Hibernate Reference Documentation
Version: 3.0alpha
Table of Contents
Preface ...........................................................................................................................................vii
1. Quickstart with Tomcat ..............................................................................................................1
1.1. Getting started with Hibernate .............................................................................................1
1.2. First persistent class ...........................................................................................................3
1.3. Mapping the cat .................................................................................................................4
1.4. Playing with cats ................................................................................................................5
1.5. Finally ...............................................................................................................................7
2. Architecture ................................................................................................................................8
2.1. Overview ...........................................................................................................................8
2.2. Instance states .................................................................................................................. 10
2.3. JMX Integration ............................................................................................................... 10
2.4. JCA Support .................................................................................................................... 10
3. Configuration ............................................................................................................................ 12
3.1. Programmatic configuration .............................................................................................. 12
3.2. Obtaining a SessionFactory ............................................................................................... 12
3.3. User provided JDBC connection ....................................................................................... 13
3.4. Hibernate provided JDBC connection ................................................................................ 13
3.5. Optional configuration properties ...................................................................................... 15
3.5.1. SQL Dialects ......................................................................................................... 18
3.5.2. Outer Join Fetching ............................................................................................... 19
3.5.3. Binary Streams ...................................................................................................... 19
3.5.4. Second-level and query cache ................................................................................. 19
3.5.5. Transaction strategy configuration .......................................................................... 19
3.5.6. JNDI-bound SessionFactory ................................................................................... 20
3.5.7. Query Language Substitution ................................................................................. 20
3.5.8. Hibernate statistics ................................................................................................ 21
3.6. Logging ........................................................................................................................... 21
3.7. Implementing a NamingStrategy ....................................................................................... 21
3.8. XML Configuration File ................................................................................................... 21
4. Persistent Classes ...................................................................................................................... 23
4.1. A simple POJO example ................................................................................................... 23
4.1.1. Declare accessors and mutators for persistent fields ................................................. 24
4.1.2. Implement a no-argument constructor ..................................................................... 24
4.1.3. Provide an identifier property (optional) .................................................................. 24
4.1.4. Prefer non-final classes (optional) ........................................................................... 25
4.2. Implementing inheritance ................................................................................................. 25
4.3. Implementing equals() and hashCode() .............................................................................. 25
4.4. Dynamic models .............................................................................................................. 26
5. Basic O/R Mapping ................................................................................................................... 28
5.1. Mapping declaration ......................................................................................................... 28
5.1.1. Doctype ................................................................................................................ 29
5.1.2. hibernate-mapping ................................................................................................. 29
5.1.3. class, dynamic-class .............................................................................................. 30
5.1.4. id .......................................................................................................................... 32
5.1.4.1. generator .................................................................................................... 32
5.1.4.2. Hi/lo algorithm ........................................................................................... 33
5.1.4.3. UUID algorithm ......................................................................................... 34
5.1.4.4. Identity columns and sequences ................................................................... 34
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5.1.4.5. Assigned identifiers .................................................................................... 34
5.1.4.6. Primary keys assigned by triggers ................................................................ 34
5.1.5. composite-id ......................................................................................................... 34
5.1.6. discriminator ......................................................................................................... 35
5.1.7. version (optional) .................................................................................................. 36
5.1.8. timestamp (optional) .............................................................................................. 36
5.1.9. property ................................................................................................................ 37
5.1.10. many-to-one ........................................................................................................ 38
5.1.11. one-to-one ........................................................................................................... 39
5.1.12. component, dynamic-component .......................................................................... 40
5.1.13. subclass .............................................................................................................. 41
5.1.14. joined-subclass .................................................................................................... 41
5.1.15. union-subclass ..................................................................................................... 42
5.1.16. join ..................................................................................................................... 43
5.1.17. key ..................................................................................................................... 43
5.1.18. map, set, list, bag ................................................................................................. 44
5.1.19. import ................................................................................................................. 44
5.2. Hibernate Types ............................................................................................................... 44
5.2.1. Entities and values ................................................................................................. 44
5.2.2. Basic value types ................................................................................................... 45
5.2.3. Custom value types ............................................................................................... 46
5.2.4. Any type mappings ................................................................................................ 46
5.3. SQL quoted identifiers ...................................................................................................... 47
5.4. Modular mapping files ...................................................................................................... 47
5.5. Using XDoclet markup ..................................................................................................... 48
6. Collection Mapping ................................................................................................................... 50
6.1. Persistent Collections ....................................................................................................... 50
6.2. Mapping a Collection ....................................................................................................... 51
6.3. Collections of Values and Many-To-Many Associations ..................................................... 52
6.4. One-To-Many Associations .............................................................................................. 54
6.5. Lazy Initialization ............................................................................................................ 54
6.6. Sorted Collections ............................................................................................................ 55
6.7. Using an <idbag> ............................................................................................................. 56
6.8. Bidirectional Associations ................................................................................................ 57
6.9. Ternary Associations ........................................................................................................ 58
6.10. Heterogeneous Associations ............................................................................................ 58
6.11. Collection examples ....................................................................................................... 58
7. Component Mapping ................................................................................................................ 61
7.1. Dependent objects ............................................................................................................ 61
7.2. Collections of dependent objects ....................................................................................... 62
7.3. Components as Map indices .............................................................................................. 63
7.4. Components as composite identifiers ................................................................................. 63
7.5. Dynamic components ....................................................................................................... 64
8. Inheritance Mapping ................................................................................................................. 66
8.1. The Three Strategies ......................................................................................................... 66
8.2. Limitations ...................................................................................................................... 68
9. Working with Persistent Data ................................................................................................... 70
9.1. Creating a persistent object ............................................................................................... 70
9.2. Loading an object ............................................................................................................. 70
9.3. Querying ......................................................................................................................... 71
9.3.1. Scalar queries ........................................................................................................ 72
9.3.2. The Query interface ............................................................................................... 72
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9.3.3. Scrollable iteration ................................................................................................ 73
9.3.4. Filtering collections ............................................................................................... 73
9.3.5. Criteria queries ...................................................................................................... 74
9.3.6. Queries in native SQL ............................................................................................ 74
9.4. Updating objects .............................................................................................................. 74
9.4.1. Updating in the same Session ................................................................................. 75
9.4.2. Updating detached objects ...................................................................................... 75
9.4.3. Reattaching detached objects .................................................................................. 76
9.5. Deleting persistent objects ................................................................................................ 76
9.6. Flush ............................................................................................................................... 77
9.7. Ending a Session .............................................................................................................. 77
9.7.1. Flushing the Session .............................................................................................. 78
9.7.2. Committing the database transaction ....................................................................... 78
9.7.3. Closing the Session ............................................................................................... 78
9.7.4. Exception handling ................................................................................................ 78
9.8. Lifecyles and object graphs ............................................................................................... 79
9.9. Parameterized application views with filters ....................................................................... 80
9.10. Interceptors .................................................................................................................... 82
9.11. Event system .................................................................................................................. 83
9.12. Metadata API ................................................................................................................. 84
10. Transactions And Concurrency ............................................................................................... 85
10.1. Configurations, Sessions and Factories ............................................................................ 85
10.2. Threads and connections ................................................................................................. 85
10.3. Considering object identity ............................................................................................. 85
10.4. Optimistic concurrency control ........................................................................................ 86
10.4.1. Long session with automatic versioning ................................................................ 86
10.4.2. Many sessions with automatic versioning .............................................................. 87
10.4.3. Application version checking ............................................................................... 87
10.5. Session disconnection ..................................................................................................... 87
10.6. Pessimistic Locking ........................................................................................................ 88
11. HQL: The Hibernate Query Language .................................................................................... 90
11.1. Case Sensitivity .............................................................................................................. 90
11.2. The from clause .............................................................................................................. 90
11.3. Associations and joins .................................................................................................... 90
11.4. The select clause ............................................................................................................ 91
11.5. Aggregate functions ....................................................................................................... 92
11.6. Polymorphic queries ....................................................................................................... 92
11.7. The where clause ............................................................................................................ 93
11.8. Expressions .................................................................................................................... 94
11.9. The order by clause ........................................................................................................ 96
11.10. The group by clause ...................................................................................................... 96
11.11. Subqueries ................................................................................................................... 97
11.12. HQL examples ............................................................................................................. 97
11.13. Tips & Tricks ............................................................................................................... 99
12. Criteria Queries .................................................................................................................... 101
12.1. Creating a Criteria instance ........................................................................................... 101
12.2. Narrowing the result set ................................................................................................ 101
12.3. Ordering the results ...................................................................................................... 102
12.4. Associations ................................................................................................................. 102
12.5. Dynamic association fetching ........................................................................................ 102
12.6. Example queries ........................................................................................................... 103
13. Native SQL ............................................................................................................................ 104
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13.1. Creating a SQL based Query ......................................................................................... 104
13.2. Alias and property references ........................................................................................ 104
13.3. Named SQL queries ..................................................................................................... 104
13.4. Custom SQL for CUD .................................................................................................. 105
13.5. Custom SQL for loading ............................................................................................... 106
14. Improving performance ........................................................................................................ 107
14.1. Understanding Collection performance .......................................................................... 107
14.1.1. Taxonomy ......................................................................................................... 107
14.1.2. Lists, maps, idbags and sets are the most efficient collections to update ................. 108
14.1.3. Bags and lists are the most efficient inverse collections ........................................ 108
14.1.4. One shot delete .................................................................................................. 108
14.2. Proxies for Lazy Initialization ....................................................................................... 109
14.3. Using batch fetching ..................................................................................................... 110
14.4. Using lazy property fetching ......................................................................................... 111
14.5. Outer join fetching ....................................................................................................... 112
14.6. The Second Level Cache ............................................................................................... 112
14.6.1. Cache mappings ................................................................................................ 113
14.6.2. Strategy: read only ............................................................................................. 113
14.6.3. Strategy: read/write ............................................................................................ 113
14.6.4. Strategy: nonstrict read/write .............................................................................. 113
14.6.5. Strategy: transactional ........................................................................................ 113
14.7. Managing the Session Cache ......................................................................................... 114
14.8. The Query Cache ......................................................................................................... 114
15. Toolset Guide ........................................................................................................................ 116
15.1. Schema Generation ....................................................................................................... 116
15.1.1. Customizing the schema ..................................................................................... 116
15.1.2. Running the tool ................................................................................................ 118
15.1.3. Properties .......................................................................................................... 118
15.1.4. Using Ant ......................................................................................................... 119
15.1.5. Incremental schema updates ............................................................................... 119
15.1.6. Using Ant for incremental schema updates .......................................................... 119
15.2. Code Generation .......................................................................................................... 120
15.2.1. The config file (optional) ................................................................................... 120
15.2.2. The meta attribute .............................................................................................. 121
15.2.3. Basic finder generator ........................................................................................ 123
15.2.4. Velocity based renderer/generator ....................................................................... 123
15.3. Mapping File Generation .............................................................................................. 124
15.3.1. Running the tool ................................................................................................ 125
16. Example: Parent/Child .......................................................................................................... 127
16.1. A note about collections ................................................................................................ 127
16.2. Bidirectional one-to-many ............................................................................................. 127
16.3. Cascading lifecycle ....................................................................................................... 128
16.4. Using cascading update() .............................................................................................. 129
16.5. Conclusion ................................................................................................................... 131
17. Example: Weblog Application ............................................................................................... 132
17.1. Persistent Classes ......................................................................................................... 132
17.2. Hibernate Mappings ..................................................................................................... 133
17.3. Hibernate Code ............................................................................................................ 134
18. Example: Various Mappings ................................................................................................. 138
18.1. Employer/Employee ..................................................................................................... 138
18.2. Author/Work ................................................................................................................ 139
18.3. Customer/Order/Product ............................................................................................... 141
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Hibernate 3.0alpha v
Preface
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 and can significantly reduce development
time otherwise spent with manual data handling in SQL and JDBC.
Hibernates 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 represent-
ation to a graph of objects.
If you are new to Hibernate and Object/Relational Mapping or even Java, please follow these steps:
1. Read Chapter 1, Quickstart with Tomcat for a 30 minute tutorial, using Tomcat.
2. Read Chapter 2, Architecture to understand the environments where Hibernate can be used.
3. Have a look at 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 correct val-
ues for your database. From a command prompt in the distribution directory, type ant eg (using Ant), or
under Windows, type build eg.
4. Use this reference documentation as your primary source of information. Consider reading Hibernate in
Action (http://www.manning.com/bauer) 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 Hibernate in Action.
5. FAQs are answered on the Hibernate website.
6. Third party demos, examples and tutorials are linked on the Hibernate website.
7. The Community Area on the Hibernate website is a good source for design patterns and various integra-
tion solutions (Tomcat, JBoss, Spring, Struts, EJB, etc.).
If you have questions, use the user forum linked on the Hibernate website. We also provide a JIRA issue track-
ings 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 project of the JBoss Professional Open
Source product suite.
Hibernate 3.0alpha vii
Chapter 1. Quickstart with Tomcat
1.1. Getting started with Hibernate
This tutorial explains a setup of Hibernate 3.0 with the Apache Tomcat servlet container for a web-based ap-
plication. Hibernate works well in a managed environment with all major J2EE application servers, or even in
standalone Java applications. The database system used in this tutorial is PostgreSQL 7.4, support for other
database is only a matter of changing the Hibernate SQL dialect configuration.
First, we have to copy all required libraries to the Tomcat installation. We use a separate web context
(webapps/quickstart) for this tutorial, so we've to consider both the global library search path (TOMCAT/com-
mon/lib) and the classloader at the context level in webapps/quickstart/WEB-INF/lib (for JAR files) and
webapps/quickstart/WEB-INF/classes. We refer to both classloader levels as the global classpath and the
context classpath.
Now, copy the libraries to the two classpaths:
1. Copy the JDBC driver for the database to the global classpath. This is required for the DBCP connection
pool software which comes bundled with Tomcat. Hibernate uses JDBC connections to execute SQL on
the database, so you either have to provide pooled JDBC connections or configure Hibernate to use one of
the directly supported pools (C3P0, Proxool). For this tutorial, copy the pg74jdbc3.jar library (for Post-
greSQL 7.4 and JDK 1.4) to the global classloaders path. If you'd like to use a different database, simply
copy its appropriate JDBC driver.
2. Never copy anything else into the global classloader path in Tomcat, or you will get problems with various
tools, including Log4j, commons-logging and others. Always use the context classpath for each web ap-
plication, that is, copy libraries to WEB-INF/lib and your own classes and configuration/property files to
WEB-INF/classes. Both directories are in the context level classpath by default.
3. Hibernate is packaged as a JAR library. The hibernate3.jar file should be copied in the context
classpath together with other classes of the application. Hibernate requires some 3rd party libraries at
runtime, these come bundled with the Hibernate distribution in the lib/ directory; see Table 1.1, “ Hibern-
ate 3rd party libraries ”. Copy the required 3rd party libraries to the context classpath.
Table 1.1. Hibernate 3rd party libraries
Library Description
dom4j (required) Hibernate uses dom4j to parse XML configuration and XML mapping
metadata files.
CGLIB (required) Hibernate uses the code generation library to enhance classes at runtime
(in combination with Java reflection).
Commons Collections, Commons
Logging (required)
Hibernate uses various utility libraries from the Apache Jakarta Com-
mons project.
EHCache (required) Hibernate can use various cache providers for the second-level cache.
EHCache is the default cache provider if not changed in the configura-
tion.
Log4j (optional) Hibernate uses the Commons Logging API, which in turn can use Log4j
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Library Description
as the underlying logging mechanism. If the Log4j library is available in
the context library directory, Commons Logging will use Log4j and the
log4j.properties configuration in the context classpath. An example
properties file for Log4j is bundled with the Hibernate distribution. So,
copy log4j.jar and the configuration file (from src/) to your context
classpath if you want to see whats going on behind the scenes.
Required or not? Have a look at the file lib/README.txt in the Hibernate distribution.
This is an up-to-date list of 3rd party libraries distributed with Hibern-
ate. You will find all required and optional libraries listed there.
We now set up the database connection pooling and sharing in both Tomcat and Hibernate. This means Tomcat
will provide pooled JDBC connections (using its builtin DBCP pooling feature), Hibernate requests theses con-
nections through JNDI. Tomcat binds the connection pool to JNDI, we add a resource declaration to Tomcats
main configuration file, TOMCAT/conf/server.xml:
<Context path="/quickstart" docBase="quickstart">
<Resource name="jdbc/quickstart" scope="Shareable" type="javax.sql.DataSource"/>
<ResourceParams name="jdbc/quickstart">
<parameter>
<name>factory</name>
<value>org.apache.commons.dbcp.BasicDataSourceFactory</value>
</parameter>
<!-- DBCP database connection settings -->
<parameter>
<name>url</name>
<value>jdbc:postgresql://localhost/quickstart</value>
</parameter>
<parameter>
<name>driverClassName</name><value>org.postgresql.Driver</value>
</parameter>
<parameter>
<name>username</name>
<value>quickstart</value>
</parameter>
<parameter>
<name>password</name>
<value>secret</value>
</parameter>
<!-- DBCP connection pooling options -->
<parameter>
<name>maxWait</name>
<value>3000</value>
</parameter>
<parameter>
<name>maxIdle</name>
<value>100</value>
</parameter>
<parameter>
<name>maxActive</name>
<value>10</value>
</parameter>
</ResourceParams>
</Context>
The context we configure in this example is named quickstart, its base is the TOMCAT/webapp/quickstart dir-
ectory. To access any servlets, call the path http://localhost:8080/quickstart in your browser (of course,
adding the name of the servlet as mapped in your web.xml). You may also go ahead and create a simple servlet
now that has an empty process() method.
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Tomcat provides connections now through JNDI at java:comp/env/jdbc/quickstart. If you have trouble get-
ting the connection pool running, refer to the Tomcat documentation. If you get JDBC driver exception mes-
sages, try to setup JDBC connection pool without Hibernate first. Tomcat & JDBC tutorials are available on the
Web.
Your next step is to configure Hibernate. Hibernate has to know how it should obtain JDBC connections We
use Hibernates XML-based configuration. The other approach, using a properties file, is equivalent in features,
but doesn't offer any advantages. We use the XML configuration because it is usually more convenient. The
XML configuration file is placed in the context classpath (WEB-INF/classes), as hibernate.cfg.xml:
<?xml version='1.0' encoding='utf-8'?>
<!DOCTYPE hibernate-configuration PUBLIC
"-//Hibernate/Hibernate Configuration DTD//EN"
"http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd">
<hibernate-configuration>
<session-factory>
<property name="connection.datasource">java:comp/env/jdbc/quickstart</property>
<property name="show_sql">false</property>
<property name="dialect">org.hibernate.dialect.PostgreSQLDialect</property>
<!-- Mapping files -->
<mapping resource="Cat.hbm.xml"/>
</session-factory>
</hibernate-configuration>
We turn logging of SQL commands off and tell Hibernate what database SQL dialect is used and where to get
the JDBC connections (by declaring the JNDI address of the Tomcat bound pool). The dialect is a required set-
ting, databases differ in their interpretation of the SQL "standard". Hibernate will take care of the differences
and comes bundled with dialects for all major commercial and open source databases.
A SessionFactory is Hibernate's concept of a single datastore, multiple databases can be used by creating mul-
tiple XML configuration files and creating multiple Configuration and SessionFactory objects in your ap-
plication.
The last element of the hibernate.cfg.xml declares Cat.hbm.xml as the name of a Hibernate XML mapping
file for the persistent class Cat. This file contains the metadata for the mapping of the POJO class Cat to a dat-
base table (or tables). We'll come back to that file soon. Let's write the POJO class first and then declare the
mapping metadata for it.
1.2. First persistent class
Hibernate works best with the Plain Old Java Objects (POJOs, sometimes called Plain Ordinary Java Objects)
programming model for persistent classes. A POJO is much like a JavaBean, with properties of the class ac-
cessible via getter and setter methods, shielding the internal representation from the publicly visible interface
(Hibernate can also access fields directly, if needed):
package org.hibernate.examples.quickstart;
public class Cat {
private String id;
private String name;
private char sex;
private float weight;
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public Cat() {
}
public String getId() {
return id;
}
private void setId(String id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public char getSex() {
return sex;
}
public void setSex(char sex) {
this.sex = sex;
}
public float getWeight() {
return weight;
}
public void setWeight(float weight) {
this.weight = weight;
}
}
Hibernate is not restricted in its usage of property types, all Java JDK types and primitives (like String, char
and Date) can be mapped, including classes from the Java collections framework. You can map them as values,
collections of values, or associations to other entities. The id is a special property that represents the database
identifer (primary key) of that class, it is highly recommended for entities like a Cat. Hibernate can use identifi-
ers only internally, but we would lose some of the flexibility in our application architecture.
No special interface has to be implemented for persistent classes nor do you have to subclass from a special
root persistent class. Hibernate also doesn't require any build time processing, such as byte-code manipulation,
it relies solely on Java reflection and runtime class enhancement (through CGLIB). So, without any depend-
ency of the POJO class on Hibernate, we can map it to a database table.
1.3. Mapping the cat
The Cat.hbm.xml mapping file contains the metadata required for the object/relational mapping. The metadata
includes declaration of persistent classes and the mapping of properties (to columns and foreign key relation-
ships to other entities) to database tables.
<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping>
<class name="org.hibernate.examples.quickstart.Cat" table="CAT">
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<!-- A 32 hex character is our surrogate key. It's automatically
generated by Hibernate with the UUID pattern. -->
<id name="id" type="string" unsaved-value="null" >
<column name="CAT_ID" sql-type="char(32)" not-null="true"/>
<generator class="uuid.hex"/>
</id>
<!-- A cat has to have a name, but it shouldn' be too long. -->
<property name="name">
<column name="NAME" length="16" not-null="true"/>
</property>
<property name="sex"/>
<property name="weight"/>
</class>
</hibernate-mapping>
Every persistent class should have an identifer attribute (actually, only classes representing entities, not depend-
ent value-typed classes, which are mapped as components of an entity). This property is used to distinguish per-
sistent objects: Two cats are equal if catA.getId().equals(catB.getId()) is true, this concept is called data-
base identity. Hibernate comes bundled with various identifer generators for different scenarios (including nat-
ive generators for database sequences, hi/lo identifier tables, and application assigned identifiers). We use the
UUID generator (only recommended for testing, as integer surrogate keys generated by the database should be
prefered) and also specify the column CAT_ID of the table CAT for the Hibernate generated identifier value (as a
primary key of the table).
All other properties of Cat are mapped to the same table. In the case of the name property, we mapped it with an
explicit database column declaration. This is especially useful when the database schema is automatically gen-
erated (as SQL DDL statements) from the mapping declaration with Hibernate's SchemaExport tool. All other
properties are mapped using Hibernate's default settings, which is what you need most of the time. The table
CAT in the database looks like this:
Column | Type | Modifiers
--------+-----------------------+-----------
cat_id | character(32) | not null
name | character varying(16) | not null
sex | character(1) |
weight | real |
Indexes: cat_pkey primary key btree (cat_id)
You should now create this table in your database manually, and later read Chapter 15, Toolset Guide if you
want to automate this step with the hbm2ddl tool. This tool can create a full SQL DDL, including table defini-
tion, custom column type constraints, unique constraints and indexes.
1.4. Playing with cats
We're now ready to start Hibernate's Session. It is the persistence manager, we use it to store and retrieve Cats
to and from the database. But first, we've to get a Session (Hibernate's unit-of-work) from the
SessionFactory:
SessionFactory sessionFactory =
new Configuration().configure().buildSessionFactory();
The call to configure() loads the hibernate.cfg.xml configuration file and initializes the Configuration in-
stance. You can set other properties (and even change the mapping metadata) by accessing the Configuration
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before you build the SessionFactory (it is immutable). Where do we create the SessionFactory and how can
we access it in our application?
A SessionFactory is usually only build once, e.g. at startup with a load-on-startup servlet. This also means
you should not keep it in an instance variable in your servlets, but in some other location. Furthermore, we need
some kind of Singleton, so we can access the SessionFactory easily in application code. The approach shown
next solves both problems: startup configuration and easy access to a SessionFactory.
We implement a HibernateUtil helper class:
import org.hibernate.*;
import org.hibernate.cfg.*;
public class HibernateUtil {
private static Log log = LogFactory.getLog(HibernateUtil.class);
private static final SessionFactory sessionFactory;
static {
try {
// Create the SessionFactory
sessionFactory = new Configuration().configure().buildSessionFactory();
} catch (Throwable ex) {
// Make sure you log the exception, as it might be swallowed
log.error("Initial SessionFactory creation failed.", ex);
throw new ExceptionInInitializerError(ex);
}
}
public static final ThreadLocal session = new ThreadLocal();
public static Session currentSession() throws HibernateException {
Session s = (Session) session.get();
// Open a new Session, if this Thread has none yet
if (s == null) {
s = sessionFactory.openSession();
session.set(s);
}
return s;
}
public static void closeSession() throws HibernateException {
Session s = (Session) session.get();
session.set(null);
if (s != null)
s.close();
}
}
This class does not only take care of the SessionFactory with its static initializer, but also has a ThreadLocal
variable which holds the Session for the current thread. Make sure you understand the Java concept of a
thread-local variable before you try to use this helper. A more complex and powerful HibernateUtil class can
be found in CaveatEmptor, http://caveatemptor.hibernate.org/
A SessionFactory is threadsafe, many threads can access it concurrently and request Sessions. A Session is a
non-threadsafe object that represents a single unit-of-work with the database. Sessions are opened by a Ses-
sionFactory and are closed when all work is completed. An example in your servlet's process() method
might look like this (sans exception handling):
Session session = HibernateUtil.currentSession();
Transaction tx = session.beginTransaction();
Cat princess = new Cat();
Quickstart with Tomcat
Hibernate 3.0alpha 6
princess.setName("Princess");
princess.setSex('F');
princess.setWeight(7.4f);
session.save(princess);
tx.commit();
HibernateUtil.closeSession();
In a Session, every database operation occurs inside a transaction that isolates the database operations (even
read-only operations). We use Hibernates Transaction API to abstract from the underlying transaction strategy
(in our case, JDBC transactions). This allows our code to be deployed with container-managed transactions
(using JTA) without any changes.
