
unique="true|false"
not-null="true|false"
optimistic-lock="true|false"
generated="never|insert|always"
node="element-name|@attribute-name|element/@attribute|."
index="index_name"
unique_key="unique_key_id"
length="L"
precision="P"
scale="S"
/>
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whose value is initialized from some other property that maps to the same column(s), or by a trigger or
other application.
formula (optional): an SQL expression that defines the value for a computed property. Computed
properties do not have a column mapping of their own.
access (optional - defaults to property): the strategy Hibernate uses for accessing the property value.
lazy (optional - defaults to false): specifies that this property should be fetched lazily when the
instance variable is first accessed. It requires build-time bytecode instrumentation.
unique (optional): enables the DDL generation of a unique constraint for the columns. Also, allow this to
be the target of a property-ref.
not-null (optional): enables the DDL generation of a nullability constraint for the columns.
optimistic-lock (optional - defaults to true): specifies that updates to this property do or do not
require acquisition of the optimistic lock. In other words, it determines if a version increment should occur
when this property is dirty.
generated (optional - defaults to never): specifies that this property value is actually generated by the
database.
typename could be:
1. The name of a Hibernate basic type: integer, string, character, date,
timestamp, float, binary, serializable, object, blob etc.
2. The name of a Java class with a default basic type: int, float, char,
java.lang.String, java.util.Date, java.lang.Integer, java.sql.Clob etc.
3. The name of a serializable Java class.
4. The class name of a custom type: com.illflow.type.MyCustomType etc.
If you do not specify a type, Hibernate will use reflection upon the named property and guess the correct
Hibernate type. Hibernate will attempt to interpret the name of the return class of the property getter
using, in order, rules 2, 3, and 4. In certain cases you will need the type attribute. For example, to
distinguish between Hibernate.DATE and Hibernate.TIMESTAMP, or to specify a custom type.
The access attribute allows you to control how Hibernate accesses the property at runtime. By default,
Hibernate will call the property get/set pair. If you specify access="field", Hibernate will bypass the
get/set pair and access the field directly using reflection. You can specify your own strategy for property
access by naming a class that implements the interface
org.hibernate.property.PropertyAccessor.
A powerful feature is derived properties. These properties are by definition read-only. The property value
is computed at load time. You declare the computation as an SQL expression. This then translates to a
SELECT clause subquery in the SQL query that loads an instance:
<property name="totalPrice"
formula="( SELECT SUM (li.quantity*p.price) FROM LineItem li, Product
p
WHERE li.productId = p.productId
AND li.customerId = customerId
AND li.orderNumber = orderNumber )"/>
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You can reference the entity table by not declaring an alias on a particular column. This would be
customerId in the given example. You can also use the nested <formula> mapping element if you do
not want to use the attribute.
Report a bug
5.1.20. Many-to-one
An ordinary association to another persistent class is declared using a many-to-one element. The
relational model is a many-to-one association; a foreign key in one table is referencing the primary key
column(s) of the target table.
name: the name of the property.
column (optional): the name of the foreign key column. This can also be specified by nested <column>
element(s).
class (optional - defaults to the property type determined by reflection): the name of the associated
class.
cascade (optional): specifies which operations should be cascaded from the parent object to the
associated object.
fetch (optional - defaults to select): chooses between outer-join fetching or sequential select
fetching.
update, insert (optional - defaults to true): specifies that the mapped columns should be included
in SQL UPDATE and/or INSERT statements. Setting both to false allows a pure "derived" association
whose value is initialized from another property that maps to the same column(s), or by a trigger or other
application.
<many-to-one
name="propertyName"
column="column_name"
class="ClassName"
cascade="cascade_style"
fetch="join|select"
update="true|false"
insert="true|false"
property-ref="propertyNameFromAssociatedClass"
access="field|property|ClassName"
unique="true|false"
not-null="true|false"
optimistic-lock="true|false"
lazy="proxy|no-proxy|false"
not-found="ignore|exception"
entity-name="EntityName"
formula="arbitrary SQL expression"
node="element-name|@attribute-name|element/@attribute|."
embed-xml="true|false"
index="index_name"
unique_key="unique_key_id"
foreign-key="foreign_key_name"
/>
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property-ref (optional): the name of a property of the associated class that is joined to this foreign
key. If not specified, the primary key of the associated class is used.
access (optional - defaults to property): the strategy Hibernate uses for accessing the property value.
unique (optional): enables the DDL generation of a unique constraint for the foreign-key column. By
allowing this to be the target of a property-ref, you can make the association multiplicity one-to-one.
not-null (optional): enables the DDL generation of a nullability constraint for the foreign key columns.
optimistic-lock (optional - defaults to true): specifies that updates to this property do or do not
require acquisition of the optimistic lock. In other words, it determines if a version increment should occur
when this property is dirty.
lazy (optional - defaults to proxy): by default, single point associations are proxied. lazy="no-
proxy" specifies that the property should be fetched lazily when the instance variable is first accessed.
This requires build-time bytecode instrumentation. lazy="false" specifies that the association will
always be eagerly fetched.
not-found (optional - defaults to exception): specifies how foreign keys that reference missing rows
will be handled. ignore will treat a missing row as a null association.
entity-name (optional): the entity name of the associated class.
formula (optional): an SQL expression that defines the value for a computed foreign key.
Setting a value of the cascade attribute to any meaningful value other than none will propagate certain
operations to the associated object. The meaningful values are divided into three categories. First, basic
operations, which include: persist, merge, delete, save-update, evict, replicate,
lock and refresh; second, special values: delete-orphan; and third,all comma-separated
combinations of operation names: cascade="persist,merge,evict" or cascade="all,delete-
orphan". Note that single valued, many-to-one and one-to-one, associations do not support orphan
delete.
Here is an example of a typical many-to-one declaration:
The property-ref attribute should only be used for mapping legacy data where a foreign key refers to
a unique key of the associated table other than the primary key. This is a complicated and confusing
relational model. For example, if the Product class had a unique serial number that is not the primary
key. The unique attribute controls Hibernate's DDL generation with the SchemaExport tool.
Then the mapping for OrderItem might use:
This is not encouraged, however.
<many-to-one name="product" class="Product" column="PRODUCT_ID"/>
<property name="serialNumber" unique="true" type="string"
column="SERIAL_NUMBER"/>
<many-to-one name="product" property-ref="serialNumber"
column="PRODUCT_SERIAL_NUMBER"/>
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If the referenced unique key comprises multiple properties of the associated entity, you should map the
referenced properties inside a named <properties> element.
If the referenced unique key is the property of a component, you can specify a property path:
Report a bug
5.1.21. One-to-one
A one-to-one association to another persistent class is declared using a one-to-one element.
name: the name of the property.
class (optional - defaults to the property type determined by reflection): the name of the associated
class.
cascade (optional): specifies which operations should be cascaded from the parent object to the
associated object.
constrained (optional): specifies that a foreign key constraint on the primary key of the mapped table
and references the table of the associated class. This option affects the order in which save() and
delete() are cascaded, and determines whether the association can be proxied. It is also used by the
schema export tool.
fetch (optional - defaults to select): chooses between outer-join fetching or sequential select
fetching.
property-ref (optional): the name of a property of the associated class that is joined to the primary
key of this class. If not specified, the primary key of the associated class is used.
access (optional - defaults to property): the strategy Hibernate uses for accessing the property value.
formula (optional): almost all one-to-one associations map to the primary key of the owning entity. If this
is not the case, you can specify another column, columns or expression to join on using an SQL formula.
See org.hibernate.test.onetooneformula for an example.
lazy (optional - defaults to proxy): by default, single point associations are proxied. lazy="no-
<many-to-one name="owner" property-ref="identity.ssn" column="OWNER_SSN"/>
<one-to-one
name="propertyName"
class="ClassName"
cascade="cascade_style"
constrained="true|false"
fetch="join|select"
property-ref="propertyNameFromAssociatedClass"
access="field|property|ClassName"
formula="any SQL expression"
lazy="proxy|no-proxy|false"
entity-name="EntityName"
node="element-name|@attribute-name|element/@attribute|."
embed-xml="true|false"
foreign-key="foreign_key_name"
/>
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proxy" specifies that the property should be fetched lazily when the instance variable is first accessed.
It requires build-time bytecode instrumentation. lazy="false" specifies that the association will always
be eagerly fetched. Note that if constrained="false", proxying is impossible and Hibernate will
eagerly fetch the association.
entity-name (optional): the entity name of the associated class.
There are two varieties of one-to-one associations:
primary key associations
unique foreign key associations
Primary key associations do not need an extra table column. If two rows are related by the association,
then the two table rows share the same primary key value. To relate two objects by a primary key
association, ensure that they are assigned the same identifier value.
For a primary key association, add the following mappings to Employee and Person respectively:
Ensure that the primary keys of the related rows in the PERSON and EMPLOYEE tables are equal. You
use a special Hibernate identifier generation strategy called foreign:
A newly saved instance of Person is assigned the same primary key value as the Employee instance
referred with the employee property of that Person.
Alternatively, a foreign key with a unique constraint, from Employee to Person, can be expressed as:
This association can be made bidirectional by adding the following to the Person mapping:
Report a bug
5.1.22. Natural-id
<one-to-one name="person" class="Person"/>
<one-to-one name="employee" class="Employee" constrained="true"/>
<class name="person" table="PERSON">
<id name="id" column="PERSON_ID">
<generator class="foreign">
<param name="property">employee</param>
</generator>
</id>
...
<one-to-one name="employee"
class="Employee"
constrained="true"/>
</class>
<many-to-one name="person" class="Person" column="PERSON_ID"
unique="true"/>
<one-to-one name="employee" class="Employee" property-ref="person"/>
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Although we recommend the use of surrogate keys as primary keys, you should try to identify natural
keys for all entities. A natural key is a property or combination of properties that is unique and non-null. It
is also immutable. Map the properties of the natural key inside the <natural-id> element. Hibernate
will generate the necessary unique key and nullability constraints and, as a result, your mapping will be
more self-documenting.
It is recommended that you implement equals() and hashCode() to compare the natural key
properties of the entity.
This mapping is not intended for use with entities that have natural primary keys.
mutable (optional - defaults to false): by default, natural identifier properties are assumed to
be immutable (constant).
Report a bug
5.1.23. Component and Dynamic-component
The <component> element maps properties of a child object to columns of the table of a parent class.
Components can, in turn, declare their own properties, components or collections. See the "Component"
examples below:
name: the name of the property.
class (optional - defaults to the property type determined by reflection): the name of the component
(child) class.
insert: do the mapped columns appear in SQL INSERTs?
update: do the mapped columns appear in SQL UPDATEs?
access (optional - defaults to property): the strategy Hibernate uses for accessing the property value.
<natural-id mutable="true|false"/>
<property ... />
<many-to-one ... />
......
</natural-id>
<component
name="propertyName"
class="className"
insert="true|false"
update="true|false"
access="field|property|ClassName"
lazy="true|false"
optimistic-lock="true|false"
unique="true|false"
node="element-name|."
>
<property ...../>
<many-to-one .... />
........
</component>
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lazy (optional - defaults to false): specifies that this component should be fetched lazily when the
instance variable is first accessed. It requires build-time bytecode instrumentation.
optimistic-lock (optional - defaults to true): specifies that updates to this component either do or
do not require acquisition of the optimistic lock. It determines if a version increment should occur when
this property is dirty.
unique (optional - defaults to false): specifies that a unique constraint exists upon all mapped
columns of the component.
The child <property> tags map properties of the child class to table columns.
The <component> element allows a <parent> subelement that maps a property of the component
class as a reference back to the containing entity.
The <dynamic-component> element allows a Map to be mapped as a component, where the property
names refer to keys of the map.
Report a bug
5.1.24. Properties
The <properties> element allows the definition of a named, logical grouping of the properties of a
class. The most important use of the construct is that it allows a combination of properties to be the
target of a property-ref. It is also a convenient way to define a multi-column unique constraint. For
example:
name: the logical name of the grouping. It is not an actual property name.
insert: do the mapped columns appear in SQL INSERTs?
update: do the mapped columns appear in SQL UPDATEs?
optimistic-lock (optional - defaults to true): specifies that updates to these properties either do or
do not require acquisition of the optimistic lock. It determines if a version increment should occur when
these properties are dirty.
unique (optional - defaults to false): specifies that a unique constraint exists upon all mapped
columns of the component.
For example, if we have the following <properties> mapping:
<properties
name="logicalName"
insert="true|false"
update="true|false"
optimistic-lock="true|false"
unique="true|false"
>
<property ...../>
<many-to-one .... />
........
</properties>
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You might have some legacy data association that refers to this unique key of the Person table, instead
of to the primary key:
The use of this outside the context of mapping legacy data is not recommended.
Report a bug
5.1.25. Subclass
Polymorphic persistence requires the declaration of each subclass of the root persistent class. For the
table-per-class-hierarchy mapping strategy, the <subclass> declaration is used. For example:
name: the fully qualified class name of the subclass.
discriminator-value (optional - defaults to the class name): a value that distinguishes individual
subclasses.
proxy (optional): specifies a class or interface used for lazy initializing proxies.
lazy (optional - defaults to true): setting lazy="false" disables the use of lazy fetching.
<class name="Person">
<id name="personNumber"/>
...
<properties name="name"
unique="true" update="false">
<property name="firstName"/>
<property name="initial"/>
<property name="lastName"/>
</properties>
</class>
<many-to-one name="person"
class="Person" property-ref="name">
<column name="firstName"/>
<column name="initial"/>
<column name="lastName"/>
</many-to-one>
<subclass
name="ClassName"
discriminator-value="discriminator_value"
proxy="ProxyInterface"
lazy="true|false"
dynamic-update="true|false"
dynamic-insert="true|false"
entity-name="EntityName"
node="element-name"
extends="SuperclassName">
<property .... />
.....
</subclass>
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Each subclass declares its own persistent properties and subclasses. <version> and <id> properties
are assumed to be inherited from the root class. Each subclass in a hierarchy must define a unique
discriminator-value. If this is not specified, the fully qualified Java class name is used.
Report a bug
5.1.26. Joined-subclass
Each subclass can also be mapped to its own table. This is called the table-per-subclass mapping
strategy. An inherited state is retrieved by joining with the table of the superclass. To do this you use the
<joined-subclass> element. For example:
name: the fully qualified class name of the subclass.
table: the name of the subclass table.
proxy (optional): specifies a class or interface to use for lazy initializing proxies.
lazy (optional, defaults to true): setting lazy="false" disables the use of lazy fetching.
A discriminator column is not required for this mapping strategy. Each subclass must, however, declare a
table column holding the object identifier using the <key> element. The mapping at the start of the
chapter would then be re-written as:
<joined-subclass
name="ClassName"
table="tablename"
proxy="ProxyInterface"
lazy="true|false"
dynamic-update="true|false"
dynamic-insert="true|false"
schema="schema"
catalog="catalog"
extends="SuperclassName"
persister="ClassName"
subselect="SQL expression"
entity-name="EntityName"
node="element-name">
<key .... >
<property .... />
.....
</joined-subclass>
<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
"-//Hibernate/Hibernate Mapping DTD//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping package="eg">
<class name="Cat" table="CATS">
<id name="id" column="uid" type="long">
<generator class="hilo"/>
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Report a bug
5.1.27. Union-subclass
A third option is to map only the concrete classes of an inheritance hierarchy to tables. This is called the
table-per-concrete-class strategy. Each table defines all persistent states of the class, including the
inherited state. In Hibernate, it is not necessary to explicitly map such inheritance hierarchies. You can
map each class with a separate <class> declaration. However, if you wish use polymorphic
associations (e.g. an association to the superclass of your hierarchy), you need to use the <union-
subclass> mapping. For example:
name: the fully qualified class name of the subclass.
</id>
<property name="birthdate" type="date"/>
<property name="color" not-null="true"/>
<property name="sex" not-null="true"/>
<property name="weight"/>
<many-to-one name="mate"/>
<set name="kittens">
<key column="MOTHER"/>
<one-to-many class="Cat"/>
</set>
<joined-subclass name="DomesticCat"
table="DOMESTIC_CATS">
<key column="CAT"/>
<property name="name" type="string"/>
</joined-subclass>
</class>
<class name="eg.Dog">
<!-- mapping for Dog could go here -->
</class>
</hibernate-mapping>
<union-subclass
name="ClassName"
table="tablename"
proxy="ProxyInterface"
lazy="true|false"
dynamic-update="true|false"
dynamic-insert="true|false"
schema="schema"
catalog="catalog"
extends="SuperclassName"
abstract="true|false"
persister="ClassName"
subselect="SQL expression"
entity-name="EntityName"
node="element-name">
<property .... />
.....
</union-subclass>
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table: the name of the subclass table.
proxy (optional): specifies a class or interface to use for lazy initializing proxies.
lazy (optional, defaults to true): setting lazy="false" disables the use of lazy fetching.
No discriminator column or key column is required for this mapping strategy.
Report a bug
5.1.28. Join
Using the <join> element, it is possible to map properties of one class to several tables that have a
one-to-one relationship. For example:
table: the name of the joined table.
schema (optional): overrides the schema name specified by the root <hibernate-mapping> element.
catalog (optional): overrides the catalog name specified by the root <hibernate-mapping> element.
fetch (optional - defaults to join): if set to join, the default, Hibernate will use an inner join to retrieve
a <join> defined by a class or its superclasses. It will use an outer join for a <join> defined by a
subclass. If set to select then Hibernate will use a sequential select for a <join> defined on a
subclass. This will be issued only if a row represents an instance of the subclass. Inner joins will still be
used to retrieve a <join> defined by the class and its superclasses.
inverse (optional - defaults to false): if enabled, Hibernate will not insert or update the properties
defined by this join.
optional (optional - defaults to false): if enabled, Hibernate will insert a row only if the properties
defined by this join are non-null. It will always use an outer join to retrieve the properties.
For example, address information for a person can be mapped to a separate table while preserving value
type semantics for all properties:
<join
table="tablename"
schema="owner"
catalog="catalog"
fetch="join|select"
inverse="true|false"
optional="true|false">
<key ... />
<property ... />
...
</join>
<class name="Person"
table="PERSON">
<id name="id" column="PERSON_ID">...</id>
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This feature is often only useful for legacy data models. We recommend fewer tables than classes and a
fine-grained domain model. However, it is useful for switching between inheritance mapping strategies in
a single hierarchy, as explained later.
Report a bug
5.1.29. Key
The <key> element has featured a few times within this guide. It appears anywhere the parent mapping
element defines a join to a new table that references the primary key of the original table. It also defines
the foreign key in the joined table:
column (optional): the name of the foreign key column. This can also be specified by nested <column>
element(s).
on-delete (optional - defaults to noaction): specifies whether the foreign key constraint has
database-level cascade delete enabled.
property-ref (optional): specifies that the foreign key refers to columns that are not the primary key of
the original table. It is provided for legacy data.
not-null (optional): specifies that the foreign key columns are not nullable. This is implied whenever
the foreign key is also part of the primary key.
update (optional): specifies that the foreign key should never be updated. This is implied whenever the
foreign key is also part of the primary key.
unique (optional): specifies that the foreign key should have a unique constraint. This is implied
whenever the foreign key is also the primary key.
For systems where delete performance is important, we recommend that all keys should be defined on-
delete="cascade". Hibernate uses a database-level ON CASCADE DELETE constraint, instead of
many individual DELETE statements. Be aware that this feature bypasses Hibernate's usual optimistic
locking strategy for versioned data.
The not-null and update attributes are useful when mapping a unidirectional one-to-many
association. If you map a unidirectional one-to-many association to a non-nullable foreign key, you must
declare the key column using <key not-null="true">.
<join table="ADDRESS">
<key column="ADDRESS_ID"/>
<property name="address"/>
<property name="zip"/>
<property name="country"/>
</join>
...
<key
column="columnname"
on-delete="noaction|cascade"
property-ref="propertyName"
not-null="true|false"
update="true|false"
unique="true|false"
/>
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Report a bug
5.1.30. Column and Formula Elements
Mapping elements which accept a column attribute will alternatively accept a <column> subelement.
Likewise, <formula> is an alternative to the formula attribute. For example:
column and formula attributes can even be combined within the same property or association
mapping to express, for example, exotic join conditions.
Report a bug
5.1.31. Import
If your application has two persistent classes with the same name, and you do not want to specify the
fully qualified package name in Hibernate queries, classes can be "imported" explicitly, rather than
relying upon auto-import="true". You can also import classes and interfaces that are not explicitly
mapped:
class: the fully qualified class name of any Java class.
rename (optional - defaults to the unqualified class name): a name that can be used in the query
language.
Report a bug
<column
name="column_name"
length="N"
precision="N"
scale="N"
not-null="true|false"
unique="true|false"
unique-key="multicolumn_unique_key_name"
index="index_name"
sql-type="sql_type_name"
check="SQL expression"
default="SQL expression"/>
<formula>SQL expression</formula>
<many-to-one name="homeAddress" class="Address"
insert="false" update="false">
<column name="person_id" not-null="true" length="10"/>
<formula>'MAILING'</formula>
</many-to-one>
<import class="java.lang.Object" rename="Universe"/>
<import
class="ClassName"
rename="ShortName"
/>
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5.1.32. Any
There is one more type of property mapping. The <any> mapping element defines a polymorphic
association to classes from multiple tables. This type of mapping requires more than one column. The
first column contains the type of the associated entity. The remaining columns contain the identifier. It is
impossible to specify a foreign key constraint for this kind of association. This is not the usual way of
mapping polymorphic associations and you should use this only in special cases. For example, for audit
logs, user session data, etc.
The meta-type attribute allows the application to specify a custom type that maps database column
values to persistent classes that have identifier properties of the type specified by id-type. You must
specify the mapping from values of the meta-type to class names.
name: the property name.
id-type: the identifier type.
meta-type (optional - defaults to string): any type that is allowed for a discriminator mapping.
cascade (optional- defaults to none): the cascade style.
access (optional - defaults to property): the strategy Hibernate uses for accessing the property value.
optimistic-lock (optional - defaults to true): specifies that updates to this property either do or do
not require acquisition of the optimistic lock. It defines whether a version increment should occur if this
property is dirty.
Report a bug
5.2. HIBERNATE TYPES
<any name="being" id-type="long" meta-type="string">
<meta-value value="TBL_ANIMAL" class="Animal"/>
<meta-value value="TBL_HUMAN" class="Human"/>
<meta-value value="TBL_ALIEN" class="Alien"/>
<column name="table_name"/>
<column name="id"/>
</any>
<any
name="propertyName"
id-type="idtypename"
meta-type="metatypename"
cascade="cascade_style"
access="field|property|ClassName"
optimistic-lock="true|false"
>
<meta-value ... />
<meta-value ... />
.....
<column .... />
<column .... />
.....
</any>
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5.2.1. Entities and Values
In relation to the persistence service, Java language-level objects are classified into two groups:
An entity exists independently of any other objects holding references to the entity. Contrast this with the
usual Java model, where an unreferenced object is garbage collected. Entities must be explicitly saved
and deleted. Saves and deletions, however, can be cascaded from a parent entity to its children. This is
different from the ODMG model of object persistence by reachability and corresponds more closely to
how application objects are usually used in large systems. Entities support circular and shared
references. They can also be versioned.
An entity's persistent state consists of references to other entities and instances of value types. Values
are primitives: collections (not what is inside a collection), components and certain immutable objects.
Unlike entities, values in particular collections and components, are persisted and deleted by
reachability. Since value objects and primitives are persisted and deleted along with their containing
entity, they cannot be independently versioned. Values have no independent identity, so they cannot be
shared by two entities or collections.
Until now, we have been using the term "persistent class" to refer to entities. We will continue to do that.
Not all user-defined classes with a persistent state, however, are entities. A component is a user-defined
class with value semantics. A Java property of type java.lang.String also has value semantics.
Given this definition, all types (classes) provided by the JDK have value type semantics in Java, while
user-defined types can be mapped with entity or value type semantics. This decision is up to the
application developer. An entity class in a domain model will normally have shared references to a single
instance of that class, while composition or aggregation usually translates to a value type.
We will revisit both concepts throughout this reference guide.
The challenge is to map the Java type system, and the developers' definition of entities and value types,
to the SQL/database type system. The bridge between both systems is provided by Hibernate. For
entities, <class>, <subclass> and so on are used. For value types we use <property>,
<component>etc., that usually have a type attribute. The value of this attribute is the name of a
Hibernate mapping type. Hibernate provides a range of mappings for standard JDK value types out of
the box. You can write your own mapping types and implement your own custom conversion strategies.
With the exception of collections, all built-in Hibernate types support null semantics.
Report a bug
5.2.2. Basic Value Types
The built-in basic mapping types can be roughly categorized into the following:
integer, long, short, float, double, character, byte, boolean, yes_no,
true_false
Type mappings from Java primitives or wrapper classes to appropriate (vendor-specific) SQL column
types. boolean, yes_no and true_false are all alternative encodings for a Java boolean or
java.lang.Boolean.
string
A type mapping from java.lang.String to VARCHAR (or Oracle VARCHAR2).
date, time, timestamp
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Type mappings from java.util.Date and its subclasses to SQL types DATE, TIME and
TIMESTAMP (or equivalent).
calendar, calendar_date
Type mappings from java.util.Calendar to SQL types TIMESTAMP and DATE (or equivalent).
big_decimal, big_integer
Type mappings from java.math.BigDecimal and java.math.BigInteger to NUMERIC (or
Oracle NUMBER).
locale, timezone, currency
Type mappings from java.util.Locale, java.util.TimeZone and java.util.Currency to
VARCHAR (or Oracle VARCHAR2). Instances of Locale and Currency are mapped to their ISO
codes. Instances of TimeZone are mapped to their ID.
class
A type mapping from java.lang.Class to VARCHAR (or Oracle VARCHAR2). A Class is mapped to
its fully qualified name.
binary
Maps byte arrays to an appropriate SQL binary type.
text
Maps long Java strings to a SQL CLOB or TEXT type.
serializable
Maps serializable Java types to an appropriate SQL binary type. You can also indicate the Hibernate
type serializable with the name of a serializable Java class or interface that does not default to a
basic type.
clob, blob
Type mappings for the JDBC classes java.sql.Clob and java.sql.Blob. These types can be
inconvenient for some applications, since the blob or clob object cannot be reused outside of a
transaction. Driver support is patchy and inconsistent.
imm_date, imm_time, imm_timestamp, imm_calendar, imm_calendar_date,
imm_serializable, imm_binary
Type mappings for what are considered mutable Java types. This is where Hibernate makes certain
optimizations appropriate only for immutable Java types, and the application treats the object as
immutable. For example, you should not call Date.setTime() for an instance mapped as
imm_timestamp. To change the value of the property, and have that change made persistent, the
application must assign a new, nonidentical, object to the property.
Unique identifiers of entities and collections can be of any basic type except binary, blob and clob.
Composite identifiers are also allowed. See below for more information.
The basic value types have corresponding Type constants defined on org.hibernate.Hibernate.
For example, Hibernate.STRING represents the string type.
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Report a bug
5.2.3. Custom Value Types
It is relatively easy for developers to create their own value types. For example, you might want to persist
properties of type java.lang.BigInteger to VARCHAR columns. Hibernate does not provide a built-in
type for this. Custom types are not limited to mapping a property, or collection element, to a single table
column. So, for example, you might have a Java property getName()/setName() of type
java.lang.String that is persisted to the columns FIRST_NAME, INITIAL, SURNAME.
To implement a custom type, implement either org.hibernate.UserType or
org.hibernate.CompositeUserType and declare properties using the fully qualified classname of
the type. View org.hibernate.test.DoubleStringType to see the kind of things that are possible.
Notice the use of <column> tags to map a property to multiple columns.
The CompositeUserType, EnhancedUserType, UserCollectionType, and UserVersionType
interfaces provide support for more specialized uses.
You can even supply parameters to a UserType in the mapping file. To do this, your UserType must
implement the org.hibernate.usertype.ParameterizedType interface. To supply parameters to
your custom type, you can use the <type> element in your mapping files.
The UserType can now retrieve the value for the parameter named default from the Properties
object passed to it.
If you regularly use a certain UserType, it is useful to define a shorter name for it. You can do this using
the <typedef> element. Typedefs assign a name to a custom type, and can also contain a list of default
parameter values if the type is parameterized.
It is also possible to override the parameters supplied in a typedef on a case-by-case basis by using type
parameters on the property mapping.
Even though Hibernate's rich range of built-in types and support for components means you will rarely
need to use a custom type, it is considered good practice to use custom types for non-entity classes that
<property name="twoStrings" type="org.hibernate.test.DoubleStringType">
<column name="first_string"/>
<column name="second_string"/>
</property>
<property name="priority">
<type name="com.mycompany.usertypes.DefaultValueIntegerType">
<param name="default">0</param>
</type>
</property>
<typedef class="com.mycompany.usertypes.DefaultValueIntegerType"
name="default_zero">
<param name="default">0</param>
</typedef>
<property name="priority" type="default_zero"/>
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occur frequently in your application. For example, a MonetaryAmount class is a good candidate for a
CompositeUserType, even though it could be mapped as a component. One reason for this is
abstraction. With a custom type, your mapping documents would be protected against changes to the
way monetary values are represented.
Report a bug
5.3. ADDITIONAL INFORMATION
5.3.1. Mapping a Class Multiple Times
It is possible to provide more than one mapping for a particular persistent class. In this case, you must
specify an entity name to disambiguate between instances of the two mapped entities. By default, the
entity name is the same as the class name. Hibernate lets you specify the entity name when working
with persistent objects, when writing queries, or when mapping associations to the named entity.
Associations are now specified using entity-name instead of class.
Report a bug
5.3.2. SQL Quoted Identifiers
You can force Hibernate to quote an identifier in the generated SQL by enclosing the table or column
name in backticks in the mapping document. Hibernate will use the correct quotation style for the SQL
Dialect. This is usually double quotes, but the SQL Server uses brackets and MySQL uses backticks.
Report a bug
5.4. METADATA ALTERNATIVES
<class name="Contract" table="Contracts"
entity-name="CurrentContract">
...
<set name="history" inverse="true"
order-by="effectiveEndDate desc">
<key column="currentContractId"/>
<one-to-many entity-name="HistoricalContract"/>
</set>
</class>
<class name="Contract" table="ContractHistory"
entity-name="HistoricalContract">
...
<many-to-one name="currentContract"
column="currentContractId"
entity-name="CurrentContract"/>
</class>
<class name="LineItem" table="`Line Item`">
<id name="id" column="`Item Id`"/><generator class="assigned"/></id>
<property name="itemNumber" column="`Item #`"/>
...
</class>
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5.4.1. About Metadata Alternatives
XML does not suit all users so there are some alternative ways to define O/R mapping metadata in
Hibernate.
Report a bug
5.4.2. Using XDoclet Markup
Many Hibernate users prefer to embed mapping information directly in sourcecode using XDoclet
@hibernate.tags. We do not cover this approach in this reference guide since it is considered part of
XDoclet. However, we include the following example of the Cat class with XDoclet mappings:
package eg;
import java.util.Set;
import java.util.Date;
/**
* @hibernate.class
* table="CATS"
*/
public class Cat {
private Long id; // identifier
private Date birthdate;
private Cat mother;
private Set kittens;
private Color color;
private char sex;
private float weight;
/*
* @hibernate.id
* generator-class="native"
* column="CAT_ID"
*/
public Long getId() {
return id;
}
private void setId(Long id) {
this.id=id;
}
/**
* @hibernate.many-to-one
* column="PARENT_ID"
*/
public Cat getMother() {
return mother;
}
void setMother(Cat mother) {
this.mother = mother;
}
/**
* @hibernate.property
* column="BIRTH_DATE"
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*/
public Date getBirthdate() {
return birthdate;
}
void setBirthdate(Date date) {
birthdate = date;
}
/**
* @hibernate.property
* column="WEIGHT"
*/
public float getWeight() {
return weight;
}
void setWeight(float weight) {
this.weight = weight;
}
/**
* @hibernate.property
* column="COLOR"
* not-null="true"
*/
public Color getColor() {
return color;
}
void setColor(Color color) {
this.color = color;
}
/**
* @hibernate.set
* inverse="true"
* order-by="BIRTH_DATE"
* @hibernate.collection-key
* column="PARENT_ID"
* @hibernate.collection-one-to-many
*/
public Set getKittens() {
return kittens;
}
void setKittens(Set kittens) {
this.kittens = kittens;
}
// addKitten not needed by Hibernate
public void addKitten(Cat kitten) {
kittens.add(kitten);
}
/**
* @hibernate.property
* column="SEX"
* not-null="true"
* update="false"
*/
public char getSex() {
return sex;
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See the Hibernate website for more examples of XDoclet and Hibernate.
Report a bug
5.4.3. Using JDK 5.0 Annotations
JDK 5.0 introduced XDoclet-style annotations at the language level that are type-safe and checked at
compile time. This mechanism is more powerful than XDoclet annotations and better supported by tools
and IDEs. IntelliJ IDEA, for example, supports auto-completion and syntax highlighting of JDK 5.0
annotations. The new revision of the EJB specification (JSR-220) uses JDK 5.0 annotations as the
primary metadata mechanism for entity beans. Hibernate3 implements the EntityManager of JSR-220
(the persistence API). Support for mapping metadata is available via the Hibernate Annotations package
as a separate download. Both EJB3 (JSR-220) and Hibernate3 metadata is supported.
This is an example of a POJO class annotated as an EJB entity bean:
Report a bug
5.4.4. Generated Properties
Generated properties are properties that have their values generated by the database. Typically,
Hibernate applications needed to refresh objects that contain any properties for which the database
was generating values. Marking properties as generated, however, lets the application delegate this
responsibility to Hibernate. When Hibernate issues an SQL INSERT or UPDATE for an entity that has
defined generated properties, it immediately issues a select afterwards to retrieve the generated values.
}
void setSex(char sex) {
this.sex=sex;
}
}
@Entity
public class Customer implements Serializable {
@Id
Long id;
String firstName;
String lastName;
Date birthday;
@Transient
Integer age;
@Embedded
private Address homeAddress;
@OneToMany(cascade=CascadeType.ALL)
@JoinColumn(name="CUSTOMER_ID")
Set<Order> orders;
// Getter/setter and business methods
}
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Properties marked as generated must additionally be non-insertable and non-updateable. Only version,
timestamp and property can be marked as generated.
never (the default): the given property value is not generated within the database.
insert: the given property value is generated on insert, but is not regenerated on subsequent updates.
Properties like created-date fall into this category. Even though the version and timestamp properties can
be marked as generated, this option is not available.
always: the property value is generated both on insert and on update.
Report a bug
5.4.5. Auxiliary Database Objects
Auxiliary database objects allow for the CREATE and DROP of arbitrary database objects. In conjunction
with Hibernate's schema evolution tools, they have the ability to fully define a user schema within the
Hibernate mapping files. Although designed specifically for creating and dropping things like triggers or
stored procedures, any SQL command that can be run via a java.sql.Statement.execute()
method is valid (for example, ALTERs, INSERTS, etc.). There are essentially two modes for defining
auxiliary database objects:
The first mode is to explicitly list the CREATE and DROP commands in the mapping file:
The second mode is to supply a custom class that constructs the CREATE and DROP commands. This
custom class must implement the org.hibernate.mapping.AuxiliaryDatabaseObject
interface.
Additionally, these database objects can be optionally scoped so that they only apply when certain
dialects are used.
<hibernate-mapping>
...
<database-object>
<create>CREATE TRIGGER my_trigger ...</create>
<drop>DROP TRIGGER my_trigger</drop>
</database-object>
</hibernate-mapping>
<hibernate-mapping>
...
<database-object>
<definition class="MyTriggerDefinition"/>
</database-object>
</hibernate-mapping>
<hibernate-mapping>
...
<database-object>
<definition class="MyTriggerDefinition"/>
<dialect-scope name="org.hibernate.dialect.Oracle9iDialect"/>
<dialect-scope name="org.hibernate.dialect.Oracle10gDialect"/>
</database-object>
</hibernate-mapping>
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CHAPTER 6. COLLECTION MAPPING
6.1. PERSISTENT COLLECTIONS
Hibernate requires that persistent collection-valued fields be declared as an interface type. For example:
The actual interface might be java.util.Set, java.util.Collection, java.util.List,
java.util.Map, java.util.SortedSet, java.util.SortedMap or anything you like ("anything
you like" means you will have to write an implementation of
org.hibernate.usertype.UserCollectionType.)
Notice how the instance variable was initialized with an instance of HashSet. This is the best way to
initialize collection valued properties of newly instantiated (non-persistent) instances. When you make
the instance persistent, by calling persist() for example, Hibernate will actually replace the HashSet
with an instance of Hibernate's own implementation of Set. Be aware of the following errors:
The persistent collections injected by Hibernate behave like HashMap, HashSet, TreeMap, TreeSet or
ArrayList, depending on the interface type.
Collections instances have the usual behavior of value types. They are automatically persisted when
referenced by a persistent object and are automatically deleted when unreferenced. If a collection is
passed from one persistent object to another, its elements might be moved from one table to another.
Two entities cannot share a reference to the same collection instance. Due to the underlying relational
model, collection-valued properties do not support null value semantics. Hibernate does not distinguish
between a null collection reference and an empty collection.
Use persistent collections the same way you use ordinary Java collections. However, please ensure you
understand the semantics of bidirectional associations (these are discussed later).
Report a bug
6.2. COLLECTION MAPPINGS
public class Product {
private String serialNumber;
private Set parts = new HashSet();
public Set getParts() { return parts; }
void setParts(Set parts) { this.parts = parts; }
public String getSerialNumber() { return serialNumber; }
void setSerialNumber(String sn) { serialNumber = sn; }
}
Cat cat = new DomesticCat();
Cat kitten = new DomesticCat();
....
Set kittens = new HashSet();
kittens.add(kitten);
cat.setKittens(kittens);
session.persist(cat);
kittens = cat.getKittens(); // Okay, kittens collection is a Set
(HashSet) cat.getKittens(); // Error!
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6.2.1. About Collection Mappings
NOTE
There are quite a range of mappings that can be generated for collections that cover
many common relational models. We suggest you experiment with the schema
generation tool so that you understand how various mapping declarations translate to
database tables.
The Hibernate mapping element used for mapping a collection depends upon the type of interface. For
example, a <set> element is used for mapping properties of type Set.
Apart from <set>, there is also <list>, <map>, <bag>, <array> and <primitive-array> mapping
elements. The <map> element is representative:
name: the collection property name
table (optional - defaults to property name): the name of the collection table. It is not used for one-to-
many associations.
schema (optional): the name of a table schema to override the schema declared on the root element
<class name="Product">
<id name="serialNumber" column="productSerialNumber"/>
<set name="parts">
<key column="productSerialNumber" not-null="true"/>
<one-to-many class="Part"/>
</set>
</class>
<map
name="propertyName"
table="table_name"
schema="schema_name"
lazy="true|extra|false"
inverse="true|false"
cascade="all|none|save-update|delete|all-delete-orphan|delete-orphan"
sort="unsorted|natural|comparatorClass"
order-by="column_name asc|desc"
where="arbitrary sql where condition"
fetch="join|select|subselect"
batch-size="N"
access="field|property|ClassName"
optimistic-lock="true|false"
mutable="true|false"
node="element-name|."
embed-xml="true|false"
>
<key .... />
<map-key .... />
<element .... />
</map>
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lazy (optional - defaults to true): disables lazy fetching and specifies that the association is always
eagerly fetched. It can also be used to enable "extra-lazy" fetching where most operations do not
initialize the collection. This is suitable for large collections.
inverse (optional - defaults to false): marks this collection as the "inverse" end of a bidirectional
association.
cascade (optional - defaults to none): enables operations to cascade to child entities.
sort (optional): specifies a sorted collection with natural sort order or a given comparator class.
order-by (optional, JDK1.4 only): specifies a table column or columns that define the iteration order of
the Map, Set or bag, together with an optional asc or desc.
where (optional): specifies an arbitrary SQL WHERE condition that is used when retrieving or removing
the collection. This is useful if the collection needs to contain only a subset of the available data.
fetch (optional, defaults to select): chooses between outer-join fetching, fetching by sequential
select, and fetching by sequential subselect.
batch-size (optional, defaults to 1): specifies a "batch size" for lazily fetching instances of this
collection.
access (optional - defaults to property): the strategy Hibernate uses for accessing the collection
property value.
optimistic-lock (optional - defaults to true): specifies that changes to the state of the collection
results in increments of the owning entity's version. For one-to-many associations you may want to
disable this setting.
mutable (optional - defaults to true): a value of false specifies that the elements of the collection
never change. This allows for minor performance optimization in some cases.
Report a bug
6.2.2. Collection Foreign Keys
Collection instances are distinguished in the database by the foreign key of the entity that owns the
collection. This foreign key is referred to as the collection key column, or columns, of the collection table.
The collection key column is mapped by the <key> element.
There can be a nullability constraint on the foreign key column. For most collections, this is implied. For
unidirectional one-to-many associations, the foreign key column is nullable by default, so you may need
to specify not-null="true".
The foreign key constraint can use ON DELETE CASCADE.
See the previous chapter for a full definition of the <key> element.
Report a bug
<key column="productSerialNumber" not-null="true"/>
<key column="productSerialNumber" on-delete="cascade"/>
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6.2.3. Collection Elements
Collections can contain almost any other Hibernate type, including: basic types, custom types,
components and references to other entities. This is an important distinction. An object in a collection
might be handled with "value" semantics (its life cycle fully depends on the collection owner), or it might
be a reference to another entity with its own life cycle. In the latter case, only the "link" between the two
objects is considered to be a state held by the collection.
The contained type is referred to as the collection element type. Collection elements are mapped by
<element> or <composite-element>, or in the case of entity references, with <one-to-many> or
<many-to-many>. The first two map elements with value semantics, the next two are used to map
entity associations.
Report a bug
6.2.4. Indexed Collections
All collection mappings, except those with set and bag semantics, need an index column in the collection
table. An index column is a column that maps to an array index, or List index, or Map key. The index of
a Map may be of any basic type, mapped with <map-key>. It can be an entity reference mapped with
<map-key-many-to-many>, or it can be a composite type mapped with <composite-map-key>.
The index of an array or list is always of type integer and is mapped using the <list-index>
element. The mapped column contains sequential integers that are numbered from zero by default.
column_name (required): the name of the column holding the collection index values.
base (optional - defaults to 0): the value of the index column that corresponds to the first element of the
list or array.
column (optional): the name of the column holding the collection index values.
formula (optional): a SQL formula used to evaluate the key of the map.
type (required): the type of the map keys.
column (optional): the name of the foreign key column for the collection index values.
<list-index
column="column_name"
base="0|1|..."/>
<map-key
column="column_name"
formula="any SQL expression"
type="type_name"
node="@attribute-name"
length="N"/>
<map-key-many-to-many
column="column_name"
formula="any SQL expression"
class="ClassName"
/>
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formula (optional): a SQ formula used to evaluate the foreign key of the map key.
class (required): the entity class used as the map key.
If your table does not have an index column, and you still wish to use List as the property type, you can
map the property as a Hibernate <bag>. A bag does not retain its order when it is persisted to the
database, but it can be optionally sorted or ordered when it is retrieved from the database.
Report a bug
6.2.5. Collections of Values and Many-to-many Associations
Any collection of values or many-to-many associations requires a dedicated collection table with a
foreign key column or columns, collection element column or columns, and possibly an index column or
columns.
For a collection of values use the <element> tag. For example:
column (optional): the name of the column holding the collection element values.
formula (optional): an SQL formula used to evaluate the element.
type (required): the type of the collection element.
A many-to-many association is specified using the <many-to-many> element.
column (optional): the name of the element foreign key column.
formula (optional): an SQL formula used to evaluate the element foreign key value.
<element
column="column_name"
formula="any SQL expression"
type="typename"
length="L"
precision="P"
scale="S"
not-null="true|false"
unique="true|false"
node="element-name"
/>
<many-to-many
column="column_name"
formula="any SQL expression"
class="ClassName"
fetch="select|join"
unique="true|false"
not-found="ignore|exception"
entity-name="EntityName"
property-ref="propertyNameFromAssociatedClass"
node="element-name"
embed-xml="true|false"
/>
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class (required): the name of the associated class.
fetch (optional - defaults to join): enables outer-join or sequential select fetching for this association.
This is a special case; for full eager fetching in a single SELECT of an entity and its many-to-many
relationships to other entities, you would enable join fetching,not only of the collection itself, but also
with this attribute on the <many-to-many> nested element.
unique (optional): enables the DDL generation of a unique constraint for the foreign-key column. This
makes the association multiplicity effectively one-to-many.
not-found (optional - defaults to exception): specifies how foreign keys that reference missing rows
will be handled: ignore will treat a missing row as a null association.
entity-name (optional): the entity name of the associated class, as an alternative to class.
property-ref (optional): the name of a property of the associated class that is joined to this foreign
key. If not specified, the primary key of the associated class is used.
Here are some examples.
A set of strings:
A bag containing integers with an iteration order determined by the order-by attribute:
An array of entities, in this case, a many-to-many association:
A map from string indices to dates:
<set name="names" table="person_names">
<key column="person_id"/>
<element column="person_name" type="string"/>
</set>
<bag name="sizes"
table="item_sizes"
order-by="size asc">
<key column="item_id"/>
<element column="size" type="integer"/>
</bag>
<array name="addresses"
table="PersonAddress"
cascade="persist">
<key column="personId"/>
<list-index column="sortOrder"/>
<many-to-many column="addressId" class="Address"/>
</array>
<map name="holidays"
table="holidays"
schema="dbo"
order-by="hol_name asc">
<key column="id"/>
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A list of components (this is discussed in the next chapter):
Report a bug
6.2.6. One-to-many Associations
A one-to-many association links the tables of two classes via a foreign key with no intervening collection
table. This mapping loses certain semantics of normal Java collections:
An instance of the contained entity class cannot belong to more than one instance of the
collection.
An instance of the contained entity class cannot appear at more than one value of the collection
index.
An association from Product to Part requires the existence of a foreign key column and possibly an
index column to the Part table. A <one-to-many> tag indicates that this is a one-to-many association.
class (required): the name of the associated class.
not-found (optional - defaults to exception): specifies how cached identifiers that reference missing
rows will be handled. ignore will treat a missing row as a null association.
entity-name (optional): the entity name of the associated class, as an alternative to class.
The <one-to-many> element does not need to declare any columns. Nor is it necessary to specify the
table name anywhere.
<map-key column="hol_name" type="string"/>
<element column="hol_date" type="date"/>
</map>
<list name="carComponents"
table="CarComponents">
<key column="carId"/>
<list-index column="sortOrder"/>
<composite-element class="CarComponent">
<property name="price"/>
<property name="type"/>
<property name="serialNumber" column="serialNum"/>
</composite-element>
</list>
<one-to-many
class="ClassName"
not-found="ignore|exception"
entity-name="EntityName"
node="element-name"
embed-xml="true|false"
/>
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WARNING
If the foreign key column of a <one-to-many> association is declared NOT NULL,
you must declare the <key> mapping not-null="true" or use a bidirectional
association with the collection mapping marked inverse="true". See the
discussion of bidirectional associations later in this chapter for more information.
The following example shows a map of Part entities by name, where partName is a persistent property
of Part. Notice the use of a formula-based index:
Report a bug
6.3. ADVANCED COLLECTION MAPPINGS
6.3.1. Sorted Collections
Hibernate supports collections implementing java.util.SortedMap and java.util.SortedSet.
You must specify a comparator in the mapping file:
Allowed values of the sort attribute are unsorted, natural and the name of a class implementing
java.util.Comparator.
Sorted collections actually behave like java.util.TreeSet or java.util.TreeMap.
If you want the database itself to order the collection elements, use the order-by attribute of set, bag
or map mappings. This solution is only available under JDK 1.4 or higher and is implemented using
LinkedHashSet or LinkedHashMap. This performs the ordering in the SQL query and not in the
memory.
<map name="parts"
cascade="all">
<key column="productId" not-null="true"/>
<map-key formula="partName"/>
<one-to-many class="Part"/>
</map>
<set name="aliases"
table="person_aliases"
sort="natural">
<key column="person"/>
<element column="name" type="string"/>
</set>
<map name="holidays" sort="my.custom.HolidayComparator">
<key column="year_id"/>
<map-key column="hol_name" type="string"/>
<element column="hol_date" type="date"/>
</map>
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NOTE
The value of the order-by attribute is an SQL ordering, not an HQL ordering.
Associations can even be sorted by arbitrary criteria at runtime using a collection filter():
Report a bug
6.3.2. Bidirectional Associations
A bidirectional association allows navigation from both "ends" of the association. Two kinds of
bidirectional association are supported:
one-to-many
set or bag valued at one end and single-valued at the other
many-to-many
set or bag valued at both ends
You can specify a bidirectional many-to-many association by mapping two many-to-many associations
to the same database table and declaring one end as inverse. You cannot select an indexed collection.
Here is an example of a bidirectional many-to-many association that illustrates how each category can
have many items and each item can be in many categories:
<set name="aliases" table="person_aliases" order-by="lower(name) asc">
<key column="person"/>
<element column="name" type="string"/>
</set>
<map name="holidays" order-by="hol_date, hol_name">
<key column="year_id"/>
<map-key column="hol_name" type="string"/>
<element column="hol_date type="date"/>
</map>
sortedUsers = s.createFilter( group.getUsers(), "order by this.name"
).list();
<class name="Category">
<id name="id" column="CATEGORY_ID"/>
...
<bag name="items" table="CATEGORY_ITEM">
<key column="CATEGORY_ID"/>
<many-to-many class="Item" column="ITEM_ID"/>
</bag>
</class>
<class name="Item">
<id name="id" column="ITEM_ID"/>
...
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Changes made only to the inverse end of the association are not persisted. This means that Hibernate
has two representations in memory for every bidirectional association: one link from A to B and another
link from B to A. This is easier to understand if you think about the Java object model and how a many-
to-many relationship in Javais created:
The non-inverse side is used to save the in-memory representation to the database.
You can define a bidirectional one-to-many association by mapping a one-to-many association to the
same table column(s) as a many-to-one association and declaring the many-valued end
inverse="true".
Mapping one end of an association with inverse="true" does not affect the operation of cascades as
these are orthogonal concepts.
Report a bug
6.3.3. Bidirectional Associations with Indexed Collections
<!-- inverse end -->
<bag name="categories" table="CATEGORY_ITEM" inverse="true">
<key column="ITEM_ID"/>
<many-to-many class="Category" column="CATEGORY_ID"/>
</bag>
</class>
category.getItems().add(item); // The category now "knows" about
the relationship
item.getCategories().add(category); // The item now "knows" about the
relationship
session.persist(item); // The relationship won't be
saved!
session.persist(category); // The relationship will be
saved
<class name="Parent">
<id name="id" column="parent_id"/>
....
<set name="children" inverse="true">
<key column="parent_id"/>
<one-to-many class="Child"/>
</set>
</class>
<class name="Child">
<id name="id" column="child_id"/>
....
<many-to-one name="parent"
class="Parent"
column="parent_id"
not-null="true"/>
</class>
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A bidirectional association where one end is represented as a <list> or <map>, requires special
consideration. If there is a property of the child class that maps to the index column you can use
inverse="true" on the collection mapping:
If there is no such property on the child class, the association cannot be considered truly bidirectional.
That is, there is information available at one end of the association that is not available at the other end.
In this case, you cannot map the collection inverse="true". Instead, you could use the following
mapping:
<class name="Parent">
<id name="id" column="parent_id"/>
....
<map name="children" inverse="true">
<key column="parent_id"/>
<map-key column="name"
type="string"/>
<one-to-many class="Child"/>
</map>
</class>
<class name="Child">
<id name="id" column="child_id"/>
....
<property name="name"
not-null="true"/>
<many-to-one name="parent"
class="Parent"
column="parent_id"
not-null="true"/>
</class>
<class name="Parent">
<id name="id" column="parent_id"/>
....
<map name="children">
<key column="parent_id"
not-null="true"/>
<map-key column="name"
type="string"/>
<one-to-many class="Child"/>
</map>
</class>
<class name="Child">
<id name="id" column="child_id"/>
....
<many-to-one name="parent"
class="Parent"
column="parent_id"
insert="false"
update="false"
not-null="true"/>
</class>
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125
Note that in this mapping, the collection-valued end of the association is responsible for updates to the
foreign key.
Report a bug
6.3.4. Ternary Associations
There are three possible approaches to mapping a ternary association. One approach is to use a Map
with an association as its index:
A second approach is to remodel the association as an entity class. This is the most common approach.
A final alternative is to use composite elements, which will be discussed later.
Report a bug
6.3.5. Using an idbag
The majority of the many-to-many associations and collections of values shown previously all map to
tables with composite keys, even though it has been have suggested that entities should have synthetic
identifiers (surrogate keys). A pure association table does not seem to benefit much from a surrogate
key, although a collection of composite values might. It is for this reason that Hibernate provides a
feature that allows you to map many-to-many associations and collections of values to a table with a
surrogate key.
The <idbag> element lets you map a List (or Collection) with bag semantics. For example:
An <idbag> has a synthetic id generator, just like an entity class. A different surrogate key is assigned
to each collection row. Hibernate does not, however, provide any mechanism for discovering the
surrogate key value of a particular row.
The update performance of an <idbag> supersedes a regular <bag>. Hibernate can locate individual
rows efficiently and update or delete them individually, similar to a list, map or set.
<map name="contracts">
<key column="employer_id" not-null="true"/>
<map-key-many-to-many column="employee_id" class="Employee"/>
<one-to-many class="Contract"/>
</map>
<map name="connections">
<key column="incoming_node_id"/>
<map-key-many-to-many column="outgoing_node_id" class="Node"/>
<many-to-many column="connection_id" class="Connection"/>
</map>
<idbag name="lovers" table="LOVERS">
<collection-id column="ID" type="long">
<generator class="sequence"/>
</collection-id>
<key column="PERSON1"/>
<many-to-many column="PERSON2" class="Person" fetch="join"/>
</idbag>
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In the current implementation, the native identifier generation strategy is not supported for <idbag>
collection identifiers.
Report a bug
6.4. COLLECTION EXAMPLE
6.4.1. Collection Examples
This section covers collection examples.
The following class has a collection of Child instances:
If each child has, at most, one parent, the most natural mapping is a one-to-many association:
This maps to the following table definitions:
package eg;
import java.util.Set;
public class Parent {
private long id;
private Set children;
public long getId() { return id; }
private void setId(long id) { this.id=id; }
private Set getChildren() { return children; }
private void setChildren(Set children) { this.children=children; }
....
....
}
<hibernate-mapping>
<class name="Parent">
<id name="id">
<generator class="sequence"/>
</id>
<set name="children">
<key column="parent_id"/>
<one-to-many class="Child"/>
</set>
</class>
<class name="Child">
<id name="id">
<generator class="sequence"/>
</id>
<property name="name"/>
</class>
</hibernate-mapping>
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If the parent is required, use a bidirectional one-to-many association:
Notice the NOT NULL constraint:
Alternatively, if this association must be unidirectional you can declare the NOT NULL constraint on the
<key> mapping:
create table parent ( id bigint not null primary key )
create table child ( id bigint not null primary key, name varchar(255),
parent_id bigint )
alter table child add constraint childfk0 (parent_id) references parent
<hibernate-mapping>
<class name="Parent">
<id name="id">
<generator class="sequence"/>
</id>
<set name="children" inverse="true">
<key column="parent_id"/>
<one-to-many class="Child"/>
</set>
</class>
<class name="Child">
<id name="id">
<generator class="sequence"/>
</id>
<property name="name"/>
<many-to-one name="parent" class="Parent" column="parent_id" not-
null="true"/>
</class>
</hibernate-mapping>
create table parent ( id bigint not null primary key )
create table child ( id bigint not null
primary key,
name varchar(255),
parent_id bigint not null )
alter table child add constraint childfk0 (parent_id) references parent
<hibernate-mapping>
<class name="Parent">
<id name="id">
<generator class="sequence"/>
</id>
<set name="children">
<key column="parent_id" not-null="true"/>
<one-to-many class="Child"/>
</set>
</class>
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On the other hand, if a child has multiple parents, a many-to-many association is appropriate:
Table definitions:
For more examples and a complete explanation of a parent/child relationship mapping, refer to the
Parent/Child Example chapter.
Even more complex association mappings are covered in the next chapter.
Report a bug
<class name="Child">
<id name="id">
<generator class="sequence"/>
</id>
<property name="name"/>
</class>
</hibernate-mapping>
<hibernate-mapping>
<class name="Parent">
<id name="id">
<generator class="sequence"/>
</id>
<set name="children" table="childset">
<key column="parent_id"/>
<many-to-many class="Child" column="child_id"/>
</set>
</class>
<class name="Child">
<id name="id">
<generator class="sequence"/>
</id>
<property name="name"/>
</class>
</hibernate-mapping>
create table parent ( id bigint not null primary key )
create table child ( id bigint not null primary key, name varchar(255) )
create table childset ( parent_id bigint not null,
child_id bigint not null,
primary key ( parent_id, child_id ) )
alter table childset add constraint childsetfk0 (parent_id) references
parent
alter table childset add constraint childsetfk1 (child_id) references
child
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CHAPTER 7. ASSOCIATION MAPPINGS
7.1. ABOUT ASSOCIATION MAPPINGS
Association mappings are often the most difficult thing to implement correctly. In this section we examine
some canonical cases one by one, starting with unidirectional mappings and then bidirectional cases.
We will use Person and Address in all the examples.
Associations will be classified by multiplicity and whether or not they map to an intervening join table.
Nullable foreign keys are not considered to be good practice in traditional data modelling, so our
examples do not use nullable foreign keys. This is not a requirement of Hibernate, and the mappings will
work if you drop the nullability constraints.
Report a bug
7.2. UNIDIRECTIONAL ASSOCIATIONS
7.2.1. Unidirectional Many-to-one
A unidirectional many-to-one association is the most common kind of unidirectional association.
Report a bug
7.2.2. Unidirectional One-to-one
A unidirectional one-to-one association on a foreign key is almost identical. The only difference is the
column unique constraint.
<class name="Person">
<id name="id" column="personId">
<generator class="native"/>
</id>
<many-to-one name="address"
column="addressId"
not-null="true"/>
</class>
<class name="Address">
<id name="id" column="addressId">
<generator class="native"/>
</id>
</class>
create table Person ( personId bigint not null primary key, addressId
bigint not null )
create table Address ( addressId bigint not null primary key )
<class name="Person">
<id name="id" column="personId">
<generator class="native"/>
</id>
<many-to-one name="address"
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A unidirectional one-to-one association on a primary key usually uses a special id generator In this
example, however, we have reversed the direction of the association:
Report a bug
7.2.3. Unidirectional One-to-many
A unidirectional one-to-many association on a foreign key is an unusual case, and is not recommended.
column="addressId"
unique="true"
not-null="true"/>
</class>
<class name="Address">
<id name="id" column="addressId">
<generator class="native"/>
</id>
</class>
create table Person ( personId bigint not null primary key, addressId
bigint not null unique )
create table Address ( addressId bigint not null primary key )
<class name="Person">
<id name="id" column="personId">
<generator class="native"/>
</id>
</class>
<class name="Address">
<id name="id" column="personId">
<generator class="foreign">
<param name="property">person</param>
</generator>
</id>
<one-to-one name="person" constrained="true"/>
</class>
create table Person ( personId bigint not null primary key )
create table Address ( personId bigint not null primary key )
<class name="Person">
<id name="id" column="personId">
<generator class="native"/>
</id>
<set name="addresses">
<key column="personId"
not-null="true"/>
<one-to-many class="Address"/>
</set>
</class>
<class name="Address">
CHAPTER 7. ASSOCIATION MAPPINGS
131
You should instead use a join table for this kind of association.
Report a bug
7.3. UNIDIRECTIONAL ASSOCIATIONS WITH JOIN TABLES
7.3.1. Unidirectional One-to-many with Join Tables
A unidirectional one-to-many association on a join table is the preferred option. Specifying
unique="true", changes the multiplicity from many-to-many to one-to-many.
Report a bug
7.3.2. Unidirectional Many-to-one with Join Tables
A unidirectional many-to-one association on a join table is common when the association is optional. For
example:
<id name="id" column="addressId">
<generator class="native"/>
</id>
</class>
create table Person ( personId bigint not null primary key )
create table Address ( addressId bigint not null primary key, personId
bigint not null )
<class name="Person">
<id name="id" column="personId">
<generator class="native"/>
</id>
<set name="addresses" table="PersonAddress">
<key column="personId"/>
<many-to-many column="addressId"
unique="true"
class="Address"/>
</set>
</class>
<class name="Address">
<id name="id" column="addressId">
<generator class="native"/>
</id>
</class>
create table Person ( personId bigint not null primary key )
create table PersonAddress ( personId not null, addressId bigint not null
primary key )
create table Address ( addressId bigint not null primary key )
<class name="Person">
<id name="id" column="personId">
<generator class="native"/>
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Report a bug
7.3.3. Unidirectional One-to-one with Join Tables
A unidirectional one-to-one association on a join table is possible, but extremely unusual.
Report a bug
</id>
<join table="PersonAddress"
optional="true">
<key column="personId" unique="true"/>
<many-to-one name="address"
column="addressId"
not-null="true"/>
</join>
</class>
<class name="Address">
<id name="id" column="addressId">
<generator class="native"/>
</id>
</class>
create table Person ( personId bigint not null primary key )
create table PersonAddress ( personId bigint not null primary key,
addressId bigint not null )
create table Address ( addressId bigint not null primary key )
<class name="Person">
<id name="id" column="personId">
<generator class="native"/>
</id>
<join table="PersonAddress"
optional="true">
<key column="personId"
unique="true"/>
<many-to-one name="address"
column="addressId"
not-null="true"
unique="true"/>
</join>
</class>
<class name="Address">
<id name="id" column="addressId">
<generator class="native"/>
</id>
</class>
create table Person ( personId bigint not null primary key )
create table PersonAddress ( personId bigint not null primary key,
addressId bigint not null unique )
create table Address ( addressId bigint not null primary key )
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7.3.4. Unidirectional Many-to-many with Join Tables
Finally, here is an example of a unidirectional many-to-many association .
Report a bug
7.4. BIDIRECTIONAL ASSOCIATIONS
7.4.1. Bidirectional One-to-many and Many-to-one
A bidirectional many-to-one association is the most common kind of association. The following example
illustrates the standard parent/child relationship.
<class name="Person">
<id name="id" column="personId">
<generator class="native"/>
</id>
<set name="addresses" table="PersonAddress">
<key column="personId"/>
<many-to-many column="addressId"
class="Address"/>
</set>
</class>
<class name="Address">
<id name="id" column="addressId">
<generator class="native"/>
</id>
</class>
create table Person ( personId bigint not null primary key )
create table PersonAddress ( personId bigint not null, addressId bigint
not null, primary key (personId, addressId) )
create table Address ( addressId bigint not null primary key )
<class name="Person">
<id name="id" column="personId">
<generator class="native"/>
</id>
<many-to-one name="address"
column="addressId"
not-null="true"/>
</class>
<class name="Address">
<id name="id" column="addressId">
<generator class="native"/>
</id>
<set name="people" inverse="true">
<key column="addressId"/>
<one-to-many class="Person"/>
</set>
</class>
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If you use a List, or other indexed collection, set the key column of the foreign key to not null.
Hibernate will manage the association from the collections side to maintain the index of each element,
making the other side virtually inverse by setting update="false" and insert="false":
If the underlying foreign key column is NOT NULL, it is important that you define not-null="true" on
the <key> element of the collection mapping. Do not only declare not-null="true" on a possible
nested <column> element, but on the <key> element.
Report a bug
7.4.2. Bidirectional One-to-one
A bidirectional one-to-one association on a foreign key is common:
create table Person ( personId bigint not null primary key, addressId
bigint not null )
create table Address ( addressId bigint not null primary key )
<class name="Person">
<id name="id"/>
...
<many-to-one name="address"
column="addressId"
not-null="true"
insert="false"
update="false"/>
</class>
<class name="Address">
<id name="id"/>
...
<list name="people">
<key column="addressId" not-null="true"/>
<list-index column="peopleIdx"/>
<one-to-many class="Person"/>
</list>
</class>
<class name="Person">
<id name="id" column="personId">
<generator class="native"/>
</id>
<many-to-one name="address"
column="addressId"
unique="true"
not-null="true"/>
</class>
<class name="Address">
<id name="id" column="addressId">
<generator class="native"/>
</id>
<one-to-one name="person"
property-ref="address"/>
</class>
CHAPTER 7. ASSOCIATION MAPPINGS
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A bidirectional one-to-one association on a primary key uses the special id generator:
Report a bug
7.5. BIDIRECTIONAL ASSOCIATIONS WITH JOIN TABLES
7.5.1. Bidirectional One-to-many and Many-to-one with Join Tables
The following is an example of a bidirectional one-to-many association on a join table. The
inverse="true" can go on either end of the association, on the collection, or on the join.
create table Person ( personId bigint not null primary key, addressId
bigint not null unique )
create table Address ( addressId bigint not null primary key )
<class name="Person">
<id name="id" column="personId">
<generator class="native"/>
</id>
<one-to-one name="address"/>
</class>
<class name="Address">
<id name="id" column="personId">
<generator class="foreign">
<param name="property">person</param>
</generator>
</id>
<one-to-one name="person"
constrained="true"/>
</class>
create table Person ( personId bigint not null primary key )
create table Address ( personId bigint not null primary key )
<class name="Person">
<id name="id" column="personId">
<generator class="native"/>
</id>
<set name="addresses"
table="PersonAddress">
<key column="personId"/>
<many-to-many column="addressId"
unique="true"
class="Address"/>
</set>
</class>
<class name="Address">
<id name="id" column="addressId">
<generator class="native"/>
</id>
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Report a bug
7.5.2. Bidirectional One to one with Join Tables
A bidirectional one-to-one association on a join table is possible, but extremely unusual.
<join table="PersonAddress"
inverse="true"
optional="true">
<key column="addressId"/>
<many-to-one name="person"
column="personId"
not-null="true"/>
</join>
</class>
create table Person ( personId bigint not null primary key )
create table PersonAddress ( personId bigint not null, addressId bigint
not null primary key )
create table Address ( addressId bigint not null primary key )
<class name="Person">
<id name="id" column="personId">
<generator class="native"/>
</id>
<join table="PersonAddress"
optional="true">
<key column="personId"
unique="true"/>
<many-to-one name="address"
column="addressId"
not-null="true"
unique="true"/>
</join>
</class>
<class name="Address">
<id name="id" column="addressId">
<generator class="native"/>
</id>
<join table="PersonAddress"
optional="true"
inverse="true">
<key column="addressId"
unique="true"/>
<many-to-one name="person"
column="personId"
not-null="true"
unique="true"/>
</join>
</class>
create table Person ( personId bigint not null primary key )
create table PersonAddress ( personId bigint not null primary key,
addressId bigint not null unique )
CHAPTER 7. ASSOCIATION MAPPINGS
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Report a bug
7.5.3. Bidirectional Many-to-many with Join Tables
Here is an example of a bidirectional many-to-many association .
Report a bug
7.6. OTHER ASSOCIATION MAPPINGS
7.6.1. Complex Association Mappings
More complex association joins are extremely rare. Hibernate handles more complex situations by using
SQL fragments embedded in the mapping document. For example, if a table with historical account
information data defines accountNumber, effectiveEndDate and effectiveStartDatecolumns,
it would be mapped as follows:
create table Address ( addressId bigint not null primary key )
<class name="Person">
<id name="id" column="personId">
<generator class="native"/>
</id>
<set name="addresses" table="PersonAddress">
<key column="personId"/>
<many-to-many column="addressId"
class="Address"/>
</set>
</class>
<class name="Address">
<id name="id" column="addressId">
<generator class="native"/>
</id>
<set name="people" inverse="true" table="PersonAddress">
<key column="addressId"/>
<many-to-many column="personId"
class="Person"/>
</set>
</class>
create table Person ( personId bigint not null primary key )
create table PersonAddress ( personId bigint not null, addressId bigint
not null, primary key (personId, addressId) )
create table Address ( addressId bigint not null primary key )
<properties name="currentAccountKey">
<property name="accountNumber" type="string" not-null="true"/>
<property name="currentAccount" type="boolean">
<formula>case when effectiveEndDate is null then 1 else 0
end</formula>
</property>
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You can then map an association to the current instance, the one with null effectiveEndDate, by
using:
In a more complex example, imagine that the association between Employee and Organization is
maintained in an Employment table full of historical employment data. An association to the employee's
most recent employer, the one with the most recent startDate, could be mapped in the following way:
This functionality allows a degree of creativity and flexibility, but it is more practical to handle these kinds
of cases using HQL or a criteria query.
Report a bug
</properties>
<property name="effectiveEndDate" type="date"/>
<property name="effectiveStateDate" type="date" not-null="true"/>
<many-to-one name="currentAccountInfo"
property-ref="currentAccountKey"
class="AccountInfo">
<column name="accountNumber"/>
<formula>'1'</formula>
</many-to-one>
<join>
<key column="employeeId"/>
<subselect>
select employeeId, orgId
from Employments
group by orgId
having startDate = max(startDate)
</subselect>
<many-to-one name="mostRecentEmployer"
class="Organization"
column="orgId"/>
</join>
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139
CHAPTER 8. COMPONENT MAPPING
8.1. ABOUT COMPONENT MAPPING
The notion of a component is re-used in several different contexts and purposes throughout Hibernate.
Report a bug
8.2. DEPENDENT OBJECTS
A component is a contained object that is persisted as a value type and not an entity reference. The term
"component" refers to the object-oriented notion of composition and not to architecture-level
components. For example, you can model a person like this:
public class Person {
private java.util.Date birthday;
private Name name;
private String key;
public String getKey() {
return key;
}
private void setKey(String key) {
this.key=key;
}
public java.util.Date getBirthday() {
return birthday;
}
public void setBirthday(java.util.Date birthday) {
this.birthday = birthday;
}
public Name getName() {
return name;
}
public void setName(Name name) {
this.name = name;
}
......
......
}
public class Name {
char initial;
String first;
String last;
public String getFirst() {
return first;
}
void setFirst(String first) {
this.first = first;
}
public String getLast() {
return last;
}
void setLast(String last) {
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Now Name can be persisted as a component of Person. Name defines getter and setter methods for its
persistent properties, but it does not need to declare any interfaces or identifier properties.
Our Hibernate mapping would look like this:
The person table would have the columns pid, birthday, initial, first and last.
Like value types, components do not support shared references. In other words, two persons could have
the same name, but the two person objects would contain two independent name objects that were only
"the same" by value. The null value semantics of a component are ad hoc. When reloading the
containing object, Hibernate will assume that if all component columns are null, then the entire
component is null. This is suitable for most purposes.
The properties of a component can be of any Hibernate type (collections, many-to-one associations,
other components, etc). Nested components should not be considered an exotic usage. Hibernate is
intended to support a fine-grained object model.
The <component> element allows a <parent> subelement that maps a property of the component
class as a reference back to the containing entity.
this.last = last;
}
public char getInitial() {
return initial;
}
void setInitial(char initial) {
this.initial = initial;
}
}
<class name="eg.Person" table="person">
<id name="Key" column="pid" type="string">
<generator class="uuid"/>
</id>
<property name="birthday" type="date"/>
<component name="Name" class="eg.Name"> <!-- class attribute optional
-->
<property name="initial"/>
<property name="first"/>
<property name="last"/>
</component>
</class>
<class name="eg.Person" table="person">
<id name="Key" column="pid" type="string">
<generator class="uuid"/>
</id>
<property name="birthday" type="date"/>
<component name="Name" class="eg.Name" unique="true">
<parent name="namedPerson"/> <!-- reference back to the Person -->
<property name="initial"/>
<property name="first"/>
<property name="last"/>
</component>
</class>
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141
Report a bug
8.3. COLLECTIONS OF DEPENDENT OBJECTS
Collections of components are supported (e.g. an array of type Name). Declare your component
collection by replacing the <element> tag with a <composite-element> tag:
IMPORTANT
If you define a Set of composite elements, it is important to implement equals() and
hashCode() correctly.
Composite elements can contain components but not collections. If your composite element contains
components, use the <nested-composite-element> tag. This case is a collection of components
which themselves have components. You may want to consider if a one-to-many association is more
appropriate. Remodel the composite element as an entity, but be aware that even though the Java model
is the same, the relational model and persistence semantics are still slightly different.
A composite element mapping does not support null-able properties if you are using a <set>. There is
no separate primary key column in the composite element table. Hibernate uses each column's value to
identify a record when deleting objects, which is not possible with null values. You have to either use
only not-null properties in a composite-element or choose a <list>, <map>, <bag> or <idbag>.
A special case of a composite element is a composite element with a nested <many-to-one> element.