Note that you may call HibernateUtil.currentSession(); as many times as you like, you will always get the
current Session of this thread. You have to make sure the Session is closed after your unit-of-work completes,
either in your servlet code or in a servlet filter before the HTTP response is send. The nice side effect of the
second option is easy lazy initialization: the Session is still open when the view is rendered, so Hibernate can
load unitialized objects while you navigate the current object graph.
Hibernate has various methods that can be used to retrieve objects from the database. The most flexible way is
using the Hibernate Query Language (HQL), which is an easy to learn and powerful object-oriented extension
to SQL:
Transaction tx = session.beginTransaction();
Query query = session.createQuery("select c from Cat as c where c.sex = :sex");
query.setCharacter("sex", 'F');
for (Iterator it = query.iterate(); it.hasNext();) {
Cat cat = (Cat) it.next();
out.println("Female Cat: " + cat.getName() );
}
tx.commit();
Hibernate also offers an object-oriented query by criteria API that can be used to formulate type-safe queries.
Hibernate of course uses PreparedStatements and parameter binding for all SQL communication with the
database. You may also use Hibernate's direct SQL query feature or get a plain JDBC connection from a Ses-
sion in rare cases.
1.5. Finally
We only scratched the surface of Hibernate in this small tutorial. Please note that we don't include any servlet
specific code in our examples. You have to create a servlet yourself and insert the Hibernate code as you see fit.
Keep in mind that Hibernate, as a data access layer, is tightly integrated into your application. Usually, all other
layers depent on the persistence mechanism. Make sure you understand the implications of this design.
For a more complex application design, see http://caveatemptor.hibernate.org/
Quickstart with Tomcat
Hibernate 3.0alpha 7
Chapter 2. Architecture
2.1. Overview
A (very) high-level view of the Hibernate architecture:
This diagram shows Hibernate using the database and configuration data to provide persistence services (and
persistent objects) to the application.
We would like to show a more detailed view of the runtime architecture. Unfortunately, Hibernate is flexible
and supports several approaches. We will show the two extremes. The "lite" architecture has the application
provide its own JDBC connections and manage its own transactions. This approach uses a minimal subset of
Hibernate's APIs:
The "full cream" architecture abstracts the application away from the underlying JDBC/JTA APIs and lets Hi-
Hibernate 3.0alpha 8
bernate take care of the details.
Heres some definitions of the objects in the diagrams:
SessionFactory (org.hibernate.SessionFactory)
A threadsafe (immutable) cache of compiled mappings for a single database. A factory for Session and a
client of ConnectionProvider. Might 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. Wraps a JDBC connection. Factory for Transaction. Holds a mandatory (first-level) cache of per-
sistent objects, 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 might be ordin-
ary JavaBeans/POJOs, the only special thing about them is that they are currently associated with (exactly
one) Session. As soon as the Session is closed, they will be detached and free to use in any application
layer (e.g. 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 instan-
tiated 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.
Abstracts application from underlying JDBC, JTA or CORBA transaction. A Session might span several
Transactions in some cases. However, transaction demarcation, either using the underlying API or Trans-
action, is never optional!
Architecture
Hibernate 3.0alpha 9
ConnectionProvider (org.hibernate.connection.ConnectionProvider)
(Optional) A factory for (and pool of) JDBC connections. Abstracts application from underlying Data-
source or DriverManager. Not exposed to application, but can be extended/implemented by the developer.
TransactionFactory (org.hibernate.TransactionFactory)
(Optional) A factory for Transaction instances. Not exposed to the application, but can be extended/
implemented by the developer.
Extension Interfaces
Hibernate offers many optional extension interfaces you can implement to customize the behavior of your
persistence layer. See the API documentation for details.
Given a "lite" architecture, the application bypasses the Transaction/TransactionFactory and/or Connec-
tionProvider APIs to talk to JTA or JDBC directly.
2.2. Instance states
An instance of a persistent classes may be in one of three different states, which are defined with respect to a
persistence context. The Hibernate Session object is the persistence context:
transient
The instance is not, and has never been associated with any persistence context. It has no persistent identity
(primary key value).
persistent
The instance is currently associated with a persistence context. It has a persistent identity (primary key
value) and, perhaps, a corresponding row in the database. For a particular persistence context, Hibernate
guarantees that persistent identity is equivalent to Java identity (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, perhaps, a corrsponding row in the data-
base. For detached instances, Hibernate makes no guarantees about the relationship between persistent
identity and Java identity.
2.3. JMX Integration
JMX is the J2EE standard for management of Java components. Hibernate may be managed via a JMX stand-
ard MBean but because most application servers do not yet support JMX, Hibernate also affords some non-
standard configuration mechanisms.
Please see the Hibernate website for more information on how to configure Hibernate to run as a JMX compon-
ent inside JBoss. Integration with other JMX containers is also possible.
TODO: More documentation about JMX integration.
2.4. JCA Support
Hibernate may also be configured as a JCA connector. Please see the website for more details. Please note that
Architecture
Hibernate 3.0alpha 10
Hibernate JCA support is still considered experimental.
TODO: More documentation about JCA integration.
Architecture
Hibernate 3.0alpha 11
Chapter 3. Configuration
Because Hibernate is designed to operate in many different environments, there are a large number of configur-
ation parameters. Fortunately, most have sensible default values and Hibernate is distributed with an example
hibernate.properties file in etc/ that shows the various options. You usually only have to put that file in
your classpath and customize it.
3.1. Programmatic configuration
An instance of org.hibernate.cfg.Configuration represents an entire set of mappings of an application's
Java types to an SQL database. The Configuration is used to build an (immutable) SessionFactory. The map-
pings are compiled from various XML mapping files.
You may obtain a Configuration instance by instantiating it directly. Heres an example of setting up a data-
store from mappings defined in two XML mapping files (in the classpath):
Configuration cfg = new Configuration()
.addFile("Item.hbm.xml")
.addFile("Bid.hbm.xml");
An alternative (sometimes better) way is to let Hibernate load a mapping file using getResourceAsStream():
Configuration cfg = new Configuration()
.addClass(org.hibernate.auction.Item.class)
.addClass(org.hibernate.auction.Bid.class);
Then Hibernate will look for mapping files named /org/hibernate/auction/Item.hbm.xml and /
org/hibernate/auction/Bid.hbm.xml in the classpath. This approach eliminates any hardcoded filenames.
A Configuration also specifies various optional properties:
Properties props = new Properties();
...
Configuration cfg = new Configuration()
.addClass(org.hibernate.auction.Item.class)
.addClass(org.hibernate.auction.Bid.class)
.setProperties(props);
A Configuration is intended as a startup-time object, to be discarded once a SessionFactory is built.
Instead of adding mapping files and setting properties programatially, you may also place Hibernate configura-
tion files in your classpath, as you will see later.
3.2. Obtaining a SessionFactory
When all mappings have been parsed by the Configuration, the application must obtain a factory for Session
instances. This factory is intended to be shared by all application threads:
SessionFactory sessions = cfg.buildSessionFactory();
However, Hibernate does allow your application to instantiate more than one SessionFactory. This is useful if
you are using more than one database.
Hibernate 3.0alpha 12
3.3. User provided JDBC connection
A SessionFactory may open a Session on a user-provided JDBC connection. This design choice frees the ap-
plication to obtain JDBC connections wherever it pleases:
java.sql.Connection conn = datasource.getConnection();
Session session = sessions.openSession(conn);
// do some data access work
The application must be careful not to open two concurrent Sessions on the same JDBC connection!
We don't recommend user-provided JDBC connections, as Hibernate will disable caching (it doesn't know what
else you might have executed on the given connection) and one of the following options is usually more appro-
priate.
3.4. Hibernate provided JDBC connection
Alternatively, you can have the SessionFactory open connections for you. The SessionFactory must be
provided with JDBC connection properties in one of the following ways:
1. Pass an instance of java.util.Properties to Configuration.setProperties().
2. Place hibernate.properties in a root directory of the classpath.
3. Set System properties using java -Dproperty=value.
4. Include <property> elements in hibernate.cfg.xml (discussed later).
If you take this approach, opening a Session is as simple as:
Session session = sessions.openSession(); // open a new Session
// do some data access work, a JDBC connection will be used on demand
All Hibernate property names and semantics are defined on the class org.hibernate.cfg.Environment. We
will now describe the most important settings for JDBC connection configuration.
Hibernate will obtain (and pool) connections using java.sql.DriverManager if you set the following proper-
ties:
Table 3.1. Hibernate JDBC Properties
Property name Purpose
hibernate.connection.driver_class jdbc driver class
hibernate.connection.url jdbc URL
hibernate.connection.username database user
hibernate.connection.password database user password
hibernate.connection.pool_size maximum number of pooled connections
Hibernate's own connection pooling algorithm is however quite rudimentary. It is intended to help you get star-
ted and is not intended for use in a production system or even for performance testing. Use a third party pool
Configuration
Hibernate 3.0alpha 13
for best performance and stability, i.e., replace the hibernate.connection.pool_size property with connec-
tion pool specific settings. This will turn off Hibernate's internal pool.
C3P0 is an open source JDBC connection pool distributed along with Hibernate in the lib directory. Hibernate
will use its C3P0ConnectionProvider for connection pooling if you set hibernate.c3p0.* properties. If you'd
like to use Proxool refer to the packaged hibernate.properties and the Hibernate web site for more informa-
tion.
This is an example using C3P0:
hibernate.connection.driver_class = org.postgresql.Driver
hibernate.connection.url = jdbc:postgresql://localhost/mydatabase
hibernate.connection.username = myuser
hibernate.connection.password = secret
hibernate.c3p0.min_size=5
hibernate.c3p0.max_size=20
hibernate.c3p0.timeout=1800
hibernate.c3p0.max_statements=50
hibernate.dialect = org.hibernate.dialect.PostgreSQLDialect
For use inside an application server, Hibernate may obtain connections from a javax.sql.Datasource re-
gistered in JNDI. Set the following properties:
Table 3.2. Hibernate Datasource Properties
Propery name Purpose
hibernate.connection.datasource datasource JNDI name
hibernate.jndi.url URL of the JNDI provider (optional)
hibernate.jndi.class class of the JNDI InitialContextFactory (optional)
hibernate.connection.username database user (optional)
hibernate.connection.password database user password (optional)
This is an example using an application server provided JNDI datasource:
hibernate.connection.datasource = java:/comp/env/jdbc/MyDB
hibernate.transaction.factory_class = \
org.hibernate.transaction.JTATransactionFactory
hibernate.transaction.manager_lookup_class = \
org.hibernate.transaction.JBossTransactionManagerLookup
hibernate.dialect = \
org.hibernate.dialect.PostgreSQLDialect
JDBC connections obtained from a JNDI datasource will automatically participate in the container-managed
transactions of the application server.
Arbitrary connection properties may be given by prepending "hibernate.connnection" to the property name.
For example, you may specify a charSet using hibernate.connnection.charSet.
You may define your own plugin strategy for obtaining JDBC connections by implementing the interface
org.hibernate.connection.ConnectionProvider. You may select a custom implementation by setting hi-
bernate.connection.provider_class.
Configuration
Hibernate 3.0alpha 14
3.5. Optional configuration properties
There are a number of other properties that control the behaviour of Hibernate at runtime. All are optional and
have reasonable default values.
System-level properties can only be set via java -Dproperty=value or be defined in hibernate.properties
and not with an instance of Properties passed to the Configuration. They are also not available in the hi-
bernate.cfg.xml file, discusse later.
Table 3.3. Hibernate Configuration Properties
Property name Purpose
hibernate.dialect The classname of a Hibernate Dialect - enables cer-
tain platform dependent features.
eg. full.classname.of.Dialect
hibernate.default_schema Qualify unqualified tablenames with the given
schema/tablespace in generated SQL.
eg. SCHEMA_NAME
hibernate.default_catalog Qualify unqualified tablenames with the given cata-
log in generated SQL.
eg. CATALOG_NAME
hibernate.session_factory_name The SessionFactory will be automatically bound to
this name in JNDI after it has been created.
eg. jndi/composite/name
hibernate.max_fetch_depth Set 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.
eg. recommended values between 0 and 3
hibernate.jdbc.fetch_size A non-zero value determines the JDBC fetch size
(calls Statement.setFetchSize()).
hibernate.jdbc.batch_size A non-zero value enables use of JDBC2 batch up-
dates by Hibernate.
eg. recommended values between 5 and 30
hibernate.jdbc.batch_versioned_data Set this property to true if your JDBC driver returns
correct row counts from executeBatch() (it is usu-
ally safe to turn this option on). Hibernate will then
use batched DML for automatically versioned data.
Defaults to false.
eg. true | false
hibernate.jdbc.factory_class Select a custom Batcher. Most applications will not
need this configuration property.
Configuration
Hibernate 3.0alpha 15
Property name Purpose
eg. classname.of.Batcher
hibernate.jdbc.use_scrollable_resultset Enables use of JDBC2 scrollable resultsets by Hi-
bernate. This property is only necessary when using
user supplied JDBC connections, Hibernate uses con-
nection metadata otherwise.
eg. true | false
hibernate.jdbc.use_streams_for_binary Use streams when writing/reading binary or serial-
izable types to/from JDBC (system-level property).
eg. true | false
hibernate.jdbc.use_get_generated_keys Enable use of JDBC3 PreparedState-
ment.getGeneratedKeys() to retrieve natively gener-
ated keys after insert. Requires JDBC3+ driver and
JRE1.4+, set to false if your driver has problems with
the Hibernate identifier generators. By default, tries to
determine the driver capabilites using connection
metadata.
eg. true|false
hibernate.cglib.use_reflection_optimizer Enables use of CGLIB instead of runtime reflection
(System-level property). Reflection can sometimes be
useful when troubleshooting, note that Hibernate al-
ways requires CGLIB even if you turn off the optim-
izer. You can not set this property in hibern-
ate.cfg.xml.
eg. true | false
hibernate.jndi.<propertyName> Pass the property propertyName to the JNDI Ini-
tialContextFactory.
hibernate.connection.isolation Set the JDBC transaction isolation level. Check
java.sql.Connection for meaningful values but note
that most databases do not support all isolation levels.
eg. 1, 2, 4, 8
hibernate.connection.<propertyName> Pass the JDBC property propertyName to DriverMan-
ager.getConnection().
hibernate.connection.provider_class The classname of a custom ConnectionProvider.
eg. classname.of.ConnectionProvider
hibernate.cache.provider_class The classname of a custom CacheProvider.
eg. classname.of.CacheProvider
hibernate.cache.use_minimal_puts Optimize second-level cache operation to minimize
writes, at the cost of more frequent reads (useful for
clustered caches).
Configuration
Hibernate 3.0alpha 16
Property name Purpose
eg. true|false
hibernate.cache.use_query_cache Enable the query cache, individual queries still have
to be set cachable.
eg. true|false
hibernate.cache.query_cache_factory The classname of a custom QueryCache interface, de-
faults to the built-in StandardQueryCache.
eg. classname.of.QueryCache
hibernate.cache.region_prefix A prefix to use for second-level cache region names.
eg. prefix
hibernate.transaction.factory_class The classname of a TransactionFactory to use with
Hibernate Transaction API (defaults to JDBCTrans-
actionFactory).
eg. classname.of.TransactionFactory
jta.UserTransaction A JNDI name used by JTATransactionFactory to
obtain the JTA UserTransaction from the applica-
tion server.
eg. jndi/composite/name
hibernate.transaction.manager_lookup_class The classname of a TransactionManagerLookup - re-
quired when JVM-level caching is enabled in a JTA
environment.
eg. classname.of.TransactionManagerLookup
hibernate.query.substitutions Mapping from tokens in Hibernate queries to SQL
tokens (tokens might be function or literal names, for
example).
eg. hqlLiteral=SQL_LITERAL, hqlFunc-
tion=SQLFUNC
hibernate.show_sql Write all SQL statements to console.
eg. true | false
hibernate.hbm2ddl.auto Automatically export 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.
eg. update | create | create-drop
hibernate.generate_statistics If enabled, Hibernate will collect statistics useful for
performance tuning.
eg. true | false
Configuration
Hibernate 3.0alpha 17
Property name Purpose
hibernate.use_identifer_rollback If enabled, generated identifier properties will be re-
set to default values when objects are deleted.
eg. true | false
hibernate.use_sql_comments If turned on, Hibernate will generate comments inside
the SQL, for easier debugging, defaults to false.
eg. true | false
3.5.1. SQL Dialects
You should always set the hibernate.dialect property to the correct org.hibernate.dialect.Dialect sub-
class for your database. This is not strictly essential unless you wish to use native or sequence primary key
generation or pessimistic locking (with, eg. Session.lock() or Query.setLockMode()). However, if you spe-
cify a dialect, Hibernate will use sensible defaults for some of the other properties listed above, saving you the
effort of specifying them manually.
Table 3.4. Hibernate SQL Dialects (hibernate.dialect)
RDBMS Dialect
DB2 org.hibernate.dialect.DB2Dialect
DB2 AS/400 org.hibernate.dialect.DB2400Dialect
DB2 OS390 org.hibernate.dialect.DB2390Dialect
PostgreSQL org.hibernate.dialect.PostgreSQLDialect
MySQL org.hibernate.dialect.MySQLDialect
Oracle (any version) org.hibernate.dialect.OracleDialect
Oracle 9/10g org.hibernate.dialect.Oracle9Dialect
Sybase org.hibernate.dialect.SybaseDialect
Sybase Anywhere org.hibernate.dialect.SybaseAnywhereDialect
Microsoft SQL Server org.hibernate.dialect.SQLServerDialect
SAP DB org.hibernate.dialect.SAPDBDialect
Informix org.hibernate.dialect.InformixDialect
HypersonicSQL org.hibernate.dialect.HSQLDialect
Ingres org.hibernate.dialect.IngresDialect
Progress org.hibernate.dialect.ProgressDialect
Mckoi SQL org.hibernate.dialect.MckoiDialect
Interbase org.hibernate.dialect.InterbaseDialect
Pointbase org.hibernate.dialect.PointbaseDialect
Configuration
Hibernate 3.0alpha 18
RDBMS Dialect
FrontBase org.hibernate.dialect.FrontbaseDialect
Firebird org.hibernate.dialect.FirebirdDialect
3.5.2. Outer Join Fetching
If your database supports ANSI, Oracle or Sybase style outer joins, outer join fetching will often increase per-
formance by limiting the number of round trips to and from the database (at the cost of possibly more work per-
formed by the database itself). Outer join fetching allows a whole graph of objects connected by many-to-one,
one-to-many, many-to-many and one-to-one associations to be retrieved in a single SQL SELECT.
By default, the fetched graph when loading an objects ends at leaf objects, collections, objects with proxies, or
where circularities occur in the case of *-to-one associations. Hibernate will however execute an immediate ad-
ditional SELECT for any persistent collection (we recommend that you turn on lazy loading for all collection
mappings).
For a particular association, fetching may be enabled or disabled (and the default behaviour overridden) by set-
ting the outer-join attribute in the XML mapping.
Outer join fetching may be disabled globally by setting the property hibernate.max_fetch_depth to 0. A set-
ting of 1 or higher enables outer join fetching for all one-to-one and many-to-one associations, which are, also
by default, set to auto outer join. However, one-to-many associations and collections are never fetched with an
outer-join, unless explicitly declared for each particular association. This behavior can also be overriden at
runtime with Hibernate queries. See the query chapters in the documentation for more details.
3.5.3. Binary Streams
Oracle limits the size of byte arrays that may be passed to/from its JDBC driver. If you wish to use large in-
stances of binary or serializable type, you should enable hibernate.jdbc.use_streams_for_binary. This
is a system-level setting only.
3.5.4. Second-level and query cache
The properties prefixed by hibernate.cache allow you to use a process or cluster scoped second-level cache
system with Hibernate. See the "Performance" chapter for more details.
3.5.5. Transaction strategy configuration
If you wish to use the Hibernate Transaction API instead of directly calling a particular transaction API, you
must specify a factory class for Transaction instances by setting the property hibern-
ate.transaction.factory_class. The Transaction API hides the underlying transaction mechanism and al-
lows Hibernate code to run in managed and non-managed environments.
There are two standard (built-in) choices:
org.hibernate.transaction.JDBCTransactionFactory
delegates to database (JDBC) transactions (default)
org.hibernate.transaction.JTATransactionFactory
Configuration
Hibernate 3.0alpha 19
delegates to JTA (if an existing transaction is underway, the Session performs its work in that context, oth-
erwise a new transaction is started)
You may also define your own transaction strategies (for a CORBA transaction service, for example).
If you wish to use a second-level cache for mutable data in a JTA environment, you must specify a strategy for
obtaining the JTA TransactionManager, since J2EE does not standardize a single mechanism:
Table 3.5. JTA TransactionManagers
Transaction Factory Application Server
org.hibernate.transaction.JBossTransactionManagerLookup JBoss
org.hibernate.transaction.WeblogicTransactionManagerLookup Weblogic
org.hibernate.transaction.WebSphereTransactionManagerLookup WebSphere
org.hibernate.transaction.OrionTransactionManagerLookup Orion
org.hibernate.transaction.ResinTransactionManagerLookup Resin
org.hibernate.transaction.JOTMTransactionManagerLookup JOTM
org.hibernate.transaction.JOnASTransactionManagerLookup JOnAS
org.hibernate.transaction.JRun4TransactionManagerLookup JRun4
org.hibernate.transaction.BESTransactionManagerLookup Borland ES
3.5.6. JNDI-bound SessionFactory
A JNDI bound Hibernate SessionFactory can simplify the lookup of the factory and the creation of new Ses-
sions. Note that this is not related to a JNDI bound Datasource in a managed environment.
If you wish to have the SessionFactory bound to a JNDI namespace, specify a name (eg.
java:hibernate/SessionFactory) using the property hibernate.session_factory_name. If this property is
omitted, the SessionFactory will not be bound to JNDI. (This is especially useful in environments with a read-
only JNDI default implementation, eg. Tomcat.)
When binding the SessionFactory to JNDI, Hibernate will use the values of hibernate.jndi.url, hibern-
ate.jndi.class to instantiate an initial context. If they are not specified, the default InitialContext will be
used.
Hibernate will automatically place the SessionFactory in JNDI after you call cfg.buildSessionFactory().
This means you will at least have this call in some startup code (or utility class) in your application.
If you use a JNDI SessionFactory, an EJB or any other class may obtain the SessionFactory using a JNDI
lookup. Note that this setup is not neccessary if you use the HibernateUtil helper class introduced in chapter
1, which acts as a Singleton registry.
3.5.7. Query Language Substitution
You may define new Hibernate query tokens using hibernate.query.substitutions. For example:
hibernate.query.substitutions true=1, false=0
Configuration
Hibernate 3.0alpha 20
would cause the tokens true and false to be translated to integer literals in the generated SQL.
hibernate.query.substitutions toLowercase=LOWER
would allow you to rename the SQL LOWER function.
3.5.8. Hibernate statistics
If you enable hibernate.generate_statistics, Hibernate will expose a number of metrics that are useful
when tuning a running system via SessionFactory.getStatistics(). Hibernate can even be configured to ex-
pose these statistics via JMX (see the website for details).
3.6. Logging
Hibernate logs various events using Apache commons-logging.
The commons-logging service will direct output to either Apache Log4j (if you include log4j.jar in your
classpath) or JDK1.4 logging (if running under JDK1.4 or above). You may download Log4j from ht-
tp://jakarta.apache.org. To use Log4j you will need to place a log4j.properties file in your classpath, an
example properties file is distributed with Hibernate in the src/ directory.
We strongly recommend that you familiarize yourself with Hibernate's log messages. A lot of work has been
put into making the Hibernate log as detailed as possible, without making it unreadable. It is an essential
troubleshooting device. Also don't forget to enable SQL logging as described above (hibernate.show_sql), it
is your first step when looking for performance problems.
3.7. Implementing a NamingStrategy
The interface org.hibernate.cfg.NamingStrategy allows you to specify a "naming standard" for database ob-
jects and schema elements.
You may provide rules for automatically generating database identifiers from Java identifiers or for processing
"logical" column and table names given in the mapping file into "physical" table and column names. This fea-
ture helps reduce the verbosity of the mapping document, eliminating repetitive noise (TBL_ prefixes, for ex-
ample). The default strategy used by Hibernate is quite minimal.
You may specify a different strategy by calling Configuration.setNamingStrategy() before adding map-
pings:
SessionFactory sf = new Configuration()
.setNamingStrategy(ImprovedNamingStrategy.INSTANCE)
.addFile("Item.hbm.xml")
.addFile("Bid.hbm.xml")
.buildSessionFactory();
org.hibernate.cfg.ImprovedNamingStrategy is a built-in strategy that might be a useful starting point for
some applications.
3.8. XML Configuration File
Configuration
Hibernate 3.0alpha 21
An alternative approach to configuration is to specify a full configuration in a file named hibernate.cfg.xml.
This file can be used as a replacement for the hibernate.properties file or, if both are present, to override
properties.
The XML configuration file is by default expected to be in the root o your CLASSPATH. Here is an example:
<?xml version='1.0' encoding='utf-8'?>
<!DOCTYPE hibernate-configuration PUBLIC
"-//Hibernate/Hibernate Configuration DTD//EN"
"http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd">
<hibernate-configuration>
<!-- a SessionFactory instance listed as /jndi/name -->
<session-factory
name="java:hibernate/SessionFactory">
<!-- properties -->
<property name="connection.datasource">java:/comp/env/jdbc/MyDB</property>
<property name="dialect">org.hibernate.dialect.MySQLDialect</property>
<property name="show_sql">false</property>
<property name="transaction.factory_class">
org.hibernate.transaction.JTATransactionFactory
</property>
<property name="jta.UserTransaction">java:comp/UserTransaction</property>
<!-- mapping files -->
<mapping resource="org/hibernate/auction/Item.hbm.xml"/>
<mapping resource="org/hibernate/auction/Bid.hbm.xml"/>
</session-factory>
</hibernate-configuration>
As you can see, the advantage of this approach is the externalization of the mapping file names to configura-
tion. The hibernate.cfg.xml is also more convenient once you have to tune the Hibernate cache.
Configuring Hibernate is then as simple as
SessionFactory sf = new Configuration().configure().buildSessionFactory();
You can pick a different XML configuration file using
SessionFactory sf = new Configuration()
.configure("catdb.cfg.xml")
.buildSessionFactory();
Configuration
Hibernate 3.0alpha 22
Chapter 4. Persistent Classes
Persistent classes are classes in an application that implement the entities of the business problem (e.g. Custom-
er and Order in an E-commerce application). Persistent classes have, as the name implies, transient and also
persistent instance stored in the database.