This mapping allows you to map extra columns of a many-to-many association table to the composite
element class. The following is a many-to-many association from Order to Item, where
purchaseDate, price and quantity are properties of the association:
<set name="someNames" table="some_names" lazy="true">
<key column="id"/>
<composite-element class="eg.Name"> <!-- class attribute required -->
<property name="initial"/>
<property name="first"/>
<property name="last"/>
</composite-element>
</set>
<class name="eg.Order" .... >
....
<set name="purchasedItems" table="purchase_items" lazy="true">
<key column="order_id">
<composite-element class="eg.Purchase">
<property name="purchaseDate"/>
<property name="price"/>
<property name="quantity"/>
<many-to-one name="item" class="eg.Item"/> <!-- class
attribute is optional -->
</composite-element>
</set>
</class>
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There cannot be a reference to the purchase on the other side for bidirectional association navigation.
Components are value types and do not allow shared references. A single Purchase can be in the set
of an Order, but it cannot be referenced by the Item at the same time.
Even ternary (or quaternary, etc) associations are possible:
Composite elements can appear in queries using the same syntax as associations to other entities.
Report a bug
8.4. COMPONENTS AS MAP INDICES
The <composite-map-key> element allows you to map a component class as the key of a Map.
Ensure that you override hashCode() and equals() correctly on the component class.
Report a bug
8.5. COMPONENTS AS COMPOSITE IDENTIFIERS
You can use a component as an identifier of an entity class. Your component class must satisfy certain
requirements:
It must implement java.io.Serializable.
It must re-implement equals() and hashCode() consistently with the database's notion of
composite key equality.
NOTE
In Hibernate3, although the second requirement is not an absolutely hard requirement of
Hibernate, it is recommended.
You cannot use an IdentifierGenerator to generate composite keys. Instead the application must
assign its own identifiers.
Use the <composite-id> tag, with nested <key-property> elements, in place of the usual <id>
declaration. For example, the OrderLine class has a primary key that depends upon the (composite)
primary key of Order.
<class name="eg.Order" .... >
....
<set name="purchasedItems" table="purchase_items" lazy="true">
<key column="order_id">
<composite-element class="eg.OrderLine">
<many-to-one name="purchaseDetails class="eg.Purchase"/>
<many-to-one name="item" class="eg.Item"/>
</composite-element>
</set>
</class>
<class name="OrderLine">
<composite-id name="id" class="OrderLineId">
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143
Any foreign keys referencing the OrderLine table are now composite. Declare this in your mappings for
other classes. An association to OrderLine is mapped like this:
NOTE
The <column> tag is an alternative to the column attribute everywhere.
A many-to-many association to OrderLine also uses the composite foreign key:
The collection of OrderLines in Order would use:
The <one-to-many> element declares no columns.
<key-property name="lineId"/>
<key-property name="orderId"/>
<key-property name="customerId"/>
</composite-id>
<property name="name"/>
<many-to-one name="order" class="Order"
insert="false" update="false">
<column name="orderId"/>
<column name="customerId"/>
</many-to-one>
....
</class>
<many-to-one name="orderLine" class="OrderLine">
<!-- the "class" attribute is optional, as usual -->
<column name="lineId"/>
<column name="orderId"/>
<column name="customerId"/>
</many-to-one>
<set name="undeliveredOrderLines">
<key column name="warehouseId"/>
<many-to-many class="OrderLine">
<column name="lineId"/>
<column name="orderId"/>
<column name="customerId"/>
</many-to-many>
</set>
<set name="orderLines" inverse="true">
<key>
<column name="orderId"/>
<column name="customerId"/>
</key>
<one-to-many class="OrderLine"/>
</set>
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If OrderLine itself owns a collection, it also has a composite foreign key.
Report a bug
8.6. DYNAMIC COMPONENTS
You can also map a property of type Map:
The semantics of a <dynamic-component> mapping are identical to <component>. The advantage of
this kind of mapping is the ability to determine the actual properties of the bean at deployment time just
by editing the mapping document. Runtime manipulation of the mapping document is also possible, using
a DOM parser. You can also access, and change, Hibernate's configuration-time metamodel via the
Configuration object.
Report a bug
<class name="OrderLine">
....
....
<list name="deliveryAttempts">
<key> <!-- a collection inherits the composite key type -->
<column name="lineId"/>
<column name="orderId"/>
<column name="customerId"/>
</key>
<list-index column="attemptId" base="1"/>
<composite-element class="DeliveryAttempt">
...
</composite-element>
</set>
</class>
<dynamic-component name="userAttributes">
<property name="foo" column="FOO" type="string"/>
<property name="bar" column="BAR" type="integer"/>
<many-to-one name="baz" class="Baz" column="BAZ_ID"/>
</dynamic-component>
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CHAPTER 9. INHERITANCE MAPPING
9.1. INHERITANCE MAPPING STRATEGIES
9.1.1. About Inheritance Mapping Strategies
Hibernate supports the three basic inheritance mapping strategies:
table per class hierarchy
table per subclass
table per concrete class
In addition, Hibernate supports a fourth, slightly different kind of polymorphism:
implicit polymorphism
It is possible to use different mapping strategies for different branches of the same inheritance hierarchy.
You can then make use of implicit polymorphism to achieve polymorphism across the whole hierarchy.
However, Hibernate does not support mixing <subclass>, <joined-subclass> and <union-
subclass> mappings under the same root <class> element. It is possible to mix together the table per
class hierarchy and table per subclass strategies under the the same <class> element, by combining
the <subclass> and <join> elements (see below for an example).
It is possible to define subclass, union-subclass, and joined-subclass mappings in separate
mapping documents directly beneath hibernate-mapping. This allows you to extend a class
hierarchy by adding a new mapping file. You must specify an extends attribute in the subclass
mapping, naming a previously mapped superclass. Previously this feature made the ordering of the
mapping documents important. Since Hibernate3, the ordering of mapping files is irrelevant when using
the extends keyword. The ordering inside a single mapping file still needs to be defined as superclasses
before subclasses.
Report a bug
9.1.2. Table Per Class Hierarchy
Suppose we have an interface Payment with the implementors CreditCardPayment, CashPayment,
and ChequePayment. The table per class hierarchy mapping would display in the following way:
<hibernate-mapping>
<subclass name="DomesticCat" extends="Cat" discriminator-value="D">
<property name="name" type="string"/>
</subclass>
</hibernate-mapping>
<class name="Payment" table="PAYMENT">
<id name="id" type="long" column="PAYMENT_ID">
<generator class="native"/>
</id>
<discriminator column="PAYMENT_TYPE" type="string"/>
<property name="amount" column="AMOUNT"/>
...
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Exactly one table is required. There is a limitation of this mapping strategy: columns declared by the
subclasses, such as CCTYPE, cannot have NOT NULL constraints.
Report a bug
9.1.3. Table Per Subclass
A table per subclass mapping looks like this:
Four tables are required. The three subclass tables have primary key associations to the superclass
table so the relational model is actually a one-to-one association.
Report a bug
9.1.4. Table Per Subclass: Using a Discriminator
Hibernate's implementation of table per subclass does not require a discriminator column. Other
object/relational mappers use a different implementation of table per subclass that requires a type
discriminator column in the superclass table. The approach taken by Hibernate is much more difficult to
implement, but arguably more correct from a relational point of view. If you want to use a discriminator
column with the table per subclass strategy, you can combine the use of <subclass> and <join>, as
follows:
<subclass name="CreditCardPayment" discriminator-value="CREDIT">
<property name="creditCardType" column="CCTYPE"/>
...
</subclass>
<subclass name="CashPayment" discriminator-value="CASH">
...
</subclass>
<subclass name="ChequePayment" discriminator-value="CHEQUE">
...
</subclass>
</class>
<class name="Payment" table="PAYMENT">
<id name="id" type="long" column="PAYMENT_ID">
<generator class="native"/>
</id>
<property name="amount" column="AMOUNT"/>
...
<joined-subclass name="CreditCardPayment" table="CREDIT_PAYMENT">
<key column="PAYMENT_ID"/>
<property name="creditCardType" column="CCTYPE"/>
...
</joined-subclass>
<joined-subclass name="CashPayment" table="CASH_PAYMENT">
<key column="PAYMENT_ID"/>
...
</joined-subclass>
<joined-subclass name="ChequePayment" table="CHEQUE_PAYMENT">
<key column="PAYMENT_ID"/>
...
</joined-subclass>
</class>
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The optional fetch="select" declaration tells Hibernate not to fetch the ChequePayment subclass
data using an outer join when querying the superclass.
Report a bug
9.1.5. Mixing Table Per Class Hierarchy with Table Per Subclass
You can even mix the table per class hierarchy and table per subclass strategies using the following
approach:
<class name="Payment" table="PAYMENT">
<id name="id" type="long" column="PAYMENT_ID">
<generator class="native"/>
</id>
<discriminator column="PAYMENT_TYPE" type="string"/>
<property name="amount" column="AMOUNT"/>
...
<subclass name="CreditCardPayment" discriminator-value="CREDIT">
<join table="CREDIT_PAYMENT">
<key column="PAYMENT_ID"/>
<property name="creditCardType" column="CCTYPE"/>
...
</join>
</subclass>
<subclass name="CashPayment" discriminator-value="CASH">
<join table="CASH_PAYMENT">
<key column="PAYMENT_ID"/>
...
</join>
</subclass>
<subclass name="ChequePayment" discriminator-value="CHEQUE">
<join table="CHEQUE_PAYMENT" fetch="select">
<key column="PAYMENT_ID"/>
...
</join>
</subclass>
</class>
<class name="Payment" table="PAYMENT">
<id name="id" type="long" column="PAYMENT_ID">
<generator class="native"/>
</id>
<discriminator column="PAYMENT_TYPE" type="string"/>
<property name="amount" column="AMOUNT"/>
...
<subclass name="CreditCardPayment" discriminator-value="CREDIT">
<join table="CREDIT_PAYMENT">
<property name="creditCardType" column="CCTYPE"/>
...
</join>
</subclass>
<subclass name="CashPayment" discriminator-value="CASH">
...
</subclass>
<subclass name="ChequePayment" discriminator-value="CHEQUE">
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For any of these mapping strategies, a polymorphic association to the root Payment class is mapped
using <many-to-one>.
Report a bug
9.1.6. Table Per Concrete Class
There are two ways we can map the table per concrete class strategy. First, you can use <union-
subclass>.
Three tables are involved for the subclasses. Each table defines columns for all properties of the class,
including inherited properties.
The limitation of this approach is that if a property is mapped on the superclass, the column name must
be the same on all subclass tables. The identity generator strategy is not allowed in union subclass
inheritance. The primary key seed has to be shared across all unioned subclasses of a hierarchy.
If your superclass is abstract, map it with abstract="true". If it is not abstract, an additional table (it
defaults to PAYMENT in the example above), is needed to hold instances of the superclass.
Report a bug
9.1.7. Table Per Concrete Class Using Implicit Polymorphism
An alternative approach is to make use of implicit polymorphism:
...
</subclass>
</class>
<many-to-one name="payment" column="PAYMENT_ID" class="Payment"/>
<class name="Payment">
<id name="id" type="long" column="PAYMENT_ID">
<generator class="sequence"/>
</id>
<property name="amount" column="AMOUNT"/>
...
<union-subclass name="CreditCardPayment" table="CREDIT_PAYMENT">
<property name="creditCardType" column="CCTYPE"/>
...
</union-subclass>
<union-subclass name="CashPayment" table="CASH_PAYMENT">
...
</union-subclass>
<union-subclass name="ChequePayment" table="CHEQUE_PAYMENT">
...
</union-subclass>
</class>
<class name="CreditCardPayment" table="CREDIT_PAYMENT">
<id name="id" type="long" column="CREDIT_PAYMENT_ID">
<generator class="native"/>
</id>
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Notice that the Payment interface is not mentioned explicitly. Also notice that properties of Payment are
mapped in each of the subclasses. If you want to avoid duplication, consider using XML entities (for
example, [ <!ENTITY allproperties SYSTEM "allproperties.xml"> ] in the DOCTYPE
declaration and &allproperties; in the mapping).
The disadvantage of this approach is that Hibernate does not generate SQL UNIONs when performing
polymorphic queries.
For this mapping strategy, a polymorphic association to Payment is usually mapped using <any>.
Report a bug
9.1.8. Mixing Implicit Polymorphism With Other Inheritance Mappings
Since the subclasses are each mapped in their own <class> element, and since Payment is just an
interface), each of the subclasses could easily be part of another inheritance hierarchy. You can still use
polymorphic queries against the Payment interface.
<property name="amount" column="CREDIT_AMOUNT"/>
...
</class>
<class name="CashPayment" table="CASH_PAYMENT">
<id name="id" type="long" column="CASH_PAYMENT_ID">
<generator class="native"/>
</id>
<property name="amount" column="CASH_AMOUNT"/>
...
</class>
<class name="ChequePayment" table="CHEQUE_PAYMENT">
<id name="id" type="long" column="CHEQUE_PAYMENT_ID">
<generator class="native"/>
</id>
<property name="amount" column="CHEQUE_AMOUNT"/>
...
</class>
<any name="payment" meta-type="string" id-type="long">
<meta-value value="CREDIT" class="CreditCardPayment"/>
<meta-value value="CASH" class="CashPayment"/>
<meta-value value="CHEQUE" class="ChequePayment"/>
<column name="PAYMENT_CLASS"/>
<column name="PAYMENT_ID"/>
</any>
<class name="CreditCardPayment" table="CREDIT_PAYMENT">
<id name="id" type="long" column="CREDIT_PAYMENT_ID">
<generator class="native"/>
</id>
<discriminator column="CREDIT_CARD" type="string"/>
<property name="amount" column="CREDIT_AMOUNT"/>
...
<subclass name="MasterCardPayment" discriminator-value="MDC"/>
<subclass name="VisaPayment" discriminator-value="VISA"/>
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Once again, Payment is not mentioned explicitly. If we execute a query against the Payment interface,
for example from Payment, Hibernate automatically returns instances of CreditCardPayment (and
its subclasses, since they also implement Payment), CashPayment and ChequePayment, but not
instances of NonelectronicTransaction.
Report a bug
9.2. LIMITATIONS
9.2.1. Inheritance Mapping Limitations
There are limitations to the "implicit polymorphism" approach to the table per concrete-class mapping
strategy. There are somewhat less restrictive limitations to <union-subclass> mappings.
The following list shows the limitations of table per concrete-class mappings, and of implicit
polymorphism, in Hibernate.
table per class-heirarchy, table per subclass
Polymorphic many-to-one: <many-to-one>
Polymorphic one-to-one: <one-to-one>
Polymorphic one-to-many: <one-to-many>
Polymorphic many-to-many: <many-to-many>
Polymorphic load() or get(): s.get(Payment.class, id)
Polymorphic queries: from Payment p
Polymorphic joins: from Order o join o.payment p
Outer join fetching is supported.
table per concrete-class (union-subclass)
</class>
<class name="NonelectronicTransaction" table="NONELECTRONIC_TXN">
<id name="id" type="long" column="TXN_ID">
<generator class="native"/>
</id>
...
<joined-subclass name="CashPayment" table="CASH_PAYMENT">
<key column="PAYMENT_ID"/>
<property name="amount" column="CASH_AMOUNT"/>
...
</joined-subclass>
<joined-subclass name="ChequePayment" table="CHEQUE_PAYMENT">
<key column="PAYMENT_ID"/>
<property name="amount" column="CHEQUE_AMOUNT"/>
...
</joined-subclass>
</class>
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Polymorphic many-to-one: <many-to-one>
Polymorphic one-to-one: <one-to-one>
Polymorphic one-to-many: <one-to-many> (for inverse="true" only)
Polymorphic many-to-many: <many-to-many>
Polymorphic load() or get(): s.get(Payment.class, id)
Polymorphic queries: from Payment p
Polymorphic joins: from Order o join o.payment p
Outer join fetching is supported.
table per concrete class (implicit polymorphism
Polymorphic many-to-one: <any>
Polymorphic many-to-many: <many-to-many>
Polymorphic load() or get(): s.createCriteria(Payment.class).add(
Restrictions.idEq(id) ).uniqueResult()
Polymorphic queries: from Payment p
Polymorphic one-to-one, polymorphic one-to-many, polymorphic joins, and outer join fetching are not
supported.
Report a bug
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CHAPTER 10. WORKING WITH OBJECTS
10.1. ABOUT WORKING WITH OBJECTS
Hibernate is a full object/relational mapping solution that not only shields the developer from the details of
the underlying database management system, but also offers state management of objects. This is,
contrary to the management of SQL statements in common JDBC/SQL persistence layers, a natural
object-oriented view of persistence in Java applications.
In other words, Hibernate application developers should always think about the state of their objects, and
not necessarily about the execution of SQL statements. This part is taken care of by Hibernate and is
only relevant for the application developer when tuning the performance of the system.
Report a bug
10.2. HIBERNATE OBJECT STATES
Hibernate defines and supports the following object states:
Transient - an object is transient if it has just been instantiated using the new operator, and it is
not associated with a Hibernate Session. It has no persistent representation in the database
and no identifier value has been assigned. Transient instances will be destroyed by the garbage
collector if the application does not hold a reference anymore. Use the Hibernate Session to
make an object persistent (and let Hibernate take care of the SQL statements that need to be
executed for this transition).
Persistent - a persistent instance has a representation in the database and an identifier value. It
might just have been saved or loaded, however, it is by definition in the scope of a Session.
Hibernate will detect any changes made to an object in persistent state and synchronize the
state with the database when the unit of work completes. Developers do not execute manual
UPDATE statements, or DELETE statements when an object should be made transient.
Detached - a detached instance is an object that has been persistent, but its Session has been
closed. The reference to the object is still valid, of course, and the detached instance might even
be modified in this state. A detached instance can be reattached to a new Session at a later
point in time, making it (and all the modifications) persistent again. This feature enables a
programming model for long running units of work that require user think-time. We call them
application transactions, i.e., a unit of work from the point of view of the user.
We will now discuss the states and state transitions (and the Hibernate methods that trigger a transition)
in more detail.
Report a bug
10.3. MAKING OBJECTS PERSISTENT
Newly instantiated instances of a a persistent class are considered transient by Hibernate. We can make
a transient instance persistent by associating it with a session:
DomesticCat fritz = new DomesticCat();
fritz.setColor(Color.GINGER);
fritz.setSex('M');
fritz.setName("Fritz");
Long generatedId = (Long) sess.save(fritz);
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If Cat has a generated identifier, the identifier is generated and assigned to the cat when save() is
called. If Cat has an assigned identifier, or a composite key, the identifier should be assigned to the
cat instance before calling save(). You can also use persist() instead of save(), with the
semantics defined in the JPA early draft.
persist() makes a transient instance persistent. However, it does not guarantee that the
identifier value will be assigned to the persistent instance immediately, the assignment might
happen at flush time. persist() also guarantees that it will not execute an INSERT statement
if it is called outside of transaction boundaries. This is useful in long-running conversations with
an extended Session/persistence context.
save() does guarantee to return an identifier. If an INSERT has to be executed to get the
identifier ( e.g. "identity" generator, not "sequence"), this INSERT happens immediately, no
matter if you are inside or outside of a transaction. This is problematic in a long-running
conversation with an extended Session/persistence context.
Alternatively, you can assign the identifier using an overloaded version of save().
If the object you make persistent has associated objects (e.g. the kittens collection in the previous
example), these objects can be made persistent in any order you like unless you have a NOT NULL
constraint upon a foreign key column. There is never a risk of violating foreign key constraints. However,
you might violate a NOT NULL constraint if you save() the objects in the wrong order.
Usually you do not bother with this detail, as you will normally use Hibernate's transitive persistence
feature to save the associated objects automatically. Then, even NOT NULL constraint violations do not
occur - Hibernate will take care of everything. Transitive persistence is discussed later in this chapter.
Report a bug
10.4. LOADING AN OBJECT
The load() methods of Session provide a way of retrieving a persistent instance if you know its
identifier. load() takes a class object and loads the state into a newly instantiated instance of that class
in a persistent state.
Alternatively, you can load state into a given instance:
DomesticCat pk = new DomesticCat();
pk.setColor(Color.TABBY);
pk.setSex('F');
pk.setName("PK");
pk.setKittens( new HashSet() );
pk.addKitten(fritz);
Cat fritz = (Cat) sess.load(Cat.class, generatedId);
// you need to wrap primitive identifiers
long id = 1234;
DomesticCat pk = (DomesticCat) sess.load( DomesticCat.class, new Long(id)
);
Cat cat = new DomesticCat();
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Be aware that load() will throw an unrecoverable exception if there is no matching database row. If the
class is mapped with a proxy, load() just returns an uninitialized proxy and does not actually hit the
database until you invoke a method of the proxy. This is useful if you wish to create an association to an
object without actually loading it from the database. It also allows multiple instances to be loaded as a
batch if batch-size is defined for the class mapping.
If you are not certain that a matching row exists, you should use the get() method which hits the
database immediately and returns null if there is no matching row.
You can even load an object using an SQL SELECT ... FOR UPDATE, using a LockMode. See the
API documentation for more information.
Any associated instances or contained collections will not be selected FOR UPDATE, unless you decide
to specify lock or all as a cascade style for the association.
It is possible to re-load an object and all its collections at any time, using the refresh() method. This is
useful when database triggers are used to initialize some of the properties of the object.
How much does Hibernate load from the database and how many SQL SELECTs will it use? This
depends on the fetching strategy. This is explained in detail in the "Fetching Strategies" section.
Report a bug
10.5. QUERYING
10.5.1. About Querying
If you do not know the identifiers of the objects you are looking for, you need a query. Hibernate supports
an easy-to-use but powerful object oriented query language (HQL). For programmatic query creation,
Hibernate supports a sophisticated Criteria and Example query feature (QBC and QBE). You can also
express your query in the native SQL of your database, with optional support from Hibernate for result
set conversion into objects.
Report a bug
10.5.2. Executing Queries
// load pk's state into cat
sess.load( cat, new Long(pkId) );
Set kittens = cat.getKittens();
Cat cat = (Cat) sess.get(Cat.class, id);
if (cat==null) {
cat = new Cat();
}
return cat;
Cat cat = (Cat) sess.get(Cat.class, id, LockMode.UPGRADE);
sess.save(cat);
sess.flush(); //force the SQL INSERT
sess.refresh(cat); //re-read the state (after the trigger executes)
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HQL and native SQL queries are represented with an instance of org.hibernate.Query. This
interface offers methods for parameter binding, result set handling, and for the execution of the actual
query. You always obtain a Query using the current Session:
A query is usually executed by invoking list(). The result of the query will be loaded completely into a
collection in memory. Entity instances retrieved by a query are in a persistent state. The
uniqueResult() method offers a shortcut if you know your query will only return a single object.
Queries that make use of eager fetching of collections usually return duplicates of the root objects, but
with their collections initialized. You can filter these duplicates through a Set.
Report a bug
10.5.3. Iterating Results
Occasionally, you might be able to achieve better performance by executing the query using the
iterate() method. This will usually be the case if you expect that the actual entity instances returned
by the query will already be in the session or second-level cache. If they are not already cached,
iterate() will be slower than list() and might require many database hits for a simple query,
usually 1 for the initial select which only returns identifiers, and n additional selects to initialize the actual
instances.
List cats = session.createQuery(
"from Cat as cat where cat.birthdate < ?")
.setDate(0, date)
.list();
List mothers = session.createQuery(
"select mother from Cat as cat join cat.mother as mother where
cat.name = ?")
.setString(0, name)
.list();
List kittens = session.createQuery(
"from Cat as cat where cat.mother = ?")
.setEntity(0, pk)
.list();
Cat mother = (Cat) session.createQuery(
"select cat.mother from Cat as cat where cat = ?")
.setEntity(0, izi)
.uniqueResult();
Query mothersWithKittens = session.createQuery(
"select mother from Cat as mother left join fetch mother.kittens");
Set uniqueMothers = new HashSet(mothersWithKittens.list());
// fetch ids
Iterator iter = sess.createQuery("from eg.Qux q order by
q.likeliness").iterate();
while ( iter.hasNext() ) {
Qux qux = (Qux) iter.next(); // fetch the object
// something we couldn't express in the query
if ( qux.calculateComplicatedAlgorithm() ) {
// delete the current instance
iter.remove();
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Report a bug
10.5.4. Queries that Return Tuples
Hibernate queries sometimes return tuples of objects. Each tuple is returned as an array:
Report a bug
10.5.5. Scalar Results
Queries can specify a property of a class in the select clause. They can even call SQL aggregate
functions. Properties or aggregates are considered "scalar" results and not entities in persistent state.
Report a bug
10.5.6. Bind Parameters
Methods on Query are provided for binding values to named parameters or JDBC-style ? parameters.
Contrary to JDBC, Hibernate numbers parameters from zero. Named parameters are identifiers of the
form :name in the query string. The advantages of named parameters are as follows:
named parameters are insensitive to the order they occur in the query string
// don't need to process the rest
break;
}
}
Iterator kittensAndMothers = sess.createQuery(
"select kitten, mother from Cat kitten join kitten.mother
mother")
.list()
.iterator();
while ( kittensAndMothers.hasNext() ) {
Object[] tuple = (Object[]) kittensAndMothers.next();
Cat kitten = (Cat) tuple[0];
Cat mother = (Cat) tuple[1];
....
}
Iterator results = sess.createQuery(
"select cat.color, min(cat.birthdate), count(cat) from Cat cat " +
"group by cat.color")
.list()
.iterator();
while ( results.hasNext() ) {
Object[] row = (Object[]) results.next();
Color type = (Color) row[0];
Date oldest = (Date) row[1];
Integer count = (Integer) row[2];
.....
}
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they can occur multiple times in the same query
they are self-documenting
Report a bug
10.5.7. Pagination
If you need to specify bounds upon your result set, that is, the maximum number of rows you want to
retrieve and/or the first row you want to retrieve, you can use methods of the Query interface:
Hibernate knows how to translate this limit query into the native SQL of your DBMS.
Report a bug
10.5.8. Scrollable Iteration
If your JDBC driver supports scrollable ResultSets, the Query interface can be used to obtain a
ScrollableResults object that allows flexible navigation of the query results.
//named parameter (preferred)
Query q = sess.createQuery("from DomesticCat cat where cat.name = :name");
q.setString("name", "Fritz");
Iterator cats = q.iterate();
//positional parameter
Query q = sess.createQuery("from DomesticCat cat where cat.name = ?");
q.setString(0, "Izi");
Iterator cats = q.iterate();
//named parameter list
List names = new ArrayList();
names.add("Izi");
names.add("Fritz");
Query q = sess.createQuery("from DomesticCat cat where cat.name in
(:namesList)");
q.setParameterList("namesList", names);
List cats = q.list();
Query q = sess.createQuery("from DomesticCat cat");
q.setFirstResult(20);
q.setMaxResults(10);
List cats = q.list();
Query q = sess.createQuery("select cat.name, cat from DomesticCat cat " +
"order by cat.name");
ScrollableResults cats = q.scroll();
if ( cats.first() ) {
// find the first name on each page of an alphabetical list of cats by
name
firstNamesOfPages = new ArrayList();
do {
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Note that an open database connection and cursor is required for this functionality. Use
setMaxResult()/setFirstResult() if you need offline pagination functionality.
Report a bug
10.5.9. Externalizing Named Queries
You can also define named queries in the mapping document. Remember to use a CDATA section if your
query contains characters that could be interpreted as markup.
Parameter binding and executing is done programatically:
The actual program code is independent of the query language that is used. You can also define native
SQL queries in metadata, or migrate existing queries to Hibernate by placing them in mapping files.
Also note that a query declaration inside a <hibernate-mapping> element requires a global unique
name for the query, while a query declaration inside a <class> element is made unique automatically
by prepending the fully qualified name of the class. For example eg.Cat.ByNameAndMaximumWeight.
Report a bug
10.5.10. Filtering Collections
A collection filter is a special type of query that can be applied to a persistent collection or array. The
query string can refer to this, meaning the current collection element.
String name = cats.getString(0);
firstNamesOfPages.add(name);
}
while ( cats.scroll(PAGE_SIZE) );
// Now get the first page of cats
pageOfCats = new ArrayList();
cats.beforeFirst();
int i=0;
while( ( PAGE_SIZE > i++ ) && cats.next() ) pageOfCats.add(
cats.get(1) );
}
cats.close();
<query name="ByNameAndMaximumWeight"><![CDATA[
from eg.DomesticCat as cat
where cat.name = ?
and cat.weight > ?
] ]></query>
Query q = sess.getNamedQuery("ByNameAndMaximumWeight");
q.setString(0, name);
q.setInteger(1, minWeight);
List cats = q.list();
Collection blackKittens = session.createFilter(
pk.getKittens(),
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The returned collection is considered a bag that is a copy of the given collection. The original collection is
not modified. This is contrary to the implication of the name "filter", but consistent with expected
behavior.
Observe that filters do not require a from clause, although they can have one if required. Filters are not
limited to returning the collection elements themselves.
Even an empty filter query is useful, e.g. to load a subset of elements in a large collection:
Report a bug
10.5.11. Criteria Queries
HQL is extremely powerful, but some developers prefer to build queries dynamically using an object-
oriented API, rather than building query strings. Hibernate provides an intuitive Criteria query API for
these cases:
Report a bug
10.5.12. Queries in Native SQL
You can express a query in SQL, using createSQLQuery() and let Hibernate manage the mapping
from result sets to objects. You can at any time call session.connection() and use the JDBC
Connection directly. If you choose to use the Hibernate API, you must enclose SQL aliases in braces:
"where this.color = ?")
.setParameter( Color.BLACK, Hibernate.custom(ColorUserType.class) )
.list()
Collection blackKittenMates = session.createFilter(
pk.getKittens(),
"select this.mate where this.color = eg.Color.BLACK.intValue")
.list();
Collection tenKittens = session.createFilter(
mother.getKittens(), "")
.setFirstResult(0).setMaxResults(10)
.list();
Criteria crit = session.createCriteria(Cat.class);
crit.add( Restrictions.eq( "color", eg.Color.BLACK ) );
crit.setMaxResults(10);
List cats = crit.list();
List cats = session.createSQLQuery("SELECT {cat.*} FROM CAT {cat} WHERE
ROWNUM<10")
.addEntity("cat", Cat.class)
.list();
List cats = session.createSQLQuery(
"SELECT {cat}.ID AS {cat.id}, {cat}.SEX AS {cat.sex}, " +
"{cat}.MATE AS {cat.mate}, {cat}.SUBCLASS AS {cat.class}, ... "
+
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SQL queries can contain named and positional parameters, just like Hibernate queries. .
Report a bug
10.6. MODIFYING OBJECTS
10.6.1. Modifying Persistent Objects
Transactional persistent instances (i.e. objects loaded, saved, created or queried by the Session) can
be manipulated by the application, and any changes to persistent state will be persisted when the
Session is flushed. This is discussed later in this chapter. There is no need to call a particular method
(like update(), which has a different purpose) to make your modifications persistent. The most
straightforward way to update the state of an object is to load() it and then manipulate it directly while
the Session is open:
Sometimes this programming model is inefficient, as it requires in the same session both an SQL
SELECT to load an object and an SQL UPDATE to persist its updated state. Hibernate offers an alternate
approach by using detached instances.
IMPORTANT
Hibernate does not offer its own API for direct execution of UPDATE or DELETE
statements. Hibernate is a state management service, you do not have to think in
statements to use it. JDBC is a perfect API for executing SQL statements, you can get a
JDBC Connection at any time by calling session.connection(). Furthermore, the
notion of mass operations conflicts with object/relational mapping for online transaction
processing-oriented applications. Future versions of Hibernate can, however, provide
special mass operation functions.
Report a bug
10.6.2. Modifying Detached Objects
Many applications need to retrieve an object in one transaction, send it to the UI layer for manipulation,
then save the changes in a new transaction. Applications that use this kind of approach in a high-
concurrency environment usually use versioned data to ensure isolation for the "long" unit of work.
Hibernate supports this model by providing for reattachment of detached instances using the
Session.update() or Session.merge() methods:
"FROM CAT {cat} WHERE ROWNUM<10")
.addEntity("cat", Cat.class)
.list();
DomesticCat cat = (DomesticCat) sess.load( Cat.class, new Long(69) );
cat.setName("PK");
sess.flush(); // changes to cat are automatically detected and persisted
// in the first session
Cat cat = (Cat) firstSession.load(Cat.class, catId);
Cat potentialMate = new Cat();
firstSession.save(potentialMate);
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If the Cat with identifier catId had already been loaded by secondSession when the application tried
to reattach it, an exception would have been thrown.
Use update() if you are certain that the session does not contain an already persistent instance with
the same identifier. Use merge() if you want to merge your modifications at any time without
consideration of the state of the session. In other words, update() is usually the first method you would
call in a fresh session, ensuring that the reattachment of your detached instances is the first operation
that is executed.
The application should individually update() detached instances that are reachable from the given
detached instance only if it wants their state to be updated. This can be automated using transitive
persistence. See the "Transitive Persistence" section for further information.
The lock() method also allows an application to reassociate an object with a new session. However,
the detached instance has to be unmodified.
Note that lock() can be used with various LockModes. See the API documentation and the chapter on
transaction handling for more information. Reattachment is not the only usecase for lock().
Other models for long units of work are discussed in the "Optimistic Concurrency Control" section.
Report a bug
10.7. OTHER OBJECT OPERATIONS
10.7.1. Automatic State Detection
Hibernate users have requested a general purpose method that either saves a transient instance by
generating a new identifier or updates/reattaches the detached instances associated with its current
identifier. The saveOrUpdate() method implements this functionality.
// in a higher layer of the application
cat.setMate(potentialMate);
// later, in a new session
secondSession.update(cat); // update cat
secondSession.update(mate); // update mate
//just reassociate:
sess.lock(fritz, LockMode.NONE);
//do a version check, then reassociate:
sess.lock(izi, LockMode.READ);
//do a version check, using SELECT ... FOR UPDATE, then reassociate:
sess.lock(pk, LockMode.UPGRADE);
// in the first session
Cat cat = (Cat) firstSession.load(Cat.class, catID);
// in a higher tier of the application
Cat mate = new Cat();
cat.setMate(mate);
// later, in a new session
secondSession.saveOrUpdate(cat); // update existing state (cat has a
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The usage and semantics of saveOrUpdate() seems to be confusing for new users. Firstly, so long as
you are not trying to use instances from one session in another new session, you should not need to use
update(), saveOrUpdate(), or merge(). Some whole applications will never use either of these
methods.
Usually update() or saveOrUpdate() are used in the following scenario:
the application loads an object in the first session
the object is passed up to the UI tier
some modifications are made to the object
the object is passed back down to the business logic tier
the application persists these modifications by calling update() in a second session
saveOrUpdate() does the following:
if the object is already persistent in this session, do nothing
if another object associated with the session has the same identifier, throw an exception
if the object has no identifier property, save() it
if the object's identifier has the value assigned to a newly instantiated object, save() it
if the object is versioned by a <version> or <timestamp>, and the version property value is
the same value assigned to a newly instantiated object, save() it
otherwise update() the object
and merge() is very different:
if there is a persistent instance with the same identifier currently associated with the session,
copy the state of the given object onto the persistent instance
if there is no persistent instance currently associated with the session, try to load it from the
database, or create a new persistent instance
the persistent instance is returned
the given instance does not become associated with the session, it remains detached
Report a bug
10.7.2. Deleting Persistent Objects
Session.delete() will remove an object's state from the database. Your application, however, can
still hold a reference to a deleted object. It is best to think of delete() as making a persistent instance,
transient.
non-null id)
secondSession.saveOrUpdate(mate); // save the new instance (mate has a
null id)
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You can delete objects in any order, without risk of foreign key constraint violations. It is still possible to
violate a NOT NULL constraint on a foreign key column by deleting objects in the wrong order, e.g. if you
delete the parent, but forget to delete the children.
Report a bug
10.7.3. Replicating an Object Between Two Datastores
It is sometimes useful to be able to take a graph of persistent instances and make them persistent in a
different datastore, without regenerating identifier values.
The ReplicationMode determines how replicate() will deal with conflicts with existing rows in the
database:
ReplicationMode.IGNORE: ignores the object when there is an existing database row with
the same identifier
ReplicationMode.OVERWRITE: overwrites any existing database row with the same identifier
ReplicationMode.EXCEPTION: throws an exception if there is an existing database row with
the same identifier
ReplicationMode.LATEST_VERSION: overwrites the row if its version number is earlier than
the version number of the object, or ignore the object otherwise
Usecases for this feature include reconciling data entered into different database instances, upgrading
system configuration information during product upgrades, rolling back changes made during non-ACID
transactions and more.
Report a bug
10.7.4. Flushing the Session
Sometimes the Session will execute the SQL statements needed to synchronize the JDBC
connection's state with the state of objects held in memory. This process, called flush, occurs by default
at the following points:
before some query executions
sess.delete(cat);
//retrieve a cat from one database
Session session1 = factory1.openSession();
Transaction tx1 = session1.beginTransaction();
Cat cat = (Cat) session1.get(Cat.class, catId);
tx1.commit();
session1.close();
//reconcile with a second database
Session session2 = factory2.openSession();
Transaction tx2 = session2.beginTransaction();
session2.replicate(cat, ReplicationMode.LATEST_VERSION);
tx2.commit();
session2.close();
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from org.hibernate.Transaction.commit()
from Session.flush()
The SQL statements are issued in the following order:
1. all entity insertions in the same order the corresponding objects were saved using
Session.save()
2. all entity updates
3. all collection deletions
4. all collection element deletions, updates and insertions
5. all collection insertions
6. all entity deletions in the same order the corresponding objects were deleted using
Session.delete()
An exception is that objects using native ID generation are inserted when they are saved.
Except when you explicitly flush(), there are absolutely no guarantees about when the Session
executes the JDBC calls, only the order in which they are executed. However, Hibernate does guarantee
that the Query.list(..) will never return stale or incorrect data.
It is possible to change the default behavior so that flush occurs less frequently. The FlushMode class
defines three different modes: only flush at commit time when the Hibernate Transaction API is used,
flush automatically using the explained routine, or never flush unless flush() is called explicitly. The
last mode is useful for long running units of work, where a Session is kept open and disconnected for a
long time (see the "Extended Session and Automatic Versioning" section for further information).
During flush, an exception might occur (e.g. if a DML operation violates a constraint). Since handling
exceptions involves some understanding of Hibernate's transactional behavior, we discuss it in the
"Transaction and Concurrency" chapter.
Report a bug
10.7.5. Transitive Persistence
sess = sf.openSession();
Transaction tx = sess.beginTransaction();
sess.setFlushMode(FlushMode.COMMIT); // allow queries to return stale
state
Cat izi = (Cat) sess.load(Cat.class, id);
izi.setName(iznizi);
// might return stale data
sess.createQuery("from Cat as cat left outer join cat.kittens kitten");
// change to izi is not flushed!
...
tx.commit(); // flush occurs
sess.close();
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It is quite cumbersome to save, delete, or reattach individual objects, especially if you deal with a graph
of associated objects. A common case is a parent/child relationship. Consider the following example:
If the children in a parent/child relationship would be value typed (e.g. a collection of addresses or
strings), their life cycle would depend on the parent and no further action would be required for
convenient "cascading" of state changes. When the parent is saved, the value-typed child objects are
saved and when the parent is deleted, the children will be deleted, etc. This works for operations such as
the removal of a child from the collection. Since value-typed objects cannot have shared references,
Hibernate will detect this and delete the child from the database.
Now consider the same scenario with parent and child objects being entities, not value-types (e.g.
categories and items, or parent and child cats). Entities have their own life cycle and support shared
references. Removing an entity from the collection does not mean it can be deleted), and there is by
default no cascading of state from one entity to any other associated entities. Hibernate does not
implement persistence by reachability by default.
For each basic operation of the Hibernate session - including persist(), merge(),
saveOrUpdate(), delete(), lock(), refresh(), evict(), replicate() - there is a
corresponding cascade style. Respectively, the cascade styles are named create, merge, save-
update, delete, lock, refresh, evict, replicate. If you want an operation to be
cascaded along an association, you must indicate that in the mapping document. For example:
Cascade styles my be combined:
You can even use cascade="all" to specify that all operations should be cascaded along the
association. The default cascade="none" specifies that no operations are to be cascaded.
A special cascade style, delete-orphan, applies only to one-to-many associations, and indicates that
the delete() operation should be applied to any child object that is removed from the association.
Recommendations:
It does not usually make sense to enable cascade on a <many-to-one> or <many-to-many>
association. Cascade is often useful for <one-to-one> and <one-to-many> associations.
If the child object's lifespan is bounded by the lifespan of the parent object, make it a life cycle
object by specifying cascade="all,delete-orphan".
Otherwise, you might not need cascade at all. But if you think that you will often be working with
the parent and children together in the same transaction, and you want to save yourself some
typing, consider using cascade="persist,merge,save-update".
Mapping an association (either a single valued association, or a collection) with cascade="all" marks
the association as a parent/child style relationship where save/update/delete of the parent results in
save/update/delete of the child or children.
Furthermore, a mere reference to a child from a persistent parent will result in save/update of the child.
This metaphor is incomplete, however. A child which becomes unreferenced by its parent is not
automatically deleted, except in the case of a <one-to-many> association mapped with
cascade="delete-orphan". The precise semantics of cascading operations for a parent/child
relationship are as follows:
<one-to-one name="person" cascade="persist"/>
<one-to-one name="person" cascade="persist,delete,lock"/>
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If a parent is passed to persist(), all children are passed to persist()
If a parent is passed to merge(), all children are passed to merge()
If a parent is passed to save(), update() or saveOrUpdate(), all children are passed to
saveOrUpdate()
If a transient or detached child becomes referenced by a persistent parent, it is passed to
saveOrUpdate()
If a parent is deleted, all children are passed to delete()
If a child is dereferenced by a persistent parent, nothing special happens - the application should
explicitly delete the child if necessary - unless cascade="delete-orphan", in which case the
"orphaned" child is deleted.
Finally, note that cascading of operations can be applied to an object graph at call time or at flush time.
All operations, if enabled, are cascaded to associated entities reachable when the operation is executed.
However, save-update and delete-orphan are transitive for all associated entities reachable during
flush of the Session.
Report a bug
10.7.6. Using Metadata
Hibernate requires a rich meta-level model of all entity and value types. This model can be useful to the
application itself. For example, the application might use Hibernate's metadata to implement a "smart"
deep-copy algorithm that understands which objects should be copied (eg. mutable value types) and
which objects that should not (e.g. immutable value types and, possibly, associated entities).
Hibernate exposes metadata via the ClassMetadata and CollectionMetadata interfaces and the
Type hierarchy. Instances of the metadata interfaces can be obtained from the SessionFactory.
Report a bug
Cat fritz = ......;
ClassMetadata catMeta = sessionfactory.getClassMetadata(Cat.class);
Object[] propertyValues = catMeta.getPropertyValues(fritz,
EntityMode.POJO);
String[] propertyNames = catMeta.getPropertyNames();
Type[] propertyTypes = catMeta.getPropertyTypes();
// get a Map of all properties which are not collections or associations
Map namedValues = new HashMap();
for ( int i=0; i<propertyNames.length; i++ ) {
if ( !propertyTypes[i].isEntityType() &&
!propertyTypes[i].isCollectionType() ) {
namedValues.put( propertyNames[i], propertyValues[i] );
}
}
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CHAPTER 11. TRANSACTIONS AND CONCURRENCY
11.1. ABOUT TRANSACTIONS AND CONCURRENCY
The most important point about Hibernate and concurrency control is that it is easy to understand.
Hibernate directly uses JDBC connections and JTA resources without adding any additional locking
behavior. It is recommended that you spend some time with the JDBC, ANSI, and transaction isolation
specification of your database management system.
Hibernate does not lock objects in memory. Your application can expect the behavior as defined by the
isolation level of your database transactions. Through Session, which is also a transaction-scoped
cache, Hibernate provides repeatable reads for lookup by identifier and entity queries and not reporting
queries that return scalar values.
In addition to versioning for automatic optimistic concurrency control, Hibernate also offers, using the
SELECT FOR UPDATE syntax, a (minor) API for pessimistic locking of rows. Optimistic concurrency
control and this API are discussed later in this chapter.
The discussion of concurrency control in Hibernate begins with the granularity of Configuration,
SessionFactory, and Session, as well as database transactions and long conversations.
Report a bug
11.2. SESSION AND TRANSACTION SCOPES
11.2.1. About Session and Transaction Scopes
A SessionFactory is an expensive-to-create, threadsafe object, intended to be shared by all
application threads. It is created once, usually on application startup, from a Configuration instance.
A Session is an inexpensive, non-threadsafe object that should be used once and then discarded for: a
single request, a conversation or a single unit of work. A Session will not obtain a JDBC Connection,
or a Datasource, unless it is needed. It will not consume any resources until used.
In order to reduce lock contention in the database, a database transaction has to be as short as
possible. Long database transactions will prevent your application from scaling to a highly concurrent
load. It is not recommended that you hold a database transaction open during user think time until the
unit of work is complete.
What is the scope of a unit of work? Can a single Hibernate Session span several database
transactions, or is this a one-to-one relationship of scopes? When should you open and close a Session
and how do you demarcate the database transaction boundaries? These questions are addressed in the
following sections.
Report a bug
11.2.2. Unit of Work
First, let's define a unit of work. A unit of work is a design pattern described by Martin Fowler as
[maintaining] a list of objects affected by a business transaction and coordinates the writing out of
changes and the resolution of concurrency problems. In other words, its a series of operations we wish to
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carry out against the database together. Basically, it is a transaction, though fulfilling a unit of work will
often span multiple physical database transactions. So really we are talking about a more abstract notion
of a transaction. The term "business transaction" is also sometimes used in lieu of unit of work.
Do not use the session-per-operation antipattern: do not open and close a Session for every simple
database call in a single thread. The same is true for database transactions. Database calls in an
application are made using a planned sequence; they are grouped into atomic units of work. This also
means that auto-commit after every single SQL statement is useless in an application as this mode is
intended for ad-hoc SQL console work. Hibernate disables, or expects the application server to disable,
auto-commit mode immediately. Database transactions are never optional. All communication with a
database has to occur inside a transaction. Auto-commit behavior for reading data should be avoided, as
many small transactions are unlikely to perform better than one clearly defined unit of work. The latter is
also more maintainable and extensible.
The most common pattern in a multi-user client/server application is session-per-request. In this model,
a request from the client is sent to the server, where the Hibernate persistence layer runs. A new
Hibernate Session is opened, and all database operations are executed in this unit of work. On
completion of the work, and once the response for the client has been prepared, the session is flushed
and closed. Use a single database transaction to serve the clients request, starting and committing it
when you open and close the Session. The relationship between the two is one-to-one and this model
is a perfect fit for many applications.
The challenge lies in the implementation. Hibernate provides built-in management of the "current
session" to simplify this pattern. Start a transaction when a server request has to be processed, and end
the transaction before the response is sent to the client. Common solutions are ServletFilter, AOP
interceptor with a pointcut on the service methods, or a proxy/interception container. An EJB container is
a standardized way to implement cross-cutting aspects such as transaction demarcation on EJB session
beans, declaratively with CMT. If you use programmatic transaction demarcation, for ease of use and
code portability use the Hibernate Transaction API shown later in this chapter.
Your application code can access a "current session" to process the request by calling
sessionFactory.getCurrentSession(). You will always get a Session scoped to the current
database transaction. This has to be configured for either resource-local or JTA environments.
You can extend the scope of a Session and database transaction until the "view has been rendered".
This is especially useful in servlet applications that utilize a separate rendering phase after the request
has been processed. Extending the database transaction until view rendering, is achieved by
implementing your own interceptor. However, this will be difficult if you rely on EJBs with container-
managed transactions. A transaction will be completed when an EJB method returns, before rendering of
any view can start. See the Hibernate website and forum for tips and examples relating to this Open
Session in View pattern.
Report a bug
11.2.3. Long Conversations
The session-per-request pattern is not the only way of designing units of work. Many business processes
require a whole series of interactions with the user that are interleaved with database accesses. In web
and enterprise applications, it is not acceptable for a database transaction to span a user interaction.
Consider the following example:
The first screen of a dialog opens. The data seen by the user has been loaded in a particular
Session and database transaction. The user is free to modify the objects.
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The user clicks "Save" after 5 minutes and expects their modifications to be made persistent.
The user also expects that they were the only person editing this information and that no
conflicting modification has occurred.
From the point of view of the user, we call this unit of work a long-running conversation or application
transaction. There are many ways to implement this in your application.
A first naive implementation might keep the Session and database transaction open during user think
time, with locks held in the database to prevent concurrent modification and to guarantee isolation and
atomicity. This is an anti-pattern, since lock contention would not allow the application to scale with the
number of concurrent users.
You have to use several database transactions to implement the conversation. In this case, maintaining
isolation of business processes becomes the partial responsibility of the application tier. A single
conversation usually spans several database transactions. It will be atomic if only one of these database
transactions (the last one) stores the updated data. All others simply read data (for example, in a wizard-
style dialog spanning several request/response cycles). This is easier to implement than it might sound,
especially if you utilize some of Hibernate's features:
Automatic Versioning: Hibernate can perform automatic optimistic concurrency control for you. It
can automatically detect if a concurrent modification occurred during user think time. Check for
this at the end of the conversation.
Detached Objects: if you decide to use the session-per-request pattern, all loaded instances will
be in the detached state during user think time. Hibernate allows you to reattach the objects and
persist the modifications. The pattern is called session-per-request-with-detached-objects.
Automatic versioning is used to isolate concurrent modifications.
Extended (or Long) Session: the Hibernate Session can be disconnected from the underlying
JDBC connection after the database transaction has been committed and reconnected when a
new client request occurs. This pattern is known as session-per-conversation and makes even
reattachment unnecessary. Automatic versioning is used to isolate concurrent modifications and
the Session will not be allowed to be flushed automatically, but explicitly.
Both session-per-request-with-detached-objects and session-per-conversation have advantages and
disadvantages. These disadvantages are discussed later in this chapter in the context of optimistic
concurrency control.
Report a bug
11.2.4. Considering Object Identity
An application can concurrently access the same persistent state in two different Sessions. However,
an instance of a persistent class is never shared between two Session instances. It is for this reason
that there are two different notions of identity:
Database Identity
foo.getId().equals( bar.getId() )
JVM Identity
foo==bar
For objects attached to a particular Session (i.e., in the scope of a Session), the two notions are
equivalent and JVM identity for database identity is guaranteed by Hibernate. While the application
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might concurrently access the "same" (persistent identity) business object in two different sessions, the
two instances will actually be "different" (JVM identity). Conflicts are resolved using an optimistic
approach and automatic versioning at flush/commit time.
This approach leaves Hibernate and the database to worry about concurrency. It also provides the best
scalability, since guaranteeing identity in single-threaded units of work means that it does not need
expensive locking or other means of synchronization. The application does not need to synchronize on
any business object, as long as it maintains a single thread per Session. Within a Session the
application can safely use == to compare objects.
However, an application that uses == outside of a Session might produce unexpected results. This
might occur even in some unexpected places. For example, if you put two detached instances into the
same Set, both might have the same database identity (i.e., they represent the same row). JVM identity,
however, is by definition not guaranteed for instances in a detached state. The developer has to override
the equals() and hashCode() methods in persistent classes and implement their own notion of object
equality. There is one caveat: never use the database identifier to implement equality. Use a business
key that is a combination of unique, usually immutable, attributes. The database identifier will change if a
transient object is made persistent. If the transient instance (usually together with detached instances) is
held in a Set, changing the hashcode breaks the contract of the Set. Attributes for business keys do not
have to be as stable as database primary keys; you only have to guarantee stability as long as the
objects are in the same Set. See the Hibernate website for a more thorough discussion of this issue.
Please note that this is not a Hibernate issue, but simply how Java object identity and equality has to be
implemented.
Report a bug
11.2.5. Common Issues
Do not use the anti-patterns session-per-user-session or session-per-application (there are, however,
rare exceptions to this rule). Some of the following issues might also arise within the recommended
patterns, so ensure that you understand the implications before making a design decision:
A Session is not thread-safe. Things that work concurrently, like HTTP requests, session
beans, or Swing workers, will cause race conditions if a Session instance is shared. If you keep
your Hibernate Session in your HttpSession (this is discussed later in the chapter), you
should consider synchronizing access to your Http session. Otherwise, a user that clicks reload
fast enough can use the same Session in two concurrently running threads.
An exception thrown by Hibernate means you have to rollback your database transaction and
close the Session immediately (this is discussed in more detail later in the chapter). If your
Session is bound to the application, you have to stop the application. Rolling back the
database transaction does not put your business objects back into the state they were at the
start of the transaction. This means that the database state and the business objects will be out
of sync. Usually this is not a problem, because exceptions are not recoverable and you will have
to start over after rollback anyway.
The Session caches every object that is in a persistent state (watched and checked for dirty
state by Hibernate). If you keep it open for a long time or simply load too much data, it will grow
endlessly until you get an OutOfMemoryException. One solution is to call clear() and
evict() to manage the Session cache, but you should consider a Stored Procedure if you
need mass data operations. Some solutions are shown in the "Batch Processing" chapter.
Keeping a Session open for the duration of a user session also means a higher probability of
stale data.
Report a bug
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11.3. DATABASE TRANSACTION DEMARCATION
11.3.1. About Database Transaction Demarcation
Database, or system, transaction boundaries are always necessary. No communication with the
database can occur outside of a database transaction (this seems to confuse many developers who are
used to the auto-commit mode). Always use clear transaction boundaries, even for read-only operations.
Depending on your isolation level and database capabilities this might not be required, but there is no
downside if you always demarcate transactions explicitly. Certainly, a single database transaction is
going to perform better than many small transactions, even for reading data.
A Hibernate application can run in non-managed (i.e., standalone, simple Web- or Swing applications)
and managed J2EE environments. In a non-managed environment, Hibernate is usually responsible for
its own database connection pool. The application developer has to manually set transaction boundaries
(begin, commit, or rollback database transactions) themselves. A managed environment usually provides
container-managed transactions (CMT), with the transaction assembly defined declaratively (in
deployment descriptors of EJB session beans, for example). Programmatic transaction demarcation is
then no longer necessary.
However, it is often desirable to keep your persistence layer portable between non-managed resource-
local environments, and systems that can rely on JTA but use BMT instead of CMT. In both cases use
programmatic transaction demarcation. Hibernate offers a wrapper API called Transaction that
translates into the native transaction system of your deployment environment. This API is actually
optional, but we strongly encourage its use unless you are in a CMT session bean.
Ending a Session usually involves four distinct phases:
flush the session
commit the transaction
close the session
handle exceptions
We discussed Flushing the session earlier, so we will now have a closer look at transaction demarcation
and exception handling in both managed and non-managed environments.
Report a bug
11.3.2. Non-managed Environment
If a Hibernate persistence layer runs in a non-managed environment, database connections are usually
handled by simple (i.e., non-DataSource) connection pools from which Hibernate obtains connections as
needed. The session/transaction handling idiom looks like this:
// Non-managed environment idiom
Session sess = factory.openSession();
Transaction tx = null;
try {
tx = sess.beginTransaction();
// do some work
...
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You do not have to flush() the Session explicitly: the call to commit() automatically triggers the
synchronization depending on the flushing set for the session. A call to close() marks the end of a
session. The main implication of close() is that the JDBC connection will be relinquished by the
session. This Java code is portable and runs in both non-managed and JTA environments.
As outlined earlier, a much more flexible solution is Hibernate's built-in "current session" context
management:
You will not see these code snippets in a regular application; fatal (system) exceptions should always be
caught at the "top". In other words, the code that executes Hibernate calls in the persistence layer, and
the code that handles RuntimeException (and usually can only clean up and exit), are in different
layers. The current context management by Hibernate can significantly simplify this design by accessing
a SessionFactory. Exception handling is discussed later in this chapter.
You should select org.hibernate.transaction.JDBCTransactionFactory, which is the default,
and for the second example select "thread" as your
hibernate.current_session_context_class.
Report a bug
11.3.3. Using JTA
If your persistence layer runs in an application server (for example, behind EJB session beans), every
datasource connection obtained by Hibernate will automatically be part of the global JTA transaction.
You can also install a standalone JTA implementation and use it without EJB. Hibernate offers two
strategies for JTA integration.
If you use bean-managed transactions (BMT), Hibernate will tell the application server to start and end a
BMT transaction if you use the Transaction API. The transaction management code is identical to the
non-managed environment.
tx.commit();
}
catch (RuntimeException e) {
if (tx != null) tx.rollback();
throw e; // or display error message
}
finally {
sess.close();
}
// Non-managed environment idiom with getCurrentSession()
try {
factory.getCurrentSession().beginTransaction();
// do some work
...
factory.getCurrentSession().getTransaction().commit();
}
catch (RuntimeException e) {
factory.getCurrentSession().getTransaction().rollback();
throw e; // or display error message
}
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If you want to use a transaction-bound Session, that is, the getCurrentSession() functionality for
easy context propagation, use the JTA UserTransaction API directly:
With CMT, transaction demarcation is completed in session bean deployment descriptors, not
programmatically. The code is reduced to:
In a CMT/EJB, even rollback happens automatically. An unhandled RuntimeException thrown by a
session bean method tells the container to set the global transaction to rollback. You do not need to use
the Hibernate Transaction API at all with BMT or CMT, and you get automatic propagation of the
"current" Session bound to the transaction.
When configuring Hibernate's transaction factory, choose
// BMT idiom
Session sess = factory.openSession();
Transaction tx = null;
try {
tx = sess.beginTransaction();
// do some work
...
tx.commit();
}
catch (RuntimeException e) {
if (tx != null) tx.rollback();
throw e; // or display error message
}
finally {
sess.close();
}
// BMT idiom with getCurrentSession()
try {
UserTransaction tx = (UserTransaction)new InitialContext()
.lookup("java:comp/UserTransaction");
tx.begin();
// Do some work on Session bound to transaction
factory.getCurrentSession().load(...);
factory.getCurrentSession().persist(...);
tx.commit();
}
catch (RuntimeException e) {
tx.rollback();
throw e; // or display error message
}
// CMT idiom
Session sess = factory.getCurrentSession();
// do some work
...
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org.hibernate.transaction.JTATransactionFactory if you use JTA directly (BMT), and
org.hibernate.transaction.CMTTransactionFactory in a CMT session bean. Remember to
also set hibernate.transaction.manager_lookup_class. Ensure that your
hibernate.current_session_context_class is either unset (backwards compatibility), or is set
to "jta".
The getCurrentSession() operation has one downside in a JTA environment. There is one caveat to
the use of after_statement connection release mode, which is then used by default. Due to a
limitation of the JTA spec, it is not possible for Hibernate to automatically clean up any unclosed
ScrollableResults or Iterator instances returned by scroll() or iterate(). You must
release the underlying database cursor by calling ScrollableResults.close() or
Hibernate.close(Iterator) explicitly from a finally block. Most applications can easily avoid
using scroll() or iterate() from the JTA or CMT code.)
Report a bug
11.3.4. Exception Handling
If the Session throws an exception, including any SQLException, immediately rollback the database
transaction, call Session.close() and discard the Session instance. Certain methods of Session
will not leave the session in a consistent state. No exception thrown by Hibernate can be treated as
recoverable. Ensure that the Session will be closed by calling close() in a finally block.
The HibernateException, which wraps most of the errors that can occur in a Hibernate persistence
layer, is an unchecked exception. It was not in older versions of Hibernate. In our opinion, we should not
force the application developer to catch an unrecoverable exception at a low layer. In most systems,
unchecked and fatal exceptions are handled in one of the first frames of the method call stack (i.e., in
higher layers) and either an error message is presented to the application user or some other
appropriate action is taken. Note that Hibernate might also throw other unchecked exceptions that are
not a HibernateException. These are not recoverable and appropriate action should be taken.
Hibernate wraps SQLExceptions thrown while interacting with the database in a JDBCException. In
fact, Hibernate will attempt to convert the exception into a more meaningful subclass of
JDBCException. The underlying SQLException is always available via
JDBCException.getCause(). Hibernate converts the SQLException into an appropriate
JDBCException subclass using the SQLExceptionConverter attached to the SessionFactory. By
default, the SQLExceptionConverter is defined by the configured dialect. However, it is also possible
to plug in a custom implementation. See the javadocs for the SQLExceptionConverterFactory
class for details. The standard JDBCException subtypes are:
JDBCConnectionException: indicates an error with the underlying JDBC communication.
SQLGrammarException: indicates a grammar or syntax problem with the issued SQL.
ConstraintViolationException: indicates some form of integrity constraint violation.
LockAcquisitionException: indicates an error acquiring a lock level necessary to perform
the requested operation.
GenericJDBCException: a generic exception which did not fall into any of the other
categories.
Report a bug
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11.3.5. Transaction Timeout
An important feature provided by a managed environment like EJB is transaction timeout, which is never
provided in non-managed code. Transaction timeouts ensure that no misbehaving transaction can
indefinitely tie up resources while returning no response to the user. Outside a managed (JTA)
environment, Hibernate cannot fully provide this functionality. However, Hibernate can at least control
data access operations, ensuring that database level deadlocks and queries with huge result sets are
limited by a defined timeout. In a managed environment, Hibernate can delegate transaction timeout to
JTA. This functionality is abstracted by the Hibernate Transaction object.
setTimeout() cannot be called in a CMT bean, where transaction timeouts must be defined
declaratively.
Report a bug
11.4. OPTIMISTIC CONCURRENCY CONTROL
11.4.1. About Optimistic Concurrency Control
The only approach that is consistent with high concurrency and high scalability, is optimistic concurrency
control with versioning. Version checking uses version numbers, or timestamps, to detect conflicting
updates and to prevent lost updates. Hibernate provides three possible approaches to writing application
code that uses optimistic concurrency. The use cases we discuss are in the context of long
conversations, but version checking also has the benefit of preventing lost updates in single database
transactions.
Report a bug
11.4.2. Application Version Checking
In an implementation without much help from Hibernate, each interaction with the database occurs in a
new Session and the developer is responsible for reloading all persistent instances from the database
before manipulating them. The application is forced to carry out its own version checking to ensure
conversation transaction isolation. This approach is the least efficient in terms of database access. It is
the approach most similar to entity EJBs.
Session sess = factory.openSession();
try {
//set transaction timeout to 3 seconds
sess.getTransaction().setTimeout(3);
sess.getTransaction().begin();
// do some work
...
sess.getTransaction().commit();
}
catch (RuntimeException e) {
sess.getTransaction().rollback();
throw e; // or display error message
}
finally {
sess.close();
}
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The version property is mapped using <version>, and Hibernate will automatically increment it
during flush if the entity is dirty.
If you are operating in a low-data-concurrency environment, and do not require version checking, you
can use this approach and skip the version check. In this case, last commit wins is the default strategy
for long conversations. Be aware that this might confuse the users of the application, as they might
experience lost updates without error messages or a chance to merge conflicting changes.
Manual version checking is only feasible in trivial circumstances and not practical for most applications.
Often not only single instances, but complete graphs of modified objects, have to be checked. Hibernate
offers automatic version checking with either an extended Session or detached instances as the design
paradigm.
Report a bug
11.4.3. Extended Session and Automatic Versioning
A single Session instance and its persistent instances that are used for the whole conversation are
known as session-per-conversation. Hibernate checks instance versions at flush time, throwing an
exception if concurrent modification is detected. It is up to the developer to catch and handle this
exception. Common options are the opportunity for the user to merge changes or to restart the business
conversation with non-stale data.
The Session is disconnected from any underlying JDBC connection when waiting for user interaction.
This approach is the most efficient in terms of database access. The application does not version check
or reattach detached instances, nor does it have to reload instances in every database transaction.
The foo object knows which Session it was loaded in. Beginning a new database transaction on an old
session obtains a new connection and resumes the session. Committing a database transaction
disconnects a session from the JDBC connection and returns the connection to the pool. After
reconnection, to force a version check on data you are not updating, you can call Session.lock() with
LockMode.READ on any objects that might have been updated by another transaction. You do not need
// foo is an instance loaded by a previous Session
session = factory.openSession();
Transaction t = session.beginTransaction();
int oldVersion = foo.getVersion();
session.load( foo, foo.getKey() ); // load the current state
if ( oldVersion != foo.getVersion() ) throw new
StaleObjectStateException("Message", foo.getId());
foo.setProperty("bar");
t.commit();
session.close();
// foo is an instance loaded earlier by the old session
Transaction t = session.beginTransaction(); // Obtain a new JDBC
connection, start transaction
foo.setProperty("bar");
session.flush(); // Only for last transaction in conversation
t.commit(); // Also return JDBC connection
session.close(); // Only for last transaction in conversation
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to lock any data that you are updating. Usually you would set FlushMode.MANUAL on an extended
Session, so that only the last database transaction cycle is allowed to actually persist all modifications
made in this conversation. Only this last database transaction will include the flush() operation, and
then close() the session to end the conversation.
This pattern is problematic if the Session is too big to be stored during user think time (for example, an
HttpSession should be kept as small as possible). As the Session is also the first-level cache and
contains all loaded objects, we can probably use this strategy only for a few request/response cycles.
Use a Session only for a single conversation as it will soon have stale data.
NOTE
Earlier versions of Hibernate required explicit disconnection and reconnection of a
Session. These methods are deprecated, as beginning and ending a transaction has the
same effect.
Keep the disconnected Session close to the persistence layer. Use an EJB stateful session bean to
hold the Session in a three-tier environment. Do not transfer it to the web layer, or even serialize it to a
separate tier, to store it in the HttpSession.
The extended session pattern, or session-per-conversation, is more difficult to implement with automatic
current session context management. You need to supply your own implementation of the
CurrentSessionContext for this. See the Hibernate Wiki for examples.
Report a bug
11.4.4. Detached Objects and Automatic Versioning
Each interaction with the persistent store occurs in a new Session. However, the same persistent
instances are reused for each interaction with the database. The application manipulates the state of
detached instances originally loaded in another Session and then reattaches them using
Session.update(), Session.saveOrUpdate(), or Session.merge().
Again, Hibernate will check instance versions during flush, throwing an exception if conflicting updates
occurred.
You can also call lock() instead of update(), and use LockMode.READ (performing a version check
and bypassing all caches) if you are sure that the object has not been modified.
Report a bug
11.4.5. Customizing Automatic Versioning
// foo is an instance loaded by a previous Session
foo.setProperty("bar");
session = factory.openSession();
Transaction t = session.beginTransaction();
session.saveOrUpdate(foo); // Use merge() if "foo" might have been loaded
already
t.commit();
session.close();
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You can disable Hibernate's automatic version increment for particular properties and collections by
setting the optimistic-lock mapping attribute to false. Hibernate will then no longer increment
versions if the property is dirty.
Legacy database schemas are often static and cannot be modified. Or, other applications might access
the same database and will not know how to handle version numbers or even timestamps. In both cases,
versioning cannot rely on a particular column in a table. To force a version check with a comparison of
the state of all fields in a row but without a version or timestamp property mapping, turn on
optimistic-lock="all" in the <class> mapping. This conceptually only works if Hibernate can
compare the old and the new state (i.e., if you use a single long Session and not session-per-request-
with-detached-objects).
Concurrent modification can be permitted in instances where the changes that have been made do not
overlap. If you set optimistic-lock="dirty" when mapping the <class>, Hibernate will only
compare dirty fields during flush.
In both cases, with dedicated version/timestamp columns or with a full/dirty field comparison, Hibernate
uses a single UPDATE statement, with an appropriate WHERE clause, per entity to execute the version
check and update the information. If you use transitive persistence to cascade reattachment to
associated entities, Hibernate may execute unnecessary updates. This is usually not a problem, but on
update triggers in the database might be executed even when no changes have been made to detached
instances. You can customize this behavior by setting select-before-update="true" in the
<class> mapping, forcing Hibernate to SELECT the instance to ensure that changes did occur before
updating the row.
Report a bug
11.5. PESSIMISTIC LOCKING
11.5.1. About Pessimistic Locking
It is not intended that users spend much time worrying about locking strategies. It is usually enough to
specify an isolation level for the JDBC connections and then simply let the database do all the work.
However, advanced users may wish to obtain exclusive pessimistic locks or re-obtain locks at the start of
a new transaction.
Hibernate will always use the locking mechanism of the database; it never lock objects in memory.
The LockMode class defines the different lock levels that can be acquired by Hibernate. A lock is
obtained by the following mechanisms:
LockMode.WRITE is acquired automatically when Hibernate updates or inserts a row.
LockMode.UPGRADE can be acquired upon explicit user request using SELECT ... FOR
UPDATE on databases which support that syntax.
LockMode.UPGRADE_NOWAIT can be acquired upon explicit user request using a SELECT ...
FOR UPDATE NOWAIT under Oracle.
LockMode.READ is acquired automatically when Hibernate reads data under Repeatable Read
or Serializable isolation level. It can be re-acquired by explicit user request.
LockMode.NONE represents the absence of a lock. All objects switch to this lock mode at the
end of a Transaction. Objects associated with the session via a call to update() or
saveOrUpdate() also start out in this lock mode.
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The "explicit user request" is expressed in one of the following ways:
A call to Session.load(), specifying a LockMode.
A call to Session.lock().
A call to Query.setLockMode().
If Session.load() is called with UPGRADE or UPGRADE_NOWAIT, and the requested object was not yet
loaded by the session, the object is loaded using SELECT ... FOR UPDATE. If load() is called for an
object that is already loaded with a less restrictive lock than the one requested, Hibernate calls lock()
for that object.
Session.lock() performs a version number check if the specified lock mode is READ, UPGRADE or
UPGRADE_NOWAIT. In the case of UPGRADE or UPGRADE_NOWAIT, SELECT ... FOR UPDATE is used.
If the requested lock mode is not supported by the database, Hibernate uses an appropriate alternate
mode instead of throwing an exception. This ensures that applications are portable.
Report a bug
11.5.2. Connection Release Modes
One of the legacies of Hibernate 2.x JDBC connection management meant that a Session would obtain
a connection when it was first required and then maintain that connection until the session was closed.
Hibernate 3.x introduced the notion of connection release modes that would instruct a session how to
handle its JDBC connections. The following discussion is pertinent only to connections provided through
a configured ConnectionProvider. User-supplied connections are outside the breadth of this
discussion. The different release modes are identified by the enumerated values of
org.hibernate.ConnectionReleaseMode:
ON_CLOSE: is the legacy behavior described above. The Hibernate session obtains a connection
when it first needs to perform some JDBC access and maintains that connection until the
session is closed.
AFTER_TRANSACTION: releases connections after a org.hibernate.Transaction has
been completed.
AFTER_STATEMENT (also referred to as aggressive release): releases connections after every
statement execution. This aggressive releasing is skipped if that statement leaves open
resources associated with the given session. Currently the only situation where this occurs is
through the use of org.hibernate.ScrollableResults.
The configuration parameter hibernate.connection.release_mode is used to specify which
release mode to use. The possible values are as follows:
auto (the default): this choice delegates to the release mode returned by the
org.hibernate.transaction.TransactionFactory.getDefaultReleaseMode()
method. For JTATransactionFactory, this returns
ConnectionReleaseMode.AFTER_STATEMENT; for JDBCTransactionFactory, this returns
ConnectionReleaseMode.AFTER_TRANSACTION. Do not change this default behavior as
failures due to the value of this setting tend to indicate bugs and/or invalid assumptions in user
code.
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on_close: uses ConnectionReleaseMode.ON_CLOSE. This setting is left for backwards
compatibility, but its use is discouraged.
after_transaction: uses ConnectionReleaseMode.AFTER_TRANSACTION. This setting
should not be used in JTA environments. Also note that with
ConnectionReleaseMode.AFTER_TRANSACTION, if a session is considered to be in auto-
commit mode, connections will be released as if the release mode were AFTER_STATEMENT.
after_statement: uses ConnectionReleaseMode.AFTER_STATEMENT. Additionally, the
configured ConnectionProvider is consulted to see if it supports this setting
(supportsAggressiveRelease()). If not, the release mode is reset to
ConnectionReleaseMode.AFTER_TRANSACTION. This setting is only safe in environments
where we can either re-acquire the same underlying JDBC connection each time you make a
call into ConnectionProvider.getConnection() or in auto-commit environments where it
does not matter if we re-establish the same connection.
Report a bug
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CHAPTER 12. INTERCEPTORS AND EVENTS
12.1. INTERCEPTORS
It is useful for the application to react to certain events that occur inside Hibernate. This allows for the
implementation of generic functionality and the extension of Hibernate functionality.
The Interceptor interface provides callbacks from the session to the application, allowing the
application to inspect and/or manipulate properties of a persistent object before it is saved, updated,
deleted or loaded. One possible use for this is to track auditing information. For example, the following
Interceptor automatically sets the createTimestamp when an Auditable is created and updates
the lastUpdateTimestamp property when an Auditable is updated.