Hibernate works best if these classes follow some simple rules, also known as the Plain Old Java Object
(POJO) programming model. However, Hibernate3 allows you to express a domain model in other ways: using
trees of Map instances, for example.
4.1. A simple POJO example
Most Java applications require a persistent class representing felines.
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;
}
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;
}
Hibernate 3.0alpha 23
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);
}
}
There are four main rules to follow here:
4.1.1. Declare accessors and mutators for persistent fields
Cat declares accessor methods for all its persistent fields. Many other ORM tools directly persist instance vari-
ables. We believe it is far better to decouple this implementation detail from the persistence mechanism. Hi-
bernate persists JavaBeans style properties, and recognizes method names of the form getFoo, isFoo and set-
Foo. You may however switch to direct field access for particular properties, if needed.
Properties need not be declared public - Hibernate can persist a property with a default, protected or private
get / set pair.
4.1.2. Implement a no-argument constructor
Cat has a no-argument constructor. All persistent classes must have a default constructor (which may be non-
public) so Hibernate can instantiate them using Constructor.newInstance(). We recommend having a con-
structor with at least package visibility for runtime proxy generation in Hibernate.
4.1.3. Provide an identifier property (optional)
Cat has a property called id. This property holds 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 even
use a user-defined class with properties of these types - see the section on composite identifiers later.)
The identifier property is optional. You can leave it off and let Hibernate keep track of object identifiers intern-
ally. However, for many applications it is still a good (and very popular) design decision.
What's more, some functionality is available only to classes which declare an identifier property:
Persistent Classes
Hibernate 3.0alpha 24
• Transitive reattachment for detached objects (cascade update) - see "Lifecycle Objects"
• Session.saveOrUpdate()
We recommend you declare consistently-named identifier properties on persistent classes. We further recom-
mend that you use a nullable (ie. non-primitive) type.
4.1.4. Prefer non-final classes (optional)
A central feature of Hibernate, proxies, depends upon the persistent class being either non-final, or the imple-
mentation of an interface that declares all public methods.
You can persist final classes that do not implement an interface with Hibernate, but you won't be able to use
proxies for lazy associationfetching - which will limit your options for performance tuning.
4.2. Implementing inheritance
A subclass must also observe the first and second rules. It inherits its identifier property from the superclass,
Cat.
package eg;
public class DomesticCat extends Cat {
private String name;
public String getName() {
return name;
}
protected void setName(String name) {
this.name=name;
}
}
4.3. 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 asso-
ciations) 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. So as soon as we mix instances retrieved in different sessions, we must implement equals() and
hashCode() if we 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, they are therefore equal (if both are added to a
Set, we will only have one element in the Set). Unfortunately, we can't use that approach with generated iden-
tifiers! Hibernate will only assign identifier values to objects that are persistent, a newly created instance will
not have any identifier value! We recommend implementing equals() and hashCode() using Business key
equality.
Business key equality means that the equals() method compares only the properties that form the business
key, a key that would identify our instance in the real world (a natural candidate key):
public class Cat {
Persistent Classes
Hibernate 3.0alpha 25
...
public boolean equals(Object other) {
if (this == other) return true;
if ( !(other instanceof Cat) ) return false;
final Cat cat = (Cat) other;
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;
}
}
4.4. Dynamic models
Hibernate also supports dynamic domain models, using Maps of Maps. With this approach, you don't write per-
sistent classes, a Hibernate mapping file for each "entity" is sufficient:
<hibernate-mapping>
<dynamic-class entity-name="TestMap">
<id name="id"
type="long"
column="ID">
<generator class="sequence"/>
</id>
<property name="name"
column="NAME"
type="string"/>
<property name="address"
column="ADDRESS"
type="string"/>
<many-to-one name="parent"
column="PARENT_ID"
class="TestMap"/>
<bag name="children"
inverse="true"
lazy="false"
cascade="all">
<key column="PARENT_ID"/>
<one-to-many class="TestMap"/>
</bag>
</dynamic-class>
</hibernate-mapping>
At runtime, you just instantiate HashMaps and use the Hibernate entity name to refer to a particular type.
Session s = openSession();
Transaction t = s.beginTransaction();
Persistent Classes
Hibernate 3.0alpha 26
Map parent = new HashMap();
parent.put("name", "foo");
parent.put("address", "bar");
Map child = new HashMap();
child.put("name", "fooTwo");
child.put("address", "barTwo");
child.put("parent", parent);
s.save("TestMap", parent);
t.commit();
s.close();
TODO: Document user-extension framework in the property and proxy package
Persistent Classes
Hibernate 3.0alpha 27
Chapter 5. Basic O/R Mapping
5.1. Mapping declaration
Object/relational mappings are defined in an XML document. The mapping document is designed to be read-
able and hand-editable. The mapping language is Java-centric, meaning that mappings are constructed around
persistent class declarations, not table declarations.
Note that, even though many Hibernate users choose to write the XML by hand, a number of tools exist to gen-
erate the mapping document, including XDoclet, Middlegen and AndroMDA.
Lets kick off with 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"
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">
Hibernate 3.0alpha 28
<property name="name"
type="string"/>
</subclass>
</class>
<class name="Dog">
<!-- mapping for Dog could go here -->
</class>
</hibernate-mapping>
We will now discuss the content of the mapping document. We will only describe the document elements and
attributes that are used by Hibernate at runtime. The mapping document also contains some extra optional at-
tributes and elements that affect the database schemas exported by the schema export tool. (For example the
not-null attribute.)
5.1.1. Doctype
All XML mappings should declare the doctype shown. The actual DTD may 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.
5.1.2. 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 specified, tablenames will be qualified by the given
schema and catalog names. If missing, tablenames will be unqualified. The default-cascade attribute specifies
what cascade style should be assumed for properties and collections which do not specify a cascade attribute.
The auto-import attribute lets us use unqualified class names in the query language, by default.
<hibernate-mapping
schema="schemaName" (1)
catalog="catalogName" (2)
default-cascade="none|save-update" (3)
default-access="field|property|ClassName" (4)
default-lazy="true|false" (5)
auto-import="true|false" (6)
package="package.name" (7)
/>
(1) schema (optional): The name of a database schema.
(2) catalog (optional): The name of a database catalog.
(3) default-cascade (optional - defaults to none): A default cascade style.
(4) default-access (optional - defaults to property): The strategy Hibernate should use for accessing all
properties. Can be a custom implementation of PropertyAccessor.
(5) default-lazy (optional - defaults to true): The default value for unspecifed lazy attributes of class and
collection mappings.
(6) auto-import (optional - defaults to true): Specifies whether we can use unqualified class names (of
classes in this mapping) in the query language.
(7) package (optional): Specifies a package prefix to assume for unqualified class names in the mapping doc-
ument.
If you have two persistent classes with the same (unqualified) name, you should set auto-import="false". Hi-
Basic O/R Mapping
Hibernate 3.0alpha 29
bernate will throw an exception if you attempt to assign two classes to the same "imported" name.
5.1.3. class, dynamic-class
You may declare a persistent class using the class element:
<class
name="ClassName" (1)
table="tableName" (2)
discriminator-value="discriminator_value" (3)
mutable="true|false" (4)
schema="owner" (5)
catalog="catalog" (6)
proxy="ProxyInterface" (7)
dynamic-update="true|false" (8)
dynamic-insert="true|false" (9)
select-before-update="true|false" (10)
polymorphism="implicit|explicit" (11)
where="arbitrary sql where condition" (12)
persister="PersisterClass" (13)
batch-size="N" (14)
optimistic-lock="none|version|dirty|all" (15)
lazy="true|false" (16)
entity-name="EntityName" (17)
catalog="catalog" (18)
check="arbitrary sql check condition" (19)
rowid="TODO" (20)
subselect="TODO" (21)
abstract="true|false" (22)
/>
(1) name: The fully qualified Java class name of the persistent class (or interface).
(2) table (optional - defaults to the unqualified class name): The name of its database table.
(3) discriminator-value (optional - defaults to the class name): A value that distiguishes individual sub-
classes, used for polymorphic behaviour. Acceptable values include null and not null.
(4) mutable (optional, defaults to true): Specifies that instances of the class are (not) mutable.
(5) schema (optional): Override the schema name specified by the root <hibernate-mapping> element.
(6) catalog (optional): Override the catalog name specified by the root <hibernate-mapping> element.
(7) proxy (optional): Specifies an interface to use for lazy initializing proxies. You may specify the name of
the class itself.
(8) dynamic-update (optional, defaults to false): Specifies that UPDATE SQL should be generated at runtime
and contain only those columns whose values have changed.
(9) 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.
(10) 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. In certain cases (actually, only when a
transient object has been associated with a new session using update()), this means that Hibernate will
perform an extra SQL SELECT to determine if an UPDATE is actually required.
(11) polymorphism (optional, defaults to implicit): Determines whether implicit or explicit query polymorph-
ism is used.
(12) where (optional) specify an arbitrary SQL WHERE condition to be used when retrieving objects of this class
(13) persister (optional): Specifies a custom ClassPersister.
(14) batch-size (optional, defaults to 1) specify a "batch size" for fetching instances of this class by identifier.
(15) optimistic-lock (optional, defaults to version): Determines the optimistic locking strategy.
(16) lazy (optional): Lazy fetching may be completely disabled by setting lazy="false".
(17) entity-name (optional): TODO
(18) catalog (optional): The name of a database catalog used for this class and its table.
(19) check (optional): A SQL expression used to generate a multi-row check constraint for automatic schema
Basic O/R Mapping
Hibernate 3.0alpha 30
generation.
(20) rowid (optional): TODO
(21) subselect (optional): TODO
(22) abstract (optional): Used to mark abstract superclasses in <union-subclass> hierarchies.
It is perfectly acceptable for the named persistent class to be an interface. You would then declare implement-
ing classes of that interface using the <subclass> element. You may persist any static inner class. You should
specify the class name using the standard form ie. eg.Foo$Bar.
Immutable classes, mutable="false", may not be updated or deleted by the application. This allows Hibernate
to make some minor performance optimizations.
The optional proxy attribute enables lazy initialization of persistent instances of the class. Hibernate will ini-
tially return CGLIB proxies which implement the named interface. The actual persistent object will be loaded
when a method of the proxy is invoked. See "Proxies for Lazy Initialization" below.
Implicit polymorphism means that instances of the class will be returned by a query that names any superclass
or implemented interface or the 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 be queries
that explicitly name that class and that 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 may, for example,
specify your own subclass of org.hibernate.persister.EntityPersister or you might even provide a com-
pletely new implementation of the interface org.hibernate.persister.ClassPersister that implements per-
sistence via, for example, stored procedure calls, serialization to flat files or LDAP. See
org.hibernate.test.CustomPersister for a simple example (of "persistence" to a Hashtable).
Note that the dynamic-update and dynamic-insert settings are not inherited by subclasses and so may also be
specified on the <subclass> or <joined-subclass> elements. These settings may increase performance in
some cases, but might actually decrease performance in others. Use judiciously.
Use of select-before-update will usually decrease performance. It is very useful to prevent a database update
trigger being called unnecessarily.
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
• none do not use optimistic locking
We very strongly recommend that you use version/timestamp columns for optimistic locking with Hibernate.
This is the optimal strategy with respect to performance and is the only strategy that correctly handles modific-
ations made to detached instances (ie. when Session.update() is used).
TODO: Document entity name and dynamic class
TODO: Document subselect and and synchronize for view simulation
Basic O/R Mapping
Hibernate 3.0alpha 31
5.1.4. id
Mapped classes must declare the primary key column of the database table. Most classes will also have a Java-
Beans-style property holding the unique identifier of an instance. The <id> element defines the mapping from
that property to the primary key column.
<id
name="propertyName" (1)
type="typename" (2)
column="column_name" (3)
unsaved-value="null|any|none|undefined|id_value" (4)
access="field|property|ClassName"> (5)
<generator class="generatorClass"/>
</id>
(1) name (optional): The name of the identifier property.
(2) type (optional): A name that indicates the Hibernate type.
(3) column (optional - defaults to the property name): The name of the primary key column.
(4) unsaved-value (optional - defaults to a "sensible" value): An identifier 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.
(5) 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 important! If the identfier property of your class does not default to the normal
Java default value (null or zero), then you should specify the actual default.
There is an alternative <composite-id> declaration to allow access to legacy data with composite keys. We
strongly discourage its use for anything else.
5.1.4.1. generator
The required <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="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>
All generators implement the interface org.hibernate.id.IdentifierGenerator. This is a very simple inter-
face; some applications may choose to provide their own specialized implementations. However, Hibernate
provides a range of built-in implementations. There are shortcut names for the built-in generators:
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
Basic O/R Mapping
Hibernate 3.0alpha 32
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 al-
gorithm generates identifiers that are unique only for a particular database. Do not use this generator with
connections enlisted with JTA or with a user-supplied connection.
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, unique within a network (the IP ad-
dress is used). The UUID is encoded as a string of hexadecimal digits of length 32.
guid
uses a database-generated GUID string on MS SQL Server and MySQL.
native
picks identity, sequence or hilo depending upon the capabilities of the underlying database.
assigned
lets the application to assign an identifier to the object before save() is called.
select
retrieves a primary key assigned by a database trigger by selecting the row by some unique key and retriev-
ing the primary key value.
foreign
uses the identifier of another associated object. Usually used in conjunction with a <one-to-one> primary
key association.
5.1.4.2. Hi/lo algorithm
The hilo and seqhilo generators provide two alternate implementations of the hi/lo algorithm, a favorite ap-
proach to identifier generation. The first implementation requires a "special" database table to hold the next
available "hi" value. The second uses an Oracle-style sequence (where supported).
<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>
Basic O/R Mapping
Hibernate 3.0alpha 33
Unfortunately, you can't use hilo when supplying your own Connection to Hibernate, or when Hibernate is us-
ing an application server datasource to obtain connections enlisted with JTA. Hibernate must be able to fetch
the "hi" value in a new transaction. A standard approach in an EJB environment is to implement the hi/lo al-
gorithm using a stateless session bean.
5.1.4.3. UUID algorithm
The UUID contains: IP address, startup time of the JVM (accurate to a quarter second), system time and a
counter value (unique within the JVM). It's not possible to obtain a MAC address or memory address from Java
code, so this is the best we can do without using JNI.
5.1.4.4. Identity columns and sequences
For databases which support identity columns (DB2, MySQL, Sybase, MS SQL), you may use identity key
generation. For databases that support sequences (DB2, Oracle, PostgreSQL, Interbase, McKoi, SAP DB) you
may use sequence style key generation. Both these strategies require two SQL queries to insert a new object.
<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>
For cross-platform development, the native strategy will choose from the identity, sequence and hilo
strategies, dependant upon the capabilities of the underlying database.
5.1.4.5. Assigned identifiers
If you want the application to assign identifiers (as opposed to having Hibernate generate them), you may use
the assigned generator. This special generator will use the identifier value already assigned to the object's iden-
tifier property. This generator is used when the primary key is a natural key instead of a surrogate key.
Choosing the assigned generator makes Hibernate use unsaved-value="undefined", forcing Hibernate to go
to the database to determine if an instance is transient or detached, unless there is a version or timestampe prop-
erty, or you define Interceptor.isUnsaved().
5.1.4.6. Primary keys assigned by triggers
For legacy schemas only (Hibernate does not generate DDL with triggers).
<id name="id" type="long" column="person_id">
<generator class="select">
<param name="key">socialSecurityNumber</param>
</generator>
</id>
In the above example, there is a unique valued property named socialSecurityNumber defined by the class, as
a natural key, and a surrogate key named person_id whose value is generated by a trigger.
5.1.5. composite-id
Basic O/R Mapping
Hibernate 3.0alpha 34
<composite-id
name="propertyName"
class="ClassName"
unsaved-value="undefined|any|none"
access="field|property|ClassName">
<key-property name="propertyName" type="typename" column="column_name"/>
<key-many-to-one name="propertyName class="ClassName" column="column_name"/>
......
</composite-id>
For a table with a composite key, you may map 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.
<composite-id>
<key-property name="medicareNumber"/>
<key-property name="dependent"/>
</composite-id>
Your persistent class must override equals() and hashCode() to implement composite identifier equality. It
must also implements Serializable.
Unfortunately, this approach to composite identifiers 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 it-
self and populate its identifier properties before you can load() the persistent state associated with a composite
key. We will describe a much more convenient approach where the composite identifier is implemented as a
seperate class in Section 7.4, “Components as composite identifiers”. The attributes described below apply only
to this alternative approach:
• name (optional): A property of component type that holds the composite identifier (see next section).
• class (optional - defaults to the property type determined by reflection): The component class used as a
composite identifier (see next section).
• unsaved-value (optional - defaults to undefined): Indicates that transient instances should be considered
newly instantiated, if set to any, or detached, if set to none. It is best to leave the default value in all cases.
5.1.6. discriminator
The <discriminator> element is required for polymorphic persistence using the table-per-class-hierarchy map-
ping strategy and declares a discriminator column of the table. The discriminator column contains marker val-
ues that tell the persistence layer what subclass to instantiate for a particular row. A restricted set of types may
be used: string, character, integer, byte, short, boolean, yes_no, true_false.
<discriminator
column="discriminator_column" (1)
type="discriminator_type" (2)
force="true|false" (3)
insert="true|false" (4)
formula="arbitrary sql expression" (5)
/>
(1) column (optional - defaults to class) the name of the discriminator column.
(2) type (optional - defaults to string) a name that indicates the Hibernate type
(3) force (optional - defaults to false) "force" Hibernate to specify allowed discriminator values even when
retrieving all instances of the root class.
(4) insert (optional - defaults to true) set this to false if your discriminator column is also part of a mapped
composite identifier. (Tells Hibernate to not include the column in SQL INSERTs.)
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(5) formula (optional) an arbitrary SQL expression that is executed when a type has to be evaluated. 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.
Using the formula attribute you can declare an arbitrary SQL expression that will be used to evaluate the type
of a row:
<discriminator formula="case when CLASS_TYPE in ('a', 'b', 'c') then 0 else 1 end" type="integer"/>
5.1.7. version (optional)
The <version> element is optional and indicates that the table contains versioned data. This is particularly use-
ful if you plan to use long transactions (see below).
<version
column="version_column" (1)
name="propertyName" (2)
type="typename" (3)
access="field|property|ClassName" (4)
unsaved-value="null|negative|undefined" (5)
/>
(1) column (optional - defaults to the property name): The name of the column holding the version number.
(2) name: The name of a property of the persistent class.
(3) type (optional - defaults to integer): The type of the version number.
(4) access (optional - defaults to property): The strategy Hibernate should use for accessing the property
value.
(5) 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.)
Version numbers may be of type long, integer, short, timestamp or calendar.
A version or timestamp property should never be null for a detached instance, so Hibernate will detact any in-
stance with a null version or timestamp as transient, no matter what other unsaved-value strategies are spe-
cified. Declaring a nullable version or timestamp property is an easy way to avoid any problems with transitive
reattachment in Hibernate, especially useful for people using assigned identiifers or composite keys!
5.1.8. timestamp (optional)
The optional <timestamp> element indicates that the table contains timestamped data. This is intended as an al-
ternative to versioning. Timestamps are by nature a less safe implementation of optimistic locking. However,
sometimes the application might use the timestamps in other ways.
<timestamp
column="timestamp_column" (1)
name="propertyName" (2)
access="field|property|ClassName" (3)
unsaved-value="null|undefined" (4)
/>
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(1) column (optional - defaults to the property name): The name of a column holding the timestamp.
(2) name: The name of a JavaBeans style property of Java type Date or Timestamp of the persistent class.
(3) access (optional - defaults to property): The strategy Hibernate should use for accessing the property
value.
(4) 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 pre-
vious session. (undefined specifies that the identifier property value should be used.)
Note that <timestamp> is equivalent to <version type="timestamp">.
5.1.9. property
The <property> element declares a persistent, JavaBean style property of the class.
<property
name="propertyName" (1)
column="column_name" (2)
type="typename" (3)
update="true|false" (4)
insert="true|false" (4)
formula="arbitrary SQL expression" (5)
access="field|property|ClassName" (6)
lazy="true|false" (7)
unique="true|false" (8)
not-null="true|false" (9)
optimistic-lock="true|false" (10)
/>
(1) name: the name of the property, with an initial lowercase letter.
(2) column (optional - defaults to the property name): the name of the mapped database table column. This
may also be specified by nested <column> element(s).
(3) type (optional): a name that indicates the Hibernate type.
(4) 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 whose
value is initialized from some other property that maps to the same colum(s) or by a trigger or other ap-
plication.
(5) formula (optional): an SQL expression that defines the value for a computed property. Computed proper-
ties do not have a column mapping of their own.
(6) access (optional - defaults to property): The strategy Hibernate should use for accessing the property
value.
(7) lazy (optional - defaults to false): Specifies that this property should be fetched lazily when the instance
variable is first accessed (requires build-time bytecode instrumentation).
(8) unique (optional): Enable the DDL generation of a unique constraint for the columns. Also, allow this to
be the target of a property-ref.
(9) not-null (optional): Enable the DDL generation of a nullability constraint for the columns.
(10) 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, define if a version check should be made if this property
is dirty.
typename could be:
1. The name of a Hibernate basic type (eg. integer, string, character, date, timestamp, float,
binary, serializable, object, blob).
2. The name of a Java class with a default basic type (eg. int, float, char, java.lang.String,
java.util.Date, java.lang.Integer, java.sql.Clob).
3. The name of a serializable Java class.
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4. The class name of a custom type (eg. com.illflow.type.MyCustomType).
If you do not specify a type, Hibernate will use reflection upon the named property to take a guess at the correct
Hibernate type. Hibernate will try to interpret the name of the return class of the property getter using rules 2, 3,
4 in that order. However, this is not always enough. In certain cases you will still need the type attribute. (For
example, to distinguish between Hibernate.DATE and Hibernate.TIMESTAMP, or to specify a custom type.)
The access attribute lets you control how Hibernate will access 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 ac-
cess the field directly, using reflection. You may specify your own strategy for property access by naming a
class that implements the interface org.hibernate.property.PropertyAccessor.
TODO: Document the nested column formula="" attribute with an example
5.1.10. 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. (Its really just an object reference.)
<many-to-one
name="propertyName" (1)
column="column_name" (2)
class="ClassName" (3)
cascade="all|none|save-update|delete" (4)
fetch="join|select" (5)
update="true|false" (6)
insert="true|false" (6)
property-ref="propertyNameFromAssociatedClass" (7)
access="field|property|ClassName" (8)
unique="true|false" (9)
not-null="true|false" (10)
optimistic-lock="true|false" (11)
/>
(1) name: The name of the property.
(2) column (optional): The name of the foreign key column. This may also be specified by nested <column>
element(s).
(3) class (optional - defaults to the property type determined by reflection): The name of the associated
class.
(4) cascade (optional): Specifies which operations should be cascaded from the parent object to the associ-
ated object.
(5) join (optional - defaults to select): Chooses between outer-join fetching or sequential select fetching.
(6) 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 some other property that maps to the same colum(s) or by a trigger or other ap-
plication.
(7) 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.
(8) access (optional - defaults to property): The strategy Hibernate should use for accessing the property
value.
(9) unique (optional): Enable the DDL generation of a unique constraint for the foreign-key column. Also, al-
low this to be the target of a property-ref.
(10) not-null (optional): Enable the DDL generation of a nullability constraint for the foreign key columns.
(11) 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, define if a version check should be made if this property
is dirty.
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The cascade attribute permits the following values: all, save-update, delete, none. Setting a value other than
none will propagate certain operations to the associated (child) object. See "Lifecycle Objects" below.
A typical many-to-one declaration looks as simple as
<many-to-one name="product" class="Product" column="PRODUCT_ID"/>
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 an ugly relational model. For example, suppose
the Product class had a unique serial number, that is not the primary key. (The unique attribute controls Hi-
bernate's DDL generation with the SchemaExport tool.)
<property name="serialNumber" unique="true" type="string" column="SERIAL_NUMBER"/>
Then the mapping for OrderItem might use:
<many-to-one name="product" property-ref="serialNumber" column="PRODUCT_SERIAL_NUMBER"/>
This is certainly not encouraged, however.
If the referenced unique key comprises multiple properties of the associated entity, you should map the refer-
enced properties inside a named <properties> element.
5.1.11. one-to-one
A one-to-one association to another persistent class is declared using a one-to-one element.
<one-to-one
name="propertyName" (1)
class="ClassName" (2)
cascade="all|none|save-update|delete" (3)
constrained="true|false" (4)
fetch="join|select" (5)
property-ref="propertyNameFromAssociatedClass" (6)
access="field|property|ClassName" (7)
/>
(1) name: The name of the property.
(2) class (optional - defaults to the property type determined by reflection): The name of the associated
class.
(3) cascade (optional) specifies which operations should be cascaded from the parent object to the associated
object.
(4) constrained (optional) specifies that a foreign key constraint on the primary key of the mapped table ref-
erences the table of the associated class. This option affects the order in which save() and delete() are
cascaded (and is also used by the schema export tool).
(5) fetch (optional - defaults to select): Chooses between outer-join fetching or sequential select fetching.
(6) 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.
(7) access (optional - defaults to property): The strategy Hibernate should use for accessing the property
value.
There are two varieties of one-to-one association:
• primary key associations
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• unique foreign key associations
Primary key associations don't need an extra table column; if two rows are related by the association then the
two table rows share the same primary key value. So if you want two objects to be related by a primary key as-
sociation, you must make sure that they are assigned the same identifier value!
For a primary key association, add the following mappings to Employee and Person, respectively.
<one-to-one name="person" class="Person"/>
<one-to-one name="employee" class="Employee" constrained="true"/>
Now we must ensure that the primary keys of related rows in the PERSON and EMPLOYEE tables are equal.
We use a special Hibernate identifier generation strategy called foreign:
<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>
A newly saved instance of Person is then assigned the same primar key value as the Employee instance refered
with the employee property of that Person.