You can either implement Interceptor directly or extend EmptyInterceptor.
package org.hibernate.test;
import java.io.Serializable;
import java.util.Date;
import java.util.Iterator;
import org.hibernate.EmptyInterceptor;
import org.hibernate.Transaction;
import org.hibernate.type.Type;
public class AuditInterceptor extends EmptyInterceptor {
private int updates;
private int creates;
private int loads;
public void onDelete(Object entity,
Serializable id,
Object[] state,
String[] propertyNames,
Type[] types) {
// do nothing
}
public boolean onFlushDirty(Object entity,
Serializable id,
Object[] currentState,
Object[] previousState,
String[] propertyNames,
Type[] types) {
if ( entity instanceof Auditable ) {
updates++;
for ( int i=0; i < propertyNames.length; i++ ) {
if ( "lastUpdateTimestamp".equals( propertyNames[i] ) ) {
currentState[i] = new Date();
return true;
}
}
}
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There are two kinds of inteceptors: Session-scoped and SessionFactory-scoped.
A Session-scoped interceptor is specified when a session is opened using one of the overloaded
SessionFactory.openSession() methods accepting an Interceptor.
A SessionFactory-scoped interceptor is registered with the Configuration object prior to building
the SessionFactory. Unless a session is opened explicitly specifying the interceptor to use, the
supplied interceptor will be applied to all sessions opened from that SessionFactory.
return false;
}
public boolean onLoad(Object entity,
Serializable id,
Object[] state,
String[] propertyNames,
Type[] types) {
if ( entity instanceof Auditable ) {
loads++;
}
return false;
}
public boolean onSave(Object entity,
Serializable id,
Object[] state,
String[] propertyNames,
Type[] types) {
if ( entity instanceof Auditable ) {
creates++;
for ( int i=0; i<propertyNames.length; i++ ) {
if ( "createTimestamp".equals( propertyNames[i] ) ) {
state[i] = new Date();
return true;
}
}
}
return false;
}
public void afterTransactionCompletion(Transaction tx) {
if ( tx.wasCommitted() ) {
System.out.println("Creations: " + creates + ", Updates: " +
updates + "Loads: " + loads);
}
updates=0;
creates=0;
loads=0;
}
}
Session session = sf.openSession( new AuditInterceptor() );
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SessionFactory-scoped interceptors must be thread safe. Ensure that you do not store session-
specific states, since multiple sessions will use this interceptor potentially concurrently.
Report a bug
12.2. EVENT SYSTEM
If you have to react to particular events in your persistence layer, you can also use the Hibernate3 event
architecture. The event system can be used in addition, or as a replacement, for interceptors.
All the methods of the Session interface correlate to an event. You have a LoadEvent, a
FlushEvent, etc. Consult the XML configuration-file DTD or the org.hibernate.event package for
the full list of defined event types. When a request is made of one of these methods, the Hibernate
Session generates an appropriate event and passes it to the configured event listeners for that type.
Out-of-the-box, these listeners implement the same processing in which those methods always resulted.
However, you are free to implement a customization of one of the listener interfaces (i.e., the LoadEvent
is processed by the registered implementation of the LoadEventListener interface), in which case
their implementation would be responsible for processing any load() requests made of the Session.
The listeners should be considered singletons. This means they are shared between requests, and
should not save any state as instance variables.
A custom listener implements the appropriate interface for the event it wants to process and/or extend
one of the convenience base classes (or even the default event listeners used by Hibernate out-of-the-
box as these are declared non-final for this purpose). Custom listeners can either be registered
programmatically through the Configuration object, or specified in the Hibernate configuration XML.
Declarative configuration through the properties file is not supported. Here is an example of a custom
load event listener:
You also need a configuration entry telling Hibernate to use the listener in addition to the default listener:
new Configuration().setInterceptor( new AuditInterceptor() );
public class MyLoadListener implements LoadEventListener {
// this is the single method defined by the LoadEventListener
interface
public void onLoad(LoadEvent event, LoadEventListener.LoadType
loadType)
throws HibernateException {
if ( !MySecurity.isAuthorized( event.getEntityClassName(),
event.getEntityId() ) ) {
throw new MySecurityException("Unauthorized access");
}
}
}
<hibernate-configuration>
<session-factory>
...
<event type="load">
<listener class="com.eg.MyLoadListener"/>
<listener
class="org.hibernate.event.def.DefaultLoadEventListener"/>
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Instead, you can register it programmatically:
Listeners registered declaratively cannot share instances. If the same class name is used in multiple
<listener/> elements, each reference will result in a separate instance of that class. If you need to
share listener instances between listener types you must use the programmatic registration approach.
Why implement an interface and define the specific type during configuration? A listener implementation
could implement multiple event listener interfaces. Having the type additionally defined during
registration makes it easier to turn custom listeners on or off during configuration.
Report a bug
12.3. HIBERNATE DECLARATIVE SECURITY
Usually, declarative security in Hibernate applications is managed in a session facade layer. Hibernate3
allows certain actions to be permissioned via JACC, and authorized via JAAS. This is an optional
functionality that is built on top of the event architecture.
First, you must configure the appropriate event listeners, to enable the use of JAAS authorization.
Note that <listener type="..." class="..."/> is shorthand for <event type="...">
<listener class="..."/></event> when there is exactly one listener for a particular event type.
Next, while still in hibernate.cfg.xml, bind the permissions to roles:
The role names are the roles understood by your JACC provider.
Report a bug
</event>
</session-factory>
</hibernate-configuration>
Configuration cfg = new Configuration();
LoadEventListener[] stack = { new MyLoadListener(), new
DefaultLoadEventListener() };
cfg.getEventListeners().setLoadEventListeners(stack);
<listener type="pre-delete"
class="org.hibernate.secure.JACCPreDeleteEventListener"/>
<listener type="pre-update"
class="org.hibernate.secure.JACCPreUpdateEventListener"/>
<listener type="pre-insert"
class="org.hibernate.secure.JACCPreInsertEventListener"/>
<listener type="pre-load"
class="org.hibernate.secure.JACCPreLoadEventListener"/>
<grant role="admin" entity-name="User" actions="insert,update,read"/>
<grant role="su" entity-name="User" actions="*"/>
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CHAPTER 13. BATCH PROCESSING
13.1. ABOUT BATCH PROCESSING
A naive approach to inserting 100,000 rows in the database using Hibernate might look like this:
This would fall over with an OutOfMemoryException somewhere around the 50,000th row. That is
because Hibernate caches all the newly inserted Customer instances in the session-level cache. In this
chapter we will show you how to avoid this problem.
If you are undertaking batch processing you will need to enable the use of JDBC batching. This is
absolutely essential if you want to achieve optimal performance. Set the JDBC batch size to a
reasonable number (10-50, for example):
Hibernate disables insert batching at the JDBC level transparently if you use an identity identifier
generator.
You can also do this kind of work in a process where interaction with the second-level cache is
completely disabled:
However, this is not absolutely necessary, since we can explicitly set the CacheMode to disable
interaction with the second-level cache.
Report a bug
13.2. BATCH INSERTS
When making new objects persistent flush() and then clear() the session regularly in order to
control the size of the first-level cache.
Session session = sessionFactory.openSession();
Transaction tx = session.beginTransaction();
for ( int i=0; i<100000; i++ ) {
Customer customer = new Customer(.....);
session.save(customer);
}
tx.commit();
session.close();
hibernate.jdbc.batch_size 20
hibernate.cache.use_second_level_cache false
Session session = sessionFactory.openSession();
Transaction tx = session.beginTransaction();
for ( int i=0; i<100000; i++ ) {
Customer customer = new Customer(.....);
session.save(customer);
if ( i % 20 == 0 ) { //20, same as the JDBC batch size
//flush a batch of inserts and release memory:
session.flush();
session.clear();
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Report a bug
13.3. BATCH UPDATES
For retrieving and updating data, the same ideas apply. In addition, you need to use scroll() to take
advantage of server-side cursors for queries that return many rows of data.
Report a bug
13.4. THE STATELESSSESSION INTERFACE
Alternatively, Hibernate provides a command-oriented API that can be used for streaming data to and
from the database in the form of detached objects. A StatelessSession has no persistence context
associated with it and does not provide many of the higher-level life cycle semantics. In particular, a
stateless session does not implement a first-level cache nor interact with any second-level or query
cache. It does not implement transactional write-behind or automatic dirty checking. Operations
performed using a stateless session never cascade to associated instances. Collections are ignored by a
stateless session. Operations performed via a stateless session bypass Hibernate's event model and
interceptors. Due to the lack of a first-level cache, Stateless sessions are vulnerable to data aliasing
effects. A stateless session is a lower-level abstraction that is much closer to the underlying JDBC.
}
}
tx.commit();
session.close();
Session session = sessionFactory.openSession();
Transaction tx = session.beginTransaction();
ScrollableResults customers = session.getNamedQuery("GetCustomers")
.setCacheMode(CacheMode.IGNORE)
.scroll(ScrollMode.FORWARD_ONLY);
int count=0;
while ( customers.next() ) {
Customer customer = (Customer) customers.get(0);
customer.updateStuff(...);
if ( ++count % 20 == 0 ) {
//flush a batch of updates and release memory:
session.flush();
session.clear();
}
}
tx.commit();
session.close();
StatelessSession session = sessionFactory.openStatelessSession();
Transaction tx = session.beginTransaction();
ScrollableResults customers = session.getNamedQuery("GetCustomers")
.scroll(ScrollMode.FORWARD_ONLY);
while ( customers.next() ) {
Customer customer = (Customer) customers.get(0);
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In this code example, the Customer instances returned by the query are immediately detached. They
are never associated with any persistence context.
The insert(), update() and delete() operations defined by the StatelessSession interface
are considered to be direct database row-level operations. They result in the immediate execution of a
SQL INSERT, UPDATE or DELETE respectively. They have different semantics to the save(),
saveOrUpdate() and delete() operations defined by the Session interface.
Report a bug
13.5. DML-STYLE OPERATIONS
As already discussed, automatic and transparent object/relational mapping is concerned with the
management of the object state. The object state is available in memory. This means that manipulating
data directly in the database (using the SQL Data Manipulation Language (DML) the statements:
INSERT, UPDATE, DELETE) will not affect in-memory state. However, Hibernate provides methods for
bulk SQL-style DML statement execution that is performed through the Hibernate Query Language
(Refer to the "Hibernate Query Language" chapter for further information).
The pseudo-syntax for UPDATE and DELETE statements is: ( UPDATE | DELETE ) FROM?
EntityName (WHERE where_conditions)?.
Some points to note:
In the from-clause, the FROM keyword is optional
There can only be a single entity named in the from-clause. It can, however, be aliased. If the
entity name is aliased, then any property references must be qualified using that alias. If the
entity name is not aliased, then it is illegal for any property references to be qualified.
No join syntax forms, either implicit or explicit, can be specified in a bulk HQL query. Sub-
queries can be used in the where-clause, where the subqueries themselves may contain joins.
The where-clause is also optional.
As an example, to execute an HQL UPDATE, use the Query.executeUpdate() method. The method
is named for those familiar with JDBC's PreparedStatement.executeUpdate():
customer.updateStuff(...);
session.update(customer);
}
tx.commit();
session.close();
Session session = sessionFactory.openSession();
Transaction tx = session.beginTransaction();
String hqlUpdate = "update Customer c set c.name = :newName where c.name =
:oldName";
// or String hqlUpdate = "update Customer set name = :newName where name =
:oldName";
int updatedEntities = session.createQuery( hqlUpdate )
.setString( "newName", newName )
.setString( "oldName", oldName )
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In keeping with the EJB3 specification, HQL UPDATE statements, by default, do not affect the version or
timestamp property values for the relevant entities. However, you can force Hibernate to reset the
version or timestamp property values through the use of a versioned update. This is achieved by
adding the VERSIONED keyword after the UPDATE keyword.
Custom version types, org.hibernate.usertype.UserVersionType, are not allowed in
conjunction with a update versioned statement.
To execute an HQL DELETE, use the same Query.executeUpdate() method:
The int value returned by the Query.executeUpdate() method indicates the number of entities
effected by the operation. This may or may not correlate to the number of rows effected in the database.
An HQL bulk operation might result in multiple actual SQL statements being executed (for joined-
subclass, for example). The returned number indicates the number of actual entities affected by the
statement. Going back to the example of joined-subclass, a delete against one of the subclasses may
actually result in deletes against not just the table to which that subclass is mapped, but also the "root"
table and potentially joined-subclass tables further down the inheritance hierarchy.
The pseudo-syntax for INSERT statements is: INSERT INTO EntityName properties_list
select_statement. Some points to note:
Only the INSERT INTO ... SELECT ... form is supported; not the INSERT INTO ... VALUES ...
form.
The properties_list is analogous to the column specification in the SQL INSERT
statement. For entities involved in mapped inheritance, only properties directly defined on that
given class-level can be used in the properties_list. Superclass properties are not allowed and
subclass properties do not make sense. In other words, INSERT statements are inherently non-
polymorphic.
.executeUpdate();
tx.commit();
session.close();
Session session = sessionFactory.openSession();
Transaction tx = session.beginTransaction();
String hqlVersionedUpdate = "update versioned Customer set name = :newName
where name = :oldName";
int updatedEntities = session.createQuery( hqlVersionedUpdate )
.setString( "newName", newName )
.setString( "oldName", oldName )
.executeUpdate();
tx.commit();
session.close();
Session session = sessionFactory.openSession();
Transaction tx = session.beginTransaction();
String hqlDelete = "delete Customer c where c.name = :oldName";
// or String hqlDelete = "delete Customer where name = :oldName";
int deletedEntities = session.createQuery( hqlDelete )
.setString( "oldName", oldName )
.executeUpdate();
tx.commit();
session.close();
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select_statement can be any valid HQL select query, with the caveat that the return types must
match the types expected by the insert. Currently, this is checked during query compilation
rather than allowing the check to relegate to the database. This might, however, cause problems
between Hibernate Types which are equivalent as opposed to equal. This might cause issues
with mismatches between a property defined as a org.hibernate.type.DateType and a
property defined as a org.hibernate.type.TimestampType, even though the database
might not make a distinction or might be able to handle the conversion.
For the id property, the insert statement gives you two options. You can either explicitly specify
the id property in the properties_list, in which case its value is taken from the corresponding
select expression, or omit it from the properties_list, in which case a generated value is used.
This latter option is only available when using id generators that operate in the database;
attempting to use this option with any "in memory" type generators will cause an exception
during parsing. For the purposes of this discussion, in-database generators are considered to be
org.hibernate.id.SequenceGenerator (and its subclasses) and any implementers of
org.hibernate.id.PostInsertIdentifierGenerator. The most notable exception here
is org.hibernate.id.TableHiLoGenerator, which cannot be used because it does not
expose a selectable way to get its values.
For properties mapped as either version or timestamp, the insert statement gives you two
options. You can either specify the property in the properties_list, in which case its value is taken
from the corresponding select expressions, or omit it from the properties_list, in which case the
seed value defined by the org.hibernate.type.VersionType is used.
The following is an example of an HQL INSERT statement execution:
Report a bug
Session session = sessionFactory.openSession();
Transaction tx = session.beginTransaction();
String hqlInsert = "insert into DelinquentAccount (id, name) select c.id,
c.name from Customer c where ...";
int createdEntities = session.createQuery( hqlInsert )
.executeUpdate();
tx.commit();
session.close();
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CHAPTER 14. THE HIBERNATE QUERY LANGUAGE (HQL)
14.1. ABOUT THE HIBERNATE QUERY LANGUAGE
Hibernate uses a powerful query language (HQL) that is similar in appearance to SQL. Compared with
SQL, however, HQL is fully object-oriented and understands notions like inheritance, polymorphism and
association.
Report a bug
14.2. CASE SENSITIVITY
With the exception of names of Java classes and properties, queries are case-insensitive. So SeLeCT is
the same as sELEct is the same as SELECT, but org.hibernate.eg.FOO is not
org.hibernate.eg.Foo, and foo.barSet is not foo.BARSET.
This manual uses lowercase HQL keywords. Some users find queries with uppercase keywords more
readable, but this convention is unsuitable for queries embedded in Java code.
Report a bug
14.3. THE FROM CLAUSE
The simplest possible Hibernate query is of the form:
This returns all instances of the class eg.Cat. You do not usually need to qualify the class name, since
auto-import is the default. For example:
In order to refer to the Cat in other parts of the query, you will need to assign an alias. For example:
This query assigns the alias cat to Cat instances, so you can use that alias later in the query. The as
keyword is optional. You could also write:
Multiple classes can appear, resulting in a cartesian product or "cross" join.
It is good practice to name query aliases using an initial lowercase as this is consistent with Java naming
standards for local variables (e.g. domesticCat).
Report a bug
from eg.Cat
from Cat
from Cat as cat
from Cat cat
from Formula, Parameter
from Formula as form, Parameter as param
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14.4. ASSOCIATIONS AND JOINS
You can also assign aliases to associated entities or to elements of a collection of values using a join.
For example:
The supported join types are borrowed from ANSI SQL:
inner join
left outer join
right outer join
full join (not usually useful)
The inner join, left outer join and right outer join constructs may be abbreviated.
You may supply extra join conditions using the HQL with keyword.
A "fetch" join allows associations or collections of values to be initialized along with their parent objects
using a single select. This is particularly useful in the case of a collection. It effectively overrides the outer
join and lazy declarations of the mapping file for associations and collections.
A fetch join does not usually need to assign an alias, because the associated objects should not be used
in the where clause (or any other clause). The associated objects are also not returned directly in the
query results. Instead, they may be accessed via the parent object. The only reason you might need an
alias is if you are recursively join fetching a further collection:
from Cat as cat
inner join cat.mate as mate
left outer join cat.kittens as kitten
from Cat as cat left join cat.mate.kittens as kittens
from Formula form full join form.parameter param
from Cat as cat
join cat.mate as mate
left join cat.kittens as kitten
from Cat as cat
left join cat.kittens as kitten
with kitten.bodyWeight > 10.0
from Cat as cat
inner join fetch cat.mate
left join fetch cat.kittens
from Cat as cat
inner join fetch cat.mate
left join fetch cat.kittens child
left join fetch child.kittens
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IMPORTANT
The fetch construct cannot be used in queries called using iterate() (though
scroll() can be used). Fetch should not be used together with setMaxResults() or
setFirstResult(), as these operations are based on the result rows which usually
contain duplicates for eager collection fetching, hence, the number of rows is not what you
would expect. Fetch should also not be used together with impromptu with condition. It
is possible to create a cartesian product by join fetching more than one collection in a
query, so take care in this case. Join fetching multiple collection roles can produce
unexpected results for bag mappings, so user discretion is advised when formulating
queries in this case. Finally, note that full join fetch and right join fetch are
not meaningful.
If you are using property-level lazy fetching (with bytecode instrumentation), it is possible
to force Hibernate to fetch the lazy properties in the first query immediately using fetch
all properties.
Report a bug
14.5. FORMS OF JOIN SYNTAX
HQL supports two forms of association joining: implicit and explicit.
The queries shown in the previous section all use the explicit form, that is, where the join keyword is
explicitly used in the from clause. This is the recommended form.
The implicit form does not use the join keyword. Instead, the associations are "dereferenced" using
dot-notation. implicit joins can appear in any of the HQL clauses. implicit join result in inner joins
in the resulting SQL statement.
Report a bug
14.6. REFERRING TO IDENTIFIER PROPERTY
There are 2 ways to refer to an entity's identifier property:
The special property (lowercase) id may be used to reference the identifier property of an entity
provided that the entity does not define a non-identifier property named id.
If the entity defines a named identifier property, you can use that property name.
References to composite identifier properties follow the same naming rules. If the entity has a non-
identifier property named id, the composite identifier property can only be referenced by its defined
named. Otherwise, the special id property can be used to reference the identifier property.
from Document fetch all properties order by name
from Document doc fetch all properties where lower(doc.name)
like '%cats%'
from Cat as cat where cat.mate.name like '%s%'
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IMPORTANT
Please note that, starting in version 3.2.2, this has changed significantly. In previous
versions, id always referred to the identifier property regardless of its actual name. A
ramification of that decision was that non-identifier properties named id could never be
referenced in Hibernate queries.
Report a bug
14.7. THE SELECT CLAUSE
The select clause picks which objects and properties to return in the query result set. Consider the
following:
The query will select mates of other Cats. You can express this query more compactly as:
Queries can return properties of any value type including properties of component type:
Queries can return multiple objects and/or properties as an array of type Object[]:
Or as a List:
Or - assuming that the class Family has an appropriate constructor - as an actual typesafe Java object:
You can assign aliases to selected expressions using as:
select mate
from Cat as cat
inner join cat.mate as mate
select cat.mate from Cat cat
select cat.name from DomesticCat cat
where cat.name like 'fri%'
select cust.name.firstName from Customer as cust
select mother, offspr, mate.name
from DomesticCat as mother
inner join mother.mate as mate
left outer join mother.kittens as offspr
select new list(mother, offspr, mate.name)
from DomesticCat as mother
inner join mother.mate as mate
left outer join mother.kittens as offspr
select new Family(mother, mate, offspr)
from DomesticCat as mother
join mother.mate as mate
left join mother.kittens as offspr
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This is most useful when used together with select new map:
This query returns a Map from aliases to selected values.
Report a bug
14.8. AGGREGATE FUNCTIONS
HQL queries can even return the results of aggregate functions on properties:
The supported aggregate functions are:
avg(...), sum(...), min(...), max(...)
count(*)
count(...), count(distinct ...), count(all...)
You can use arithmetic operators, concatenation, and recognized SQL functions in the select clause:
The distinct and all keywords can be used and have the same semantics as in SQL.
Report a bug
14.9. POLYMORPHIC QUERIES
A query like:
returns instances not only of Cat, but also of subclasses like DomesticCat. Hibernate queries can
select max(bodyWeight) as max, min(bodyWeight) as min, count(*) as n
from Cat cat
select new map( max(bodyWeight) as max, min(bodyWeight) as min, count(*)
as n )
from Cat cat
select avg(cat.weight), sum(cat.weight), max(cat.weight), count(cat)
from Cat cat
select cat.weight + sum(kitten.weight)
from Cat cat
join cat.kittens kitten
group by cat.id, cat.weight
select firstName||' '||initial||' '||upper(lastName) from Person
select distinct cat.name from Cat cat
select count(distinct cat.name), count(cat) from Cat cat
from Cat as cat
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name any Java class or interface in the from clause. The query will return instances of all persistent
classes that extend that class or implement the interface. The following query would return all persistent
objects:
The interface Named might be implemented by various persistent classes:
These last two queries will require more than one SQL SELECT. This means that the order by clause
does not correctly order the whole result set. It also means you cannot call these queries using
Query.scroll().
Report a bug
14.10. THE WHERE CLAUSE
The where clause allows you to refine the list of instances returned. If no alias exists, you can refer to
properties by name:
If there is an alias, use a qualified property name:
This returns instances of Cat named 'Fritz'.
The following query:
returns all instances of Foo with an instance of bar with a date property equal to the startDate
property of the Foo. Compound path expressions make the where clause extremely powerful. Consider
the following:
This query translates to an SQL query with a table (inner) join. For example:
would result in a query that would require four table joins in SQL.
The = operator can be used to compare not only properties, but also instances:
from java.lang.Object o
from Named n, Named m where n.name = m.name
from Cat where name='Fritz'
from Cat as cat where cat.name='Fritz'
select foo
from Foo foo, Bar bar
where foo.startDate = bar.date
from Cat cat where cat.mate.name is not null
from Foo foo
where foo.bar.baz.customer.address.city is not null
from Cat cat, Cat rival where cat.mate = rival.mate
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The special property (lowercase) id can be used to reference the unique identifier of an object.
The second query is efficient and does not require a table join.
Properties of composite identifiers can also be used. Consider the following example where Person has
composite identifiers consisting of country and medicareNumber:
Once again, the second query does not require a table join.
Refer to the "Referring to Identifier Property" section for more information regarding referencing identifier
properties)
The special property class accesses the discriminator value of an instance in the case of polymorphic
persistence. A Java class name embedded in the where clause will be translated to its discriminator
value.
You can also use components or composite user types, or properties of said component types.
An "any" type has the special properties id and class that allows you to express a join in the following
way (where AuditLog.item is a property mapped with <any>):
The log.item.class and payment.class would refer to the values of completely different database
columns in the above query.
Report a bug
14.11. EXPRESSIONS
Expressions used in the where clause include the following:
mathematical operators: +, -, *, /
select cat, mate
from Cat cat, Cat mate
where cat.mate = mate
from Cat as cat where cat.id = 123
from Cat as cat where cat.mate.id = 69
from bank.Person person
where person.id.country = 'AU'
and person.id.medicareNumber = 123456
from bank.Account account
where account.owner.id.country = 'AU'
and account.owner.id.medicareNumber = 123456
from Cat cat where cat.class = DomesticCat
from AuditLog log, Payment payment
where log.item.class = 'Payment' and log.item.id = payment.id
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binary comparison operators: =, >=, <=, <>, !=, like
logical operations and, or, not
Parentheses ( ) that indicates grouping
in, not in, between, is null, is not null, is empty, is not empty, member of
and not member of
"Simple" case, case ... when ... then ... else ... end, and "searched" case, case
when ... then ... else ... end
string concatenation ...||... or concat(...,...)
current_date(), current_time(), and current_timestamp()
second(...), minute(...), hour(...), day(...), month(...), and year(...)
Any function or operator defined by EJB-QL 3.0: substring(), trim(), lower(),
upper(), length(), locate(), abs(), sqrt(), bit_length(), mod()
coalesce() and nullif()
str() for converting numeric or temporal values to a readable string
cast(... as ...), where the second argument is the name of a Hibernate type, and
extract(... from ...) if ANSI cast() and extract() is supported by the underlying
database
the HQL index() function, that applies to aliases of a joined indexed collection
HQL functions that take collection-valued path expressions: size(), minelement(),
maxelement(), minindex(), maxindex(), along with the special elements() and
indices functions that can be quantified using some, all, exists, any, in.
Any database-supported SQL scalar function like sign(), trunc(), rtrim(), and sin()
JDBC-style positional parameters ?
named parameters :name, :start_date, and :x1
SQL literals 'foo', 69, 6.66E+2, '1970-01-01 10:00:01.0'
Java public static final constants eg.Color.TABBY
in and between can be used as follows:
The negated forms can be written as follows:
from DomesticCat cat where cat.name between 'A' and 'B'
from DomesticCat cat where cat.name in ( 'Foo', 'Bar', 'Baz' )
from DomesticCat cat where cat.name not between 'A' and 'B'
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Similarly, is null and is not null can be used to test for null values.
Booleans can be easily used in expressions by declaring HQL query substitutions in Hibernate
configuration:
This will replace the keywords true and false with the literals 1 and 0 in the translated SQL from this
HQL:
You can test the size of a collection with the special property size or the special size() function.
For indexed collections, you can refer to the minimum and maximum indices using minindex and
maxindex functions. Similarly, you can refer to the minimum and maximum elements of a collection of
basic type using the minelement and maxelement functions. For example:
The SQL functions any, some, all, exists, in are supported when passed the element or index
set of a collection (elements and indices functions) or the result of a subquery (see below):
Note that these constructs - size, elements, indices, minindex, maxindex, minelement,
maxelement - can only be used in the where clause in Hibernate3.
from DomesticCat cat where cat.name not in ( 'Foo', 'Bar', 'Baz' )
<property name="hibernate.query.substitutions">true 1, false 0</property>
from Cat cat where cat.alive = true
from Cat cat where cat.kittens.size > 0
from Cat cat where size(cat.kittens) > 0
from Calendar cal where maxelement(cal.holidays) > current_date
from Order order where maxindex(order.items) > 100
from Order order where minelement(order.items) > 10000
select mother from Cat as mother, Cat as kit
where kit in elements(foo.kittens)
select p from NameList list, Person p
where p.name = some elements(list.names)
from Cat cat where exists elements(cat.kittens)
from Player p where 3 > all elements(p.scores)
from Show show where 'fizard' in indices(show.acts)
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Elements of indexed collections (arrays, lists, and maps) can be referred to by index in a where clause
only:
The expression inside [] can even be an arithmetic expression:
HQL also provides the built-in index() function for elements of a one-to-many association or collection
of values.
Scalar SQL functions supported by the underlying database can be used:
Consider how much longer and less readable the following query would be in SQL:
Hint: something like
from Order order where order.items[0].id = 1234
select person from Person person, Calendar calendar
where calendar.holidays['national day'] = person.birthDay
and person.nationality.calendar = calendar
select item from Item item, Order order
where order.items[ order.deliveredItemIndices[0] ] = item and order.id =
11
select item from Item item, Order order
where order.items[ maxindex(order.items) ] = item and order.id = 11
select item from Item item, Order order
where order.items[ size(order.items) - 1 ] = item
select item, index(item) from Order order
join order.items item
where index(item) < 5
from DomesticCat cat where upper(cat.name) like 'FRI%'
select cust
from Product prod,
Store store
inner join store.customers cust
where prod.name = 'widget'
and store.location.name in ( 'Melbourne', 'Sydney' )
and prod = all elements(cust.currentOrder.lineItems)
SELECT cust.name, cust.address, cust.phone, cust.id, cust.current_order
FROM customers cust,
stores store,
locations loc,
store_customers sc,
product prod
WHERE prod.name = 'widget'
AND store.loc_id = loc.id
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Report a bug
14.12. THE ORDER BY CLAUSE
The list returned by a query can be ordered by any property of a returned class or components:
The optional asc or desc indicate ascending or descending order respectively.
Report a bug
14.13. THE GROUP BY CLAUSE
A query that returns aggregate values can be grouped by any property of a returned class or
components:
A having clause is also allowed.
SQL functions and aggregate functions are allowed in the having and order by clauses if they are
supported by the underlying database (i.e., not in MySQL).
AND loc.name IN ( 'Melbourne', 'Sydney' )
AND sc.store_id = store.id
AND sc.cust_id = cust.id
AND prod.id = ALL(
SELECT item.prod_id
FROM line_items item, orders o
WHERE item.order_id = o.id
AND cust.current_order = o.id
)
from DomesticCat cat
order by cat.name asc, cat.weight desc, cat.birthdate
select cat.color, sum(cat.weight), count(cat)
from Cat cat
group by cat.color
select foo.id, avg(name), max(name)
from Foo foo join foo.names name
group by foo.id
select cat.color, sum(cat.weight), count(cat)
from Cat cat
group by cat.color
having cat.color in (eg.Color.TABBY, eg.Color.BLACK)
select cat
from Cat cat
join cat.kittens kitten
group by cat.id, cat.name, cat.other, cat.properties
having avg(kitten.weight) > 100
order by count(kitten) asc, sum(kitten.weight) desc
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Neither the group by clause nor the order by clause can contain arithmetic expressions. Hibernate
also does not currently expand a grouped entity, so you cannot write group by cat if all properties of
cat are non-aggregated. You have to list all non-aggregated properties explicitly.
Report a bug
14.14. SUBQUERIES
For databases that support subselects, Hibernate supports subqueries within queries. A subquery must
be surrounded by parentheses (often by an SQL aggregate function call). Even correlated subqueries
(subqueries that refer to an alias in the outer query) are allowed.
Note that HQL subqueries can occur only in the select or where clauses.
Note that subqueries can also utilize row value constructor syntax. Refer to the "Row Value
Constructor Syntax" section for further information.
Report a bug
14.15. HQL EXAMPLES
Hibernate queries can be quite powerful and complex. In fact, the power of the query language is one of
Hibernate's main strengths. The following example queries are similar to queries that have been used on
recent projects. Please note that most queries you will write will be much simpler than the following
examples.
The following query returns the order id, number of items, the given minimum total value and the total
value of the order for all unpaid orders for a particular customer. The results are ordered by total value. In
determining the prices, it uses the current catalog. The resulting SQL query, against the ORDER,
from Cat as fatcat
where fatcat.weight > (
select avg(cat.weight) from DomesticCat cat
)
from DomesticCat as cat
where cat.name = some (
select name.nickName from Name as name
)
from Cat as cat
where not exists (
from Cat as mate where mate.mate = cat
)
from DomesticCat as cat
where cat.name not in (
select name.nickName from Name as name
)
select cat.id, (select max(kit.weight) from cat.kitten kit)
from Cat as cat
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ORDER_LINE, PRODUCT, CATALOG and PRICE tables has four inner joins and an (uncorrelated)
subselect.
What a monster! Actually, in real life, I'm not very keen on subqueries, so my query was really more like
this:
The next query counts the number of payments in each status, excluding all payments in the
AWAITING_APPROVAL status where the most recent status change was made by the current user. It
translates to an SQL query with two inner joins and a correlated subselect against the PAYMENT,
PAYMENT_STATUS and PAYMENT_STATUS_CHANGE tables.
select order.id, sum(price.amount), count(item)
from Order as order
join order.lineItems as item
join item.product as product,
Catalog as catalog
join catalog.prices as price
where order.paid = false
and order.customer = :customer
and price.product = product
and catalog.effectiveDate < sysdate
and catalog.effectiveDate >= all (
select cat.effectiveDate
from Catalog as cat
where cat.effectiveDate < sysdate
)
group by order
having sum(price.amount) > :minAmount
order by sum(price.amount) desc
select order.id, sum(price.amount), count(item)
from Order as order
join order.lineItems as item
join item.product as product,
Catalog as catalog
join catalog.prices as price
where order.paid = false
and order.customer = :customer
and price.product = product
and catalog = :currentCatalog
group by order
having sum(price.amount) > :minAmount
order by sum(price.amount) desc
select count(payment), status.name
from Payment as payment
join payment.currentStatus as status
join payment.statusChanges as statusChange
where payment.status.name <> PaymentStatus.AWAITING_APPROVAL
or (
statusChange.timeStamp = (
select max(change.timeStamp)
from PaymentStatusChange change
where change.payment = payment
)
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If the statusChanges collection was mapped as a list, instead of a set, the query would have been
much simpler to write.
The next query uses the MS SQL Server isNull() function to return all the accounts and unpaid
payments for the organization to which the current user belongs. It translates to an SQL query with three
inner joins, an outer join and a subselect against the ACCOUNT, PAYMENT, PAYMENT_STATUS,
ACCOUNT_TYPE, ORGANIZATION and ORG_USER tables.
For some databases, we would need to do away with the (correlated) subselect.
Report a bug
14.16. BULK UPDATE AND DELETE
HQL now supports update, delete and insert ... select ... statements. Refer to the the
"DML-style Operations" section for further information.
Report a bug
14.17. HQL TIPS
You can count the number of query results without returning them:
and statusChange.user <> :currentUser
)
group by status.name, status.sortOrder
order by status.sortOrder
select count(payment), status.name
from Payment as payment
join payment.currentStatus as status
where payment.status.name <> PaymentStatus.AWAITING_APPROVAL
or payment.statusChanges[ maxIndex(payment.statusChanges) ].user <>
:currentUser
group by status.name, status.sortOrder
order by status.sortOrder
select account, payment
from Account as account
left outer join account.payments as payment
where :currentUser in elements(account.holder.users)
and PaymentStatus.UNPAID = isNull(payment.currentStatus.name,
PaymentStatus.UNPAID)
order by account.type.sortOrder, account.accountNumber, payment.dueDate
select account, payment
from Account as account
join account.holder.users as user
left outer join account.payments as payment
where :currentUser = user
and PaymentStatus.UNPAID = isNull(payment.currentStatus.name,
PaymentStatus.UNPAID)
order by account.type.sortOrder, account.accountNumber, payment.dueDate
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To order a result by the size of a collection, use the following query:
If your database supports subselects, you can place a condition upon selection size in the where clause
of your query:
If your database does not support subselects, use the following query:
As this solution cannot return a User with zero messages because of the inner join, the following form is
also useful:
Properties of a JavaBean can be bound to named query parameters:
Collections are pageable by using the Query interface with a filter:
Collection elements can be ordered or grouped using a query filter:
( (Integer) session.createQuery("select count(*) from
....").iterate().next() ).intValue()
select usr.id, usr.name
from User as usr
left join usr.messages as msg
group by usr.id, usr.name
order by count(msg)
from User usr where size(usr.messages) >= 1
select usr.id, usr.name
from User usr
join usr.messages msg
group by usr.id, usr.name
having count(msg) >= 1
select usr.id, usr.name
from User as usr
left join usr.messages as msg
group by usr.id, usr.name
having count(msg) = 0
Query q = s.createQuery("from foo Foo as foo where foo.name=:name and
foo.size=:size");
q.setProperties(fooBean); // fooBean has getName() and getSize()
List foos = q.list();
Query q = s.createFilter( collection, "" ); // the trivial filter
q.setMaxResults(PAGE_SIZE);
q.setFirstResult(PAGE_SIZE * pageNumber);
List page = q.list();
Collection orderedCollection = s.createFilter( collection, "order by
this.amount" ).list();
Collection counts = s.createFilter( collection, "select this.type,
count(this) group by this.type" ).list();
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205
You can find the size of a collection without initializing it:
Report a bug
14.18. COMPONENTS
Components can be used similarly to the simple value types that are used in HQL queries. They can
appear in the select clause as follows:
where the Person's name property is a component. Components can also be used in the where clause:
Components can also be used in the order by clause:
Another common use of components is detailed in the "Row Value Constrictor Syntax" section.
Report a bug
14.19. ROW VALUE CONSTRUCTOR SYNTAX
HQL supports the use of ANSI SQL row value constructor syntax, sometimes referred to AS
tuple syntax, even though the underlying database may not support that notion. Here, we are generally
referring to multi-valued comparisons, typically associated with components. Consider an entity Person
which defines a name component:
That is valid syntax although it is a little verbose. You can make this more concise by using row value
constructor syntax:
It can also be useful to specify this in the select clause:
( (Integer) session.createQuery("select count(*) from
....").iterate().next() ).intValue();
select p.name from Person p
select p.name.first from Person p
from Person p where p.name = :name
from Person p where p.name.first = :firstName
from Person p order by p.name
from Person p order by p.name.first
from Person p where p.name.first='John' and p.name.last='Jingleheimer-
Schmidt'
from Person p where p.name=('John', 'Jingleheimer-Schmidt')
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Using row value constructor syntax can also be beneficial when using subqueries that need to
compare against multiple values:
One thing to consider when deciding if you want to use this syntax, is that the query will be dependent
upon the ordering of the component sub-properties in the metadata.
Report a bug
select p.name from Person p
from Cat as cat
where not ( cat.name, cat.color ) in (
select cat.name, cat.color from DomesticCat cat
)
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CHAPTER 15. CRITERIA QUERIES
15.1. CREATING A CRITERIA INSTANCE
The interface org.hibernate.Criteria represents a query against a particular persistent class. The
Session is a factory for Criteria instances.
Report a bug
15.2. NARROWING THE RESULT SET
An individual query criterion is an instance of the interface org.hibernate.criterion.Criterion.
The class org.hibernate.criterion.Restrictions defines factory methods for obtaining certain
built-in Criterion types.
Restrictions can be grouped logically.
There are a range of built-in criterion types (Restrictions subclasses). One of the most useful allows
you to specify SQL directly.
Criteria crit = sess.createCriteria(Cat.class);
crit.setMaxResults(50);
List cats = crit.list();
List cats = sess.createCriteria(Cat.class)
.add( Restrictions.like("name", "Fritz%") )
.add( Restrictions.between("weight", minWeight, maxWeight) )
.list();
List cats = sess.createCriteria(Cat.class)
.add( Restrictions.like("name", "Fritz%") )
.add( Restrictions.or(
Restrictions.eq( "age", new Integer(0) ),
Restrictions.isNull("age")
) )
.list();
List cats = sess.createCriteria(Cat.class)
.add( Restrictions.in( "name", new String[] { "Fritz", "Izi", "Pk" } )
)
.add( Restrictions.disjunction()
.add( Restrictions.isNull("age") )
.add( Restrictions.eq("age", new Integer(0) ) )
.add( Restrictions.eq("age", new Integer(1) ) )
.add( Restrictions.eq("age", new Integer(2) ) )
)
.list();
List cats = sess.createCriteria(Cat.class)
.add( Restrictions.sqlRestriction("lower({alias}.name) like lower(?)",
"Fritz%", Hibernate.STRING) )
.list();
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The {alias} placeholder with be replaced by the row alias of the queried entity.
You can also obtain a criterion from a Property instance. You can create a Property by calling
Property.forName():
Report a bug
15.3. ORDERING THE RESULTS
You can order the results using org.hibernate.criterion.Order.
Report a bug
15.4. ASSOCIATIONS
By navigating associations using createCriteria() you can specify constraints upon related entities:
The second createCriteria() returns a new instance of Criteria that refers to the elements of the
kittens collection.
There is also an alternate form that is useful in certain circumstances:
Property age = Property.forName("age");
List cats = sess.createCriteria(Cat.class)
.add( Restrictions.disjunction()
.add( age.isNull() )
.add( age.eq( new Integer(0) ) )
.add( age.eq( new Integer(1) ) )
.add( age.eq( new Integer(2) ) )
)
.add( Property.forName("name").in( new String[] { "Fritz", "Izi", "Pk"
} ) )
.list();
List cats = sess.createCriteria(Cat.class)
.add( Restrictions.like("name", "F%") )
.addOrder( Order.asc("name") )
.addOrder( Order.desc("age") )
.setMaxResults(50)
.list();
List cats = sess.createCriteria(Cat.class)
.add( Property.forName("name").like("F%") )
.addOrder( Property.forName("name").asc() )
.addOrder( Property.forName("age").desc() )
.setMaxResults(50)
.list();
List cats = sess.createCriteria(Cat.class)
.add( Restrictions.like("name", "F%") )
.createCriteria("kittens")
.add( Restrictions.like("name", "F%") )
.list();
CHAPTER 15. CRITERIA QUERIES
209
(createAlias() does not create a new instance of Criteria.)
The kittens collections held by the Cat instances returned by the previous two queries are not pre-
filtered by the criteria. If you want to retrieve just the kittens that match the criteria, you must use a
ResultTransformer.
Report a bug
15.5. DYNAMIC ASSOCIATION FETCHING
You can specify association fetching semantics at runtime using setFetchMode().
This query will fetch both mate and kittens by outer join.
Report a bug
15.6. EXAMPLE QUERIES
The class org.hibernate.criterion.Example allows you to construct a query criterion from a
given instance.
List cats = sess.createCriteria(Cat.class)
.createAlias("kittens", "kt")
.createAlias("mate", "mt")
.add( Restrictions.eqProperty("kt.name", "mt.name") )
.list();
List cats = sess.createCriteria(Cat.class)
.createCriteria("kittens", "kt")
.add( Restrictions.eq("name", "F%") )
.setResultTransformer(Criteria.ALIAS_TO_ENTITY_MAP)
.list();
Iterator iter = cats.iterator();
while ( iter.hasNext() ) {
Map map = (Map) iter.next();
Cat cat = (Cat) map.get(Criteria.ROOT_ALIAS);
Cat kitten = (Cat) map.get("kt");
}
List cats = sess.createCriteria(Cat.class)
.add( Restrictions.like("name", "Fritz%") )
.setFetchMode("mate", FetchMode.EAGER)
.setFetchMode("kittens", FetchMode.EAGER)
.list();
Cat cat = new Cat();
cat.setSex('F');
cat.setColor(Color.BLACK);
List results = session.createCriteria(Cat.class)
.add( Example.create(cat) )
.list();
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Version properties, identifiers and associations are ignored. By default, null valued properties are
excluded.
You can adjust how the Example is applied.
You can even use examples to place criteria upon associated objects.
Report a bug
15.7. PROJECTIONS, AGGREGATION AND GROUPING
The class org.hibernate.criterion.Projections is a factory for Projection instances. You
can apply a projection to a query by calling setProjection().
There is no explicit "group by" necessary in a criteria query. Certain projection types are defined to be
grouping projections, which also appear in the SQL group by clause.
An alias can be assigned to a projection so that the projected value can be referred to in restrictions or
orderings. Here are two different ways to do this:
Example example = Example.create(cat)
.excludeZeroes() //exclude zero valued properties
.excludeProperty("color") //exclude the property named "color"
.ignoreCase() //perform case insensitive string
comparisons
.enableLike(); //use like for string comparisons
List results = session.createCriteria(Cat.class)
.add(example)
.list();
List results = session.createCriteria(Cat.class)
.add( Example.create(cat) )
.createCriteria("mate")
.add( Example.create( cat.getMate() ) )
.list();
List results = session.createCriteria(Cat.class)
.setProjection( Projections.rowCount() )
.add( Restrictions.eq("color", Color.BLACK) )
.list();
List results = session.createCriteria(Cat.class)
.setProjection( Projections.projectionList()
.add( Projections.rowCount() )
.add( Projections.avg("weight") )
.add( Projections.max("weight") )
.add( Projections.groupProperty("color") )
)
.list();
List results = session.createCriteria(Cat.class)
.setProjection( Projections.alias( Projections.groupProperty("color"),
"colr" ) )
.addOrder( Order.asc("colr") )
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211
The alias() and as() methods simply wrap a projection instance in another, aliased, instance of
Projection. As a shortcut, you can assign an alias when you add the projection to a projection list:
You can also use Property.forName() to express projections:
Report a bug
15.8. DETACHED QUERIES AND SUBQUERIES
.list();
List results = session.createCriteria(Cat.class)
.setProjection( Projections.groupProperty("color").as("colr") )
.addOrder( Order.asc("colr") )
.list();
List results = session.createCriteria(Cat.class)
.setProjection( Projections.projectionList()
.add( Projections.rowCount(), "catCountByColor" )
.add( Projections.avg("weight"), "avgWeight" )
.add( Projections.max("weight"), "maxWeight" )
.add( Projections.groupProperty("color"), "color" )
)
.addOrder( Order.desc("catCountByColor") )
.addOrder( Order.desc("avgWeight") )
.list();
List results = session.createCriteria(Domestic.class, "cat")
.createAlias("kittens", "kit")
.setProjection( Projections.projectionList()
.add( Projections.property("cat.name"), "catName" )
.add( Projections.property("kit.name"), "kitName" )
)
.addOrder( Order.asc("catName") )
.addOrder( Order.asc("kitName") )
.list();
List results = session.createCriteria(Cat.class)
.setProjection( Property.forName("name") )
.add( Property.forName("color").eq(Color.BLACK) )
.list();
List results = session.createCriteria(Cat.class)
.setProjection( Projections.projectionList()
.add( Projections.rowCount() )
.add( Property.forName("weight").avg().as("avgWeight") )
.add( Property.forName("weight").max().as("maxWeight") )
.add( Property.forName("color").group().as("color" )
) )
.addOrder( Order.desc("catCountByColor") )
.addOrder( Order.desc("avgWeight") )
.list();
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The DetachedCriteria class allows you to create a query outside the scope of a session and then
execute it using an arbitrary Session.
A DetachedCriteria can also be used to express a subquery. Criterion instances involving
subqueries can be obtained via Subqueries or Property.
Correlated subqueries are also possible:
Report a bug
15.9. QUERIES BY NATURAL IDENTIFIER
For most queries, including criteria queries, the query cache is not efficient because query cache
invalidation occurs too frequently. However, there is a special kind of query where you can optimize the
cache invalidation algorithm: lookups by a constant natural key. In some applications, this kind of query
occurs frequently. The criteria API provides special provision for this use case.
First, map the natural key of your entity using <natural-id> and enable use of the second-level cache.
DetachedCriteria query = DetachedCriteria.forClass(Cat.class)
.add( Property.forName("sex").eq('F') );
Session session = ....;
Transaction txn = session.beginTransaction();
List results =
query.getExecutableCriteria(session).setMaxResults(100).list();
txn.commit();
session.close();
DetachedCriteria avgWeight = DetachedCriteria.forClass(Cat.class)
.setProjection( Property.forName("weight").avg() );
session.createCriteria(Cat.class)
.add( Property.forName("weight").gt(avgWeight) )
.list();
DetachedCriteria weights = DetachedCriteria.forClass(Cat.class)
.setProjection( Property.forName("weight") );
session.createCriteria(Cat.class)
.add( Subqueries.geAll("weight", weights) )
.list();
DetachedCriteria avgWeightForSex = DetachedCriteria.forClass(Cat.class,
"cat2")
.setProjection( Property.forName("weight").avg() )
.add( Property.forName("cat2.sex").eqProperty("cat.sex") );
session.createCriteria(Cat.class, "cat")
.add( Property.forName("weight").gt(avgWeightForSex) )
.list();
<class name="User">
<cache usage="read-write"/>
<id name="id">
<generator class="increment"/>
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213
This functionality is not intended for use with entities with mutable natural keys.
Once you have enabled the Hibernate query cache, the Restrictions.naturalId() allows you to
make use of the more efficient cache algorithm.
Report a bug
</id>
<natural-id>
<property name="name"/>
<property name="org"/>
</natural-id>
<property name="password"/>
</class>
session.createCriteria(User.class)
.add( Restrictions.naturalId()
.set("name", "gavin")
.set("org", "hb")
).setCacheable(true)
.uniqueResult();
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CHAPTER 16. NATIVE SQL
16.1. ABOUT NATIVE SQL
Execution of native SQL queries is controlled via the SQLQuery interface, which is obtained by calling
Session.createSQLQuery(). The following sections describe how to use this API for querying.
Report a bug
16.2. USING SQLQUERIES
16.2.1. Using a SQLQuery
The most basic SQL query is to get a list of scalars (values).
These will return a List of Object arrays (Object[]) with scalar values for each column in the CATS table.
Hibernate will use ResultSetMetadata to deduce the actual order and types of the returned scalar values.
To avoid the overhead of using ResultSetMetadata, or simply to be more explicit in what is returned,
one can use addScalar():
This query specified:
the SQL query string
the columns and types to return
This will return Object arrays, but now it will not use ResultSetMetadata but will instead explicitly get
the ID, NAME and BIRTHDATE column as respectively a Long, String and a Short from the underlying
resultset. This also means that only these three columns will be returned, even though the query is using
* and could return more than the three listed columns.
It is possible to leave out the type information for all or some of the scalars.
This is essentially the same query as before, but now ResultSetMetaData is used to determine the
type of NAME and BIRTHDATE, where as the type of ID is explicitly specified.
How the java.sql.Types returned from ResultSetMetaData is mapped to Hibernate types is controlled by
the Dialect. If a specific type is not mapped, or does not result in the expected type, it is possible to
customize it via calls to registerHibernateType in the Dialect.
sess.createSQLQuery("SELECT * FROM CATS").list();
sess.createSQLQuery("SELECT ID, NAME, BIRTHDATE FROM CATS").list();
sess.createSQLQuery("SELECT * FROM CATS")
.addScalar("ID", Hibernate.LONG)
.addScalar("NAME", Hibernate.STRING)
.addScalar("BIRTHDATE", Hibernate.DATE);
sess.createSQLQuery("SELECT * FROM CATS")
.addScalar("ID", Hibernate.LONG)
.addScalar("NAME")
.addScalar("BIRTHDATE");
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215
Report a bug
16.2.2. Scalar Queries
The most basic SQL query is to get a list of scalars (values).
These will return a List of Object arrays (Object[]) with scalar values for each column in the CATS table.
Hibernate will use ResultSetMetadata to deduce the actual order and types of the returned scalar values.
To avoid the overhead of using ResultSetMetadata, or simply to be more explicit in what is returned,
one can use addScalar():
This query specified:
the SQL query string
the columns and types to return
This will return Object arrays, but now it will not use ResultSetMetadata but will instead explicitly get
the ID, NAME and BIRTHDATE column as respectively a Long, String and a Short from the underlying
resultset. This also means that only these three columns will be returned, even though the query is using
* and could return more than the three listed columns.
It is possible to leave out the type information for all or some of the scalars.
This is essentially the same query as before, but now ResultSetMetaData is used to determine the
type of NAME and BIRTHDATE, where as the type of ID is explicitly specified.
How the java.sql.Types returned from ResultSetMetaData is mapped to Hibernate types is controlled by
the Dialect. If a specific type is not mapped, or does not result in the expected type, it is possible to
customize it via calls to registerHibernateType in the Dialect.
Report a bug
16.2.3. Entity Queries
The above queries were all about returning scalar values, basically returning the "raw" values from the
resultset. The following shows how to get entity objects from a native sql query via addEntity().
sess.createSQLQuery("SELECT * FROM CATS").list();
sess.createSQLQuery("SELECT ID, NAME, BIRTHDATE FROM CATS").list();
sess.createSQLQuery("SELECT * FROM CATS")
.addScalar("ID", Hibernate.LONG)
.addScalar("NAME", Hibernate.STRING)
.addScalar("BIRTHDATE", Hibernate.DATE);
sess.createSQLQuery("SELECT * FROM CATS")
.addScalar("ID", Hibernate.LONG)
.addScalar("NAME")
.addScalar("BIRTHDATE");
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This query specified:
the SQL query string
the entity returned by the query
Assuming that Cat is mapped as a class with the columns ID, NAME and BIRTHDATE the above queries
will both return a List where each element is a Cat entity.
If the entity is mapped with a many-to-one to another entity it is required to also return this when
performing the native query, otherwise a database specific "column not found" error will occur. The
additional columns will automatically be returned when using the * notation, but we prefer to be explicit as
in the following example for a many-to-one to a Dog:
This will allow cat.getDog() to function properly.
Report a bug
16.2.4. Handling Associations and Collections
It is possible to eagerly join in the Dog to avoid the possible extra roundtrip for initializing the proxy. This
is done via the addJoin() method, which allows you to join in an association or collection.
In this example, the returned Cat's will have their dog property fully initialized without any extra roundtrip
to the database. Notice that you added an alias name ("cat") to be able to specify the target property path
of the join. It is possible to do the same eager joining for collections, e.g. if the Cat had a one-to-many to
Dog instead.
At this stage you are reaching the limits of what is possible with native queries, without starting to
enhance the sql queries to make them usable in Hibernate. Problems can arise when returning multiple
entities of the same type or when the default alias/column names are not enough.
Report a bug
16.2.5. Returning Multiple Entities
sess.createSQLQuery("SELECT * FROM CATS").addEntity(Cat.class);
sess.createSQLQuery("SELECT ID, NAME, BIRTHDATE FROM
CATS").addEntity(Cat.class);
sess.createSQLQuery("SELECT ID, NAME, BIRTHDATE, DOG_ID FROM
CATS").addEntity(Cat.class);
sess.createSQLQuery("SELECT c.ID, NAME, BIRTHDATE, DOG_ID, D_ID, D_NAME
FROM CATS c, DOGS d WHERE c.DOG_ID = d.D_ID")
.addEntity("cat", Cat.class)
.addJoin("dog", "cat.dog");
sess.createSQLQuery("SELECT ID, NAME, BIRTHDATE, D_ID, D_NAME, CAT_ID FROM
CATS c, DOGS d WHERE c.ID = d.CAT_ID")
.addEntity("cat", Cat.class)
.addJoin("dog", "cat.dogs");
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217
Until now, the result set column names are assumed to be the same as the column names specified in
the mapping document. This can be problematic for SQL queries that join multiple tables, since the same
column names can appear in more than one table.
Column alias injection is needed in the following query (which most likely will fail):
The query was intended to return two Cat instances per row: a cat and its mother. The query will,
however, fail because there is a conflict of names; the instances are mapped to the same column names.
Also, on some databases the returned column aliases will most likely be on the form "c.ID", "c.NAME",
etc. which are not equal to the columns specified in the mappings ("ID" and "NAME").
The following form is not vulnerable to column name duplication:
This query specified:
the SQL query string, with placeholders for Hibernate to inject column aliases
the entities returned by the query
The {cat.*} and {mother.*} notation used above is a shorthand for "all properties". Alternatively, you can
list the columns explicitly, but even in this case Hibernate injects the SQL column aliases for each
property. The placeholder for a column alias is just the property name qualified by the table alias. In the
following example, you retrieve Cats and their mothers from a different table (cat_log) to the one
declared in the mapping metadata. You can even use the property aliases in the where clause.
Report a bug
16.2.6. Returning Non-managed Entities
In most cases the above alias injection is needed. For queries relating to more complex mappings, like
composite properties, inheritance discriminators, collections etc., you can use specific aliases that allow
Hibernate to inject the proper aliases.
The following table shows the different ways you can use the alias injection. Please note that the alias
names in the result are simply examples; each alias will have a unique and probably different name
when used.
sess.createSQLQuery("SELECT c.*, m.* FROM CATS c, CATS m WHERE
c.MOTHER_ID = c.ID")
.addEntity("cat", Cat.class)
.addEntity("mother", Cat.class)
sess.createSQLQuery("SELECT {cat.*}, {mother.*} FROM CATS c, CATS m WHERE
c.MOTHER_ID = c.ID")
.addEntity("cat", Cat.class)
.addEntity("mother", Cat.class)
String sql = "SELECT ID as {c.id}, NAME as {c.name}, " +
"BIRTHDATE as {c.birthDate}, MOTHER_ID as {c.mother}, {mother.*}
" +
"FROM CAT_LOG c, CAT_LOG m WHERE {c.mother} = c.ID";
List loggedCats = sess.createSQLQuery(sql)
.addEntity("cat", Cat.class)
.addEntity("mother", Cat.class).list()
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Table 16.1. Alias injection names
Description Syntax Example
A simple property {[aliasname].
[propertyname]
A_NAME as {item.name}
A composite property {[aliasname].
[componentname].
[propertyname]}
CURRENCY as
{item.amount.currency},
VALUE as
{item.amount.value}
Discriminator of an entity {[aliasname].class} DISC as {item.class}
All properties of an entity {[aliasname].*} {item.*}
A collection key {[aliasname].key} ORGID as {coll.key}
The id of an collection {[aliasname].id} EMPID as {coll.id}
The element of an collection {[aliasname].element} XID as {coll.element}
property of the element in the
collection
{[aliasname].element.
[propertyname]}
NAME as
{coll.element.name}
All properties of the element in the
collection
{[aliasname].element.*} {coll.element.*}
All properties of the the collection {[aliasname].*} {coll.*}
Report a bug
16.2.7. Handling Inheritance
Native SQL queries which query for entities that are mapped as part of an inheritance must include all
properties for the baseclass and all its subclasses.
Report a bug
16.2.8. Parameters
Native SQL queries support positional as well as named parameters:
Query query = sess.createSQLQuery("SELECT * FROM CATS WHERE NAME like
?").addEntity(Cat.class);
List pusList = query.setString(0, "Pus%").list();
query = sess.createSQLQuery("SELECT * FROM CATS WHERE NAME like
:name").addEntity(Cat.class);
List pusList = query.setString("name", "Pus%").list();
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Report a bug
16.3. NAMED SQL QUERIES
16.3.1. About Named SQL Queries
Named SQL queries can be defined in the mapping document and called in exactly the same way as a
named HQL query. In this case, you do not need to call addEntity().
The <return-join> element is use to join associations and the <load-collection> element is
used to define queries which initialize collections,
A named SQL query may return a scalar value. You must declare the column alias and Hibernate type
using the <return-scalar> element:
<sql-query name="persons">
<return alias="person" class="eg.Person"/>
SELECT person.NAME AS {person.name},
person.AGE AS {person.age},
person.SEX AS {person.sex}
FROM PERSON person
WHERE person.NAME LIKE :namePattern
</sql-query>
List people = sess.getNamedQuery("persons")
.setString("namePattern", namePattern)
.setMaxResults(50)
.list();
<sql-query name="personsWith">
<return alias="person" class="eg.Person"/>
<return-join alias="address" property="person.mailingAddress"/>
SELECT person.NAME AS {person.name},
person.AGE AS {person.age},
person.SEX AS {person.sex},
address.STREET AS {address.street},
address.CITY AS {address.city},
address.STATE AS {address.state},
address.ZIP AS {address.zip}
FROM PERSON person
JOIN ADDRESS address
ON person.ID = address.PERSON_ID AND address.TYPE='MAILING'
WHERE person.NAME LIKE :namePattern
</sql-query>
<sql-query name="mySqlQuery">
<return-scalar column="name" type="string"/>
<return-scalar column="age" type="long"/>
SELECT p.NAME AS name,
p.AGE AS age,
FROM PERSON p WHERE p.NAME LIKE 'Hiber%'
</sql-query>
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You can externalize the resultset mapping information in a <resultset> element which will allow you
to either reuse them across several named queries or through the setResultSetMapping() API.
You can, alternatively, use the resultset mapping information in your hbm files directly in java code.
Report a bug
16.3.2. Using return-property to Explicitly Specify Column/Alias Names
You can explicitly tell Hibernate what column aliases to use with <return-property>, instead of using
the {}-syntax to let Hibernate inject its own aliases.For example:
<return-property> also works with multiple columns. This solves a limitation with the {}-syntax
which cannot allow fine grained control of multi-column properties.
<resultset name="personAddress">
<return alias="person" class="eg.Person"/>
<return-join alias="address" property="person.mailingAddress"/>
</resultset>
<sql-query name="personsWith" resultset-ref="personAddress">
SELECT person.NAME AS {person.name},
person.AGE AS {person.age},
person.SEX AS {person.sex},
address.STREET AS {address.street},
address.CITY AS {address.city},
address.STATE AS {address.state},
address.ZIP AS {address.zip}
FROM PERSON person
JOIN ADDRESS address
ON person.ID = address.PERSON_ID AND address.TYPE='MAILING'
WHERE person.NAME LIKE :namePattern
</sql-query>
List cats = sess.createSQLQuery(
"select {cat.*}, {kitten.*} from cats cat, cats kitten where
kitten.mother = cat.id"
)
.setResultSetMapping("catAndKitten")
.list();
<sql-query name="mySqlQuery">
<return alias="person" class="eg.Person">
<return-property name="name" column="myName"/>
<return-property name="age" column="myAge"/>
<return-property name="sex" column="mySex"/>
</return>
SELECT person.NAME AS myName,
person.AGE AS myAge,
person.SEX AS mySex,
FROM PERSON person WHERE person.NAME LIKE :name
</sql-query>
<sql-query name="organizationCurrentEmployments">
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In this example <return-property> was used in combination with the {}-syntax for injection. This
allows users to choose how they want to refer column and properties.
If your mapping has a discriminator you must use <return-discriminator> to specify the
discriminator column.
Report a bug
16.3.3. Using Stored Procedures for Querying
Hibernate3 provides support for queries via stored procedures and functions. Most of the following
documentation is equivalent for both. The stored procedure/function must return a resultset as the first
out-parameter to be able to work with Hibernate. An example of such a stored function in Oracle 9 and
higher is as follows:
To use this query in Hibernate you need to map it via a named query.
<return alias="emp" class="Employment">
<return-property name="salary">
<return-column name="VALUE"/>
<return-column name="CURRENCY"/>
</return-property>
<return-property name="endDate" column="myEndDate"/>
</return>
SELECT EMPLOYEE AS {emp.employee}, EMPLOYER AS {emp.employer},
STARTDATE AS {emp.startDate}, ENDDATE AS {emp.endDate},
REGIONCODE as {emp.regionCode}, EID AS {emp.id}, VALUE, CURRENCY
FROM EMPLOYMENT
WHERE EMPLOYER = :id AND ENDDATE IS NULL
ORDER BY STARTDATE ASC
</sql-query>
CREATE OR REPLACE FUNCTION selectAllEmployments
RETURN SYS_REFCURSOR
AS
st_cursor SYS_REFCURSOR;
BEGIN
OPEN st_cursor FOR
SELECT EMPLOYEE, EMPLOYER,
STARTDATE, ENDDATE,
REGIONCODE, EID, VALUE, CURRENCY
FROM EMPLOYMENT;
RETURN st_cursor;
END;
<sql-query name="selectAllEmployees_SP" callable="true">
<return alias="emp" class="Employment">
<return-property name="employee" column="EMPLOYEE"/>
<return-property name="employer" column="EMPLOYER"/>
<return-property name="startDate" column="STARTDATE"/>
<return-property name="endDate" column="ENDDATE"/>
<return-property name="regionCode" column="REGIONCODE"/>
<return-property name="id" column="EID"/>
<return-property name="salary">
<return-column name="VALUE"/>
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Stored procedures currently only return scalars and entities. <return-join> and <load-
collection> are not supported.
Report a bug
16.3.4. Rules/limitations for Using Stored Procedures
You cannot use stored procedures with Hibernate unless you follow some procedure/function rules. If
they do not follow those rules they are not usable with Hibernate. If you still want to use these
procedures you have to execute them via session.connection(). The rules are different for each
database, since database vendors have different stored procedure semantics/syntax.
Stored procedure queries cannot be paged with setFirstResult()/setMaxResults().
The recommended call form is standard SQL92: { ? = call functionName(<parameters>) } or
{ ? = call procedureName(<parameters>}. Native call syntax is not supported.
For Oracle the following rules apply:
A function must return a result set. The first parameter of a procedure must be an OUT that
returns a result set. This is done by using a SYS_REFCURSOR type in Oracle 9 or 10. In Oracle
you need to define a REF CURSOR type. See Oracle literature for further information.
For Sybase or MS SQL server the following rules apply:
The procedure must return a result set. Note that since these servers can return multiple result
sets and update counts, Hibernate will iterate the results and take the first result that is a result
set as its return value. Everything else will be discarded.
If you can enable SET NOCOUNT ON in your procedure it will probably be more efficient, but this
is not a requirement.
Report a bug
16.4. ADDITIONAL SQL FUNCTIONS
16.4.1. Custom SQL for Create, Update and Delete
Hibernate3 can use custom SQL statements for create, update, and delete operations. The class and
collection persisters in Hibernate already contain a set of configuration time generated strings (insertsql,
deletesql, updatesql etc.). The mapping tags <sql-insert>, <sql-delete>, and <sql-update>
override these strings:
<return-column name="CURRENCY"/>
</return-property>
</return>
{ ? = call selectAllEmployments() }
</sql-query>
<class name="Person">
<id name="id">
<generator class="increment"/>
</id>
<property name="name" not-null="true"/>
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The SQL is directly executed in your database, so you can use any dialect you like. This will reduce the
portability of your mapping if you use database specific SQL.
Stored procedures are supported if the callable attribute is set:
The order of the positional parameters is vital, as they must be in the same sequence as Hibernate
expects them.
You can view the expected order by enabling debug logging for the
org.hibernate.persister.entity level. With this level enabled, Hibernate will print out the static
SQL that is used to create, update, delete etc. entities. To view the expected sequence, do not include
your custom SQL in the mapping files, as this will override the Hibernate generated static SQL.
The stored procedures are in most cases required to return the number of rows inserted, updated and
deleted, as Hibernate has some runtime checks for the success of the statement. Hibernate always
registers the first statement parameter as a numeric output parameter for the CUD operations:
Report a bug
16.4.2. Custom SQL for Loading
You can also declare your own SQL (or HQL) queries for entity loading:
<sql-insert>INSERT INTO PERSON (NAME, ID) VALUES ( UPPER(?), ? )</sql-
insert>
<sql-update>UPDATE PERSON SET NAME=UPPER(?) WHERE ID=?</sql-update>
<sql-delete>DELETE FROM PERSON WHERE ID=?</sql-delete>
</class>
<class name="Person">
<id name="id">
<generator class="increment"/>
</id>
<property name="name" not-null="true"/>
<sql-insert callable="true">{call createPerson (?, ?)}</sql-insert>
<sql-delete callable="true">{? = call deletePerson (?)}</sql-delete>
<sql-update callable="true">{? = call updatePerson (?, ?)}</sql-
update>
</class>
CREATE OR REPLACE FUNCTION updatePerson (uid IN NUMBER, uname IN VARCHAR2)
RETURN NUMBER IS
BEGIN
update PERSON
set
NAME = uname,
where
ID = uid;
return SQL%ROWCOUNT;
END updatePerson;
<sql-query name="person">
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This is just a named query declaration, as discussed earlier. You can reference this named query in a
class mapping:
This even works with stored procedures.
You can even define a query for collection loading:
You can also define an entity loader that loads a collection by join fetching:
Report a bug
<return alias="pers" class="Person" lock-mode="upgrade"/>
SELECT NAME AS {pers.name}, ID AS {pers.id}
FROM PERSON
WHERE ID=?
FOR UPDATE
</sql-query>
<class name="Person">
<id name="id">
<generator class="increment"/>
</id>
<property name="name" not-null="true"/>
<loader query-ref="person"/>
</class>
<set name="employments" inverse="true">
<key/>
<one-to-many class="Employment"/>
<loader query-ref="employments"/>
</set>
<sql-query name="employments">
<load-collection alias="emp" role="Person.employments"/>
SELECT {emp.*}
FROM EMPLOYMENT emp
WHERE EMPLOYER = :id
ORDER BY STARTDATE ASC, EMPLOYEE ASC
</sql-query>
<sql-query name="person">
<return alias="pers" class="Person"/>
<return-join alias="emp" property="pers.employments"/>
SELECT NAME AS {pers.*}, {emp.*}
FROM PERSON pers
LEFT OUTER JOIN EMPLOYMENT emp
ON pers.ID = emp.PERSON_ID
WHERE ID=?
</sql-query>
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CHAPTER 17. FILTERING DATA
17.1. ABOUT FILTERING DATA
Hibernate3 provides an innovative new approach to handling data with "visibility" rules. A Hibernate filter
is a global, named, parameterized filter that can be enabled or disabled for a particular Hibernate
session.
Report a bug
17.2. ABOUT HIBERNATE FILTERS
Hibernate3 has the ability to pre-define filter criteria and attach those filters at both a class level and a
collection level. A filter criteria allows you to define a restriction clause similar to the existing "where"
attribute available on the class and various collection elements. These filter conditions, however, can be
parameterized. The application can then decide at runtime whether certain filters should be enabled and
what their parameter values should be. Filters can be used like database views, but they are
parameterized inside the application.
Report a bug
17.3. USING HIBERNATE FILTERS
In order to use filters, they must first be defined and then attached to the appropriate mapping elements.
To define a filter, use the <filter-def/> element within a <hibernate-mapping/> element:
This filter can then be attached to a class:
Or, to a collection:
Or, to both or multiples of each at the same time.
The methods on Session are: enableFilter(String filterName),
getEnabledFilter(String filterName), and disableFilter(String filterName). By
default, filters are not enabled for a given session. Filters must be enabled through use of the
Session.enableFilter() method, which returns an instance of the Filter interface. If you used
the simple filter defined above, it would look like this:
<filter-def name="myFilter">
<filter-param name="myFilterParam" type="string"/>
</filter-def>
<class name="myClass" ...>
...
<filter name="myFilter" condition=":myFilterParam =
MY_FILTERED_COLUMN"/>
</class>
<set ...>
<filter name="myFilter" condition=":myFilterParam =
MY_FILTERED_COLUMN"/>
</set>
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Methods on the org.hibernate.Filter interface do allow the method-chaining common to much of
Hibernate.
Report a bug
17.4. HIBERNATE FILTERS EXAMPLE
The following is a full example, using temporal data with an effective record date pattern:
In order to ensure that you are provided with currently effective records, enable the filter on the session
prior to retrieving employee data:
session.enableFilter("myFilter").setParameter("myFilterParam", "some-
value");
<filter-def name="effectiveDate">
<filter-param name="asOfDate" type="date"/>
</filter-def>
<class name="Employee" ...>
...
<many-to-one name="department" column="dept_id" class="Department"/>
<property name="effectiveStartDate" type="date"
column="eff_start_dt"/>
<property name="effectiveEndDate" type="date" column="eff_end_dt"/>
...
<!--
Note that this assumes non-terminal records have an eff_end_dt set
to
a max db date for simplicity-sake
-->
<filter name="effectiveDate"
condition=":asOfDate BETWEEN eff_start_dt and eff_end_dt"/>
</class>
<class name="Department" ...>
...
<set name="employees" lazy="true">
<key column="dept_id"/>
<one-to-many class="Employee"/>
<filter name="effectiveDate"
condition=":asOfDate BETWEEN eff_start_dt and
eff_end_dt"/>
</set>
</class>
Session session = ...;
session.enableFilter("effectiveDate").setParameter("asOfDate", new
Date());
List results = session.createQuery("from Employee as e where e.salary >
:targetSalary")
.setLong("targetSalary", new Long(1000000))
.list();
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227
Even though a salary constraint was mentioned explicitly on the results in the above HQL, because of
the enabled filter, the query will return only currently active employees who have a salary greater than
one million dollars.
If you want to use filters with outer joining, either through HQL or load fetching, be careful of the direction
of the condition expression. It is safest to set this up for left outer joining. Place the parameter first
followed by the column name(s) after the operator.
After being defined, a filter might be attached to multiple entities and/or collections each with its own
condition. This can be problematic when the conditions are the same each time. Using <filter-def/>
allows you to definine a default condition, either as an attribute or CDATA:
This default condition will be used whenever the filter is attached to something without specifying a
condition. This means you can give a specific condition as part of the attachment of the filter that
overrides the default condition in that particular case.
Report a bug
<filter-def name="myFilter" condition="abc > xyz">...</filter-def>
<filter-def name="myOtherFilter">abc=xyz</filter-def>
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CHAPTER 18. XML MAPPING
18.1. WORKING WITH XML DATA
18.1.1. About Working with XML Data
Hibernate allows you to work with persistent XML data in much the same way you work with persistent
POJOs. A parsed XML tree can be thought of as another way of representing the relational data at the
object level, instead of POJOs.