Alternatively, a foreign key with a unique constraint, from Employee to Person, may be expressed as:
<many-to-one name="person" class="Person" column="PERSON_ID" unique="true"/>
And this association may be made bidirectional by adding the following to the Person mapping:
<one-to-one name"employee" class="Employee" property-ref="person"/>
5.1.12. component, dynamic-component
The <component> element maps properties of a child object to columns of the table of a parent class. Compon-
ents may, in turn, declare their own properties, components or collections. See "Components" below.
<component
name="propertyName" (1)
class="className" (2)
insert="true|false" (3)
upate="true|false" (4)
access="field|property|ClassName" (5)
lazy="true|false" (6)
optimistic-lock="true|false" (7)
>
<property ...../>
<many-to-one .... />
........
</component>
(1) name: The name of the property.
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(2) class (optional - defaults to the property type determined by reflection): The name of the component
(child) class.
(3) insert: Do the mapped columns appear in SQL INSERTs?
(4) update: Do the mapped columns appear in SQL UPDATEs?
(5) access (optional - defaults to property): The strategy Hibernate should use for accessing the property
value.
(6) lazy (optional - defaults to false): Specifies that this component should be fetched lazily when the in-
stance variable is first accessed (requires build-time bytecode instrumentation).
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 ref-
erence 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.
5.1.13. subclass
Finally, polymorphic persistence requires the declaration of each subclass of the root persistent class. For the
(recommended) table-per-class-hierarchy mapping strategy, the <subclass> declaration is used.
<subclass
name="ClassName" (1)
discriminator-value="discriminator_value" (2)
proxy="ProxyInterface" (3)
lazy="true|false" (4)
dynamic-update="true|false"
dynamic-insert="true|false">
<property .... />
.....
</subclass>
(1) name: The fully qualified class name of the subclass.
(2) discriminator-value (optional - defaults to the class name): A value that distiguishes individual sub-
classes.
(3) proxy (optional): Specifies a class or interface to use for lazy initializing proxies.
(4) lazy (optional, defaults to true): Setting lazy="false" disables the use of lazy fetching.
Each subclass should declare its own persistent properties and subclasses. <version> and <id> properties are
assumed to be inherited from the root class. Each subclass in a heirarchy must define a unique discriminator-
value. If none is specified, the fully qualified Java class name is used.
5.1.14. joined-subclass
Alternatively, each subclass may be mapped to its own table (table-per-subclass mapping strategy). Inherited
state is retrieved by joining with the table of the superclass. We use the <joined-subclass> element.
<joined-subclass
name="ClassName" (1)
table="tablename" (2)
proxy="ProxyInterface" (3)
lazy="true|false" (4)
dynamic-update="true|false"
dynamic-insert="true|false"
schema="schema"
catalog="catalog"
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extends="SuperclassName"
persister="ClassName"
subselect="TODO">
<key .... >
<property .... />
.....
</joined-subclass>
(1) name: The fully qualified class name of the subclass.
(2) table: The name of the subclass table.
(3) proxy (optional): Specifies a class or interface to use for lazy initializing proxies.
(4) lazy (optional, defaults to true): Setting lazy="false" disables the use of lazy fetching.
TODO
No discriminator column is 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 be
re-written as:
<?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"/>
</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>
5.1.15. union-subclass
A third option is to map only the concrete classes of an inheritance hierarchy to tables, (the table-
per-concrete-class strategy) where each table defines all persistent state of the class, including inherited state. In
Hibernate, it is not absolutely necessary to explicitly map such inheritance hierarchies. You can simply map
each class with a separate <class> declaration. However, if you wish use polymorphic associations, you need
to use the <union-subclass> mapping.
<union-subclass
name="ClassName" (1)
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table="tablename" (2)
proxy="ProxyInterface" (3)
lazy="true|false" (4)
dynamic-update="true|false" (5)
dynamic-insert="true|false" (6)
schema="schema" (7)
catalog="catalog" (8)
extends="SuperclassName" (9)
abstract="true|false" (10)
persister="ClassName" (11)
subselect="TODO"> (12)
<property .... />
.....
</union-subclass>
(1) name: The fully qualified class name of the subclass.
(2) table: The name of the subclass table.
(3) proxy (optional): Specifies a class or interface to use for lazy initializing proxies.
(4) lazy (optional, defaults to true): Setting lazy="false" disables the use of lazy fetching.
TODO
No discriminator column or key column is required for this mapping strategy.
5.1.16. join
Using the <join> element, it is possible to map properties of one class to several tables.
<join
table="tablename" (1)
schema="owner" (2)
catalog="catalog" (3)
fetch="join|select" (4)
inverse="true|false" (5)
optional="true|false"> (6)
<key ... />
<property ... />
...
</join>
(1) tabe: The name of the joined table.
(2) schema (optional): Override the schema name specified by the root <hibernate-mapping> element.
(3) catalog (optional): Override the catalog name specified by the root <hibernate-mapping> element.
(4) fetch (optional - defaults to join): If set to select for a join defined on a subclass, then rather than join-
ing, a sequential select will be issued only if a row turns out to represent an instance of the subclass.
(5) inverse (optional - defaults to false): If enabled, Hibernate will not try to insert or update the properties
defined by this join.
(6) optional (optional - defaults to false): If enabled, Hibernate will insert a row only if the properties
defined by this join are non-null .
TODO: Document join with an example
5.1.17. key
We've seen the <key> element crop up a few times now. It appears anywhere the parent mapping element
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defines a join to a new table, and defines the foreign key in the joined table, that references the primary key of
the original table.
<key
column="columnname" (1)
on-delete="noaction|cascade" (2)
property-ref="propertyName" (3)
/>
(1) column (optional): The name of the foreign key column. This may also be specified by nested <column>
element(s).
(2) on-delete (optional, defaults to noaction): Specifies whether the foreign key constraint has database-
level cascade delete enabled.
(3) property-ref (optional): Specifies that the foreign key refers to columns that are not the primary key of
the orginal table. (Provided for legacy data.)
We recommend that for systems where delete performance is important, all keys should be defined on-de-
lete="cascade", and Hibernate will use a database-level ON CASCADE DELETE constraint, instead of many indi-
vidual DELETE statements. Be aware that this feature bypasses Hibernate's usual optimistic locking strategy for
versioned data.
5.1.18. map, set, list, bag
Collections are discussed later.
5.1.19. import
Suppose your application has two persistent classes with the same name, and you don't want to specify the fully
qualified (package) name in Hibernate queries. Classes may be "imported" explicitly, rather than relying upon
auto-import="true". You may even import classes and interfaces that are not explicitly mapped.
<import class="java.lang.Object" rename="Universe"/>
<import
class="ClassName" (1)
rename="ShortName" (2)
/>
(1) class: The fully qualified class name of of any Java class.
(2) rename (optional - defaults to the unqualified class name): A name that may be used in the query lan-
guage.
5.2. Hibernate Types
5.2.1. Entities and values
To understand the behaviour of various Java language-level objects with respect to the persistence service, we
need to classify them 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
(except that saves and deletions may be cascaded from a parent entity to its children). This is different from the
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ODMG model of object persistence by reachablity - and corresponds more closely to how application objects
are usually used in large systems. Entities support circular and shared references. They may also be versioned.
An entity's persistent state consists of references to other entities and instances of value types. Values are prim-
itives, collections, 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 may not be independently versioned. Values have no inde-
pendent identity, so they cannot be shared by two entities or collections.
All Hibernate types except collections support null semantics.
Up until now, we've been using the term "persistent class" to refer to entities. We will continue to do that.
Strictly speaking, however, not all user-defined classes with persistent state are entities. A component is a user
defined class with value semantics.
5.2.2. Basic value types
The basic types may be roughly categorized into
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
Type mappings from java.util.Date and its subclasses to SQL types DATE, TIME and TIMESTAMP (or equi-
valent).
calendar, calendar_date
Type mappings from java.util.Calendar to SQL types TIMESTAMP and DATE (or equivalent).
big_decimal
A type mapping from java.math.BigDecimal 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 may also indicate the Hibernate type
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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 may be inconveni-
ent for some applications, since the blob or clob object may not be reused outside of a transaction.
(Furthermore, driver support is patchy and inconsistent.)
Unique identifiers of entities and collections may be of any basic type except binary, blob and clob.
(Composite identifiers are also allowed, see below.)
The basic value types have corresponding Type constants defined on org.hibernate.Hibernate. For example,
Hibernate.STRING represents the string type.
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 prop-
erties of type java.lang.BigInteger to VARCHAR columns. Hibernate does not provide a built-in type for this.
But 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.
Check out org.hibernate.test.DoubleStringType to see the kind of things that are possible.
<property name="twoStrings" type="org.hibernate.test.DoubleStringType">
<column name="first_string"/>
<column name="second_string"/>
</property>
Notice the use of <column> tags to map a property to multiple columns.
Even though Hibernate's rich range of built-in types and support for components means you will very rarely
need to use a custom type, it is nevertheless considered good form to use custom types for (non-entity) classes
that occur frequently in your application. For example, a MonetoryAmount class is a good candidate for a Com-
positeUserType, even though it could easily be mapped as a component. One motivation for this is abstraction.
With a custom type, your mapping documents would be future-proofed against possible changes in your way of
representing monetory values.
5.2.4. Any type mappings
There is one further type of property mapping. The <any> mapping element defines a polymorphic association
to classes from multiple tables. This type of mapping always requires more than one column. The first column
holds the type of the associated entity. The remaining columns hold the identifier. It is impossible to specify a
foreign key constraint for this kind of association, so this is most certainly not meant as the usual way of map-
ping (polymorphic) associations. You should use this only in very special cases (eg. audit logs, user session
data, etc).
The meta-type attribute lets the application specify a custom type that maps database column values to persist-
ent classes which have identifier properties of the type specified by id-type. You must specify the mapping
from values of the meta-type to class names.
<any name="being" id-type="long" meta-type="string">
<meta-value value="TBL_ANIMAL" class="Animal"/>
<meta-value value="TBL_HUMAN" class="Human"/>
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<meta-value value="TBL_ALIEN" class="Alien"/>
<column name="table_name"/>
<column name="id"/>
</any>
<any
name="propertyName" (1)
id-type="idtypename" (2)
meta-type="metatypename" (3)
cascade="none|all|save-update" (4)
access="field|property|ClassName" (5)
optimistic-lock="true|false" (6)
>
<meta-value ... />
<meta-value ... />
.....
<column .... />
<column .... />
.....
</any>
(1) name: the property name.
(2) id-type: the identifier type.
(3) meta-type (optional - defaults to string): Any type that is allowed for a discriminator mapping.
(4) cascade (optional- defaults to none): the cascade style.
(5) access (optional - defaults to property): The strategy Hibernate should use for accessing the property
value.
(6) 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, define if a version check should be made if this property
is dirty.
5.3. SQL quoted identifiers
You may 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 (usually
double quotes, but brackets for SQL Server and backticks for MySQL).
<class name="LineItem" table="`Line Item`">
<id name="id" column="`Item Id`"/><generator class="assigned"/></id>
<property name="itemNumber" column="`Item #`"/>
...
</class>
5.4. Modular mapping files
It is possible to define subclass and joined-subclass mappings in seperate mapping documents, directly be-
neath hibernate-mapping. This allows you to extend a class hierachy just by adding a new mapping file. You
must specify an extends attribute in the subclass mapping, naming a previously mapped superclass. Note: Pre-
viously this feature made the ordering of the mapping documents important. Since Hibernate 3, the ordering of
mapping files does not matter when using the extends keyword. The ordering inside a single mapping file still
needs to be defined as superclasses before subclasses.
<hibernate-mapping>
<subclass name="eg.subclass.DomesticCat" extends="eg.Cat" discriminator-value="D">
<property name="name" type="string"/>
</subclass>
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Hibernate 3.0alpha 47
</hibernate-mapping>
5.5. Using XDoclet markup
Many Hibernate users prefer to embed mapping information directly in sourcecode using XDoclet
@hibernate.tags. We will not cover this approach in this document, since strictly 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"
*/
public Date getBirthdate() {
return birthdate;
}
void setBirthdate(Date date) {
birthdate = date;
}
/**
* @hibernate.property
* column="WEIGHT"
*/
public float getWeight() {
return weight;
}
void setWeight(float weight) {
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Hibernate 3.0alpha 48
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;
}
void setSex(char sex) {
this.sex=sex;
}
}
See the Hibernate web site for more examples of XDoclet and Hibernate.
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Hibernate 3.0alpha 49
Chapter 6. Collection Mapping
6.1. Persistent Collections
This section does not contain much example Java code. We assume you already know how to use Java's collec-
tions framework. If so, there's not really anything more to know - with a single caveat, you may use Java col-
lections the same way you always have.
Hibernate can persist instances of java.util.Map, java.util.Set, java.util.SortedMap,
java.util.SortedSet, java.util.List, and any array of persistent entities or values. Properties of type
java.util.Collection or java.util.List may also be persisted with "bag" semantics.
Now the caveat: persistent collections do not retain any extra semantics added by the class implementing the
collection interface (eg. iteration order of a LinkedHashSet). The persistent collections actually behave like
HashMap, HashSet, TreeMap, TreeSet and ArrayList respectively. Furthermore, the Java type of a property
holding a collection must be the interface type (ie. Map, Set or List; never HashMap, TreeSet or ArrayList).
This restriction exists because, when you're not looking, Hibernate sneakily replaces your instances of Map, Set
and List with instances of its own persistent implementations of Map, Set or List. (So also be careful when us-
ing == on your collections.)
Cat cat = new DomesticCat();
Cat kitten = new DomesticCat();
....
Set kittens = new HashSet();
kittens.add(kitten);
cat.setKittens(kittens);
session.save(cat);
kittens = cat.getKittens(); //Okay, kittens collection is a Set
(HashSet) cat.getKittens(); //Error!
Collections obey the usual rules for value types: no shared references, created and deleted along with contain-
ing entity. Due to the underlying relational model, they do not support null value semantics; Hibernate does not
distinguish between a null collection reference and an empty collection.
Collections are automatically persisted when referenced by a persistent object and 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. You shouldn't have to worry much about any of this. Just use Hibernate's collections the
same way you use ordinary Java collections, but make sure you understand the semantics of bidirectional asso-
ciations (discussed later) before using them.
Collection instances are distinguished in the database by a foreign key to the owning entity. This foreign key is
referred to as the collection key . The collection key is mapped by the <key> element. If you have a foreign-key
constraint set in the database, and have chosen the ON DELETE CASCADE option, always use the on-delete at-
tribute on your <key> mappings:
<key column="CHILD_ID" on-delete="cascade"/>
Collections may contain almost any other Hibernate type, including all basic types, custom types, entity types
and components. This is an important definition: An object in a collection can either be handled with "pass by
value" semantics (it therefore fully depends on the collection owner) or it can be a reference to another entity
with an own lifecycle. Collections may not contain other collections. The contained type is referred to as the
collection element type. Collection elements are mapped by <element>, <composite-element>,
<one-to-many>, <many-to-many> or <many-to-any>. The first two map elements with value semantics, the oth-
Hibernate 3.0alpha 50
er three are used to map entity associations.
All collection types except Set and bag have an index column - a column that maps to an array or List index or
Map key. The index of a Map may be of any basic type, an entity type or even a composite type (it may not be a
collection). The index of an array or list is always of type integer. Indexes are mapped using <index>,
<index-many-to-many>, <composite-index> or <index-many-to-any>.
There are quite a range of mappings that can be generated for collections, covering many common relational
models. We suggest you experiment with the schema generation tool to get a feeling for how various mapping
declarations translate to database tables.
6.2. Mapping a Collection
Collections are declared by the <set>, <list>, <map>, <bag>, <array> and <primitive-array> elements.
<map> is representative:
<map
name="propertyName" (1)
table="table_name" (2)
schema="schema_name" (3)
lazy="true|false" (4)
inverse="true|false" (5)
cascade="all|none|save-update|delete|all-delete-orphan" (6)
sort="unsorted|natural|comparatorClass" (7)
order-by="column_name asc|desc" (8)
where="arbitrary sql where condition" (9)
fetch="join|select" (10)
batch-size="N" (11)
access="field|property|ClassName" (12)
>
<key .... />
<index .... />
<element .... />
</map>
(1) name the collection property name
(2) table (optional - defaults to property name) the name of the collection table (not used for one-to-many
associations)
(3) schema (optional) the name of a table schema to override the schema declared on the root element
(4) lazy (optional - defaults to true) enable lazy initialization (not available for arrays)
(5) inverse (optional - defaults to false) mark this collection as the "inverse" end of a bidirectional associ-
ation
(6) cascade (optional - defaults to none) enable operations to cascade to child entities
(7) sort (optional) specify a sorted collection with natural sort order, or a given comparator class
(8) order-by (optional, JDK1.4 only) specify a table column (or columns) that define the iteration order of
the Map, Set or bag, together with an optional asc or desc
(9) where (optional) specify an arbitrary SQL WHERE condition to be used when retrieving or removing the
collection (useful if the collection should contain only a subset of the available data)
(10) fetch (optional, defaults to select) Choose between outer-join fetching and fetching by sequential select.
Only one collection may be fetched by outer join per SQL SELECT.
(11) batch-size (optional, defaults to 1) specify a "batch size" for lazily fetching instances of this collection.
(12) access (optional - defaults to property): The strategy Hibernate should use for accessing the property
value.
The mapping of a List or array requires a seperate table column holding the array or list index (the i in
foo[i]). If your relational model doesn't have an index column, e.g. if you're working with legacy data, use an
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Hibernate 3.0alpha 51
unordered Set instead. This seems to put people off who assume that List should just be a more convenient
way of accessing an unordered collection. Hibernate collections strictly obey the actual semantics attached to
the Set, List and Map interfaces. List elements don't just spontaneously rearrange themselves!
On the other hand, people who planned to use the List to emulate bag semantics have a legitimate grievance
here. A bag is an unordered, unindexed collection which may contain the same element multiple times. The
Java collections framework lacks a Bag interface, hence you have to emulate it with a List. Hibernate lets you
map properties of type List or Collection with the <bag> element. Note that bag semantics are not really part
of the Collection contract and they actually conflict with the semantics of the List contract (however, you can
sort the bag arbitrarily, discussed later in this chapter).
Note: Large Hibernate bags mapped with inverse="false" are inefficient and should be avoided; Hibernate
can't create, delete or update rows individually, because there is no key that may be used to identify an indi-
vidual row.
6.3. Collections of Values and Many-To-Many Associations
A collection table is required for any collection of values and any collection of references to other entities
mapped as a many-to-many association (the natural semantics for a Java collection). The table requires
(foreign) key column(s), element column(s) and possibly index column(s).
The foreign key from the collection table to the table of the owning class is declared using a <key> element.
<key column="column_name"/>
(1) column (required): The name of the foreign key column.
For indexed collections like maps and lists, we require an <index> element. For lists, this column contains se-
quential integers numbered from zero. Make sure that your index really starts from zero if you have to deal with
legacy data. For maps, the column may contain any values of any Hibernate type.
<index
column="column_name" (1)
type="typename" (2)
/>
(1) column (required): The name of the column holding the collection index values.
(2) type (optional, defaults to integer): The type of the collection index.
Alternatively, a map may be indexed by objects of entity type. We use the <index-many-to-many> element.
<index-many-to-many
column="column_name" (1)
class="ClassName" (2)
/>
(1) column (required): The name of the foreign key column for the collection index values.
(2) class (required): The entity class used as the collection index.
For a collection of values, we use the <element> tag.
<element
column="column_name" (1)
type="typename" (2)
/>
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Hibernate 3.0alpha 52
(1) column (required): The name of the column holding the collection element values.
(2) type (required): The type of the collection element.
A collection of entities with its own table corresponds to the relational notion of many-to-many association. A
many to many association is the most natural mapping of a Java collection but is not usually the best relational
model.
<many-to-many
column="column_name" (1)
class="ClassName" (2)
outer-join="true|false|auto" (3)
/>
(1) column (required): The name of the element foreign key column.
(2) class (required): The name of the associated class.
(3) outer-join (optional - defaults to auto): enables outer-join fetching for this association when hibern-
ate.use_outer_join is set.
Some examples, first, a set of strings:
<set name="names" table="NAMES">
<key column="GROUPID"/>
<element column="NAME" type="string"/>
</set>
A bag containing integers (with an iteration order determined by the order-by attribute):
<bag name="sizes" table="SIZES" order-by="SIZE ASC">
<key column="OWNER"/>
<element column="SIZE" type="integer"/>
</bag>
An array of entities - in this case, a many to many association (note that the entities are lifecycle objects, cas-
cade="all"):
<array name="foos" table="BAR_FOOS" cascade="all">
<key column="BAR_ID"/>
<index column="I"/>
<many-to-many column="FOO_ID" class="org.hibernate.Foo"/>
</array>
A map from string indices to dates:
<map name="holidays" table="holidays" schema="dbo" order-by="hol_name asc">
<key column="id"/>
<index column="hol_name" type="string"/>
<element column="hol_date" type="date"/>
</map>
A list of components (discussed in the next chapter):
<list name="carComponents" table="car_components">
<key column="car_id"/>
<index column="posn"/>
<composite-element class="org.hibernate.car.CarComponent">
<property name="price" type="float"/>
<property name="type" type="org.hibernate.car.ComponentType"/>
<property name="serialNumber" column="serial_no" type="string"/>
</composite-element>
</list>
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6.4. One-To-Many Associations
A one to many association links the tables of two classes directly, with no intervening collection table. (This
implements a one-to-many relational model.) This relational model loses some of the semantics of Java collec-
tions:
• No null values may be contained in a map, set or list
• An instance of the contained entity class may not belong to more than one instance of the collection
• An instance of the contained entity class may not appear at more than one value of the collection index
An association from Foo to Bar requires the addition of a key column and possibly an index column to the table
of the contained entity class, Bar. These columns are mapped using the <key> and <index> elements described
above.
The <one-to-many> tag indicates a one to many association.
<one-to-many class="ClassName"/>
(1) class (required): The name of the associated class.
Example:
<set name="bars">
<key column="foo_id"/>
<one-to-many class="org.hibernate.Bar"/>
</set>
Notice that the <one-to-many> element does not need to declare any columns. Nor is it necessary to specify the
table name anywhere.
Very Important Note: If the <key> column of a <one-to-many> association is declared NOT NULL, Hibernate
may cause constraint violations when it creates or updates the association. To prevent this problem, you must
use a bidirectional association with the many valued end (the set or bag) marked as inverse="true". See the
discussion of bidirectional associations later in this chapter.
6.5. Lazy Initialization
Collections (other than arrays) may be lazily initialized, meaning they load their state from the database only
when the application needs to access it. Initialization of collections owned by persistent instances happens
transparently to the user, so the application would not normally need to worry about this (in fact, transparent
lazy initialization is the main reason why Hibernate needs its own collection implementations). However, if the
application tries something like this:
s = sessions.openSession();
User u = (User) s.find("from User u where u.name=?", userName, Hibernate.STRING).get(0);
Map permissions = u.getPermissions();
s.connection().commit();
s.close();
Integer accessLevel = (Integer) permissions.get("accounts"); // Error!
It could be in for a nasty surprise. 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 de-
tached objects. The fix is to move the line that reads from the collection to just before the commit. (There are
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Hibernate 3.0alpha 54
other more advanced ways to solve this problem, however.)
It's possible to use a non-lazy collection. However, it is intended that lazy initialization be used for almost all
collections, especially for collections of entities, and is now the default. If you define too many non-lazy associ-
ations in your object model, Hibernate will end up needing to fetch the entire database into memory in every
transaction!
Exceptions that occur while lazily initializing a collection are wrapped in a LazyInitializationException.
In some application architectures, particularly where the code that accesses data using Hibernate, and the code
that uses it are in different application layers, it can be a problem to ensure that the Session is open when a col-
lection is initialized. They 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 very end of a user
request, once the rendering of the view is complete. Of course, this places heavy demands upon the correct-
ness 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 render-
ing of the view. The servlet filter has to be able to access the Session for this approach. We recommend
that a ThreadLocal variable be used to hold the current Session (see chapter 1, Section 1.4, “Playing with
cats”, for an example implementation).
• In an application with a seperate business tier, the business logic must "prepare" all collections that will be
needed by the web tier 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 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.
• You may also attach a previously loaded object to a new Session with update() or lock() before access-
ing unitialized collections (or other proxies). Hibernate can not do this automatically, as it would introduce
ad hoc transaction semantics!
You can use a collection filter to get the size of a collection without initializing it:
( (Integer) s.createFilter( collection, "select count(*)" ).list().get(0) ).intValue()
The createFilter() method is also used to efficiently retrieve subsets of a collection without needing to ini-
tialize the whole collection. (And the new <filter> functionality is a more powerful approach.)
6.6. Sorted Collections
Hibernate supports collections implementing java.util.SortedMap and java.util.SortedSet. You must spe-
cify a comparator in the mapping file:
<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"/>
<index column="hol_name" type="string"/>
<element column="hol_date" type="date"/>
</map>
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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 (it is implemented using LinkedHashSet or
LinkedHashMap). This performs the ordering in the SQL query, not in memory.
<set name="aliases" table="person_aliases" order-by="name asc">
<key column="person"/>
<element column="name" type="string"/>
</set>
<map name="holidays" order-by="hol_date, hol_name">
<key column="year_id"/>
<index column="hol_name" type="string"/>
<element column="hol_date type="date"/>
</map>
Note that the value of the order-by attribute is an SQL ordering, not a HQL ordering!
Associations may even be sorted by some arbitrary criteria at runtime using a filter().
sortedUsers = s.filter( group.getUsers(), "order by this.name" );
6.7. Using an <idbag>
If you've fully embraced our view that composite keys are a bad thing and that entities should have synthetic
identifiers (surrogate keys), then you might find it a bit odd that the many to many associations and collections
of values that we've shown so far all map to tables with composite keys! Now, this point is quite arguable; a
pure association table doesn't seem to benefit much from a surrogate key (though a collection of composite val-
ues might). Nevertheless, 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.
<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="eg.Person" outer-join="true"/>
</idbag>
As you can see, 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 provide any mechanism to discover the surrogate key value
of a particular row, however.
Note that the update performance of an <idbag> is much better than a regular <bag>! Hibernate can locate indi-
vidual rows efficiently and update or delete them individually, just like a list, map or set.
In the current implementation, the native identifier generation strategy is not supported for <idbag> collection
identifiers.