Hibernate supports dom4j as API for manipulating XML trees. You can write queries that retrieve dom4j
trees from the database and have any modification you make to the tree automatically synchronized to
the database. You can even take an XML document, parse it using dom4j, and write it to the database
with any of Hibernate's basic operations: persist(), saveOrUpdate(), merge(), delete(),
replicate() (merging is not yet supported).
This feature has many applications including data import/export, externalization of entity data via JMS or
SOAP and XSLT-based reporting.
A single mapping can be used to simultaneously map properties of a class and nodes of an XML
document to the database, or, if there is no class to map, it can be used to map just the XML.
Report a bug
18.1.2. Specifying XML and Class Mapping Together
Here is an example of mapping a POJO and XML simultaneously:
Report a bug
18.1.3. Specifying Only an XML Mapping
<class name="Account"
table="ACCOUNTS"
node="account">
<id name="accountId"
column="ACCOUNT_ID"
node="@id"/>
<many-to-one name="customer"
column="CUSTOMER_ID"
node="customer/@id"
embed-xml="false"/>
<property name="balance"
column="BALANCE"
node="balance"/>
...
</class>
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Here is an example where there is no POJO class:
This mapping allows you to access the data as a dom4j tree, or as a graph of property name/value pairs
or java Maps. The property names are purely logical constructs that can be referred to in HQL queries.
Report a bug
18.2. XML MAPPING METADATA
18.2.1. About XML Mapping Metadata
A range of Hibernate mapping elements accept the node attribute. This lets you specify the name of an
XML attribute or element that holds the property or entity data. The format of the node attribute must be
one of the following:
"element-name": map to the named XML element
"@attribute-name": map to the named XML attribute
".": map to the parent element
"element-name/@attribute-name": map to the named attribute of the named element
For collections and single valued associations, there is an additional embed-xml attribute. If embed-
xml="true", the default, the XML tree for the associated entity (or collection of value type) will be
embedded directly in the XML tree for the entity that owns the association. Otherwise, if embed-
xml="false", then only the referenced identifier value will appear in the XML for single point
associations and collections will not appear at all.
<class entity-name="Account"
table="ACCOUNTS"
node="account">
<id name="id"
column="ACCOUNT_ID"
node="@id"
type="string"/>
<many-to-one name="customerId"
column="CUSTOMER_ID"
node="customer/@id"
embed-xml="false"
entity-name="Customer"/>
<property name="balance"
column="BALANCE"
node="balance"
type="big_decimal"/>
...
</class>
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Do not leave embed-xml="true" for too many associations, since XML does not deal well with
circularity.
In this case, the collection of account ids is embedded, but not the actual account data. The following
HQL query:
would return datasets such as this:
<class name="Customer"
table="CUSTOMER"
node="customer">
<id name="id"
column="CUST_ID"
node="@id"/>
<map name="accounts"
node="."
embed-xml="true">
<key column="CUSTOMER_ID"
not-null="true"/>
<map-key column="SHORT_DESC"
node="@short-desc"
type="string"/>
<one-to-many entity-name="Account"
embed-xml="false"
node="account"/>
</map>
<component name="name"
node="name">
<property name="firstName"
node="first-name"/>
<property name="initial"
node="initial"/>
<property name="lastName"
node="last-name"/>
</component>
...
</class>
from Customer c left join fetch c.accounts where c.lastName like :lastName
<customer id="123456789">
<account short-desc="Savings">987632567</account>
<account short-desc="Credit Card">985612323</account>
<name>
<first-name>Gavin</first-name>
<initial>A</initial>
<last-name>King</last-name>
</name>
...
</customer>
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If you set embed-xml="true" on the <one-to-many> mapping, the data might look more like this:
Report a bug
18.2.2. Manipulating XML Data
You can also re-read and update XML documents in the application. You can do this by obtaining a
dom4j session:
<customer id="123456789">
<account id="987632567" short-desc="Savings">
<customer id="123456789"/>
<balance>100.29</balance>
</account>
<account id="985612323" short-desc="Credit Card">
<customer id="123456789"/>
<balance>-2370.34</balance>
</account>
<name>
<first-name>Gavin</first-name>
<initial>A</initial>
<last-name>King</last-name>
</name>
...
</customer>
Document doc = ....;
Session session = factory.openSession();
Session dom4jSession = session.getSession(EntityMode.DOM4J);
Transaction tx = session.beginTransaction();
List results = dom4jSession
.createQuery("from Customer c left join fetch c.accounts where
c.lastName like :lastName")
.list();
for ( int i=0; i<results.size(); i++ ) {
//add the customer data to the XML document
Element customer = (Element) results.get(i);
doc.add(customer);
}
tx.commit();
session.close();
Session session = factory.openSession();
Session dom4jSession = session.getSession(EntityMode.DOM4J);
Transaction tx = session.beginTransaction();
Element cust = (Element) dom4jSession.get("Customer", customerId);
for ( int i=0; i<results.size(); i++ ) {
Element customer = (Element) results.get(i);
//change the customer name in the XML and database
Element name = customer.element("name");
name.element("first-name").setText(firstName);
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CHAPTER 19. IMPROVING PERFORMANCE
19.1. FETCHING STRATEGIES
19.1.1. About Fetching Strategies
Hibernate uses a fetching strategy to retrieve associated objects if the application needs to navigate the
association. Fetch strategies can be declared in the O/R mapping metadata, or over-ridden by a
particular HQL or Criteria query.
Hibernate3 defines the following fetching strategies:
Join fetching: Hibernate retrieves the associated instance or collection in the same SELECT,
using an OUTER JOIN.
Select fetching: a second SELECT is used to retrieve the associated entity or collection. Unless
you explicitly disable lazy fetching by specifying lazy="false", this second select will only be
executed when you access the association.
Subselect fetching: a second SELECT is used to retrieve the associated collections for all entities
retrieved in a previous query or fetch. Unless you explicitly disable lazy fetching by specifying
lazy="false", this second select will only be executed when you access the association.
Batch fetching: an optimization strategy for select fetching. Hibernate retrieves a batch of entity
instances or collections in a single SELECT by specifying a list of primary or foreign keys.
Hibernate also distinguishes between:
Immediate fetching: an association, collection or attribute is fetched immediately when the owner
is loaded.
Lazy collection fetching: a collection is fetched when the application invokes an operation upon
that collection. This is the default for collections.
"Extra-lazy" collection fetching: individual elements of the collection are accessed from the
database as needed. Hibernate tries not to fetch the whole collection into memory unless
absolutely needed. It is suitable for large collections.
Proxy fetching: a single-valued association is fetched when a method other than the identifier
getter is invoked upon the associated object.
"No-proxy" fetching: a single-valued association is fetched when the instance variable is
accessed. Compared to proxy fetching, this approach is less lazy; the association is fetched
even when only the identifier is accessed. It is also more transparent, since no proxy is visible to
the application. This approach requires buildtime bytecode instrumentation and is rarely
necessary.
Lazy attribute fetching: an attribute or single valued association is fetched when the instance
variable is accessed. This approach requires buildtime bytecode instrumentation and is rarely
necessary.
We have two orthogonal notions here: when is the association fetched and how is it fetched. It is
important that you do not confuse them. We use fetch to tune performance. We can use lazy to
define a contract for what data is always available in any detached instance of a particular class.
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Report a bug
19.1.2. Working with Lazy Associations
By default, Hibernate3 uses lazy select fetching for collections and lazy proxy fetching for single-valued
associations. These defaults make sense for most associations in the majority of applications.
If you set hibernate.default_batch_fetch_size, Hibernate will use the batch fetch optimization
for lazy fetching. This optimization can also be enabled at a more granular level.
Please be aware that access to a lazy association outside of the context of an open Hibernate session
will result in an exception. For example:
Since the permissions collection was not initialized when the Session was closed, the collection will not
be able to load its state. Hibernate does not support lazy initialization for detached objects . This can be
fixed by moving the code that reads from the collection to just before the transaction is committed.
Alternatively, you can use a non-lazy collection or association, by specifying lazy="false" for the
association mapping. However, it is intended that lazy initialization be used for almost all collections and
associations. If you define too many non-lazy associations in your object model, Hibernate will fetch the
entire database into memory in every transaction.
On the other hand, you can use join fetching, which is non-lazy by nature, instead of select fetching in a
particular transaction. We will now explain how to customize the fetching strategy. In Hibernate3, the
mechanisms for choosing a fetch strategy are identical for single-valued associations and collections.
Report a bug
19.1.3. Tuning Fetch Strategies
Select fetching (the default) is extremely vulnerable to N+1 selects problems, so we might want to
enable join fetching in the mapping document:
The fetch strategy defined in the mapping document affects:
s = sessions.openSession();
Transaction tx = s.beginTransaction();
User u = (User) s.createQuery("from User u where u.name=:userName")
.setString("userName", userName).uniqueResult();
Map permissions = u.getPermissions();
tx.commit();
s.close();
Integer accessLevel = (Integer) permissions.get("accounts"); // Error!
<set name="permissions"
fetch="join">
<key column="userId"/>
<one-to-many class="Permission"/>
</set
<many-to-one name="mother" class="Cat" fetch="join"/>
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235
retrieval via get() or load()
retrieval that happens implicitly when an association is navigated
Criteria queries
HQL queries if subselect fetching is used
Irrespective of the fetching strategy you use, the defined non-lazy graph is guaranteed to be loaded into
memory. This might, however, result in several immediate selects being used to execute a particular
HQL query.
Usually, the mapping document is not used to customize fetching. Instead, we keep the default behavior,
and override it for a particular transaction, using left join fetch in HQL. This tells Hibernate to
fetch the association eagerly in the first select, using an outer join. In the Criteria query API, you
would use setFetchMode(FetchMode.JOIN).
If you want to change the fetching strategy used by get() or load(), you can use a Criteria query.
For example:
This is Hibernate's equivalent of what some ORM solutions call a "fetch plan".
A completely different approach to problems with N+1 selects is to use the second-level cache.
Report a bug
19.1.4. Single-ended Association Proxies
Lazy fetching for collections is implemented using Hibernate's own implementation of persistent
collections. However, a different mechanism is needed for lazy behavior in single-ended associations.
The target entity of the association must be proxied. Hibernate implements lazy initializing proxies for
persistent objects using runtime bytecode enhancement which is accessed via the CGLIB library.
At startup, Hibernate3 generates proxies by default for all persistent classes and uses them to enable
lazy fetching of many-to-one and one-to-one associations.
The mapping file may declare an interface to use as the proxy interface for that class, with the proxy
attribute. By default, Hibernate uses a subclass of the class. The proxied class must implement a default
constructor with at least package visibility. This constructor is recommended for all persistent classes.
There are potential problems to note when extending this approach to polymorphic classes.For example:
User user = (User) session.createCriteria(User.class)
.setFetchMode("permissions", FetchMode.JOIN)
.add( Restrictions.idEq(userId) )
.uniqueResult();
<class name="Cat" proxy="Cat">
......
<subclass name="DomesticCat">
.....
</subclass>
</class>
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Firstly, instances of Cat will never be castable to DomesticCat, even if the underlying instance is an
instance of DomesticCat:
Secondly, it is possible to break proxy ==:
However, the situation is not quite as bad as it looks. Even though we now have two references to
different proxy objects, the underlying instance will still be the same object:
Third, you cannot use a CGLIB proxy for a final class or a class with any final methods.
Finally, if your persistent object acquires any resources upon instantiation (e.g. in initializers or default
constructor), then those resources will also be acquired by the proxy. The proxy class is an actual
subclass of the persistent class.
These problems are all due to fundamental limitations in Java's single inheritance model. To avoid these
problems your persistent classes must each implement an interface that declares its business methods.
You should specify these interfaces in the mapping file where CatImpl implements the interface Cat
and DomesticCatImpl implements the interface DomesticCat. For example:
Then proxies for instances of Cat and DomesticCat can be returned by load() or iterate().
Cat cat = (Cat) session.load(Cat.class, id); // instantiate a proxy (does
not hit the db)
if ( cat.isDomesticCat() ) { // hit the db to initialize
the proxy
DomesticCat dc = (DomesticCat) cat; // Error!
....
}
Cat cat = (Cat) session.load(Cat.class, id); // instantiate a
Cat proxy
DomesticCat dc =
(DomesticCat) session.load(DomesticCat.class, id); // acquire new
DomesticCat proxy!
System.out.println(cat==dc); // false
cat.setWeight(11.0); // hit the db to initialize the proxy
System.out.println( dc.getWeight() ); // 11.0
<class name="CatImpl" proxy="Cat">
......
<subclass name="DomesticCatImpl" proxy="DomesticCat">
.....
</subclass>
</class>
Cat cat = (Cat) session.load(CatImpl.class, catid);
Iterator iter = session.createQuery("from CatImpl as cat where
cat.name='fritz'").iterate();
Cat fritz = (Cat) iter.next();
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NOTE
list() does not usually return proxies.
Relationships are also lazily initialized. This means you must declare any properties to be of type Cat,
not CatImpl.
Certain operations do not require proxy initialization:
equals(): if the persistent class does not override equals()
hashCode(): if the persistent class does not override hashCode()
The identifier getter method
Hibernate will detect persistent classes that override equals() or hashCode().
By choosing lazy="no-proxy" instead of the default lazy="proxy", you can avoid problems
associated with typecasting. However, buildtime bytecode instrumentation is required, and all operations
will result in immediate proxy initialization.
Report a bug
19.1.5. Initializing Collections and Proxies
A LazyInitializationException will be thrown by Hibernate if an uninitialized collection or proxy
is accessed outside of the scope of the Session, i.e., when the entity owning the collection or having the
reference to the proxy is in the detached state.
Sometimes a proxy or collection needs to be initialized before closing the Session. You can force
initialization by calling cat.getSex() or cat.getKittens().size(), for example. However, this
can be confusing to readers of the code and it is not convenient for generic code.
The static methods Hibernate.initialize() and Hibernate.isInitialized(), provide the
application with a convenient way of working with lazily initialized collections or proxies.
Hibernate.initialize(cat) will force the initialization of a proxy, cat, as long as its Session is
still open. Hibernate.initialize( cat.getKittens() ) has a similar effect for the collection of
kittens.
Another option is to keep the Session open until all required collections and proxies have been loaded.
In some application architectures, particularly where the code that accesses data using Hibernate, and
the code that uses it are in different application layers or different physical processes, it can be a
problem to ensure that the Session is open when a collection is initialized. There are two basic ways to
deal with this issue:
In a web-based application, a servlet filter can be used to close the Session only at the end of a
user request, once the rendering of the view is complete (the Open Session in View pattern). Of
course, this places heavy demands on the correctness of the exception handling of your
application infrastructure. It is vitally important that the Session is closed and the transaction
ended before returning to the user, even when an exception occurs during rendering of the view.
See the Hibernate Wiki for examples of this "Open Session in View" pattern.
In an application with a separate business tier, the business logic must "prepare" all collections
that the web tier needs before returning. This means that the business tier should load all the
data and return all the data already initialized to the presentation/web tier that is required for a
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particular use case. Usually, the application calls Hibernate.initialize() for each
collection that will be needed in the web tier (this call must occur before the session is closed) or
retrieves the collection eagerly using a Hibernate query with a FETCH clause or a
FetchMode.JOIN in Criteria. This is usually easier if you adopt the Command pattern
instead of a Session Facade.
You can also attach a previously loaded object to a new Session with merge() or lock()
before accessing uninitialized collections or other proxies. Hibernate does not, and certainly
should not, do this automatically since it would introduce impromptu transaction semantics.
Sometimes you do not want to initialize a large collection, but still need some information about it, like its
size, for example, or a subset of the data.
You can use a collection filter to get the size of a collection without initializing it:
The createFilter() method is also used to efficiently retrieve subsets of a collection without needing
to initialize the whole collection:
Report a bug
19.1.6. Using Batch Fetching
Using batch fetching, Hibernate can load several uninitialized proxies if one proxy is accessed. Batch
fetching is an optimization of the lazy select fetching strategy. There are two ways you can configure
batch fetching: on the class level and the collection level.
Batch fetching for classes/entities is easier to understand. Consider the following example: at runtime
you have 25 Cat instances loaded in a Session, and each Cat has a reference to its owner, a
Person. The Person class is mapped with a proxy, lazy="true". If you now iterate through all cats
and call getOwner() on each, Hibernate will, by default, execute 25 SELECT statements to retrieve the
proxied owners. You can tune this behavior by specifying a batch-size in the mapping of Person:
Hibernate will now execute only three queries: the pattern is 10, 10, 5.
You can also enable batch fetching of collections. For example, if each Person has a lazy collection of
Cats, and 10 persons are currently loaded in the Session, iterating through all persons will generate 10
SELECTs, one for every call to getCats(). If you enable batch fetching for the cats collection in the
mapping of Person, Hibernate can pre-fetch collections:
With a batch-size of 3, Hibernate will load 3, 3, 3, 1 collections in four SELECTs. Again, the value of
( (Integer) s.createFilter( collection, "select count(*)" ).list().get(0)
).intValue()
s.createFilter( lazyCollection,
"").setFirstResult(0).setMaxResults(10).list();
<class name="Person" batch-size="10">...</class>
<class name="Person">
<set name="cats" batch-size="3">
...
</set>
</class>
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the attribute depends on the expected number of uninitialized collections in a particular Session.
Batch fetching of collections is particularly useful if you have a nested tree of items, i.e. the typical bill-of-
materials pattern. However, a nested set or a materialized path might be a better option for read-mostly
trees.
Report a bug
19.1.7. Using Subselect Fetching
If one lazy collection or single-valued proxy has to be fetched, Hibernate will load all of them, re-running
the original query in a subselect. This works in the same way as batch-fetching but without the
piecemeal loading.
Report a bug
19.1.8. Using Lazy Property Fetching
Hibernate3 supports the lazy fetching of individual properties. This optimization technique is also known
as fetch groups. Please note that this is mostly a marketing feature; optimizing row reads is much more
important than optimization of column reads. However, only loading some properties of a class could be
useful in extreme cases. For example, when legacy tables have hundreds of columns and the data
model cannot be improved.
To enable lazy property loading, set the lazy attribute on your particular property mappings:
Lazy property loading requires buildtime bytecode instrumentation. If your persistent classes are not
enhanced, Hibernate will ignore lazy property settings and return to immediate fetching.
For bytecode instrumentation, use the following Ant task:
<class name="Document">
<id name="id">
<generator class="native"/>
</id>
<property name="name" not-null="true" length="50"/>
<property name="summary" not-null="true" length="200" lazy="true"/>
<property name="text" not-null="true" length="2000" lazy="true"/>
</class>
<target name="instrument" depends="compile">
<taskdef name="instrument"
classname="org.hibernate.tool.instrument.InstrumentTask">
<classpath path="${jar.path}"/>
<classpath path="${classes.dir}"/>
<classpath refid="lib.class.path"/>
</taskdef>
<instrument verbose="true">
<fileset dir="${testclasses.dir}/org/hibernate/auction/model">
<include name="*.class"/>
</fileset>
</instrument>
</target>
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A different way of avoiding unnecessary column reads, at least for read-only transactions, is to use the
projection features of HQL or Criteria queries. This avoids the need for buildtime bytecode processing
and is certainly a preferred solution.
You can force the usual eager fetching of properties using fetch all properties in HQL.
Report a bug
19.2. UNDERSTANDING COLLECTION PERFORMANCE
19.2.1. Taxonomy
Hibernate defines three basic kinds of collections:
collections of values
one-to-many associations
many-to-many associations
This classification distinguishes the various table and foreign key relationships but does not tell us quite
everything we need to know about the relational model. To fully understand the relational structure and
performance characteristics, we must also consider the structure of the primary key that is used by
Hibernate to update or delete collection rows. This suggests the following classification:
indexed collections
sets
bags
All indexed collections (maps, lists, and arrays) have a primary key consisting of the <key> and
<index> columns. In this case, collection updates are extremely efficient. The primary key can be
efficiently indexed and a particular row can be efficiently located when Hibernate tries to update or delete
it.
Sets have a primary key consisting of <key> and element columns. This can be less efficient for some
types of collection element, particularly composite elements or large text or binary fields, as the database
may not be able to index a complex primary key as efficiently. However, for one-to-many or many-to-
many associations, particularly in the case of synthetic identifiers, it is likely to be just as efficient. If you
want SchemaExport to actually create the primary key of a <set>, you must declare all columns as
not-null="true".
<idbag> mappings define a surrogate key, so they are efficient to update. In fact, they are the best
case.
Bags are the worst case since they permit duplicate element values and, as they have no index column,
no primary key can be defined. Hibernate has no way of distinguishing between duplicate rows.
Hibernate resolves this problem by completely removing in a single DELETE and recreating the
collection whenever it changes. This can be inefficient.
For a one-to-many association, the "primary key" may not be the physical primary key of the database
table. Even in this case, the above classification is still useful. It reflects how Hibernate "locates"
individual rows of the collection.
Report a bug
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19.2.2. Lists, Maps, idbags and Sets
From the discussion above, it should be clear that indexed collections and sets allow the most efficient
operation in terms of adding, removing and updating elements.
There is, arguably, one more advantage that indexed collections have over sets for many-to-many
associations or collections of values. Because of the structure of a Set, Hibernate does not UPDATE a
row when an element is "changed". Changes to a Set always work via INSERT and DELETE of
individual rows. Once again, this consideration does not apply to one-to-many associations.
After observing that arrays cannot be lazy, you can conclude that lists, maps and idbags are the most
performant (non-inverse) collection types, with sets not far behind. You can expect sets to be the most
common kind of collection in Hibernate applications. This is because the "set" semantics are most
natural in the relational model.
However, in well-designed Hibernate domain models, most collections are in fact one-to-many
associations with inverse="true". For these associations, the update is handled by the many-to-one
end of the association, and so considerations of collection update performance simply do not apply.
Report a bug
19.2.3. Bags and Lists as Inverse Collections
There is a particular case, however, in which bags, and also lists, are much more performant than sets.
For a collection with inverse="true", the standard bidirectional one-to-many relationship idiom, for
example, we can add elements to a bag or list without needing to initialize (fetch) the bag elements. This
is because, unlike a set, Collection.add() or Collection.addAll() must always return true for
a bag or List. This can make the following common code much faster:
Report a bug
19.2.4. One Shot Delete
Deleting collection elements one by one can sometimes be extremely inefficient. Hibernate knows not to
do that in the case of an newly-empty collection (if you called list.clear(), for example). In this case,
Hibernate will issue a single DELETE.
Suppose you added a single element to a collection of size twenty and then remove two elements.
Hibernate will issue one INSERT statement and two DELETE statements, unless the collection is a bag.
This is certainly desirable.
However, suppose that we remove eighteen elements, leaving two and then add thee new elements.
There are two possible ways to proceed
delete eighteen rows one by one and then insert three rows
remove the whole collection in one SQL DELETE and insert all five current elements one by one
Parent p = (Parent) sess.load(Parent.class, id);
Child c = new Child();
c.setParent(p);
p.getChildren().add(c); //no need to fetch the collection!
sess.flush();
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Hibernate cannot know that the second option is probably quicker. It would probably be undesirable for
Hibernate to be that intuitive as such behavior might confuse database triggers, etc.
Fortunately, you can force this behavior (i.e. the second strategy) at any time by discarding (i.e.
dereferencing) the original collection and returning a newly instantiated collection with all the current
elements.
One-shot-delete does not apply to collections mapped inverse="true".
Report a bug
19.3. MONITORING PERFORMANCE
19.3.1. About Monitoring Performance
Optimization is not much use without monitoring and access to performance numbers. Hibernate
provides a full range of figures about its internal operations. Statistics in Hibernate are available per
SessionFactory.
Report a bug
19.3.2. Monitoring a SessionFactory
You can access SessionFactory metrics in two ways. Your first option is to call
sessionFactory.getStatistics() and read or display the Statistics yourself.
Hibernate can also use JMX to publish metrics if you enable the StatisticsService MBean. You
can enable a single MBean for all your SessionFactory or one per factory. See the following code for
minimalistic configuration examples:
You can activate and deactivate the monitoring for a SessionFactory:
at configuration time, set hibernate.generate_statistics to false
// MBean service registration for a specific SessionFactory
Hashtable tb = new Hashtable();
tb.put("type", "statistics");
tb.put("sessionFactory", "myFinancialApp");
ObjectName on = new ObjectName("hibernate", tb); // MBean object name
StatisticsService stats = new StatisticsService(); // MBean implementation
stats.setSessionFactory(sessionFactory); // Bind the stats to a
SessionFactory
server.registerMBean(stats, on); // Register the Mbean on the server
// MBean service registration for all SessionFactory's
Hashtable tb = new Hashtable();
tb.put("type", "statistics");
tb.put("sessionFactory", "all");
ObjectName on = new ObjectName("hibernate", tb); // MBean object name
StatisticsService stats = new StatisticsService(); // MBean implementation
server.registerMBean(stats, on); // Register the MBean on the server
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at runtime: sf.getStatistics().setStatisticsEnabled(true) or
hibernateStatsBean.setStatisticsEnabled(true)
Statistics can be reset programmatically using the clear() method. A summary can be sent to a logger
(info level) using the logSummary() method.
Report a bug
19.3.3. Performance Metrics
Hibernate provides a number of metrics, from basic information to more specialized information that is
only relevant in certain scenarios. All available counters are described in the Statistics interface API,
in three categories:
Metrics related to the general Session usage, such as number of open sessions, retrieved
JDBC connections, etc.
Metrics related to the entities, collections, queries, and caches as a whole (aka global metrics).
Detailed metrics related to a particular entity, collection, query or cache region.
For example, you can check the cache hit, miss, and put ratio of entities, collections and queries, and the
average time a query needs. Be aware that the number of milliseconds is subject to approximation in
Java. Hibernate is tied to the JVM precision and on some platforms this might only be accurate to 10
seconds.
Simple getters are used to access the global metrics (i.e. not tied to a particular entity, collection, cache
region, etc.). You can access the metrics of a particular entity, collection or cache region through its
name, and through its HQL or SQL representation for queries. Please refer to the Statistics,
EntityStatistics, CollectionStatistics, SecondLevelCacheStatistics, and
QueryStatistics API Javadoc for more information. The following code is a simple example:
You can work on all entities, collections, queries and region caches, by retrieving the list of names of
entities, collections, queries and region caches using the following methods: getQueries(),
getEntityNames(), getCollectionRoleNames(), and
getSecondLevelCacheRegionNames().
Report a bug
Statistics stats = HibernateUtil.sessionFactory.getStatistics();
double queryCacheHitCount = stats.getQueryCacheHitCount();
double queryCacheMissCount = stats.getQueryCacheMissCount();
double queryCacheHitRatio =
queryCacheHitCount / (queryCacheHitCount + queryCacheMissCount);
log.info("Query Hit ratio:" + queryCacheHitRatio);
EntityStatistics entityStats =
stats.getEntityStatistics( Cat.class.getName() );
long changes =
entityStats.getInsertCount()
+ entityStats.getUpdateCount()
+ entityStats.getDeleteCount();
log.info(Cat.class.getName() + " changed " + changes + "times" );
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CHAPTER 20. TOOLSET GUIDE
20.1. ABOUT THE TOOLSET GUIDE
Roundtrip engineering with Hibernate is possible using a set of Eclipse plugins, commandline tools, and
Ant tasks.
Hibernate Tools currently include plugins for the Eclipse IDE as well as Ant tasks for reverse engineering
of existing databases:
Mapping Editor: an editor for Hibernate XML mapping files that supports auto-completion and
syntax highlighting. It also supports semantic auto-completion for class names and property/field
names, making it more versatile than a normal XML editor.
Console: the console is a new view in Eclipse. In addition to a tree overview of your console
configurations, you are also provided with an interactive view of your persistent classes and their
relationships. The console allows you to execute HQL queries against your database and
browse the result directly in Eclipse.
Development Wizards: several wizards are provided with the Hibernate Eclipse tools. You can
use a wizard to quickly generate Hibernate configuration (cfg.xml) files, or to reverse engineer
an existing database schema into POJO source files and Hibernate mapping files. The reverse
engineering wizard supports customizable templates.
Please refer to the Hibernate Tools package documentation for more information.
However, the Hibernate main package comes bundled with an integrated tool : SchemaExport aka
hbm2ddl.It can even be used from "inside" Hibernate.
Report a bug
20.2. AUTOMATIC SCHEMA GENERATION
20.2.1. About Automatic Schema Generation
Roundtrip engineering with Hibernate is possible using a set of Eclipse plugins, commandline tools, and
Ant tasks.
Hibernate Tools currently include plugins for the Eclipse IDE as well as Ant tasks for reverse engineering
of existing databases:
Mapping Editor: an editor for Hibernate XML mapping files that supports auto-completion and
syntax highlighting. It also supports semantic auto-completion for class names and property/field
names, making it more versatile than a normal XML editor.
Console: the console is a new view in Eclipse. In addition to a tree overview of your console
configurations, you are also provided with an interactive view of your persistent classes and their
relationships. The console allows you to execute HQL queries against your database and
browse the result directly in Eclipse.
Development Wizards: several wizards are provided with the Hibernate Eclipse tools. You can
use a wizard to quickly generate Hibernate configuration (cfg.xml) files, or to reverse engineer
an existing database schema into POJO source files and Hibernate mapping files. The reverse
engineering wizard supports customizable templates.
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245
Please refer to the Hibernate Tools package documentation for more information.
However, the Hibernate main package comes bundled with an integrated tool : SchemaExport aka
hbm2ddl.It can even be used from "inside" Hibernate.
Report a bug
20.2.2. Customizing the Schema
Many Hibernate mapping elements define optional attributes named length, precision and scale.
You can set the length, precision and scale of a column with this attribute.
Some tags also accept a not-null attribute for generating a NOT NULL constraint on table columns,
and a unique attribute for generating UNIQUE constraint on table columns.
A unique-key attribute can be used to group columns in a single, unique key constraint. Currently, the
specified value of the unique-key attribute is not used to name the constraint in the generated DDL. It
is only used to group the columns in the mapping file.
An index attribute specifies the name of an index that will be created using the mapped column or
columns. Multiple columns can be grouped into the same index by simply specifying the same index
name.
A foreign-key attribute can be used to override the name of any generated foreign key constraint.
Many mapping elements also accept a child <column> element. This is particularly useful for mapping
multi-column types:
<property name="zip" length="5"/>
<property name="balance" precision="12" scale="2"/>
<many-to-one name="bar" column="barId" not-null="true"/>
<element column="serialNumber" type="long" not-null="true" unique="true"/>
<many-to-one name="org" column="orgId" unique-key="OrgEmployeeId"/>
<property name="employeeId" unique-key="OrgEmployee"/>
<property name="lastName" index="CustName"/>
<property name="firstName" index="CustName"/>
<many-to-one name="bar" column="barId" foreign-key="FKFooBar"/>
<property name="name" type="my.customtypes.Name"/>
<column name="last" not-null="true" index="bar_idx" length="30"/>
<column name="first" not-null="true" index="bar_idx" length="20"/>
<column name="initial"/>
</property>
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The default attribute allows you to specify a default value for a column.You should assign the same
value to the mapped property before saving a new instance of the mapped class.
The sql-type attribute allows the user to override the default mapping of a Hibernate type to SQL
datatype.
The check attribute allows you to specify a check constraint.
The following table summarizes these optional attributes.
Table 20.1. Summary
Attribute Values Interpretation
length number column length
precision number column decimal precision
scale number column decimal scale
not-null true|false specifies that the column should
be non-nullable
unique true|false specifies that the column should
have a unique constraint
index index_name specifies the name of a (multi-
column) index
<property name="credits" type="integer" insert="false">
<column name="credits" default="10"/>
</property>
<version name="version" type="integer" insert="false">
<column name="version" default="0"/>
</property>
<property name="balance" type="float">
<column name="balance" sql-type="decimal(13,3)"/>
</property>
<property name="foo" type="integer">
<column name="foo" check="foo > 10"/>
</property>
<class name="Foo" table="foos" check="bar < 100.0">
...
<property name="bar" type="float"/>
</class>
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unique-key unique_key_name specifies the name of a multi-
column unique constraint
foreign-key foreign_key_name specifies the name of the foreign
key constraint generated for an
association, for a <one-to-
one>, <many-to-one>,
<key>, or <many-to-many>
mapping element. Note that
inverse="true" sides will not
be considered by
SchemaExport.
sql-type SQL column type overrides the default column type
(attribute of <column> element
only)
default SQL expression specify a default value for the
column
check SQL expression create an SQL check constraint
on either column or table
Attribute Values Interpretation
The <comment> element allows you to specify comments for the generated schema.
This results in a comment on table or comment on column statement in the generated DDL where
supported.
Report a bug
20.2.3. Running the Tool
The SchemaExport tool writes a DDL script to standard out and/or executes the DDL statements.
The following table displays the SchemaExport command line options
java -cphibernate_classpathsorg.hibernate.tool.hbm2ddl.SchemaExport options
mapping_files
<class name="Customer" table="CurCust">
<comment>Current customers only</comment>
...
</class>
<property name="balance">
<column name="bal">
<comment>Balance in USD</comment>
</column>
</property>
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Table 20.2. SchemaExport Command Line Options
Option Description
--quiet do not output the script to stdout
--drop only drop the tables
--create only create the tables
--text do not export to the database
--output=my_schema.ddl output the ddl script to a file
--
naming=eg.MyNamingStra
tegy
select a NamingStrategy
--
config=hibernate.cfg.x
ml
read Hibernate configuration from an XML file
--
properties=hibernate.p
roperties
read database properties from a file
--format format the generated SQL nicely in the script
--delimiter=; set an end of line delimiter for the script
You can even embed SchemaExport in your application:
Report a bug
20.2.4. Database Properties
Database properties can be specified:
as system properties with -D<property>
in hibernate.properties
in a named properties file with --properties
The needed properties are:
Table 20.3. SchemaExport Connection Properties
Configuration cfg = ....;
new SchemaExport(cfg).create(false, true);
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Property Name Description
hibernate.connection.d
river_class
jdbc driver class
hibernate.connection.u
rl
jdbc url
hibernate.connection.u
sername
database user
hibernate.connection.p
assword
user password
hibernate.dialect dialect
Report a bug
20.2.5. Using Ant
You can call SchemaExport from your Ant build script:
Report a bug
20.2.6. Incremental Schema Updates
The SchemaUpdate tool will update an existing schema with "incremental" changes. The
SchemaUpdate depends upon the JDBC metadata API and, as such, will not work with all JDBC drivers.
java -cphibernate_classpathsorg.hibernate.tool.hbm2ddl.SchemaUpdate options
mapping_files
Table 20.4. SchemaUpdate Command Line Options
<target name="schemaexport">
<taskdef name="schemaexport"
classname="org.hibernate.tool.hbm2ddl.SchemaExportTask"
classpathref="class.path"/>
<schemaexport
properties="hibernate.properties"
quiet="no"
text="no"
drop="no"
delimiter=";"
output="schema-export.sql">
<fileset dir="src">
<include name="**/*.hbm.xml"/>
</fileset>
</schemaexport>
</target>
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Option Description
--quiet do not output the script to stdout
--text do not export the script to the database
--
naming=eg.MyNamingStra
tegy
select a NamingStrategy
--
properties=hibernate.p
roperties
read database properties from a file
--
config=hibernate.cfg.x
ml
specify a .cfg.xml file
You can embed SchemaUpdate in your application:
Report a bug
20.2.7. Using Ant for Incremental Schema Updates
You can call SchemaUpdate from the Ant script:
Report a bug
20.2.8. Schema Validation
The SchemaValidator tool will validate that the existing database schema "matches" your mapping
documents. The SchemaValidator depends heavily upon the JDBC metadata API and, as such, will
not work with all JDBC drivers. This tool is extremely useful for testing.
Configuration cfg = ....;
new SchemaUpdate(cfg).execute(false);
<target name="schemaupdate">
<taskdef name="schemaupdate"
classname="org.hibernate.tool.hbm2ddl.SchemaUpdateTask"
classpathref="class.path"/>
<schemaupdate
properties="hibernate.properties"
quiet="no">
<fileset dir="src">
<include name="**/*.hbm.xml"/>
</fileset>
</schemaupdate>
</target>
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java -cphibernate_classpathsorg.hibernate.tool.hbm2ddl.SchemaValidator options
mapping_files
The following table displays the SchemaValidator command line options:
Table 20.5. SchemaValidator Command Line Options
Option Description
--
naming=eg.MyNamingStra
tegy
select a NamingStrategy
--
properties=hibernate.p
roperties
read database properties from a file
--
config=hibernate.cfg.x
ml
specify a .cfg.xml file
You can embed SchemaValidator in your application:
Report a bug
20.2.9. Using Ant for Schema Validation
You can call SchemaValidator from the Ant script:
Report a bug
Configuration cfg = ....;
new SchemaValidator(cfg).validate();
<target name="schemavalidate">
<taskdef name="schemavalidator"
classname="org.hibernate.tool.hbm2ddl.SchemaValidatorTask"
classpathref="class.path"/>
<schemavalidator
properties="hibernate.properties">
<fileset dir="src">
<include name="**/*.hbm.xml"/>
</fileset>
</schemavalidator>
</target>
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CHAPTER 21. A PARENT/CHILD EXAMPLE
21.1. ABOUT THE PARENT/CHILD EXAMPLE
One of the first things that new users want to do with Hibernate is to model a parent/child type
relationship. There are two different approaches to this. The most convenient approach, especially for
new users, is to model both Parent and Child as entity classes with a <one-to-many> association
from Parent to Child. The alternative approach is to declare the Child as a <composite-
element>. The default semantics of a one-to-many association in Hibernate are much less close to the
usual semantics of a parent/child relationship than those of a composite element mapping. We will
explain how to use a bidirectional one-to-many association with cascades to model a parent/child
relationship efficiently and elegantly.
Report a bug
21.2. ABOUT COLLECTIONS
Hibernate collections are considered to be a logical part of their owning entity and not of the contained
entities. Be aware that this is a critical distinction that has the following consequences:
When you remove/add an object from/to a collection, the version number of the collection owner
is incremented.
If an object that was removed from a collection is an instance of a value type (e.g. a composite
element), that object will cease to be persistent and its state will be completely removed from the
database. Likewise, adding a value type instance to the collection will cause its state to be
immediately persistent.
Conversely, if an entity is removed from a collection (a one-to-many or many-to-many
association), it will not be deleted by default. This behavior is completely consistent; a change to
the internal state of another entity should not cause the associated entity to vanish. Likewise,
adding an entity to a collection does not cause that entity to become persistent, by default.
Adding an entity to a collection, by default, merely creates a link between the two entities. Removing the
entity will remove the link. This is appropriate for all sorts of cases. However, it is not appropriate in the
case of a parent/child relationship. In this case, the life of the child is bound to the life cycle of the parent.
Report a bug
21.3. BIDIRECTIONAL ONE-TO-MANY EXAMPLE
Suppose we start with a simple <one-to-many> association from Parent to Child.
If we were to execute the following code:
<set name="children">
<key column="parent_id"/>
<one-to-many class="Child"/>
</set>
Parent p = .....;
Child c = new Child();
p.getChildren().add(c);
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Hibernate would issue two SQL statements:
an INSERT to create the record for c
an UPDATE to create the link from p to c
This is not only inefficient, but also violates any NOT NULL constraint on the parent_id column. You
can fix the nullability constraint violation by specifying not-null="true" in the collection mapping:
However, this is not the recommended solution.
The underlying cause of this behavior is that the link (the foreign key parent_id) from p to c is not
considered part of the state of the Child object and is therefore not created in the INSERT. The solution
is to make the link part of the Child mapping.
You also need to add the parent property to the Child class.
Now that the Child entity is managing the state of the link, we tell the collection not to update the link.
We use the inverse attribute to do this:
The following code would be used to add a new Child:
Only one SQL INSERT would now be issued.
You could also create an addChild() method of Parent.
session.save(c);
session.flush();
<set name="children">
<key column="parent_id" not-null="true"/>
<one-to-many class="Child"/>
</set>
<many-to-one name="parent" column="parent_id" not-null="true"/>
<set name="children" inverse="true">
<key column="parent_id"/>
<one-to-many class="Child"/>
</set>
Parent p = (Parent) session.load(Parent.class, pid);
Child c = new Child();
c.setParent(p);
p.getChildren().add(c);
session.save(c);
session.flush();
public void addChild(Child c) {
c.setParent(this);
children.add(c);
}
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The code to add a Child looks like this:
Report a bug
21.4. CASCADING LIFE CYCLE
You can address the frustrations of the explicit call to save() by using cascades.
This simplifies the code above to:
Similarly, we do not need to iterate over the children when saving or deleting a Parent. The following
removes p and all its children from the database.
However, the following code:
will not remove c from the database. In this case, it will only remove the link to p and cause a NOT NULL
constraint violation. You need to explicitly delete() the Child.
In our case, a Child cannot exist without its parent. So if we remove a Child from the collection, we do
want it to be deleted. To do this, we must use cascade="all-delete-orphan".
Parent p = (Parent) session.load(Parent.class, pid);
Child c = new Child();
p.addChild(c);
session.save(c);
session.flush();
<set name="children" inverse="true" cascade="all">
<key column="parent_id"/>
<one-to-many class="Child"/>
</set>
Parent p = (Parent) session.load(Parent.class, pid);
Child c = new Child();
p.addChild(c);
session.flush();
Parent p = (Parent) session.load(Parent.class, pid);
session.delete(p);
session.flush();
Parent p = (Parent) session.load(Parent.class, pid);
Child c = (Child) p.getChildren().iterator().next();
p.getChildren().remove(c);
c.setParent(null);
session.flush();
Parent p = (Parent) session.load(Parent.class, pid);
Child c = (Child) p.getChildren().iterator().next();
p.getChildren().remove(c);
session.delete(c);
session.flush();
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Even though the collection mapping specifies inverse="true", cascades are still processed by
iterating the collection elements. If you need an object be saved, deleted or updated by cascade, you
must add it to the collection. It is not enough to simply call setParent().
Report a bug
21.5. CASCADES AND UNSAVED-VALUE
Suppose we loaded up a Parent in one Session, made some changes in a UI action and wanted to
persist these changes in a new session by calling update(). The Parent will contain a collection of
children and, since the cascading update is enabled, Hibernate needs to know which children are newly
instantiated and which represent existing rows in the database. We will also assume that both Parent
and Child have generated identifier properties of type Long. Hibernate will use the identifier and
version/timestamp property value to determine which of the children are new. (Refer to the "Automatic
State Detection: section for further information) In Hibernate3, it is no longer necessary to specify an
unsaved-value explicitly.
The following code will update parent and child and insert newChild:
This may be suitable for the case of a generated identifier, but what about assigned identifiers and
composite identifiers? This is more difficult, since Hibernate cannot use the identifier property to
distinguish between a newly instantiated object, with an identifier assigned by the user, and an object
loaded in a previous session. In this case, Hibernate will either use the timestamp or version property, or
will actually query the second-level cache or, worst case, the database, to see if the row exists.
Report a bug
21.6. CONCLUSION
The sections we have just covered can be a bit confusing. However, in practice, it all works out nicely.
Most Hibernate applications use the parent/child pattern in many places.
We mentioned an alternative in the first paragraph. None of the above issues exist in the case of
<composite-element> mappings, which have exactly the semantics of a parent/child relationship.
Unfortunately, there are two big limitations with composite element classes: composite elements cannot
own collections and they should not be the child of any entity other than the unique parent.
Report a bug
<set name="children" inverse="true" cascade="all-delete-orphan">
<key column="parent_id"/>
<one-to-many class="Child"/>
</set>
//parent and child were both loaded in a previous session
parent.addChild(child);
Child newChild = new Child();
parent.addChild(newChild);
session.update(parent);
session.flush();
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CHAPTER 22. WEBLOG APPLICATION EXAMPLE
22.1. PERSISTENT CLASSES
The persistent classes here represent a weblog and an item posted in a weblog. They are to be
modelled as a standard parent/child relationship, but we will use an ordered bag, instead of a set:
package eg;
import java.util.List;
public class Blog {
private Long _id;
private String _name;
private List _items;
public Long getId() {
return _id;
}
public List getItems() {
return _items;
}
public String getName() {
return _name;
}
public void setId(Long long1) {
_id = long1;
}
public void setItems(List list) {
_items = list;
}
public void setName(String string) {
_name = string;
}
}
package eg;
import java.text.DateFormat;
import java.util.Calendar;
public class BlogItem {
private Long _id;
private Calendar _datetime;
private String _text;
private String _title;
private Blog _blog;
public Blog getBlog() {
return _blog;
}
public Calendar getDatetime() {
return _datetime;
}
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Report a bug
22.2. HIBERNATE MAPPINGS
The XML mappings are now straightforward. For example:
public Long getId() {
return _id;
}
public String getText() {
return _text;
}
public String getTitle() {
return _title;
}
public void setBlog(Blog blog) {
_blog = blog;
}
public void setDatetime(Calendar calendar) {
_datetime = calendar;
}
public void setId(Long long1) {
_id = long1;
}
public void setText(String string) {
_text = string;
}
public void setTitle(String string) {
_title = string;
}
}
<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
"-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping package="eg">
<class
name="Blog"
table="BLOGS">
<id
name="id"
column="BLOG_ID">
<generator class="native"/>
</id>
<property
name="name"
column="NAME"
not-null="true"
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unique="true"/>
<bag
name="items"
inverse="true"
order-by="DATE_TIME"
cascade="all">
<key column="BLOG_ID"/>
<one-to-many class="BlogItem"/>
</bag>
</class>
</hibernate-mapping>
<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
"-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping package="eg">
<class
name="BlogItem"
table="BLOG_ITEMS"
dynamic-update="true">
<id
name="id"
column="BLOG_ITEM_ID">
<generator class="native"/>
</id>
<property
name="title"
column="TITLE"
not-null="true"/>
<property
name="text"
column="TEXT"
not-null="true"/>
<property
name="datetime"
column="DATE_TIME"
not-null="true"/>
<many-to-one
name="blog"
column="BLOG_ID"
not-null="true"/>
CHAPTER 22. WEBLOG APPLICATION EXAMPLE
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Report a bug
22.3. HIBERNATE CODE
The following class demonstrates some of the kinds of things we can do with these classes using
Hibernate:
</class>
</hibernate-mapping>
package eg;
import java.util.ArrayList;
import java.util.Calendar;
import java.util.Iterator;
import java.util.List;
import org.hibernate.HibernateException;
import org.hibernate.Query;
import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.Transaction;
import org.hibernate.cfg.Configuration;
import org.hibernate.tool.hbm2ddl.SchemaExport;
public class BlogMain {
private SessionFactory _sessions;
public void configure() throws HibernateException {
_sessions = new Configuration()
.addClass(Blog.class)
.addClass(BlogItem.class)
.buildSessionFactory();
}
public void exportTables() throws HibernateException {
Configuration cfg = new Configuration()
.addClass(Blog.class)
.addClass(BlogItem.class);
new SchemaExport(cfg).create(true, true);
}
public Blog createBlog(String name) throws HibernateException {
Blog blog = new Blog();
blog.setName(name);
blog.setItems( new ArrayList() );
Session session = _sessions.openSession();
Transaction tx = null;
try {
tx = session.beginTransaction();
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session.persist(blog);
tx.commit();
}
catch (HibernateException he) {
if (tx!=null) tx.rollback();
throw he;
}
finally {
session.close();
}
return blog;
}
public BlogItem createBlogItem(Blog blog, String title, String text)
throws HibernateException {
BlogItem item = new BlogItem();
item.setTitle(title);
item.setText(text);
item.setBlog(blog);
item.setDatetime( Calendar.getInstance() );
blog.getItems().add(item);
Session session = _sessions.openSession();
Transaction tx = null;
try {
tx = session.beginTransaction();
session.update(blog);
tx.commit();
}
catch (HibernateException he) {
if (tx!=null) tx.rollback();
throw he;
}
finally {
session.close();
}
return item;
}
public BlogItem createBlogItem(Long blogid, String title, String text)
throws HibernateException {
BlogItem item = new BlogItem();
item.setTitle(title);
item.setText(text);
item.setDatetime( Calendar.getInstance() );
Session session = _sessions.openSession();
Transaction tx = null;
try {
tx = session.beginTransaction();
Blog blog = (Blog) session.load(Blog.class, blogid);
item.setBlog(blog);
blog.getItems().add(item);
tx.commit();
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}
catch (HibernateException he) {
if (tx!=null) tx.rollback();
throw he;
}
finally {
session.close();
}
return item;
}
public void updateBlogItem(BlogItem item, String text)
throws HibernateException {
item.setText(text);
Session session = _sessions.openSession();
Transaction tx = null;
try {
tx = session.beginTransaction();
session.update(item);
tx.commit();
}
catch (HibernateException he) {
if (tx!=null) tx.rollback();
throw he;
}
finally {
session.close();
}
}
public void updateBlogItem(Long itemid, String text)
throws HibernateException {
Session session = _sessions.openSession();
Transaction tx = null;
try {
tx = session.beginTransaction();
BlogItem item = (BlogItem) session.load(BlogItem.class,
itemid);
item.setText(text);
tx.commit();
}
catch (HibernateException he) {
if (tx!=null) tx.rollback();
throw he;
}
finally {
session.close();
}
}
public List listAllBlogNamesAndItemCounts(int max)
throws HibernateException {
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Session session = _sessions.openSession();
Transaction tx = null;
List result = null;
try {
tx = session.beginTransaction();
Query q = session.createQuery(
"select blog.id, blog.name, count(blogItem) " +
"from Blog as blog " +
"left outer join blog.items as blogItem " +
"group by blog.name, blog.id " +
"order by max(blogItem.datetime)"
);
q.setMaxResults(max);
result = q.list();
tx.commit();
}
catch (HibernateException he) {
if (tx!=null) tx.rollback();
throw he;
}
finally {
session.close();
}
return result;
}
public Blog getBlogAndAllItems(Long blogid)
throws HibernateException {
Session session = _sessions.openSession();
Transaction tx = null;
Blog blog = null;
try {
tx = session.beginTransaction();
Query q = session.createQuery(
"from Blog as blog " +
"left outer join fetch blog.items " +
"where blog.id = :blogid"
);
q.setParameter("blogid", blogid);
blog = (Blog) q.uniqueResult();
tx.commit();
}
catch (HibernateException he) {
if (tx!=null) tx.rollback();
throw he;
}
finally {
session.close();
}
return blog;
}
public List listBlogsAndRecentItems() throws HibernateException {
Session session = _sessions.openSession();
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263
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Transaction tx = null;
List result = null;
try {
tx = session.beginTransaction();
Query q = session.createQuery(
"from Blog as blog " +
"inner join blog.items as blogItem " +
"where blogItem.datetime > :minDate"
);
Calendar cal = Calendar.getInstance();
cal.roll(Calendar.MONTH, false);
q.setCalendar("minDate", cal);
result = q.list();
tx.commit();
}
catch (HibernateException he) {
if (tx!=null) tx.rollback();
throw he;
}
finally {
session.close();
}
return result;
}
}
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CHAPTER 23. VARIOUS MAPPINGS EXAMPLE
23.1. EMPLOYER/EMPLOYEE
The following model of the relationship between Employer and Employee uses an entity class
(Employment) to represent the association. You can do this when there might be more than one period
of employment for the same two parties. Components are used to model monetary values and employee
names.
Here is a possible mapping document:
<hibernate-mapping>
<class name="Employer" table="employers">
<id name="id">
<generator class="sequence">
<param name="sequence">employer_id_seq</param>
</generator>
</id>
<property name="name"/>
</class>
<class name="Employment" table="employment_periods">
<id name="id">
<generator class="sequence">
<param name="sequence">employment_id_seq</param>
</generator>
</id>
<property name="startDate" column="start_date"/>
<property name="endDate" column="end_date"/>
<component name="hourlyRate" class="MonetaryAmount">
<property name="amount">
<column name="hourly_rate" sql-type="NUMERIC(12, 2)"/>
</property>
<property name="currency" length="12"/>
</component>
<many-to-one name="employer" column="employer_id" not-
null="true"/>
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265
Here is the table schema generated by SchemaExport.
<many-to-one name="employee" column="employee_id" not-
null="true"/>
</class>
<class name="Employee" table="employees">
<id name="id">
<generator class="sequence">
<param name="sequence">employee_id_seq</param>
</generator>
</id>
<property name="taxfileNumber"/>
<component name="name" class="Name">
<property name="firstName"/>
<property name="initial"/>
<property name="lastName"/>
</component>
</class>
</hibernate-mapping>
create table employers (
id BIGINT not null,
name VARCHAR(255),
primary key (id)
)
create table employment_periods (
id BIGINT not null,
hourly_rate NUMERIC(12, 2),
currency VARCHAR(12),
employee_id BIGINT not null,
employer_id BIGINT not null,
end_date TIMESTAMP,
start_date TIMESTAMP,
primary key (id)
)
create table employees (
id BIGINT not null,
firstName VARCHAR(255),
initial CHAR(1),
lastName VARCHAR(255),
taxfileNumber VARCHAR(255),
primary key (id)
)
alter table employment_periods
add constraint employment_periodsFK0 foreign key (employer_id)
references employers
alter table employment_periods
add constraint employment_periodsFK1 foreign key (employee_id)
references employees
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23.2. AUTHOR/WORK
Consider the following model of the relationships between Work, Author and Person. In the example,
the relationship between Work and Author is represented as a many-to-many association and the
relationship between Author and Person is represented as one-to-one association. Another possibility
would be to have Author extend Person.
The following mapping document correctly represents these relationships:
create sequence employee_id_seq
create sequence employment_id_seq
create sequence employer_id_seq
<hibernate-mapping>
<class name="Work" table="works" discriminator-value="W">
<id name="id" column="id">
<generator class="native"/>
</id>
<discriminator column="type" type="character"/>
<property name="title"/>
<set name="authors" table="author_work">
<key column name="work_id"/>
<many-to-many class="Author" column name="author_id"/>
</set>
<subclass name="Book" discriminator-value="B">
<property name="text"/>
</subclass>
<subclass name="Song" discriminator-value="S">
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267
There are four tables in this mapping: works, authors and persons hold work, author and person data
respectively. author_work is an association table linking authors to works. Here is the table schema,
as generated by SchemaExport:
<property name="tempo"/>
<property name="genre"/>
</subclass>
</class>
<class name="Author" table="authors">
<id name="id" column="id">
<!-- The Author must have the same identifier as the Person --
>
<generator class="assigned"/>
</id>
<property name="alias"/>
<one-to-one name="person" constrained="true"/>
<set name="works" table="author_work" inverse="true">
<key column="author_id"/>
<many-to-many class="Work" column="work_id"/>
</set>
</class>
<class name="Person" table="persons">
<id name="id" column="id">
<generator class="native"/>
</id>
<property name="name"/>
</class>
</hibernate-mapping>
create table works (
id BIGINT not null generated by default as identity,
tempo FLOAT,
genre VARCHAR(255),
text INTEGER,
title VARCHAR(255),
type CHAR(1) not null,
primary key (id)
)
create table author_work (
author_id BIGINT not null,
work_id BIGINT not null,
primary key (work_id, author_id)
)
create table authors (
id BIGINT not null generated by default as identity,
alias VARCHAR(255),
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23.3. CUSTOMER/ORDER/PRODUCT
In this section we consider a model of the relationships between Customer, Order, Line Item and
Product. There is a one-to-many association between Customer and Order, but how can you
represent Order / LineItem / Product? In the example, LineItem is mapped as an association class
representing the many-to-many association between Order and Product. In Hibernate this is called a
composite element.
The mapping document will look like this:
primary key (id)
)
create table persons (
id BIGINT not null generated by default as identity,
name VARCHAR(255),
primary key (id)
)
alter table authors
add constraint authorsFK0 foreign key (id) references persons
alter table author_work
add constraint author_workFK0 foreign key (author_id) references
authors
alter table author_work
add constraint author_workFK1 foreign key (work_id) references works
<hibernate-mapping>
<class name="Customer" table="customers">
<id name="id">
<generator class="native"/>
</id>
<property name="name"/>
<set name="orders" inverse="true">
<key column="customer_id"/>
<one-to-many class="Order"/>
</set>
</class>
<class name="Order" table="orders">
<id name="id">
<generator class="native"/>
</id>
CHAPTER 23. VARIOUS MAPPINGS EXAMPLE
269
customers, orders, line_items and products hold customer, order, order line item and product
data respectively. line_items also acts as an association table linking orders with products.
<property name="date"/>
<many-to-one name="customer" column="customer_id"/>
<list name="lineItems" table="line_items">
<key column="order_id"/>
<list-index column="line_number"/>
<composite-element class="LineItem">
<property name="quantity"/>
<many-to-one name="product" column="product_id"/>
</composite-element>
</list>
</class>
<class name="Product" table="products">
<id name="id">
<generator class="native"/>
</id>
<property name="serialNumber"/>
</class>
</hibernate-mapping>
create table customers (
id BIGINT not null generated by default as identity,
name VARCHAR(255),
primary key (id)
)
create table orders (
id BIGINT not null generated by default as identity,
customer_id BIGINT,
date TIMESTAMP,
primary key (id)
)
create table line_items (
line_number INTEGER not null,
order_id BIGINT not null,
product_id BIGINT,
quantity INTEGER,
primary key (order_id, line_number)
)
create table products (
id BIGINT not null generated by default as identity,
serialNumber VARCHAR(255),
primary key (id)
)
alter table orders
add constraint ordersFK0 foreign key (customer_id) references
customers
alter table line_items
add constraint line_itemsFK0 foreign key (product_id) references
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23.4. MISCELLANEOUS EXAMPLE MAPPINGS
23.4.1. About the Miscellaneous Example Mappings
These examples are available from the Hibernate test suite. You will find many other useful example
mappings there by searching in the test folder of the Hibernate distribution.
Report a bug
23.4.2. Typed One-to-one Association
Report a bug
23.4.3. Composite Key Example
products
alter table line_items
add constraint line_itemsFK1 foreign key (order_id) references orders
<class name="Person">
<id name="name"/>
<one-to-one name="address"
cascade="all">
<formula>name</formula>
<formula>'HOME'</formula>
</one-to-one>
<one-to-one name="mailingAddress"
cascade="all">
<formula>name</formula>
<formula>'MAILING'</formula>
</one-to-one>
</class>
<class name="Address" batch-size="2"
check="addressType in ('MAILING', 'HOME', 'BUSINESS')">
<composite-id>
<key-many-to-one name="person"
column="personName"/>
<key-property name="type"
column="addressType"/>
</composite-id>
<property name="street" type="text"/>
<property name="state"/>
<property name="zip"/>
</class>
<class name="Customer">
<id name="customerId"
length="10">
<generator class="assigned"/>
</id>
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<property name="name" not-null="true" length="100"/>
<property name="address" not-null="true" length="200"/>
<list name="orders"
inverse="true"
cascade="save-update">
<key column="customerId"/>
<index column="orderNumber"/>
<one-to-many class="Order"/>
</list>
</class>
<class name="Order" table="CustomerOrder" lazy="true">
<synchronize table="LineItem"/>
<synchronize table="Product"/>
<composite-id name="id"
class="Order$Id">
<key-property name="customerId" length="10"/>
<key-property name="orderNumber"/>
</composite-id>
<property name="orderDate"
type="calendar_date"
not-null="true"/>
<property name="total">
<formula>
( select sum(li.quantity*p.price)
from LineItem li, Product p
where li.productId = p.productId
and li.customerId = customerId
and li.orderNumber = orderNumber )
</formula>
</property>
<many-to-one name="customer"
column="customerId"
insert="false"
update="false"
not-null="true"/>
<bag name="lineItems"
fetch="join"
inverse="true"
cascade="save-update">
<key>
<column name="customerId"/>
<column name="orderNumber"/>
</key>
<one-to-many class="LineItem"/>
</bag>
</class>
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Report a bug
23.4.4. Many-to-many with Shared Composite Key Attribute
<class name="LineItem">
<composite-id name="id"
class="LineItem$Id">
<key-property name="customerId" length="10"/>
<key-property name="orderNumber"/>
<key-property name="productId" length="10"/>
</composite-id>
<property name="quantity"/>
<many-to-one name="order"
insert="false"
update="false"
not-null="true">
<column name="customerId"/>
<column name="orderNumber"/>
</many-to-one>
<many-to-one name="product"
insert="false"
update="false"
not-null="true"
column="productId"/>
</class>
<class name="Product">
<synchronize table="LineItem"/>
<id name="productId"
length="10">
<generator class="assigned"/>
</id>
<property name="description"
not-null="true"
length="200"/>
<property name="price" length="3"/>
<property name="numberAvailable"/>
<property name="numberOrdered">
<formula>
( select sum(li.quantity)
from LineItem li
where li.productId = productId )
</formula>
</property>
</class>
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Report a bug
23.4.5. Content Based Discrimination
<class name="User" table="`User`">
<composite-id>
<key-property name="name"/>
<key-property name="org"/>
</composite-id>
<set name="groups" table="UserGroup">
<key>
<column name="userName"/>
<column name="org"/>
</key>
<many-to-many class="Group">
<column name="groupName"/>
<formula>org</formula>
</many-to-many>
</set>
</class>
<class name="Group" table="`Group`">
<composite-id>
<key-property name="name"/>
<key-property name="org"/>
</composite-id>
<property name="description"/>
<set name="users" table="UserGroup" inverse="true">
<key>
<column name="groupName"/>
<column name="org"/>
</key>
<many-to-many class="User">
<column name="userName"/>
<formula>org</formula>
</many-to-many>
</set>
</class>
<class name="Person"
discriminator-value="P">
<id name="id"
column="person_id"
unsaved-value="0">
<generator class="native"/>
</id>
<discriminator
type="character">
<formula>
case
when title is not null then 'E'
when salesperson is not null then 'C'
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Report a bug
23.4.6. Associations on Alternate Keys
else 'P'
end
</formula>
</discriminator>
<property name="name"
not-null="true"
length="80"/>
<property name="sex"
not-null="true"
update="false"/>
<component name="address">
<property name="address"/>
<property name="zip"/>
<property name="country"/>
</component>
<subclass name="Employee"
discriminator-value="E">
<property name="title"
length="20"/>
<property name="salary"/>
<many-to-one name="manager"/>
</subclass>
<subclass name="Customer"
discriminator-value="C">
<property name="comments"/>
<many-to-one name="salesperson"/>
</subclass>
</class>
<class name="Person">
<id name="id">
<generator class="hilo"/>
</id>
<property name="name" length="100"/>
<one-to-one name="address"
property-ref="person"
cascade="all"
fetch="join"/>
<set name="accounts"
inverse="true">
<key column="userId"
property-ref="userId"/>
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Report a bug
<one-to-many class="Account"/>
</set>
<property name="userId" length="8"/>
</class>
<class name="Address">
<id name="id">
<generator class="hilo"/>
</id>
<property name="address" length="300"/>
<property name="zip" length="5"/>
<property name="country" length="25"/>
<many-to-one name="person" unique="true" not-null="true"/>
</class>
<class name="Account">
<id name="accountId" length="32">
<generator class="uuid"/>
</id>
<many-to-one name="user"
column="userId"
property-ref="userId"/>
<property name="type" not-null="true"/>
</class>
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CHAPTER 24. BEST PRACTICES
24.1. HIBERNATE BEST PRACTICES
Write fine-grained classes and map them using <component>:
Use an Address class to encapsulate street, suburb, state, postcode. This encourages code
reuse and simplifies refactoring.
Declare identifier properties on persistent classes:
Hibernate makes identifier properties optional. There are a range of reasons why you should use
them. We recommend that identifiers be 'synthetic', that is, generated with no business meaning.
Identify natural keys:
Identify natural keys for all entities, and map them using <natural-id>. Implement equals() and
hashCode() to compare the properties that make up the natural key.
Place each class mapping in its own file:
Do not use a single monolithic mapping document. Map com.eg.Foo in the file
com/eg/Foo.hbm.xml. This makes sense, particularly in a team environment.
Load mappings as resources:
Deploy the mappings along with the classes they map.
Consider externalizing query strings:
This is recommended if your queries call non-ANSI-standard SQL functions. Externalizing the query
strings to mapping files will make the application more portable.
Use bind variables.
As in JDBC, always replace non-constant values by "?". Do not use string manipulation to bind a
non-constant value in a query. You should also consider using named parameters in queries.
Do not manage your own JDBC connections:
Hibernate allows the application to manage JDBC connections, but his approach should be
considered a last-resort. If you cannot use the built-in connection providers, consider providing your
own implementation of org.hibernate.connection.ConnectionProvider.
Consider using a custom type:
Suppose you have a Java type from a library that needs to be persisted but does not provide the
accessors needed to map it as a component. You should consider implementing
org.hibernate.UserType. This approach frees the application code from implementing
transformations to/from a Hibernate type.
Use hand-coded JDBC in bottlenecks:
In performance-critical areas of the system, some kinds of operations might benefit from direct JDBC.
Do not assume, however, that JDBC is necessarily faster. Please wait until you know something is a
bottleneck. If you need to use direct JDBC, you can open a Hibernate Session and wrap your JDBC
operation as a Work object with session.doWork(Work). This way you can still use the same
transaction strategy and underlying connection provider.
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Understand Session flushing:
Sometimes the Session synchronizes its persistent state with the database. Performance will be
affected if this process occurs too often. You can sometimes minimize unnecessary flushing by
disabling automatic flushing, or even by changing the order of queries and other operations within a
particular transaction.
In a three tiered architecture, consider using detached objects:
When using a servlet/session bean architecture, you can pass persistent objects loaded in the
session bean to and from the servlet/JSP layer. Use a new session to service each request. Use
Session.merge() or Session.saveOrUpdate() to synchronize objects with the database.
In a two tiered architecture, consider using long persistence contexts:
Database Transactions have to be as short as possible for best scalability. However, it is often
necessary to implement long running application transactions, a single unit-of-work from the point of
view of a user. An application transaction might span several client request/response cycles. It is
common to use detached objects to implement application transactions. An appropriate alternative in
a two tiered architecture, is to maintain a single open persistence contact session for the whole life
cycle of the application transaction. Then simply disconnect from the JDBC connection at the end of
each request and reconnect at the beginning of the subsequent request. Never share a single
session across more than one application transaction or you will be working with stale data.
Do not treat exceptions as recoverable:
This is more of a necessary practice than a "best" practice. When an exception occurs, roll back the
Transaction and close the Session. If you do not do this, Hibernate cannot guarantee that in-
memory state accurately represents the persistent state. For example, do not use Session.load()
to determine if an instance with the given identifier exists on the database; use Session.get() or a
query instead.
Prefer lazy fetching for associations:
Use eager fetching sparingly. Use proxies and lazy collections for most associations to classes that
are not likely to be completely held in the second-level cache. For associations to cached classes,
where there is an a extremely high probability of a cache hit, explicitly disable eager fetching using
lazy="false". When join fetching is appropriate to a particular use case, use a query with a left
join fetch.
Use the open session in view pattern, or a disciplined assembly phase to avoid problems with
unfetched data:
Hibernate frees the developer from writing tedious Data Transfer Objects (DTO). In a traditional EJB
architecture, DTOs serve dual purposes: first, they work around the problem that entity beans are not
serializable; second, they implicitly define an assembly phase where all data to be used by the view is
fetched and marshalled into the DTOs before returning control to the presentation tier. Hibernate
eliminates the first purpose. Unless you are prepared to hold the persistence context (the session)
open across the view rendering process, you will still need an assembly phase. Think of your business
methods as having a strict contract with the presentation tier about what data is available in the
detached objects. This is not a limitation of Hibernate. It is a fundamental requirement of safe
transactional data access.
Consider abstracting your business logic from Hibernate:
Hide Hibernate data-access code behind an interface. Combine the DAO and Thread Local Session
patterns. You can even have some classes persisted by handcoded JDBC associated to Hibernate
via a UserType. This advice is, however, intended for "sufficiently large" applications. It is not
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appropriate for an application with five tables.
Do not use exotic association mappings:
Practical test cases for real many-to-many associations are rare. Most of the time you need additional
information stored in the "link table". In this case, it is much better to use two one-to-many
associations to an intermediate link class. In fact, most associations are one-to-many and many-to-
one. For this reason, you should proceed cautiously when using any other association style.
Prefer bidirectional associations:
Unidirectional associations are more difficult to query. In a large application, almost all associations
must be navigable in both directions in queries.
Report a bug
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CHAPTER 25. DATABASE PORTABILITY CONSIDERATIONS
25.1. PORTABILITY BASICS
One of the selling points of Hibernate (and really Object/Relational Mapping as a whole) is the notion of
database portability. This could mean an internal IT user migrating from one database vendor to another,
or it could mean a framework or deployable application consuming Hibernate to simultaneously target
multiple database products by their users. Regardless of the exact scenario, the basic idea is that you
want Hibernate to help you run against any number of databases without changes to your code, and
ideally without any changes to the mapping metadata.
Report a bug
25.2. DIALECT
The first line of portability for Hibernate is the dialect, which is a specialization of the
org.hibernate.dialect.Dialect contract. A dialect encapsulates all the differences in how
Hibernate must communicate with a particular database to accomplish some task like getting a
sequence value or structuring a SELECT query. Hibernate bundles a wide range of dialects for many of
the most popular databases. If you find that your particular database is not among them, it is not terribly
difficult to write your own.
Report a bug
25.3. DIALECT RESOLUTION
Originally, Hibernate would always require that users specify which dialect to use. In the case of users
looking to simultaneously target multiple databases with their build that was problematic. Generally this
required their users to configure the Hibernate dialect or defining their own method of setting that value.
Starting with version 3.2, Hibernate introduced the notion of automatically detecting the dialect to use
based on the java.sql.DatabaseMetaData obtained from a java.sql.Connection to that
database. This was much better, expect that this resolution was limited to databases Hibernate know
about ahead of time and was in no way configurable or overrideable.
Starting with version 3.3, Hibernate has a fare more powerful way to automatically determine which
dialect to should be used by relying on a series of delegates which implement the
org.hibernate.dialect.resolver.DialectResolver which defines only a single method:
. The basic contract here is that if the resolver 'understands' the given database metadata then it returns
the corresponding Dialect; if not it returns null and the process continues to the next resolver. The
signature also identifies org.hibernate.exception.JDBCConnectionException as possibly
being thrown. A JDBCConnectionException here is interpreted to imply a "non transient" (aka non-
recoverable) connection problem and is used to indicate an immediate stop to resolution attempts. All
other exceptions result in a warning and continuing on to the next resolver.
The cool part about these resolvers is that users can also register their own custom resolvers which will
be processed ahead of the built-in Hibernate ones. This might be useful in a number of different
situations: it allows easy integration for auto-detection of dialects beyond those shipped with HIbernate
itself; it allows you to specify to use a custom dialect when a particular database is recognized; etc. To
public Dialect resolveDialect(DatabaseMetaData metaData) throws
JDBCConnectionException
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register one or more resolvers, simply specify them (seperated by commas, tabs or spaces) using the
'hibernate.dialect_resolvers' configuration setting (see the DIALECT_RESOLVERS constant on
org.hibernate.cfg.Environment).
Report a bug
25.4. IDENTIFIER GENERATION
When considering portability between databases, another important decision is selecting the identifier
generation stratagy you want to use. Originally Hibernate provided the native generator for this purpose,
which was intended to select between a sequence, identity, or table strategy depending on the capability
of the underlying database. However, an insidious implication of this approach comes about when
targtetting some databases which support identity generation and some which do not. identity generation
relies on the SQL definition of an IDENTITY (or auto-increment) column to manage the identifier value; it
is what is known as a post-insert generation strategy becauase the insert must actually happen before
we can know the identifier value. Because Hibernate relies on this identifier value to uniquely reference
entities within a persistence context it must then issue the insert immediately when the users requests
the entitiy be associated with the session (like via save() e.g.) regardless of current transactional
semantics. The underlying issue is that the semanctics of the application itself changes in these cases.
NOTE
Hibernate has been improved so that the insert is delayed in cases where that is feasible.
Starting with version 3.2.3, Hibernate comes with a set of enhanced identifier generators targetting
portability in a much different way.
NOTE
There are specifically 2 bundled enhancedgenerators:
org.hibernate.id.enhanced.SequenceStyleGenerator
org.hibernate.id.enhanced.TableGenerator
The idea behind these generators is to port the actual semantics of the identifer value generation to the
different databases. For example, the org.hibernate.id.enhanced.SequenceStyleGenerator
mimics the behavior of a sequence on databases which do not support sequences by using a table.
Report a bug
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APPENDIX A. REVISION HISTORY
Revision 2.1.0-2.2 Wed Feb 11 2015 Lucas Costi
Updated the Product Name to reflect the new name grouping for the product. No update was made to details in the guide.
Revision 2.1.0-2 Wed Aug 20 2014 Mandar Joshi
Updated for release.
Revision 2.1.0-1 Fri Aug 08 2014 Misha Husnain Ali
Built from Content Specification: 10441, Revision: 691395 by mhusnain
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