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6.8. 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, single-valued at the other
many-to-many
set or bag valued at both ends
Please note that Hibernate does not support bidirectional one-to-many associations with an indexed collection
(list, map or array) as the "many" end, you have to use a set or bag mapping.
You may specify a bidirectional many-to-many association simply by mapping two many-to-many associations
to the same database table and declaring one end as inverse (which one is your choice). Here's an example of a
bidirectional many-to-many association from a class back to itself (each category can have many items and
each item can be in many categories):
<class name="org.hibernate.auction.Category">
<id name="id" column="ID"/>
...
<bag name="items" table="CATEGORY_ITEM">
<key column="CATEGORY_ID"/>
<many-to-many class="org.hibernate.auction.Item" column="ITEM_ID"/>
</bag>
</class>
<class name="org.hibernate.auction.Item">
<id name="id" column="ID"/>
...
<!-- inverse end -->
<bag name="categories" table="CATEGORY_ITEM" inverse="true">
<key column="ITEM_ID"/>
<many-to-many class="org.hibernate.auction.Category" column="CATEGORY_ID"/>
</bag>
</class>
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 we create a many-to-many re-
lationship in Java:
category.getItems().add(item); // The category now "knows" about the relationship
item.getCategories().add(category); // The item now "knows" about the relationship
session.update(item); // No effect, nothing will be saved!
session.update(category); // The relationship will be saved
The non-inverse side is used to save the in-memory representation to the database. We would get an unnec-
cessary INSERT/UPDATE and probably even a foreign key violation if both would trigger changes! The same
is of course also true for bidirectional one-to-many associations.
You may map 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".
<class name="eg.Parent">
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Hibernate 3.0alpha 57
<id name="id" column="id"/>
....
<set name="children" inverse="true">
<key column="parent_id"/>
<one-to-many class="eg.Child"/>
</set>
</class>
<class name="eg.Child">
<id name="id" column="id"/>
....
<many-to-one name="parent" class="eg.Parent" column="parent_id"/>
</class>
Mapping one end of an association with inverse="true" doesn't affect the operation of cascades, both are dif-
ferent concepts!
6.9. Ternary Associations
There are three possible approaches to mapping a ternary association. One approach is to use composite ele-
ments (discussed below). Another is to use a Map with an association as its index:
<map name="contracts">
<key column="employer_id"/>
<index-many-to-many column="employee_id" class="Employee"/>
<one-to-many column="contract_id" class="Contract"/>
</map>
<map name="connections">
<key column="node1_id"/>
<index-many-to-many column="node2_id" class="Node"/>
<many-to-many column="connection_id" class="Connection"/>
</map>
A final alternative is to simply remodel the association as an entity class. This is the approach we use most
commonly.
6.10. Heterogeneous Associations
The <many-to-any> and <index-many-to-any> elements provide for true heterogeneous associations. These
mapping elements work in the same way as the <any> element - and should also be used rarely, if ever.
6.11. Collection examples
The previous sections are pretty confusing. So lets look at an example. This class:
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; }
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private void setChildren(Set children) { this.children=children; }
....
....
}
has a collection of eg.Child instances. If each child has at most one parent, the most natural mapping is a one-
to-many association:
<hibernate-mapping>
<class name="eg.Parent">
<id name="id">
<generator class="sequence"/>
</id>
<set name="children">
<key column="parent_id"/>
<one-to-many class="eg.Child"/>
</set>
</class>
<class name="eg.Child">
<id name="id">
<generator class="sequence"/>
</id>
<property name="name"/>
</class>
</hibernate-mapping>
This maps to the following table definitions:
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
If the parent is required, use a bidirectional one-to-many association:
<hibernate-mapping>
<class name="eg.Parent">
<id name="id">
<generator class="sequence"/>
</id>
<set name="children" inverse="true">
<key column="parent_id"/>
<one-to-many class="eg.Child"/>
</set>
</class>
<class name="eg.Child">
<id name="id">
<generator class="sequence"/>
</id>
<property name="name"/>
<many-to-one name="parent" class="eg.Parent" column="parent_id" not-null="true"/>
</class>
</hibernate-mapping>
Notice the NOT NULL constraint:
create table parent ( id bigint not null primary key )
create table child ( id bigint not null
primary key,
name varchar(255),
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Hibernate 3.0alpha 59
parent_id bigint not null )
alter table child add constraint childfk0 (parent_id) references parent
On the other hand, if a child might have multiple parents, a many-to-many association is appropriate:
<hibernate-mapping>
<class name="eg.Parent">
<id name="id">
<generator class="sequence"/>
</id>
<set name="children" table="childset">
<key column="parent_id"/>
<many-to-many class="eg.Child" column="child_id"/>
</set>
</class>
<class name="eg.Child">
<id name="id">
<generator class="sequence"/>
</id>
<property name="name"/>
</class>
</hibernate-mapping>
Table definitions:
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
Collection Mapping
Hibernate 3.0alpha 60
Chapter 7. Component Mapping
The notion of a component is re-used in several different contexts, for different purposes, throughout Hibernate.
7.1. Dependent objects
A component is a contained object that is persisted as a value type, not an entity. The term "component" refers
to the object-oriented notion of composition (not to architecture-level components). For example, you might
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) {
this.last = last;
}
public char getInitial() {
return initial;
}
void setInitial(char initial) {
this.initial = initial;
}
}
Now Name may be persisted as a component of Person. Notice that Name defines getter and setter methods for
its persistent properties, but doesn't need to declare any interfaces or identifier properties.
Hibernate 3.0alpha 61
Our Hibernate mapping would look like:
<class name="eg.Person" table="person">
<id name="Key" column="pid" type="string">
<generator class="uuid.hex"/>
</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>
The person table would have the columns pid, birthday, initial, first and last.
Like all value types, components do not support shared references. 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 should be okay for most purposes.
The properties of a component may 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 sup-
port a very fine-grained object model.
The <component> element allows a <parent> subelement that maps a property of the component class as a ref-
erence back to the containing entity.
<class name="eg.Person" table="person">
<id name="Key" column="pid" type="string">
<generator class="uuid.hex"/>
</id>
<property name="birthday" type="date"/>
<component name="Name" class="eg.Name">
<parent name="namedPerson"/> <!-- reference back to the Person -->
<property name="initial"/>
<property name="first"/>
<property name="last"/>
</component>
</class>
7.2. Collections of dependent objects
Collections of components are supported (eg. an array of type Name). Declare your component collection by re-
placing the <element> tag with a <composite-element> tag.
<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>
Note: if you define a Set of composite elements, it is very important to implement equals() and hashCode()
correctly.
Composite elements may contain components but not collections. If your composite element itself contains
components, use the <nested-composite-element> tag. This is a pretty exotic case - a collection of compon-
Component Mapping
Hibernate 3.0alpha 62
ents which themselves have components. By this stage you should be asking yourself if a one-to-many associ-
ation is more appropriate. Try remodelling the composite element as an entity - but note that even though the
Java model is the same, the relational model and persistence semantics are still slightly different.
Please note that a composite element mapping doesn't support null-able properties if you're using a <set>. Hi-
bernate has to use each columns value to identify a record when deleting objects (there is no separate primary
key column in the composite element table), 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. A map-
ping like this 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:
<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>
Even ternary (or quaternary, etc) associations are possible:
<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>
Composite elements may appear in queries using the same syntax as associations to other entities.
7.3. Components as Map indices
The <composite-index> element lets you map a component class as the key of a Map. Make sure you override
hashCode() and equals() correctly on the component class.
7.4. Components as composite identifiers
You may use a component as an identifier of an entity class. Your component class must satisfy certain require-
ments:
• It must implement java.io.Serializable.
• It must re-implement equals() and hashCode(), consistently with the database's notion of composite key
equality.
You can't use an IdentifierGenerator to generate composite keys. Instead the application must assign its own
Component Mapping
Hibernate 3.0alpha 63
identifiers.
Since a composite identifier must be assigned to the object before saving it, we can't use unsaved-value of the
identifier to distinguish between newly instantiated transient instances and detached instances from a previous
session.
So, if you wish to use transitive reattachment (you don't have to), you must either implement Intercept-
or.isUnsaved() or define the unsaved-value of a <version> or <timestamp> element.
Use the <composite-id> tag (with nested <key-property> elements) in place of the usual <id> declaration:
<class name="eg.Foo" table"FOOS">
<composite-id name="compId" class="eg.FooCompositeID">
<key-property name="string"/>
<key-property name="short"/>
<key-property name="date" column="date_" type="date"/>
</composite-id>
<property name="name"/>
....
</class>
Now, any foreign keys into the table FOOS are also composite. You must declare this in your mappings for other
classes. An association to Foo would be declared like this:
<many-to-one name="foo" class="eg.Foo">
<!-- the "class" attribute is optional, as usual -->
<column name="foo_string"/>
<column name="foo_short"/>
<column name="foo_date"/>
</many-to-one>
This new <column> tag is also used by multi-column custom types. Actually it is an alternative to the column at-
tribute everywhere. A collection with elements of type Foo would use:
<set name="foos">
<key column="owner_id"/>
<many-to-many class="eg.Foo">
<column name="foo_string"/>
<column name="foo_short"/>
<column name="foo_date"/>
</many-to-many>
</set>
On the other hand, <one-to-many>, as usual, declares no columns.
If Foo itself contains collections, they will also need a composite foreign key.
<class name="eg.Foo">
....
....
<set name="dates" lazy="true">
<key> <!-- a collection inherits the composite key type -->
<column name="foo_string"/>
<column name="foo_short"/>
<column name="foo_date"/>
</key>
<element column="foo_date" type="date"/>
</set>
</class>
7.5. Dynamic components
Component Mapping
Hibernate 3.0alpha 64
You may even map a property of type Map:
<dynamic-component name="userAttributes">
<property name="foo" column="FOO"/>
<property name="bar" column="BAR"/>
<many-to-one name="baz" class="eg.Baz" column="BAZ"/>
</dynamic-component>
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.
Even better, you can access (and change) Hibernate's configuration-time metamodel via the Configuration ob-
ject.
Component Mapping
Hibernate 3.0alpha 65
Chapter 8. Inheritance Mapping
8.1. The Three Strategies
Hibernate supports the three basic inheritance mapping strategies.
• table per class hierarchy
• table per subclass
• table per concrete class (some limitations)
It is even possible to use different mapping strategies for different branches of the same inheritance hierarchy,
but the same limitations apply as apply to table-per-concrete class mappings. Hibernate does not support mix-
ing <subclass> mappings and <joined-subclass> mappings inside the same <class> element. However, it is
possible to use a <join> element to map this.
Suppose we have an interface Payment, with implementors CreditCardPayment, CashPayment, ChequePayment.
The table-per-hierarchy mapping would look like:
<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">
...
</subclass>
<subclass name="CashPayment" discriminator-value="CASH">
...
</subclass>
<subclass name="ChequePayment" discriminator-value="CHEQUE">
...
</subclass>
</class>
Exactly one table is required. There is one big limitation of this mapping strategy: columns declared by the sub-
classes may not have NOT NULL constraints.
A table-per-subclass mapping would look like:
<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"/>
...
</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"/>
...
Hibernate 3.0alpha 66
</joined-subclass>
</class>
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).
Note that Hibernate's implementation of table-per-subclass requires no discriminator column. Other object/
relational mappers use a different implementation of table-per-subclass which requires a type discriminator
column in the superclass table. The approach taken by Hibernate is much more difficult to implement but argu-
ably more correct from a relational point of view.
TODO: document usage of join for discriminators in table-per-subclass
TODO: document usage of join for mixing inheritance mapping strategies
For either of these two mapping strategies, a polymorphic association to Payment is mapped using
<many-to-one>.
<many-to-one name="payment"
column="PAYMENT"
class="Payment"/>
The table-per-concrete-class strategy is very different.
<class name="CreditCardPayment" table="CREDIT_PAYMENT">
<id name="id" type="long" column="CREDIT_PAYMENT_ID">
<generator class="native"/>
</id>
<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>
Three tables were required. Notice that nowhere do we mention the Payment interface explicitly. Instead, we
make use of Hibernate's implicit polymorphism. Also notice that properties of Payment are mapped in each of
the subclasses.
In this case, a polymorphic association to Payment is mapped using <any>.
<any name="payment"
meta-type="class"
id-type="long">
<column name="PAYMENT_CLASS"/>
<column name="PAYMENT_ID"/>
</any>
Inheritance Mapping
Hibernate 3.0alpha 67
It would be better if we defined a UserType as the meta-type, to handle the mapping from type discriminator
strings to Payment subclass.
<any name="payment"
meta-type="PaymentMetaType"
id-type="long">
<column name="PAYMENT_TYPE"/> <!-- CREDIT, CASH or CHEQUE -->
<column name="PAYMENT_ID"/>
</any>
There is one further thing to notice about this mapping. 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
table-per-class or table-per-subclass inheritance hierarchy! (And you can still use polymorphic queries against
the Payment interface.)
<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"/>
</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>
Once again, we don't mention Payment explicitly. If we execute a query against the Payment interface - for ex-
ample, from Payment - Hibernate automatically returns instances of CreditCardPayment (and its subclasses,
since they also implement Payment), CashPayment and ChequePayment but not instances of Nonelectronic-
Transaction.
TODO: Document union-subclass for polymorphic-table-per-concrete-class mappings
8.2. Limitations
There are certain limitations to the "implicit polymorphism" approach to the table-per-concrete-class mapping
strategy. There are somewhat less restrictive limitations to <union-subclass> mappings. (TODO)
The following table shows the limitations of table-per-concrete-class mappings, and of implicit polymorphism,
in Hibernate.
Table 8.1. Features of inheritance mappings
Inheritance Mapping
Hibernate 3.0alpha 68
Inherit-
ance
strategy
Poly-
morphic
many-
to-one
Poly-
morphic
one-to-one
Poly-
morphic
one-
to-many
Poly-
morphic
many-
to-many
Poly-
morphic
load()/get
()
Poly-
morphic
queries
Poly-
morphic
joins
table-
per-
class-
hierarchy
<many-to-o
ne>
<one-to-on
e>
<one-to-ma
ny>
<many-to-m
any>
s.get(Paym
ent.class,
id)
from Pay-
ment p
from Order
o join
o.payment
p
table-
per-
subclass
<many-to-o
ne>
<one-to-on
e>
<one-to-ma
ny>
<many-to-m
any>
s.get(Paym
ent.class,
id)
from Pay-
ment p
from Order
o join
o.payment
p
table-
per-
concrete-
class
(implicit
polymorph-
ism)
<any> not suppor-
ted
not suppor-
ted
<many-to-a
ny>
use a query from Pay-
ment p
not suppor-
ted
table-
per-
concrete-
class
(union-subc
lass)
<many-to-o
ne>
<one-to-on
e>
<one-to-ma
ny> (for
in-
verse="tru
e" only)
<many-to-m
any>
s.get(Paym
ent.class,
id)
from Pay-
ment p
from Order
o join
o.payment
p
Inheritance Mapping
Hibernate 3.0alpha 69
Chapter 9. Working with Persistent Data
9.1. Creating a persistent object
Newly instantiated instances of a a persistent class are considered transient by Hibernate. We can make a tran-
sient 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);
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().
Alternatively, you may assign the identifier using an overloaded version of save().
DomesticCat pk = new DomesticCat();
pk.setColor(Color.TABBY);
pk.setSex('F');
pk.setName("PK");
pk.setKittens( new HashSet() );
pk.addKitten(fritz);
sess.save( pk, new Long(1234) );
Associated objects may 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.
If you enable cascade save on your associations, even NOT NULL constraint violations are impossible - Hibernate
will take care of everything.
9.2. Loading an object
The load() methods of Session give you a way to retrieve a persistent instance if you already know its identi-
fier. load() takes a class object and will load the state into a newly instantiated instance of that class.
Cat fritz = (Cat) sess.load(Cat.class, generatedId);
// you need to wrap primitive identifiers
long pkId = 1234;
DomesticCat pk = (DomesticCat) sess.load( Cat.class, new Long(pkId) );
Alternatively, you can load state into a given instance:
Cat cat = new DomesticCat();
// load pk's state into cat
sess.load( cat, new Long(pkId) );
Set kittens = cat.getKittens();
Note 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
Hibernate 3.0alpha 70
invoke a method of the proxy. This behaviour is very 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 im-
mediately and returns null if there is no matching row.
Cat cat = (Cat) sess.get(Cat.class, id);
if (cat==null) {
cat = new Cat();
sess.save(cat, id);
}
return cat;
You may even load an object using an SQL SELECT ... FOR UPDATE. See the next section for a discussion of
Hibernate LockModes.
Cat cat = (Cat) sess.get(Cat.class, id, LockMode.UPGRADE);
Note that any associated instances or contained collections are not selected FOR UPDATE.
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.
sess.save(cat);
sess.flush(); //force the SQL INSERT
sess.refresh(cat); //re-read the state (after the trigger executes)
9.3. Querying
If you don't 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.
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();
The call to createQuery() returns an instance of org.hibernate.Query which may be used to bind arguments
to the ? parameter placeholders. (which map to IN parameters of a JDBC PreparedStatement). Just as in JD-
BC, you should always use this binding mechanism in preference to string manipulation.
A query is usually executed by invoking list().
Working with Persistent Data
Hibernate 3.0alpha 71
Occasionally, you might be able to achieve better performance by executing the query using the iterate()
method. This will only 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 find() and might require many database hits for a simple query.
Hibernate queries sometimes return tuples of objects, in which case each tuple is returned as an array:
Iterator kittensAndMothers = sess.createQuery(
"select kitten, mother from Cat kitten join kitten.mother mother")
.list()
.iterator();
while ( kittensAndMothers.hasNext() ) {
Object[] tuple = (Object[]) kittensAndMothers.next();
Cate kittem = tuple[0]; Cat mother = tuple[1];
....
}
9.3.1. Scalar queries
Queries may specify a property of a class in the select clause. They may even call SQL aggregate functions.
Properties or aggregates are considered "scalar" results.
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 = results.next();
Color type = (Color) row[0];
Date oldest = (Date) row[1];
Integer count = (Integer) row[2];
.....
}
List results = sess.createQuery(
"select cat.type, cat.birthdate, cat.name from DomesticCat cat")
.list();
9.3.2. The Query interface
If you need to specify bounds upon your result set (the maximum number of rows you want to retrieve and / or
the first row you want to retrieve) you should use methods of the Query interface:
Query q = sess.createQuery("from DomesticCat cat");
q.setFirstResult(20);
q.setMaxResults(10);
List cats = q.list();
You may even define a named query in the mapping document. (Remember to use a CDATA section if your
query contains characters that could be interpreted as markup.)
<query name="eg.DomesticCat.by.name.and.minimum.weight"><![CDATA[
from eg.DomesticCat as cat
where cat.name = ?
and cat.weight > ?
] ]></query>
Working with Persistent Data
Hibernate 3.0alpha 72
Query q = sess.getNamedQuery("eg.DomesticCat.by.name.and.minimum.weight");
q.setString(0, name);
q.setInt(1, minWeight);
List cats = q.list();
The query interface supports the use of named parameters. Named parameters are identifiers of the form :name
in the query string. There are methods on Query for binding values to named parameters or JDBC-style ? para-
meters. Contrary to JDBC, Hibernate numbers parameters from zero. The advantages of named parameters
are:
• named parameters are insensitive to the order they occur in the query string
• they may occur multiple times in the same query
• they are self-documenting
//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();
9.3.3. Scrollable iteration
If your JDBC driver supports scrollable ResultSets, the Query interface may be used to obtain a Scrolla-
bleResults which allows more flexible navigation of the query results.
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 {
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) );
}
9.3.4. Filtering collections
Working with Persistent Data
Hibernate 3.0alpha 73
A collection filter is a special type of query that may be applied to a persistent collection or array. The query
string may refer to this, meaning the current collection element.
Collection blackKittens = session.createFilter(
pk.getKittens(),
"where this.color = ?")
.setParameter( Color.BLACK, Hibernate.custom(ColorUserType.class) )
.list()
);
The returned collection is considered a bag.
Observe that filters do not require a from clause (though they may have one if required). Filters are not limited
to returning the collection elements themselves.
Collection blackKittenMates = session.createFilter(
pk.getKittens(),
"select this.mate where this.color = eg.Color.BLACK.intValue")
.list();
9.3.5. Criteria queries
HQL is extremely powerful but some people prefer to build queries dynamically, using an object oriented API,
rather than embedding strings in their Java code. For these people, Hibernate provides an intuitive Criteria
query API.
Criteria crit = session.createCriteria(Cat.class);
crit.add( Expression.eq( "color", eg.Color.BLACK ) );
crit.setMaxResults(10);
List cats = crit.list();
If you are uncomfortable with SQL-like syntax, this is perhaps the easiest way to get started with Hibernate.
This API is also more extensible than HQL. Applications might provide their own implementations of the Cri-
terion interface.
9.3.6. Queries in native SQL
You may express a query in SQL, using createSQLQuery(). You must enclose SQL aliases in braces.
List cats = session.createSQLQuery(
"SELECT {cat.*} FROM CAT {cat} WHERE ROWNUM<10",
"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}, ... " +
"FROM CAT {cat} WHERE ROWNUM<10",
"cat",
Cat.class
).list()
SQL queries may contain named and positional parameters, just like Hibernate queries.
9.4. Updating objects
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9.4.1. Updating in the same Session
Transactional persistent instances (ie. objects loaded, saved, created or queried by the Session) may be manip-
ulated by the application and any changes to persistent state will be persisted when the Session is flushed
(discussed later in this chapter). So 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:
DomesticCat cat = (DomesticCat) sess.load( Cat.class, new Long(69) );
cat.setName("PK");
sess.flush(); // changes to cat are automatically detected and persisted
Sometimes this programming model is inefficient since it would require both an SQL SELECT (to load an ob-
ject) and an SQL UPDATE (to persist its updated state) in the same session. Therefore Hibernate offers an altern-
ate approach.
9.4.2. Updating 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 envir-
onment usually use versioned data to ensure transaction isolation.) This approach requires a slightly different
programming model to the one described in the last section. Hibernate supports this model by providing for
reattachment of detached instances using the the method Session.update().
// in the first session
Cat cat = (Cat) firstSession.load(Cat.class, catId);
Cat potentialMate = new Cat();
firstSession.save(potentialMate);
// in a higher tier of the application
cat.setMate(potentialMate);
// later, in a new session
secondSession.update(cat); // update cat
secondSession.update(mate); // update mate
If the Cat with identifier catId had already been loaded by secondSession when the application tried to update
it, an exception would have been thrown.
The application should individually update() detached instances reachable from the given detached instance if
and only if it wants their state also updated. (Except for lifecycle objects, discussed later.)
Hibernate users have requested a general purpose method that either saves a transient instance by generating a
new identifier or update the persistent state associated with its current identifier. The saveOrUpdate() method
now implements this functionality.
Hibernate distinguishes "new" transient instances from detached instances by the value of the identifier (or ver-
sion, or timestamp) property. The unsaved-value attribute of the <id> (or <version>, or <timestamp>) map-
ping specifies which values should be interpreted as representing a new transient instance.
<id name="id" type="long" column="uid" unsaved-value="null">
<generator class="hilo"/>
</id>
The allowed values of unsaved-value are:
• any - always save
• none - always update
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• null - save when identifier is null (this is the default)
• valid identifier value - save when identifier is null or the given value
• undefined - the default for version or timestamp, then identifier check is used
// 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 non-null id)
secondSession.saveOrUpdate(mate); // save the new instance (mate has a null id)
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() or
saveOrUpdate(). 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 the object has no identifier property, save() it
• if the object's identifier matches the criteria specified by unsaved-value, save() it
• if the object is versioned (version or timestamp), then the version will take precedence to identifier check,
unless the versions unsaved-value="undefined" (default value)
• if another object associated with the session has the same identifier, throw an exception
The last case can be avoided by using saveOrUpdateCopy(Object o). This method copies the state of the given
object onto the persistent object with the same identifier. If there is no persistent instance currently associated
with the session, it will be loaded. The method return the persistent instance. If the given instance is unsaved or
does not exist in the database, Hibernate will save it and return it as a newly persistent instance. Otherwise, the
given instance does not become associated with the session. In most applications with detached objects, you
need both methods, saveOrUpdate() and saveOrUpdateCopy().
9.4.3. Reattaching detached objects
The lock() method allows the application to reassociate an unmodified object with a new session.
//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);
9.5. Deleting persistent objects
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Session.delete() will remove an object's state from the database. Of course, your application might still hold
a reference to it. So it's best to think of delete() as making a persistent instance transient.
sess.delete(cat);
You may also delete many objects at once by passing a Hibernate query string to delete().
You may now delete objects in any order you like, without risk of foreign key constraint violations. Of course,
it is still possible to violate a NOT NULL constraint on a foreign key column by deleting objects in the wrong or-
der.
9.6. Flush
From time to time 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, flush, occurs by default at the following points
• from some invocations of find() or iterate()
• 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 explicity 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 Ses-
sion.find(..) methods will never return stale data; nor will they return the wrong data.
It is possible to change the default behavior so that flush occurs less frequently. The FlushMode class defines
three different modes. This is most useful in the case of "readonly" transactions, where it might be used to
achieve a (very) slight performance increase.
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);
// execute some queries....
sess.find("from Cat as cat left outer join cat.kittens kitten");
//change to izi is not flushed!
...
tx.commit(); //flush occurs
9.7. Ending a Session
Ending a session involves four distinct phases:
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• flush the session
• commit the transaction
• close the session
• handle exceptions
9.7.1. Flushing the Session
If you happen to be using the Transaction API, you don't need to worry about this step. It will be performed
implicitly when the transaction is committed. Otherwise you should call Session.flush() to ensure that all
changes are synchronized with the database.
9.7.2. Committing the database transaction
If you are using the Hibernate Transaction API, this looks like:
tx.commit(); // flush the Session and commit the transaction
If you are managing JDBC transactions yourself you should manually commit() the JDBC connection.
sess.flush();
sess.connection().commit(); // not necessary for JTA datasource
If you decide not to commit your changes:
tx.rollback(); // rollback the transaction
or:
// not necessary for JTA datasource, important otherwise
sess.connection().rollback();
If you rollback the transaction you should immediately close and discard the current session to ensure that Hi-
bernate's internal state is consistent.
9.7.3. Closing the Session
A call to Session.close() marks the end of a session. The main implication of close() is that the JDBC con-
nection will be relinquished by the session.
tx.commit();
sess.close();
sess.flush();
sess.connection().commit(); // not necessary for JTA datasource
sess.close();
If you provided your own connection, close() returns a reference to it, so you can manually close it or return it
to the pool. Otherwise close() returns it to the pool.
9.7.4. Exception handling
If the Session throws an exception (including any SQLException), you should immediately rollback the trans-
action, call Session.close() and discard the Session instance. Certain methods of Session will not leave the
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session in a consistent state.
The following exception handling idiom is recommended:
Session sess = factory.openSession();
Transaction tx = null;
try {
tx = sess.beginTransaction();
// do some work
...
tx.commit();
}
catch (Exception e) {
if (tx!=null) tx.rollback();
throw e;
}
finally {
sess.close();
}
Or, when manually managing JDBC transactions:
Session sess = factory.openSession();
try {
// do some work
...
sess.flush();
sess.connection().commit();
}
catch (Exception e) {
sess.connection().rollback();
throw e;
}
finally {
sess.close();
}
Or, when using a datasource enlisted with JTA:
UserTransaction ut = .... ;
Session sess = factory.openSession();
try {
// do some work
...
sess.flush();
}
catch (Exception e) {
ut.setRollbackOnly();
throw e;
}
finally {
sess.close();
}
9.8. Lifecyles and object graphs
To save or update all objects in a graph of associated objects, you must either
• save(), saveOrUpdate() or update() each individual object OR
• map associated objects using or cascade="save-update", cascade="all" or cas-
cade="all-delete-orphan".
Likewise, to delete all objects in a graph, either
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• delete() each individual object OR
• map associated objects using cascade="all", cascade="all-delete-orphan" or cascade="delete".
Recommendation:
• If the child object's lifespan is bounded by the lifespan of the of the parent object make it a lifecycle object
by specifying cascade="all".
• Otherwise, save() and delete() it explicitly from application code. If you really want to save yourself
some extra typing, use cascade="save-update" and explicit delete().
Mapping an association (many-to-one, or collection) with cascade="all" marks the association as a parent/
child style relationship where save/update/deletion of the parent results in save/update/deletion of the
child(ren). Futhermore, a mere reference to a child from a persistent parent will result in save / update of the
child. The metaphor is incomplete, however. A child which becomes unreferenced by its parent is not automat-
ically deleted, except in the case of a <one-to-many> association mapped with cascade="all-delete-orphan".
The precise semantics of cascading operations are as follows:
• If a parent is saved, all children are passed to saveOrUpdate()
• If a parent is passed to 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="all-delete-orphan", in which case the "orphaned" child is
deleted.
Hibernate does not fully implement "persistence by reachability", which would imply (inefficient) persistent
garbage collection. However, due to popular demand, Hibernate does support the notion of entities becoming
persistent when referenced by another persistent object. Associations marked cascade="save-update" behave
in this way. If you wish to use this approach throughout your application, its easier to specify the default-cas-
cade attribute of the <hibernate-mapping> element.
9.9. Parameterized application views with filters
Hibernate3 adds the ability to pre-define filter criteria and attach those filters at both a class and a collection
level. A filter criteria is the ability to define a restriction clause very similiar to the existing "where" attribute
available on the class and various collection elements. Except these filter conditions can be parameterized. The
application can then make the decision at runtime whether given filters should be enabled and what their para-
meter values should be. Filters can be used like database views, but parameterized inside the application.
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:
<filter-def name="myFilter">
<filter-param name="myFilterParam" type="string"/>
</filter-def>
Then, this filter can be attached to a class:
<class name="myClass" ...>
...
<filter name="myFilter" condition=":myFilterParam = MY_FILTERED_COLUMN"/>
</class>
or, to a collection:
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<set ...>
<filter name="myFilter" condition=":myFilterParam = MY_FILTERED_COLUMN"/>
</set>
or, even 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; they must be
explcitly enabled through use of the Session.enabledFilter() method, which returns an instance of the Fil-
ter interface. Using the simple filter defined above, this would look like:
session.enableFilter("myFilter").setParameter("myFilterParam", "some-value");
Note that methods on the org.hibernate.Filter interface do allow the method-chaining common to much of Hi-
bernate.
A full example, using temporal data with an effective record date pattern:
<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>
Then, in order to ensure that you always get back currently effective records, simply enable the filter on the ses-
sion prior to retrieving employee data:
Session session = ...;
session.enabledFilter("effectiveDate").setParameter("asOfDate", new Date());
List results = session.createQuery("from Employee as e where e.salary > :targetSalary")
.setLong("targetSalary", new Long(1000000))
.list();
In the HQL above, even though we only explicitly mentioned a salary constraint on the results, because of the
enabled filter the query will return only currently active employees who have a salary greater than a million
dollars.
Note: if you plan on using filters with outer joining (either through HQL or load fetching) be careful of the dir-
ection of the condition expression. Its safest to set this up for left outer joining; in general, place the parameter
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first followed by the column name(s) after the operator.
9.10. Interceptors
The Interceptor interface provides callbacks from the session to the application allowing the application to in-
spect 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.
package org.hibernate.test;
import java.io.Serializable;
import java.util.Date;
import java.util.Iterator;
import org.hibernate.Interceptor;
import org.hibernate.type.Type;
public class AuditInterceptor implements Interceptor, Serializable {
private int updates;
private int creates;
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;
}
}
}
return false;
}
public boolean onLoad(Object entity,
Serializable id,
Object[] state,
String[] propertyNames,
Type[] types) {
return false;
}
public boolean onSave(Object entity,
Serializable id,
Object[] state,
String[] propertyNames,
Type[] types) {
if ( entity instanceof Auditable ) {
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creates++;
for ( int i=0; i<propertyNames.length; i++ ) {
if ( "createTimestamp".equals( propertyNames[i] ) ) {
state[i] = new Date();
return true;
}
}
}
return false;
}
public void postFlush(Iterator entities) {
System.out.println("Creations: " + creates + ", Updates: " + updates);
}
public void preFlush(Iterator entities) {
updates=0;
creates=0;
}
......
......
}
The interceptor would be specified when a session is created.
Session session = sf.openSession( new AuditInterceptor() );
You may also set an interceptor on a global level, using the Configuration:
new Configuration().setInterceptor( new AuditInterceptor() );
9.11. Event system
If you have to react to particular events in your persistence layer, you may also use the Hibernate3 event archi-
tecture. The event system can be used in addition or as a replacement for interceptors.
Essentially all of 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 listener 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 implemenation
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 effectively singletons; meaning, they are shared between requests, and thus
should not save any state as instance variables. The event objects themselves, however, do hold a lot of the con-
text needed for processing as they are unique to each request. Custom event listeners may also make use of the
event's context for storage of any needed processing variables. The context is a simple map, but the default
listeners don't use the context map at all, so don't worry about over-writing internally required context vari-
ables.
A custom listener should implement 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 programatically through
the Configuration object, or specified in the Hibernate configuration XML (declarative configuration through
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the properties file is not supported). Here's an example of a custom load event listener:
public class MyLoadListener extends DefaultLoadEventListener {
// this is the single method defined by the LoadEventListener interface
public Object onLoad(LoadEvent event, LoadEventListener.LoadType loadType)
throws HibernateException {
if ( !MySecurity.isAuthorized( event.getEntityName(), event.getEntityId() ) ) {
throw MySecurityException("Unauthorized access");
}
return super.onLoad(event, loadType);
}
}
You also need a configuration entry telling Hibernate to use the listener instead of the default listener:
<hibernate-configuration>
<session-factory>
...
<listener type="load" class="MyLoadListener"/>
</session-factory>
</hibernate-configuration>
Instead, you may register it programatically:
Configuration cfg = new Configuration();
cfg.getSessionEventListenerConfig().setLoadEventListener( new MyLoadListener() );
Listeners registered declaratively cannot share instances. If the same class name is used in multiple
<listener/> elements, each reference will result in a seperate instance of that class. If you need the capability
to share listener instances between listener types you must use the programatic registration approach.
Why implement an interface and define the specific type during configuration? Well, 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.
9.12. Metadata API
Hibernate requires a very rich meta-level model of all entity and value types. From time to time, this model is
very 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 should not (eg. immutable value types and, possibly, associated entities).
Hibernate exposes metadata via the ClassMetadata and CollectionMetadata interfaces and the Type hier-
archy. Instances of the metadata interfaces may be obtained from the SessionFactory.
Cat fritz = ......;
Long id = (Long) catMeta.getIdentifier(fritz);
ClassMetadata catMeta = sessionfactory.getClassMetadata(Cat.class);
Object[] propertyValues = catMeta.getPropertyValues(fritz);
String[] propertyNames = catMeta.getPropertyNames();
Type[] propertyTypes = catMeta.getPropertyTypes();
// get a Map of all properties which are not collections or associations
// TODO: what about components?
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 10. Transactions And Concurrency
Hibernate is not itself a database. It is a lightweight object-relational mapping tool. Transaction management is
delegated to the underlying database connection. If the connection is enlisted with JTA, operations performed
by the Session are atomically part of the wider JTA transaction. Hibernate can be seen as a thin adapter to JD-
BC, adding object- oriented semantics.
10.1. Configurations, Sessions and Factories
A SessionFactory is an expensive-to-create, threadsafe object intended to be shared by all application threads.
A Session is an inexpensive, non-threadsafe object that should be used once, for a single business process, and
then discarded. For example, when using Hibernate in a servlet-based application, servlets could obtain a Ses-
sionFactory using
SessionFactory sf = (SessionFactory)getServletContext().getAttribute("my.session.factory");
Each call to a service method could create a new Session, flush() it, commit() its connection, close() it and
finally discard it. (The SessionFactory may also be kept in JNDI or in a static Singleton helper variable.)
In a stateless session bean, a similar approach could be used. The bean would obtain a SessionFactory in set-
SessionContext(). Then each business method would create a Session, flush() it and close() it. Of course,
the application should not commit() the connection. (Leave that to JTA, the database connection participates
automatically in container-managed transactions.)
We use the Hibernate Transaction API as discussed previously, a single commit() of a Hibernate Transaction
flushes the state and commits any underlying database connection (with special handling of JTA transactions).
Ensure you understand the semantics of flush(). Flushing synchronizes the persistent store with in-memory
changes but not vice-versa. Note that for all Hibernate JDBC connections/transactions, the transaction isolation
level for that connection applies to all operations executed by Hibernate!
The next few sections will discuss alternative approaches that utilize versioning to ensure transaction atomicity.
These are considered "advanced" approaches to be used with care.
10.2. Threads and connections
You should observe the following practices when creating Hibernate Sessions:
• Never create more than one concurrent Session or Transaction instance per database connection.
• Be extremely careful when creating more than one Session per database per transaction. The Session itself
keeps track of updates made to loaded objects, so a different Session might see stale data.
• The Session is not threadsafe! Never access the same Session in two concurrent threads. A Session is
usually only a single unit-of-work!
10.3. Considering object identity
The application may concurrently access the same persistent state in two different units-of-work. However, an
instance of a persistent class is never shared between two Session instances. Hence there are two different no-
tions of identity:
Hibernate 3.0alpha 85
Database Identity
foo.getId().equals( bar.getId() )
JVM Identity
foo==bar
Then for objects attached to a particular Session, the two notions are equivalent. However, while the applica-
tion might concurrently access the "same" (persistent identity) business object in two different sessions, the two
instances will actually be "different" (JVM identity).
This approach leaves Hibernate and the database to worry about concurrency. The application never needs to
synchronize on any business object, as long as it sticks to a single thread per Session or object identity (within
a Session the application may safely use == to compare objects).
10.4. Optimistic concurrency control
Many business processes require a whole series of interactions with the user interleaved with database accesses.
In web and enterprise applications it is not acceptable for a database transaction to span a user interaction.
Maintaining isolation of business processes becomes the partial responsibility of the application tier, hence we
call this process a long running application transaction. A single application transaction usually spans several
database transactions. It will be atomar if only one of these database transactions (the last one) stores the up-
dated data, all others simply read data.
The only approach that is consistent with high concurrency and high scalability is optimistic concurrency con-
trol with versioning. Hibernate provides for three possible approaches to writing application code that uses op-
timistic concurrency.
10.4.1. Long session with automatic versioning
A single Session instance and its persistent instances are used for the whole application transaction.
The Session uses optimistic locking with versioning to ensure that many database transactions appear to the
application as a single logical application transaction. 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 need not concern itself with version checking or with reattaching detached instances.
// foo is an instance loaded earlier by the Session
session.reconnect();
foo.setProperty("bar");
session.flush();
session.connection().commit();
session.disconnect();
The foo object still knows which Session it was loaded it. As soon as the Session has a JDBC connection, we
commit the changes to the object.
This pattern is problematic if our Session is too big to be stored during user think time, e.g. an HttpSession
should be kept as small as possible. As the Session is also the (mandatory) first-level cache and contains all
loaded objects, we can propably use this strategy only for a few request/response cycles. This is indeed recom-
mended, as the Session will soon also have stale data.
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Hibernate 3.0alpha 86
10.4.2. Many sessions with 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 ori-
ginally loaded in another Session and then "reassociates" them using Session.update() or Ses-
sion.saveOrUpdate().
// foo is an instance loaded by a previous Session
foo.setProperty("bar");
session = factory.openSession();
session.saveOrUpdate(foo);
session.flush();
session.connection().commit();
session.close();
You may also call lock() instead of update() and use LockMode.READ (performing a version check, bypassing
all caches) if you are sure that the object has not been modified.
10.4.3. Application version checking
Each interaction with the database occurs in a new Session that reloads all persistent instances from the data-
base before manipulating them. This approach forces the application to carry out its own version checking to
ensure application transaction isolation. (Of course, Hibernate will still update version numbers for you.) This
approach is the least efficient in terms of database access. It is the approach most similar to entity EJBs.
// foo is an instance loaded by a previous Session
session = factory.openSession();
int oldVersion = foo.getVersion();
session.load( foo, foo.getKey() );
if ( oldVersion!=foo.getVersion ) throw new StaleObjectStateException();
foo.setProperty("bar");
session.flush();
session.connection().commit();
session.close();
Of course, if you are operating in a low-data-concurrency environment and don't require version checking, you
may use this approach and just skip the version check.
10.5. Session disconnection
The first approach described above is to maintain a single Session for a whole business process thats spans
user think time. (For example, a servlet might keep a Session in the user's HttpSession.) For performance
reasons you should
1. commit the Transaction (or JDBC connection) and then
2. disconnect the Session from the JDBC connection
before waiting for user activity. The method Session.disconnect() will disconnect the session from the JD-
BC connection and return the connection to the pool (unless you provided the connection).
Session.reconnect() obtains a new connection (or you may supply one) and restarts the session. After recon-
nection, to force a version check on data you aren't updating, you may call Session.lock() on any objects that
might have been updated by another transaction. You don't need to lock any data that you are updating.
Heres an example:
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SessionFactory sessions;
List fooList;
Bar bar;
....
Session s = sessions.openSession();
Transaction tx = null;
try {
tx = s.beginTransaction();
fooList = s.find(
"select foo from eg.Foo foo where foo.Date = current date"
// uses db2 date function
);
bar = (Bar) s.create(Bar.class);
tx.commit();
}
catch (Exception e) {
if (tx!=null) tx.rollback();
s.close();
throw e;
}
s.disconnect();
Later on:
s.reconnect();
try {
tx = s.beginTransaction();
bar.setFooTable( new HashMap() );
Iterator iter = fooList.iterator();
while ( iter.hasNext() ) {
Foo foo = (Foo) iter.next();
s.lock(foo, LockMode.READ); //check that foo isn't stale
bar.getFooTable().put( foo.getName(), foo );
}
tx.commit();
}
catch (Exception e) {
if (tx!=null) tx.rollback();
throw e;
}
finally {
s.close();
}
You can see from this how the relationship between Transactions and Sessions is many-to-one, A Session
represents a conversation between the application and the database. The Transaction breaks that conversation
up into atomic units of work at the database level.
10.6. Pessimistic Locking
It is not intended that users spend much time worring about locking strategies. Its 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 sometimes wish to obtain exclusive pessimistic locks, or re-obtain locks at the start of a new transac-
tion.
Hibernate will always use the locking mechanism of the database, never lock objects in memory!
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The LockMode class defines the different lock levels that may 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 may be acquired upon explicit user request using SELECT ... FOR UPDATE on databases
which support that syntax.
• LockMode.UPGRADE_NOWAIT may 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. May 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 Trans-
action. Objects associated with the session via a call to update() or saveOrUpdate() also start out in this
lock mode.
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 UP-
GRADE_NOWAIT. (In the case of UPGRADE or UPGRADE_NOWAIT, SELECT ... FOR UPDATE is used.)
If the database does not support the requested lock mode, Hibernate will use an appropriate alternate mode
(instead of throwing an exception). This ensures that applications will be portable.
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Chapter 11. HQL: The Hibernate Query Language
Hibernate is equiped with an extremely powerful query language that (quite intentionally) looks very much like
SQL. But don't be fooled by the syntax; HQL is fully object-oriented, understanding notions like inheritence,
polymorphism and association.
11.1. Case Sensitivity
Queries are case-insensitive, except for names of Java classes and properties. 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 we find this convention ugly when embedded in Java code.
11.2. The from clause
The simplest possible Hibernate query is of the form:
from eg.Cat
which simply returns all instances of the class eg.Cat.
Most of the time, you will need to assign an alias, since you will want to refer to the Cat in other parts of the
query.
from eg.Cat as cat
This query assigns the alias cat to Cat instances, so we could use that alias later in the query. The as keyword
is optional; we could also write:
from eg.Cat cat
Multiple classes may appear, resulting in a cartesian product or "cross" join.
from Formula, Parameter
from Formula as form, Parameter as param
It is considered good practice to name query aliases using an initial lowercase, consistent with Java naming
standards for local variables (eg. domesticCat).
11.3. Associations and joins
We may also assign aliases to associated entities, or even to elements of a collection of values, using a join.
from eg.Cat as cat
inner join cat.mate as mate
left outer join cat.kittens as kitten
from eg.Cat as cat left join cat.mate.kittens as kittens
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from Formula form full join form.parameter param
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.
from eg.Cat as cat
join cat.mate as mate
left join cat.kittens as kitten
In addition, 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.
from eg.Cat as cat
inner join fetch cat.mate
left join fetch cat.kittens
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). Also, the associated objects are not returned directly in the query results. In-
stead, they may be accessed via the parent object.
Note that, in the current implementation, only one collection role may be fetched in a query (everything else
would be non-performant). Note also that the fetch construct may not be used in queries called using scroll()
or iterate(). Finally, note that full join fetch and right join fetch are not meaningful.
11.4. The select clause
The select clause picks which objects and properties to return in the query result set. Consider:
select mate
from eg.Cat as cat
inner join cat.mate as mate
The query will select mates of other Cats. Actually, you may express this query more compactly as:
select cat.mate from eg.Cat cat
You may even select collection elements, using the special elements function. The following query returns all
kittens of any cat.
select elements(cat.kittens) from eg.Cat cat
Queries may return properties of any value type including properties of component type:
select cat.name from eg.DomesticCat cat
where cat.name like 'fri%'
select cust.name.firstName from Customer as cust
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Queries may return multiple objects and/or properties as an array of type Object[]
select mother, offspr, mate.name
from eg.DomesticCat as mother
inner join mother.mate as mate
left outer join mother.kittens as offspr
or as an actual typesafe Java object
select new Family(mother, mate, offspr)
from eg.DomesticCat as mother
join mother.mate as mate
left join mother.kittens as offspr
assuming that the class Family has an appropriate constructor.
11.5. Aggregate functions
HQL queries may even return the results of aggregate functions on properties:
select avg(cat.weight), sum(cat.weight), max(cat.weight), count(cat)
from eg.Cat cat
Collections may also appear inside aggregate functions in the select clause.
select cat, count( elements(cat.kittens) )
from eg.Cat cat group by cat
The supported aggregate functions are
• avg(...), sum(...), min(...), max(...)
• count(*)
• count(...), count(distinct ...), count(all...)
The distinct and all keywords may be used and have the same semantics as in SQL.
select distinct cat.name from eg.Cat cat
select count(distinct cat.name), count(cat) from eg.Cat cat
11.6. Polymorphic queries
A query like:
from eg.Cat as cat
returns instances not only of Cat, but also of subclasses like DomesticCat. Hibernate queries may 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:
from java.lang.Object o
The interface Named might be implemented by various persistent classes:
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from eg.Named n, eg.Named m where n.name = m.name
Note that 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 can't call these queries using Query.scroll().)
11.7. The where clause
The where clause allows you to narrow the list of instances returned.
from eg.Cat as cat where cat.name='Fritz'
returns instances of Cat named 'Fritz'.
select foo
from eg.Foo foo, eg.Bar bar
where foo.startDate = bar.date
will return all instances of Foo for which there exists 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. Con-
sider:
from eg.Cat cat where cat.mate.name is not null
This query translates to an SQL query with a table (inner) join. If you were to write something like
from eg.Foo foo
where foo.bar.baz.customer.address.city is not null
you would end up with a query that would require four table joins in SQL.
The = operator may be used to compare not only properties, but also instances:
from eg.Cat cat, eg.Cat rival where cat.mate = rival.mate
select cat, mate
from eg.Cat cat, eg.Cat mate
where cat.mate = mate
The special property (lowercase) id may be used to reference the unique identifier of an object. (You may also
use its property name.)
from eg.Cat as cat where cat.id = 123
from eg.Cat as cat where cat.mate.id = 69
The second query is efficient. No table join is required!
Properties of composite identifiers may also be used. Suppose Person has a composite identifier consisting of
country and medicareNumber.
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
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Once again, the second query requires no table join.
Likewise, 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.
from eg.Cat cat where cat.class = eg.DomesticCat
You may also specify properties of components or composite user types (and of components of components,
etc). Never try to use a path-expression that ends in a property of component type (as opposed to a property of a
component). For example, if store.owner is an entity with a component address
store.owner.address.city // okay
store.owner.address // error!
An "any" type has the special properties id and class, allowing us to express a join in the following way
(where AuditLog.item is a property mapped with <any>).
from eg.AuditLog log, eg.Payment payment
where log.item.class = 'eg.Payment' and log.item.id = payment.id
Notice that log.item.class and payment.class would refer to the values of completely different database
columns in the above query.
11.8. Expressions
Expressions allowed in the where clause include most of the kind of things you could write in SQL:
• mathematical operators +, -, *, /
• binary comparison operators =, >=, <=, <>, !=, like
• logical operations and, or, not
• string concatenation ||
• SQL scalar functions like upper() and lower()
• Parentheses ( ) indicate grouping
• in, between, is null
• JDBC IN parameters ?
• named parameters :name, :start_date, :x1
• SQL literals 'foo', 69, '1970-01-01 10:00:01.0'
• Java public static final constants eg.Color.TABBY
in and between may be used as follows:
from eg.DomesticCat cat where cat.name between 'A' and 'B'
from eg.DomesticCat cat where cat.name in ( 'Foo', 'Bar', 'Baz' )
and the negated forms may be written
from eg.DomesticCat cat where cat.name not between 'A' and 'B'
from eg.DomesticCat cat where cat.name not in ( 'Foo', 'Bar', 'Baz' )
Likewise, is null and is not null may be used to test for null values.
Booleans may be easily used in expressions by declaring HQL query substitutions in Hibernate configuration:
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<property name="hibernate.query.substitutions">true 1, false 0</property>
This will replace the keywords true and false with the literals 1 and 0 in the translated SQL from this HQL:
from eg.Cat cat where cat.alive = true
You may test the size of a collection with the special property size, or the special size() function.
from eg.Cat cat where cat.kittens.size > 0
from eg.Cat cat where size(cat.kittens) > 0
For indexed collections, you may refer to the minimum and maximum indices using minIndex and maxIndex.
Similarly, you may refer to the minimum and maximum elements of a collection of basic type using minEle-
ment and maxElement.
from Calendar cal where cal.holidays.maxElement > current date
There are also functional forms (which, unlike the constructs above, are not case sensitive):
from Order order where maxindex(order.items) > 100
from Order order where minelement(order.items) > 10000
The SQL functions any, some, all, exists, in are supported when passed the element or index set of a col-
lection (elements and indices functions) or the result of a subquery (see below).
select mother from eg.Cat as mother, eg.Cat as kit
where kit in elements(foo.kittens)
select p from eg.NameList list, eg.Person p
where p.name = some elements(list.names)
from eg.Cat cat where exists elements(cat.kittens)
from eg.Player p where 3 > all elements(p.scores)
from eg.Show show where 'fizard' in indices(show.acts)
Note that these constructs - size, elements, indices, minIndex, maxIndex, minElement, maxElement - have
certain usage restrictions:
• in a where clause: only for databases with subselects
• in a select clause: only elements and indices make sense
Elements of indexed collections (arrays, lists, maps) may be referred to by index (in a where clause only):
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
The expression inside [] may even be an arithmetic expression.
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select item from Item item, Order order
where order.items[ size(order.items) - 1 ] = item
HQL also provides the built-in index() function, for elements of a one-to-many association or collection of
values.
select item, index(item) from Order order
join order.items item
where index(item) < 5
Scalar SQL functions supported by the underlying database may be used
from eg.DomesticCat cat where upper(cat.name) like 'FRI%'
If you are not yet convinced by all this, think how much longer and less readable the following query would be
in SQL:
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)
Hint: something like
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
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
)
11.9. The order by clause
The list returned by a query may be ordered by any property of a returned class or components:
from eg.DomesticCat cat
order by cat.name asc, cat.weight desc, cat.birthdate
The optional asc or desc indicate ascending or descending order respectively.
11.10. The group by clause
A query that returns aggregate values may be grouped by any property of a returned class or components:
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select cat.color, sum(cat.weight), count(cat)
from eg.Cat cat
group by cat.color
select foo.id, avg( elements(foo.names) ), max( indices(foo.names) )
from eg.Foo foo
group by foo.id
Note: You may use the elements and indices constructs inside a select clause, even on databases with no
subselects.
A having clause is also allowed.
select cat.color, sum(cat.weight), count(cat)
from eg.Cat cat
group by cat.color
having cat.color in (eg.Color.TABBY, eg.Color.BLACK)
SQL functions and aggregate functions are allowed in the having and order by clauses, if supported by the un-
derlying database (ie. not in MySQL).
select cat
from eg.Cat cat
join cat.kittens kitten
group by cat
having avg(kitten.weight) > 100
order by count(kitten) asc, sum(kitten.weight) desc
Note that neither the group by clause nor the order by clause may contain arithmetic expressions.
11.11. Subqueries
For databases that support subselects, Hibernate supports subqueries within queries. A subquery must be sur-
rounded by parentheses (often by an SQL aggregate function call). Even correlated subqueries (subqueries that
refer to an alias in the outer query) are allowed.
from eg.Cat as fatcat
where fatcat.weight > (
select avg(cat.weight) from eg.DomesticCat cat
)
from eg.DomesticCat as cat
where cat.name = some (
select name.nickName from eg.Name as name
)
from eg.Cat as cat
where not exists (
from eg.Cat as mate where mate.mate = cat
)
from eg.DomesticCat as cat
where cat.name not in (
select name.nickName from eg.Name as name
)
11.12. HQL examples
Hibernate queries can be quite powerful and complex. In fact, the power of the query language is one of Hi-
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bernate's main selling points. Here are some example queries very similar to queries that I used on a recent
project. Note that most queries you will write are much simpler than these!
The following query returns the order id, number of items and total value of the order for all unpaid orders for a
particular customer and given minimum total value, ordering the results by total value. In determining the
prices, it uses the current catalog. The resulting SQL query, against the ORDER, ORDER_LINE, PRODUCT, CATALOG
and PRICE tables has four inner joins and an (uncorrelated) subselect.
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
What a monster! Actually, in real life, I'm not very keen on subqueries, so my query was really more like this:
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
The next query counts the number of payments in each status, excluding all payments in the AWAIT-
ING_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 PAY-
MENT_STATUS_CHANGE tables.
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
)
and statusChange.user <> :currentUser
)
group by status.name, status.sortOrder
order by status.sortOrder
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If I would have mapped the statusChanges collection as a list, instead of a set, the query would have been
much simpler to write.
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
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.
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
For some databases, we would need to do away with the (correlated) subselect.
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
11.13. Tips & Tricks
You can count the number of query results without actually returning them:
( (Integer) session.iterate("select count(*) from ....").next() ).intValue()
To order a result by the size of a collection, use the following query:
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)
If your database supports subselects, you can place a condition upon selection size in the where clause of your
query:
from User usr where size(usr.messages) >= 1
If your database doesn't support subselects, use the following query:
select usr.id, usr.name
from User usr.name
join usr.messages msg
group by usr.id, usr.name
having count(msg) >= 1
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As this solution can't return a User with zero messages because of the inner join, the following form is also use-
ful:
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
Properties of a JavaBean can be bound to named query parameters:
Query q = s.createQuery("from foo in class Foo where foo.name=:name and foo.size=:size");
q.setProperties(fooBean); // fooBean has getName() and getSize()
List foos = q.list();
Collections are pageable by using the Query interface with a filter:
Query q = s.createFilter( collection, "" ); // the trivial filter
q.setMaxResults(PAGE_SIZE);
q.setFirstResult(PAGE_SIZE * pageNumber);
List page = q.list();
Collection elements may be ordered or grouped using a query filter:
Collection orderedCollection = s.filter( collection, "order by this.amount" );
Collection counts = s.filter( collection, "select this.type, count(this) group by this.type" );
You can find the size of a collection without initializing it:
( (Integer) session.iterate("select count(*) from ....").next() ).intValue();
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Chapter 12. Criteria Queries
Hibernate now features an intuitive, extensible criteria query API. For now, this API is less powerful and than
the more mature HQL query facilities. In particular, criteria queries do not support projection or aggregation.
12.1. 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.
Criteria crit = sess.createCriteria(Cat.class);
crit.setMaxResults(50);
List cats = crit.list();
12.2. Narrowing the result set
An individual query criterion is an instance of the interface org.hibernate.expression.Criterion. The class
org.hibernate.expression.Expression defines factory methods for obtaining certain built-in Criterion
types.
List cats = sess.createCriteria(Cat.class)
.add( Expression.like("name", "Fritz%") )
.add( Expression.between("weight", minWeight, maxWeight) )
.list();
Expressions may be grouped logically.
List cats = sess.createCriteria(Cat.class)
.add( Expression.like("name", "Fritz%") )
.add( Expression.or(
Expression.eq( "age", new Integer(0) ),
Expression.isNull("age")
) )
.list();
List cats = sess.createCriteria(Cat.class)
.add( Expression.in( "name", new String[] { "Fritz", "Izi", "Pk" } ) )
.add( Expression.disjunction()
.add( Expression.isNull("age") )
.add( Expression.eq("age", new Integer(0) ) )
.add( Expression.eq("age", new Integer(1) ) )
.add( Expression.eq("age", new Integer(2) ) )
) )
.list();
There are quite a range of built-in criterion types (Expression subclasses), but one that is especially useful lets
you specify SQL directly.
List cats = sess.createCriteria(Cat.class)
.add( Expression.sql("lower({alias}.name) like lower(?)", "Fritz%", Hibernate.STRING) )
.list();
The {alias} placeholder with be replaced by the row alias of the queried entity.
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12.3. Ordering the results
You may order the results using org.hibernate.expression.Order.
List cats = sess.createCriteria(Cat.class)
.add( Expression.like("name", "F%")
.addOrder( Order.asc("name") )
.addOrder( Order.desc("age") )
.setMaxResults(50)
.list();
12.4. Associations
You may easily specify constraints upon related entities by navigating associations using createCriteria().
List cats = sess.createCriteria(Cat.class)
.add( Expression.like("name", "F%")
.createCriteria("kittens")
.add( Expression.like("name", "F%")
.list();
note that the second createCriteria() returns a new instance of Criteria, which refers to the elements of the
kittens collection.
The following, alternate form is useful in certain circumstances.
List cats = sess.createCriteria(Cat.class)
.createAlias("kittens", "kt")
.createAlias("mate", "mt")
.add( Expression.eqProperty("kt.name", "mt.name") )
.list();
(createAlias() does not create a new instance of Criteria.)
Note that the kittens collections held by the Cat instances returned by the previous two queries are not pre-
filtered by the criteria! If you wish to retrieve just the kittens that match the criteria, you must use return-
Maps().
List cats = sess.createCriteria(Cat.class)
.createCriteria("kittens", "kt")
.add( Expression.eq("name", "F%") )
.returnMaps()
.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");
}
12.5. Dynamic association fetching
You may specify association fetching semantics at runtime using setFetchMode().
List cats = sess.createCriteria(Cat.class)
.add( Expression.like("name", "Fritz%") )
.setFetchMode("mate", FetchMode.EAGER)
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.setFetchMode("kittens", FetchMode.EAGER)
.list();
This query will fetch both mate and kittens by outer join.
12.6. Example queries
The class org.hibernate.expression.Example allows you to construct a query criterion from a given in-
stance.
Cat cat = new Cat();
cat.setSex('F');
cat.setColor(Color.BLACK);
List results = session.createCriteria(Cat.class)
.add( Example.create(cat) )
.list();
Version properties, identifiers and associations are ignored. By default, null valued properties are excluded.
You can adjust how the Example is applied.
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();
You can even use examples to place criteria upon associated objects.
List results = session.createCriteria(Cat.class)
.add( Example.create(cat) )
.createCriteria("mate")
.add( Example.create( cat.getMate() ) )
.list();
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Chapter 13. Native SQL
You may also express queries in the native SQL dialect of your database. This is useful if you want to utilize
database specific features such as the CONNECT keyword in Oracle. This also allows for a cleaner migration
path from a direct SQL/JDBC based application to Hibernate.
Hibernate3 also supports native SQL statements for all create, update, delete, and load operations.
13.1. Creating a SQL based Query
SQL queries are exposed through the same Query interface, just like ordinary HQL queries. The only difference
is the use of Session.createSQLQuery().
Query sqlQuery = sess.createSQLQuery("select {cat.*} from cats {cat}", "cat", Cat.class);
sqlQuery.setMaxResults(50);
List cats = sqlQuery.list();
The three parameters provided to createSQLQuery() are:
• the SQL query string
• a table alias name
• the persistent class returned by the query
The alias name is used inside the sql string to refer to the properties of the mapped class (in this case Cat). You
may retrieve multiple objects per row by supplying a String array of alias names and a Class array of corres-
ponding classes.
13.2. Alias and property references
The {cat.*} notation used above is a shorthand for "all properties". You may even list the properties explicity,
but you must let Hibernate provide SQL column aliases for each property. The placeholders for these column
aliases are the property name qualified by the table alias. In the following example, we retrieve Cats from a dif-
ferent table (cat_log) to the one declared in the mapping metadata. Notice that we may even use the property
aliases in the where clause.
String sql = "select cat.originalId as {cat.id}, "
+ " cat.mateid as {cat.mate}, cat.sex as {cat.sex}, "
+ " cat.weight*10 as {cat.weight}, cat.name as {cat.name}"
+ " from cat_log cat where {cat.mate} = :catId"
List loggedCats = sess.createSQLQuery(sql, "cat", Cat.class)
.setLong("catId", catId)
.list();
Note: if you list each property explicitly, you must include all properties of the class and its subclasses!
13.3. Named SQL queries
Named SQL queries may be defined in the mapping document and called in exactly the same way as a named
HQL query.
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List people = sess.getNamedQuery("mySqlQuery")
.setMaxResults(50)
.list();
<sql-query name="mySqlQuery">
<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 'Hiber%'
</sql-query>
13.4. Custom SQL for CUD
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, updat-
esql etc.). The mapping tags <sql-insert>, <sql-delete>, and <sql-update> override these strings:
<class name="Person">
<id name="id">
<generator class="increment"/>
</id>
<property name="name" not-null="true"/>
<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>
The SQL is directly execute in your database, so you are free to use any dialect you like.
Stored procedures are support if the callable attribute is set:
<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>
The stored procedures are in most cases (read: better do it than not) required to return the number of rows inser-
ted/updated/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:
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;
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13.5. Custom SQL for loading
You may also declare your own SQL (or HQL) queries for entity loading:
<sql-query name="person">
<return alias="p" class="Person" lock-mode="upgrade"/>
SELECT NAME AS {p.name}, ID AS {p.id} FROM PERSON WHERE ID=? FOR UPDATE
</sql-query>
This is just a named query declaration, as discussed earlier. You may reference this named query in a class
mapping:
<class name="Person">
<id name="id">
<generator class="increment"/>
</id>
<property name="name" not-null="true"/>
<loader query-ref="person"/>
</class>
TODO: Document synchronized mapping element in named queries
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Chapter 14. Improving performance
14.1. Understanding Collection performance
We've already spent quite some time talking about collections. In this section we will highlight a couple more
issues about how collections behave at runtime.
14.1.1. 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 per-
formance characteristics, we must also consider the structure of the primary key that is used by Hibernate to up-
date or delete collection rows. This suggests the following classification:
• indexed collections
• sets
• bags
All indexed collections (maps, lists, arrays) have a primary key consisting of the <key> and <index> columns.
In this case collection updates are usually extremely efficient - the primary key may be efficiently indexed and
a particular row may be efficiently located when Hibernate tries to update or delete it.
Sets have a primary key consisting of <key> and element columns. This may be less efficient for some types of
collection element, particularly composite elements or large text or binary fields; the database may not be able
to index a complex primary key as efficently. On the other hand, for one to many or many to many associ-
ations, particularly in the case of synthetic identifiers, it is likely to be just as efficient. (Side-note: if you want
SchemaExport to actually create the primary key of a <set> for you, you must declare all columns as not-
null="true".)
<idbag> mappings define a surrogate key, so they are always very efficient to update. In fact, they are the best
case.
Bags are the worst case. Since a bag permits duplicate element values and has no index column, no primary key
may be defined. Hibernate has no way of distinguishing between duplicate rows. Hibernate resolves this prob-
lem by completely removing (in a single DELETE) and recreating the collection whenever it changes. This might
be very inefficient.
Note that for a one-to-many association, the "primary key" may not be the physical primary key of the database
table - but even in this case, the above classification is still useful. (It still reflects how Hibernate "locates" indi-
vidual rows of the collection.)
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14.1.2. Lists, maps, idbags and sets are the most efficient collections to up-
date
From the discussion above, it should be clear that indexed collections and (usually) 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 doesn't ever UPDATE a row when an ele-
ment 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, we would conclude that lists, maps and idbags are the most perform-
ant (non-inverse) collection types, with sets not far behind. Sets are expected to be the most common kind of
collection in Hibernate applications. This is because the "set" semantics are most natural in the relational mod-
el.
However, in well-designed Hibernate domain models, we usually see that 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.
14.1.3. Bags and lists are the most efficient inverse collections
Just before you ditch bags forever, there is a particular case in which bags (and also lists) are much more per-
formant 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 Collection.add() or Collection.addAll() must always return true for a bag or List (unlike
a Set). This can make the following common code much faster.
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();
14.1.4. One shot delete
Occasionally, deleting collection elements one by one can be extremely inefficient. Hibernate isn't completely
stupid, so it knows not to do that in the case of an newly-empty collection (if you called list.clear(), for ex-
ample). In this case, Hibernate will issue a single DELETE and we are done!
Suppose we add 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 desir-
able.
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)
Hibernate isn't smart enough to know that the second option is probably quicker in this case. (And it would
probably be undesirable for Hibernate to be that smart; such behaviour might confuse database triggers, etc.)
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Fortunately, you can force this behaviour (ie. the second strategy) at any time by discarding (ie. dereferencing)
the original collection and returning a newly instantiated collection with all the current elements. This can be
very useful and powerful from time to time.
Of course, one-shot-delete does not apply to collections mapped inverse="true".
We have already shown how you can use lazy initialization for persistent collections in the chapter about col-
lection mappings. A similar effect is achievable for ordinary object references, using CGLIB proxies. We have
also mentioned how Hibernate caches persistent objects at the level of a Session. More aggressive caching
strategies may be configured upon a class-by-class basis.
In the next section, we show you how to use these features, which may be used to achieve much higher per-
formance, where necessary.
14.2. Proxies for Lazy Initialization
Hibernate implements lazy initializing proxies for persistent objects using runtime bytecode enhancement (via
the excellent CGLIB library).
The mapping file may declare an interface to use as the proxy interface for that class. By default, Hibernate
uses a subclass of the class itself. (The proxied class must implement a default constructor with at least package
visibility.)
There are some gotchas to be aware of when extending this approach to polymorphic classes, eg.
<class name="Cat" proxy="Cat">
......
<subclass name="DomesticCat">
.....
</subclass>
</class>
Firstly, instances of Cat will never be castable to DomesticCat, even if the underlying instance is an instance of
DomesticCat.
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!
....
}
Secondly, it is possible to break proxy ==.
Cat cat = (Cat) session.load(Cat.class, id); // instantiate a Cat proxy
DomesticCat dc =
(DomesticCat) session.load(DomesticCat.class, id); // required new DomesticCat proxy!
System.out.println(cat==dc); // false
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:
cat.setWeight(11.0); // hit the db to initialize the proxy
System.out.println( dc.getWeight() ); // 11.0
Third, you may not use a CGLIB proxy for a final class or a class with any final methods.
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Finally, if your persistent object acquires any resources upon instantiation (eg. in initializers or default con-
structor), 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 inheritence model. If you wish 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. eg.
<class name="CatImpl" proxy="Cat">
......
<subclass name="DomesticCatImpl" proxy="DomesticCat">
.....
</subclass>
</class>
where Cat implements the interface ICat and DomesticCat implements the interface IDomesticCat. Then prox-
ies for instances of Cat and DomesticCat may be returned by load() or iterate(). (Note that find() does not
usually return proxies.)
Cat cat = (Cat) session.load(CatImpl.class, catid);
Iterator iter = session.iterate("from cat in class CatImpl where cat.name='fritz'");
Cat fritz = (Cat) iter.next();
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().
Exceptions that occur while initializing a proxy are wrapped in a LazyInitializationException.
Sometimes we need to ensure that a proxy or collection is initialized before closing the Session. Of course, we
can alway force initialization by calling cat.getSex() or cat.getKittens().size(), for example. But that is
confusing to readers of the code and is not convenient for generic code. The static methods Hibern-
ate.initialize() and Hibernate.isInitialized() provide the application with a convenient way of work-
ing with lazyily 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 ef-
fect for the collection of kittens.
14.3. Using batch fetching
Hibernate can make efficient use of batch fetching, that is, Hibernate can load several uninitialized proxies if
one proxy is accessed. Batch fetching is an optimization for the lazy loading strategy. There are two ways you
can tune batch fetching: on the class and the collection level.
Batch fetching for classes/entities is easier to understand. Imagine you have the following situation at runtime:
You have 25 Cat instances loaded in a Session, 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 beha-
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vior by specifying a batch-size in the mapping of Person:
<class name="Person" batch-size="10">...</class>
Hibernate will now execute only three queries, the pattern is 10, 10, 5. You can see that batch fetching is a
blind guess, as far as performance optimization goes, it depends on the number of unitilized proxies in a partic-
ular Session.
You may 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 Sesssion, 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, Hi-
bernate can pre-fetch collections:
<class name="Person">
<set name="cats" batch-size="3">
...
</set>
</class>
With a batch-size of 3, Hibernate will load 3, 3, 3, 1 collections in 4 SELECTs. Again, the value of 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, ie. the typical bill-
of-materials pattern.
14.4. 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, as in practice, optimizing row reads is much
more important than optimization of column reads. However, only loading some properties of a class might be
useful in extreme cases, when legacy tables have hundreds of columns and the data model can not be improved.
To enable lazy property loading, set the lazy attribute on your particular property mappings:
<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>
Lazy property loading requires buildtime bytecode instrumentation! If your persistent classes are not enhanced,
Hibernate will silently ignore lazy property settings and fall back to immediate fetching.
For bytecode instrumentation, use the following Ant task:
<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>
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</instrument>
</target>
A different (better?) way to avoid unnecessary column reads, at least for read-only transactons is to use the pro-
jection features of HQL. This avoids the need for buildtime bytecode processing.
TODO: Document issues with lazy property loading
14.5. Outer join fetching
Any kind of lazy fetching is extremely vulnerable to N+1 selects problems. So usually, we choose lazy fetching
only as a "default" strategy, and override it for a particular transaction, using the HQL LEFT JOIN FETCH
clause. This tells Hibernate to fetch the association in the first select, using an outer join. In the Criteria API,
you would use setFetchMode(FetchMode.EAGER).
You can always force outer join association fetching in the mapping file, by setting outer-join="true". We
don't recommend this setting, especially not for collections, since it is incredibly rare to find an entity which is
always used when an associated entity is used, at least in a sufficiently large system.
A completely different way to avoid problems with N+1 selects is to use the second-level cache.
14.6. The Second Level Cache
A Hibernate Session is a transaction-level cache of persistent data. It is possible to configure a cluster or JVM-
level (SessionFactory-level) cache on a class-by-class and collection-by-collection basis. You may even plug
in a clustered cache. Be careful. Caches are never aware of changes made to the persistent store by another ap-
plication (though they may be configured to regularly expire cached data).
By default, Hibernate uses EHCache for JVM-level caching. (JCS support is now deprecated and will be re-
moved in a future version of Hibernate.) You may choose a different implementation by specifying the name of
a class that implements org.hibernate.cache.CacheProvider using the property hibern-
ate.cache.provider_class.
Table 14.1. Cache Providers
Cache Provider class Type Cluster Safe Query Cache
Supported
Hashtable
(not intended
for produc-
tion use)
org.hibernate.cache.HashtableCacheProv
ider
memory yes
EHCache org.hibernate.cache.EhCacheProvider memory, disk yes
OSCache org.hibernate.cache.OSCacheProvider memory, disk yes
SwarmCache org.hibernate.cache.SwarmCacheProvider clustered (ip
multicast)
yes (clustered
invalidation)
JBoss
TreeCache
org.hibernate.cache.TreeCacheProvider clustered (ip
multicast),
transactional
yes
(replication)
yes (clock
sync req.)
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14.6.1. Cache mappings
The <cache> element of a class or collection mapping has the following form:
<cache
usage="transactional|read-write|nonstrict-read-write|read-only" (1)
/>
(1) usage specifies the caching strategy: transactional, read-write, nonstrict-read-write or read-only
Alternatively (preferrably?), you may specify <class-cache> and <collection-cache> elements in hibern-
ate.cfg.xml.
The usage attribute specifies a cache concurrency strategy.
14.6.2. Strategy: read only
If your application needs to read but never modify instances of a persistent class, a read-only cache may be
used. This is the simplest and best performing strategy. Its even perfectly safe for use in a cluster.
<class name="eg.Immutable" mutable="false">
<cache usage="read-only"/>
....
</class>
14.6.3. Strategy: read/write
If the application needs to update data, a read-write cache might be appropriate. This cache strategy should
never be used if serializable transaction isolation level is required. If the cache is used in a JTA environment,
you must specify the property hibernate.transaction.manager_lookup_class, naming a strategy for obtain-
ing the JTA TransactionManager. In other environments, you should ensure that the transaction is completed
when Session.close() or Session.disconnect() is called. If you wish to use this strategy in a cluster, you
should ensure that the underlying cache implementation supports locking. The built-in cache providers do not.
<class name="eg.Cat" .... >
<cache usage="read-write"/>
....
<set name="kittens" ... >
<cache usage="read-write"/>
....
</set>
</class>
14.6.4. Strategy: nonstrict read/write
If the application only occasionally needs to update data (ie. if it is extremely unlikely that two transactions
would try to update the same item simultaneously) and strict transaction isolation is not required, a nonstrict-
read-write cache might be appropriate. If the cache is used in a JTA environment, you must specify hibern-
ate.transaction.manager_lookup_class. In other environments, you should ensure that the transaction is
completed when Session.close() or Session.disconnect() is called.
14.6.5. Strategy: transactional
The transactional cache strategy provides support for fully transactional cache providers such as JBoss
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TreeCache. Such a cache may only be used in a JTA environment and you must specify hibern-
ate.transaction.manager_lookup_class.
None of the cache providers support all of the cache concurrency strategies. The following table shows which
providers are compatible with which concurrency strategies.
Table 14.2. Cache Concurrency Strategy Support
Cache read-only nonstrict-
read-write
read-write transactional
Hashtable (not in-
tended for produc-
tion use)
yes yes yes
EHCache yes yes yes
OSCache yes yes yes
SwarmCache yes yes
JBoss TreeCache yes yes
14.7. Managing the Session Cache
Whenever you pass an object to save(), update() or saveOrUpdate() and whenever you retrieve an object us-
ing load(), get(), list(), iterate() or scroll(), that object is added to the internal cache of the Session.
When flush() is subsequently called, the state of that object will be synchronized with the database. If you do
not want this synchronization to occur or if you are processing a huge number of objects and need to manage
memory efficiently, the evict() method may be used to remove the object and its collections from the cache.
Iterator cats = sess.iterate("from eg.Cat as cat"); //a huge result set
while ( cats.hasNext() ) {
Cat cat = (Cat) iter.next();
doSomethingWithACat(cat);
sess.evict(cat);
}
Hibernate will evict associated entities automatically if the association is mapped with cascade="all" or cas-
cade="all-delete-orphan".
The Session also provides a contains() method to determine if an instance belongs to the session cache.
To completely evict all objects from the session cache, call Session.clear()
For the second-level cache, there are methods defined on SessionFactory for evicting the cached state of an
instance, entire class, collection instance or entire collection role.
14.8. The Query Cache
Query result sets may also be cached. This is only useful for queries that are run frequently with the same para-
meters. To use the query cache you must first enable it by setting the property hibern-
ate.cache.use_query_cache=true. This causes the creation of two cache regions - one holding cached query
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result sets (org.hibernate.cache.QueryCache), the other holding timestamps of most recent updates to quer-
ied tables (org.hibernate.cache.UpdateTimestampsCache). Note that the query cache does not cache the state
of any entities in the result set; it caches only identifier values and results of value type. So the query cache is
usually used in conjunction with the second-level cache.
Most queries do not benefit from caching, so by default queries are not cached. To enable caching, call
Query.setCacheable(true). This call allows the query to look for existing cache results or add its results to
the cache when it is executed.
If you require fine-grained control over query cache expiration policies, you may specify a named cache region
for a particular query by calling Query.setCacheRegion().
List blogs = sess.createQuery("from Blog blog where blog.blogger = :blogger")
.setEntity("blogger", blogger)
.setMaxResults(15)
.setCacheable(true)
.setCacheRegion("frontpages")
.list();
If the query should force a refresh of its query cache region, you may call Query.setForceCacheRefresh() to
true. This is particularly useful in cases where underlying data may have been updated via a seperate process
(i.e., not modified through Hibernate) and allows the application to selectively refresh the query cache regions
based on its knowledge of those events. This is an alternative to eviction of a query cache region. If you need
fine-grained refresh control for many queries, use this function instead of a new region for each query.
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Chapter 15. Toolset Guide
Roundtrip engineering with Hibernate is possible using a set of commandline tools maintained as part of the
Hibernate project, along with Hibernate support built into XDoclet, Middlegen and AndroMDA.
The Hibernate main package comes bundled with the most important tool (it can even be used from "inside"
Hibernate on-the-fly):
• DDL schema generation from a mapping file (aka SchemaExport, hbm2ddl)
Other tools directly provided by the Hibernate project are delivered with a separate package, Hibernate Exten-
sions. This package includes tools for the following tasks:
• Java source generation from a mapping file (aka CodeGenerator, hbm2java)
• mapping file generation from compiled Java classes or from Java source with XDoclet markup (aka Map-
Generator, class2hbm)
There's actually another utitily living in Hibernate Extensions: ddl2hbm. It is considered deprecated and will no
longer be maintained, Middlegen does a better job for the same task.
Third party tools with Hibernate support are:
• Middlegen (mapping file generation from an existing database schema)
• AndroMDA (MDA (Model-Driven Architecture) approach generating code for persistent classes from
UML diagrams and their XML/XMI representation)
These 3rd party tools are not documented in this reference. Please refer to the Hibernate website for up-to-date
information (a snapshot of the site is included in the Hibernate main package).
15.1. Schema Generation
DDL may be generated from your mapping files by a command line utility. A batch file is located in the hi-
bernate-x.x.x/bin directory of the core Hibernate package.
The generated schema include referential integrity constraints (primary and foreign keys) for entity and collec-
tion tables. Tables and sequences are also created for mapped identifier generators.
You must specify a SQL Dialect via the hibernate.dialect property when using this tool.
15.1.1. Customizing the schema
Many Hibernate mapping elements define an optional attribute named length. You may set the length of a
column with this attribute. (Or, for numeric/decimal data types, the precision.)
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).
Some tags accept an index attribute for specifying the name of an index for that column. A unique-key attrib-
ute can be used to group columns in a single unit key constraint. Currently, the specified value of the unique-
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key attribute is not used to name the constraint, only to group the columns in the mapping file.
Examples:
<property name="foo" type="string" length="64" not-null="true"/>
<many-to-one name="bar" foreign-key="fk_foo_bar" not-null="true"/>
<element column="serial_number" type="long" not-null="true" unique="true"/>
Alternatively, these elements also accept a child <column> element. This is particularly useful for multi-column
types:
<property name="foo" type="string">
<column name="foo" length="64" not-null="true" sql-type="text"/>
</property>
<property name="bar" type="my.customtypes.MultiColumnType"/>
<column name="fee" not-null="true" index="bar_idx"/>
<column name="fi" not-null="true" index="bar_idx"/>
<column name="fo" not-null="true" index="bar_idx"/>
</property>
The sql-type attribute allows the user to override the default mapping of Hibernate type to SQL datatype.
The check attribute allows you to specify a check constraint.
<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>
Table 15.1. Summary
Attribute Values Interpretation
length number column length/decimal precision
not-null true|false specfies 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
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, use it on <one-to-one>, <many-to-one>,
<key>, and <many-to-many> mapping elements. Note that
inverse="true" sides will not be considered by SchemaEx-
port.
sql-type column_type overrides the default column type (attribute of <column>
element only)
check SQL expression create an SQL check constraint on either column or table
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15.1.2. Running the tool
The SchemaExport tool writes a DDL script to standard out and/or executes the DDL statements.
java -cp hibernate_classpaths org.hibernate.tool.hbm2ddl.SchemaExport options mapping_files
Table 15.2. SchemaExport Command Line Options
Option Description
--quiet don't output the script to stdout
--drop only drop the tables
--text don't export to the database
--output=my_schema.ddl output the ddl script to a file
--config=hibernate.cfg.xml read Hibernate configuration from an XML file
--properties=hibernate.properties read database properties from a file
--format format the generated SQL nicely in the script
--delimiter=x set an end of line delimiter for the script
You may even embed SchemaExport in your application:
Configuration cfg = ....;
new SchemaExport(cfg).create(false, true);
15.1.3. Properties
Database properties may be specified
• as system properties with -D<property>
• in hibernate.properties
• in a named properties file with --properties
The needed properties are:
Table 15.3. SchemaExport Connection Properties
Property Name Description
hibernate.connection.driver_class jdbc driver class
hibernate.connection.url jdbc url
hibernate.connection.username database user
hibernate.connection.password user password
hibernate.dialect dialect
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15.1.4. Using Ant
You can call SchemaExport from your Ant build script:
<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>
15.1.5. Incremental schema updates
The SchemaUpdate tool will update an existing schema with "incremental" changes. Note that SchemaUpdate
depends heavily upon the JDBC metadata API, so it will not work with all JDBC drivers.
java -cp hibernate_classpaths org.hibernate.tool.hbm2ddl.SchemaUpdate options mapping_files
Table 15.4. SchemaUpdate Command Line Options
Option Description
--quiet don't output the script to stdout
--properties=hibernate.properties read database properties from a file
You may embed SchemaUpdate in your application:
Configuration cfg = ....;
new SchemaUpdate(cfg).execute(false);
15.1.6. Using Ant for incremental schema updates
You can call SchemaUpdate from the Ant script:
<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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15.2. Code Generation
The Hibernate code generator may be used to generate skeletal Java implementation classes from a Hibernate
mapping file. This tool is included in the Hibernate Extensions package (a seperate download).
hbm2java parses the mapping files and generates fully working Java source files from these. Thus with
hbm2java one could "just" provide the .hbm files, and then don't worry about hand-writing/coding the Java files.
java -cp hibernate_classpaths org.hibernate.tool.hbm2java.CodeGenerator options mapping_files
Table 15.5. Code Generator Command Line Options
Option Description
--output=output_dir root directory for generated code
--config=config_file optional file for configuring hbm2java
15.2.1. The config file (optional)
The config file provides for a way to specify multiple "renderers" for the source code and to declare <meta> at-
tributes that is "global" in scope. See more about this in the <meta> attribute section.
<codegen>
<meta attribute="implements">codegen.test.IAuditable</meta>
<generate renderer="org.hibernate.tool.hbm2java.BasicRenderer"/>
<generate
package="autofinders.only"
suffix="Finder"
renderer="org.hibernate.tool.hbm2java.FinderRenderer"/>
</codegen>
This config file declares a global meta attribute "implements" and specify two renderers, the default one
(BasicRenderer) and a renderer that generates Finder's (See more in "Basic Finder generation" below).
The second renderer is provided with a package and suffix attribute.
The package attribute specifies that the generated source files from this renderer should be placed here instead
of the package scope specified in the .hbm files.
The suffix attribute specifies the suffix for generated files. E.g. here a file named Foo.java would be FooFind-
er.java instead.
It is also possible to send down arbitrary parameters to the renders by adding <param> attributes to the
<generate> elements.
hbm2java currently has support for one such parameter, namely generate-concrete-empty-classes which in-
forms the BasicRenderer to only generate empty concrete classes that extends a base class for all your classes.
The following config.xml example illustrate this feature
<codegen>
<generate prefix="Base" renderer="org.hibernate.tool.hbm2java.BasicRenderer"/>
<generate renderer="org.hibernate.tool.hbm2java.BasicRenderer">
<param name="generate-concrete-empty-classes">true</param>
<param name="baseclass-prefix">Base</param>
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</generate>
</codegen>
Notice that this config.xml configure 2 (two) renderers. One that generates the Base classes, and a second one
that just generates empty concrete classes.
15.2.2. The meta attribute
The <meta> tag is a simple way of annotating the hbm.xml with information, so tools have a natural place to
store/read information that is not directly related to the Hibernate core.
You can use the <meta> tag to tell hbm2java to only generate "protected" setters, have classes always imple-
ment a certain set of interfaces or even have them extend a certain base class and even more.
The following example:
<class name="Person">
<meta attribute="class-description">
Javadoc for the Person class
@author Frodo
</meta>
<meta attribute="implements">IAuditable</meta>
<id name="id" type="long">
<meta attribute="scope-set">protected</meta>
<generator class="increment"/>
</id>
<property name="name" type="string">
<meta attribute="field-description">The name of the person</meta>
</property>
</class>
will produce something like the following (code shortened for better understanding). Notice the Javadoc com-
ment and the protected set methods:
// default package
import java.io.Serializable;
import org.apache.commons.lang.builder.EqualsBuilder;
import org.apache.commons.lang.builder.HashCodeBuilder;
import org.apache.commons.lang.builder.ToStringBuilder;
/**
* Javadoc for the Person class
* @author Frodo
*
*/
public class Person implements Serializable, IAuditable {
/** identifier field */
public Long id;
/** nullable persistent field */
public String name;
/** full constructor */
public Person(java.lang.String name) {
this.name = name;
}
/** default constructor */
public Person() {
}
public java.lang.Long getId() {
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return this.id;
}
protected void setId(java.lang.Long id) {
this.id = id;
}
/**
* The name of the person
*/
public java.lang.String getName() {
return this.name;
}
public void setName(java.lang.String name) {
this.name = name;
}
}
Table 15.6. Supported meta tags
Attribute Description
class-description inserted into the javadoc for classes
field-description inserted into the javadoc for fields/properties
interface If true an interface is generated instead of an class.
implements interface the class should implement
extends class the class should extend (ignored for subclasses)
generated-class overrule the name of the actual class generated
scope-class scope for class
scope-set scope for setter method
scope-get scope for getter method
scope-field scope for actual field
use-in-tostring include this property in the toString()
implement-equals include a equals() and hashCode() method in this class.
use-in-equals include this property in the equals() and hashCode() meth-
od.
bound add propertyChangeListener support for a property
constrained bound + vetoChangeListener support for a property
gen-property property will not be generated if false (use with care)
property-type Overrides the default type of property. Use this with any tag's
to specify the concrete type instead of just Object.
class-code Extra code that will inserted at the end of the class
extra-import Extra import that will inserted at the end of all other imports
finder-method see "Basic finder generator" below
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Attribute Description
session-method see "Basic finder generator" below
Attributes declared via the <meta> tag are per default "inherited" inside an hbm.xml file.
What does that mean? It means that if you e.g want to have all your classes implement IAuditable then you
just add an <meta attribute="implements">IAuditable</meta> in the top of the hbm.xml file, just after
<hibernate-mapping>. Now all classes defined in that hbm.xml file will implement IAuditable! (Except if a
class also has an "implements" meta attribute, because local specified meta tags always overrules/replaces any
inherited meta tags).
Note: This applies to all <meta>-tags. Thus it can also e.g. be used to specify that all fields should be declare
protected, instead of the default private. This is done by adding <meta attrib-
ute="scope-field">protected</meta> at e.g. just under the <class> tag and all fields of that class will be
protected.
To avoid having a <meta>-tag inherited then you can simply specify inherit="false" for the attribute, e.g.
<meta attribute="scope-class" inherit="false">public abstract</meta> will restrict the "class-scope"
to the current class, not the subclasses.
15.2.3. Basic finder generator
It is now possible to have hbm2java generate basic finders for Hibernate properties. This requires two things in
the hbm.xml files.
The first is an indication of which fields you want to generate finders for. You indicate that with a meta block
inside a property tag such as:
<property name="name" column="name" type="string">
<meta attribute="finder-method">findByName</meta>
</property>
The finder method name will be the text enclosed in the meta tags.
The second is to create a config file for hbm2java of the format:
<codegen>
<generate renderer="org.hibernate.tool.hbm2java.BasicRenderer"/>
<generate suffix="Finder" renderer="org.hibernate.tool.hbm2java.FinderRenderer"/>
</codegen>
And then use the param to hbm2java --config=xxx.xml where xxx.xml is the config file you just created.
An optional parameter is meta tag at the class level of the format:
<meta attribute="session-method">
com.whatever.SessionTable.getSessionTable().getSession();
</meta>
Which would be the way in which you get sessions if you use the Thread Local Session pattern (documented in
the Design Patterns area of the Hibernate website).
15.2.4. Velocity based renderer/generator
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It is now possible to use velocity as an alternative rendering mechanism. The follwing config.xml shows how to
configure hbm2java to use its velocity renderer.
<codegen>
<generate renderer="org.hibernate.tool.hbm2java.VelocityRenderer">
<param name="template">pojo.vm</param>
</generate>
</codegen>
The parameter named template is a resource path to the velocity macro file you want to use. This file must be
available via the classpath for hbm2java. Thus remember to add the directory where pojo.vm is located to your
ant task or shell script. (The default location is ./tools/src/velocity)
Be aware that the current pojo.vm generates only the most basic parts of the java beans. It is not as complete
and feature rich as the default renderer - primarily a lot of the meta tags are not supported.
15.3. Mapping File Generation
A skeletal mapping file may be generated from compiled persistent classes using a command line utility called
MapGenerator. This utility is part of the Hibernate Extensions package.
The Hibernate mapping generator provides a mechanism to produce mappings from compiled classes. It uses
Java reflection to find properties and uses heuristics to guess an appropriate mapping from the property type.
The generated mapping is intended to be a starting point only. There is no way to produce a full Hibernate map-
ping without extra input from the user. However, the tool does take away some of the repetitive "grunt" work
involved in producing a mapping.
Classes are added to the mapping one at a time. The tool will reject classes that it judges are are not Hibernate
persistable.
To be Hibernate persistable a class
• must not be a primitive type
• must not be an array
• must not be an interface
• must not be a nested class
• must have a default (zero argument) constructor.
Note that interfaces and nested classes actually are persistable by Hibernate, but this would not usually be in-
tended by the user.
MapGenerator will climb the superclass chain of all added classes attempting to add as many Hibernate persist-
able superclasses as possible to the same database table. The search stops as soon as a property is found that has
a name appearing on a list of candidate UID names.
The default list of candidate UID property names is: uid, UID, id, ID, key, KEY, pk, PK.
Properties are discovered when there are two methods in the class, a setter and a getter, where the type of the
setter's single argument is the same as the return type of the zero argument getter, and the setter returns void.
Furthermore, the setter's name must start with the string set and either the getter's name starts with get or the
getter's name starts with is and the type of the property is boolean. In either case, the remainder of their names
must match. This matching portion is the name of the property, except that the initial character of the property
name is made lower case if the second letter is lower case.
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The rules for determining the database type of each property are as follows:
1. If the Java type is Hibernate.basic(), then the property is a simple column of that type.
2. For hibernate.type.Type custom types a simple column is used as well.
3. If the property type is an array, then a Hibernate array is used, and MapGenerator attempts to reflect on the
array element type.
4. If the property has type java.util.List, java.util.Map, or java.util.Set, then the corresponding Hi-
bernate types are used, but MapGenerator cannot further process the insides of these types.
5. If the property's type is any other class, MapGenerator defers the decision on the database representation
until all classes have been processed. At this point, if the class was discovered through the superclass
search described above, then the property is an many-to-one association. If the class has any properties,
then it is a component. Otherwise it is serializable, or not persistable.
15.3.1. Running the tool
The tool writes XML mappings to standard out and/or to a file.
When invoking the tool you must place your compiled classes on the classpath.
java -cp hibernate_and_your_class_classpaths org.hibernate.tool.class2hbm.MapGenerator options and
classnames
There are two modes of operation: command line or interactive.
The interactive mode is selected by providing the single command line argument --interact. This mode
provides a prompt response console. Using it you can set the UID property name for each class using the
uid=XXX command where XXX is the UID property name. Other command alternatives are simply a fully quali-
fied class name, or the command done which emits the XML and terminates.
In command line mode the arguments are the options below interspersed with fully qualified class names of the
classes to be processed. Most of the options are meant to be used multiple times; each use affects subsequently
added classes.
Table 15.7. MapGenerator Command Line Options
Option Description
--quiet don't output the O-R Mapping to stdout
--setUID=uid set the list of candidate UIDs to the singleton uid
--addUID=uid add uid to the front of the list of candidate UIDs
--select=mode mode use select mode mode(e.g., distinct or all) for subsequently added
classes
--depth=<small-int> limit the depth of component data recursion for subsequently added
classes
--output=my_mapping.xml output the O-R Mapping to a file
full.class.Name add the class to the mapping
--abstract=full.class.Name see below
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The abstract switch directs the map generator tool to ignore specific super classes so that classes with common
inheritance are not mapped to one large table. For instance, consider these class hierarchies:
Animal-->Mammal-->Human
Animal-->Mammal-->Marsupial-->Kangaroo
If the --abstractswitch is not used, all classes will be mapped as subclasses of Animal, resulting in one large
table containing all the properties of all the classes plus a discriminator column to indicate which subclass is ac-
tually stored. If Mammal is marked as abstract, Human and Marsupial will be mapped to separate <class> de-
clarations and stored in separate tables. Kangaroo will still be a subclass of Marsupial unless Marsupial is also
marked as abstract.
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Chapter 16. Example: Parent/Child
One of the very first things that new users try to do with Hibernate is to model a parent / child type relationship.
There are two different approaches to this. For various reasons 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>.) Now, it turns out that
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 bidirec-
tional one to many association with cascades to model a parent / child relationship efficiently and elegantly. It's
not at all difficult!
16.1. A note about collections
Hibernate collections are considered to be a logical part of their owning entity; never of the contained entities.
This is a crucial distinction! It has the following consequences:
• When we remove / add an object from / to a collection, the version number of the collection owner is incre-
mented.
• If an object that was removed from a collection is an instance of a value type (eg, 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.
• On the other hand, 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 behaviour 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.
Instead, the default behaviour is that adding an entity to a collection merely creates a link between the two en-
tities, while removing it removes the link. This is very appropriate for all sorts of cases. Where it is not appro-
priate at all is the case of a parent / child relationship, where the life of the child is bound to the lifecycle of the
parent.
16.2. Bidirectional one-to-many
Suppose we start with a simple <one-to-many> association from Parent to Child.
<set name="children">
<key column="parent_id"/>
<one-to-many class="Child"/>
</set>
If we were to execute the following code
Parent p = .....;
Child c = new Child();
p.getChildren().add(c);
session.save(c);
session.flush();
Hibernate would issue two SQL statements:
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• 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.
The underlying cause 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. So the solution is to make the link part of the
Child mapping.
<many-to-one name="parent" column="parent_id" not-null="true"/>
(We 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.
<set name="children" inverse="true">
<key column="parent_id"/>
<one-to-many class="Child"/>
</set>
The following code would be used to add a new Child
Parent p = (Parent) session.load(Parent.class, pid);
Child c = new Child();
c.setParent(p);
p.getChildren().add(c);
session.save(c);
session.flush();
And now, only one SQL INSERT would be issued!
To tighten things up a bit, we could create an addChild() method of Parent.
public void addChild(Child c) {
c.setParent(this);
children.add(c);
}
Now, the code to add a Child looks like
Parent p = (Parent) session.load(Parent.class, pid);
Child c = new Child();
p.addChild(c);
session.save(c);
session.flush();
16.3. Cascading lifecycle
The explicit call to save() is still annoying. We will address this by using cascades.
<set name="children" inverse="true" cascade="all">
<key column="parent_id"/>
<one-to-many class="Child"/>
</set>
Example: Parent/Child
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This simplifies the code above to
Parent p = (Parent) session.load(Parent.class, pid);
Child c = new Child();
p.addChild(c);
session.flush();
Similarly, we don't need to iterate over the children when saving or deleting a Parent. The following removes p
and all its children from the database.
Parent p = (Parent) session.load(Parent.class, pid);
session.delete(p);
session.flush();
However, this code
Parent p = (Parent) session.load(Parent.class, pid);
Child c = (Child) p.getChildren().iterator().next();
p.getChildren().remove(c);
c.setParent(null);
session.flush();
will not remove c from the database; it will ony remove the link to p (and cause a NOT NULL constraint viola-
tion, in this case). You need to explicitly delete() the Child.
Parent p = (Parent) session.load(Parent.class, pid);
Child c = (Child) p.getChildren().iterator().next();
p.getChildren().remove(c);
session.delete(c);
session.flush();
Now, in our case, a Child can't really exist without its parent. So if we remove a Child from the collection, we
really do want it to be deleted. For this, we must use cascade="all-delete-orphan".
<set name="children" inverse="true" cascade="all-delete-orphan">
<key column="parent_id"/>
<one-to-many class="Child"/>
</set>
Note: even though the collection mapping specifies inverse="true", cascades are still processed by iterating
the collection elements. So if you require that an object be saved, deleted or updated by cascade, you must add
it to the collection. It is not enough to simply call setParent().
16.4. Using cascading update()
Suppose we loaded up a Parent in one Session, made some changes in a UI action and wish to persist these
changes in a new Session (by calling update()). The Parent will contain a collection of childen and, since cas-
cading update is enabled, Hibernate needs to know which children are newly instantiated and which represent
existing rows in the database. Lets assume that both Parent and Child have (synthetic) identifier properties of
type java.lang.Long. Hibernate will use the identifier property value to determine which of the children are
new. (You may also use the version or timestamp property, see Section 9.4.2, “Updating detached objects”.)
The unsaved-value attribute is used to specify the identifier value of a newly instantiated instance. unsaved-
value defaults to "null", which is perfect for a Long identifier type. If we would have used a primitive identitifi-
er property, we would need to specify
<id name="id" type="long" unsaved-value="0">
Example: Parent/Child
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for the Child mapping. (There is also an unsaved-value attribute for version and timestamp property map-
pings.)
The following code will update parent and child and insert newChild.
//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();
Well, thats all very well for the case of a generated identifier, but what about assigned identifiers and composite
identifiers? This is more difficult, since unsaved-value can't distinguish between a newly instantiated object
(with an identifier assigned by the user) and an object loaded in a previous session. In these cases, you will
probably need to give Hibernate a hint; either
• define unsaved-value="null" or unsaved-value="negative" on a <version> or <timestamp> property
mapping for the class.
• set unsaved-value="none" and explicitly save() newly instantiated children before calling up-
date(parent)
• set unsaved-value="any" and explicitly update() previously persistent children before calling up-
date(parent)
none is the default unsaved-value for assigned and composite identifiers.
There is one further possibility. There is a new Interceptor method named isUnsaved() which lets the applic-
ation implement its own strategy for distinguishing newly instantiated objects. For example, you could define a
base class for your persistent classes.
public class Persistent {
private boolean _saved = false;
public void onSave() {
_saved=true;
}
public void onLoad() {
_saved=true;
}
......
public boolean isSaved() {
return _saved;
}
}
(The saved property is non-persistent.) Now implement isUnsaved(), along with onLoad() and onSave() as
follows.
public Boolean isUnsaved(Object entity) {
if (entity instanceof Persistent) {
return new Boolean( !( (Persistent) entity ).isSaved() );
}
else {
return null;
}
}
public boolean onLoad(Object entity,
Serializable id,
Object[] state,
Example: Parent/Child
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String[] propertyNames,
Type[] types) {
if (entity instanceof Persistent) ( (Persistent) entity ).onLoad();
return false;
}
public boolean onSave(Object entity,
Serializable id,
Object[] state,
String[] propertyNames,
Type[] types) {
if (entity instanceof Persistent) ( (Persistent) entity ).onSave();
return false;
}
16.5. Conclusion
There is quite a bit to digest here and it might look confusing first time around. However, in practice, it all
works out quite 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. Unfortu-
nately, there are two big limitations to composite element classes: composite elements may not own collections,
and they should not be the child of any entity other than the unique parent. (However, they may have a surrog-
ate primary key, using an <idbag> mapping.)
Example: Parent/Child
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Chapter 17. Example: Weblog Application
17.1. Persistent Classes
The persistent classes represent a weblog, and an item posted in a weblog. They are to be modelled as a stand-
ard 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;
}
public Long getId() {
return _id;
}
public String getText() {
return _text;
}
public String getTitle() {
return _title;
}
public void setBlog(Blog blog) {
_blog = blog;
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}
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;
}
}
17.2. Hibernate Mappings
The XML mappings should now be quite straightforward.
<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
"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"
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
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping package="eg">
<class
name="BlogItem"
Example: Weblog Application
Hibernate 3.0alpha 133
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"/>
</class>
</hibernate-mapping>
17.3. Hibernate Code
The following class demonstrates some of the kinds of things we can do with these classes, using Hibernate.
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()
Example: Weblog Application
Hibernate 3.0alpha 134
.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();
session.save(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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Hibernate 3.0alpha 135
}
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 {
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();
}
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Hibernate 3.0alpha 136
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.list().get(0);
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();
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 18. Example: Various Mappings
This chapters shows off some more complex association mappings.
18.1. Employer/Employee
The following model of the relationship between Employer and Employee uses an actual entity class (Employ-
ment) to represent the association. This is done because there might be more than one period of employment for
the same two parties. Components are used to model monetory values and employee names.
Heres 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="MonetoryAmount">
<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"/>
<many-to-one name="employee" column="employee_id" not-null="true"/>
</class>
<class name="Employee" table="employees">
<id name="id">
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<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>
And heres the table schema generated by SchemaExport.
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
create sequence employee_id_seq
create sequence employment_id_seq
create sequence employer_id_seq
18.2. Author/Work
Consider the following model of the relationships between Work, Author and Person. We represent the relation-
ship between Work and Author as a many-to-many association. We choose to represent the relationship between
Author and Person as one-to-one association. Another possibility would be to have Author extend Person.
Example: Various Mappings
Hibernate 3.0alpha 139
The following mapping document correctly represents these relationships:
<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" not-null="true"/>
</key>
<many-to-many class="Author">
<column name="author_id" not-null="true"/>
</many-to-many>
</set>
<subclass name="Book" discriminator-value="B">
<property name="text"/>
</subclass>
<subclass name="Song" discriminator-value="S">
<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"/>
Example: Various Mappings
Hibernate 3.0alpha 140
</set>
</class>
<class name="Person" table="persons">
<id name="id" column="id">
<generator class="native"/>
</id>
<property name="name"/>
</class>
</hibernate-mapping>
There are four tables in this mapping. works, authors and persons hold work, author and person data respect-
ively. author_work is an association table linking authors to works. Heres the table schema, as generated by
SchemaExport.
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),
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
18.3. Customer/Order/Product
Now consider a model of the relationships between Customer, Order and LineItem and Product. There is a
one-to-many association between Customer and Order, but how should we represent Order / LineItem /
Product? I've chosen to map LineItem as an association class representing the many-to-many association
between Order and Product. In Hibernate, this is called a composite element.
Example: Various Mappings
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The mapping document:
<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>
<property name="date"/>
<many-to-one name="customer" column="customer_id"/>
<list name="lineItems" table="line_items">
<key column="order_id"/>
<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>
customers, orders, line_items and products hold customer, order, order line item and product data respect-
ively. line_items also acts as an association table linking orders with products.
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,
Example: Various Mappings
Hibernate 3.0alpha 142
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 products
alter table line_items
add constraint line_itemsFK1 foreign key (order_id) references orders
Example: Various Mappings
Hibernate 3.0alpha 143
Chapter 19. 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 all sorts of reasons why you should use them. We
recommend that identifiers be 'synthetic' (generated, with no business meaning) and of a non-primitive
type. For maximum flexibility, use java.lang.Long or java.lang.String.
Place each class mapping in its own file.
Don't use a single monolithic mapping document. Map com.eg.Foo in the file com/eg/Foo.hbm.xml. This
makes particularly good sense in a team environment.
Load mappings as resources.
Deploy the mappings along with the classes they map.
Consider externalising query strings.
This is a good practice if your queries call non-ANSI-standard SQL functions. Externalising the query
strings to mapping files will make the application more portable.
Use bind variables.
As in JDBC, always replace non-constant values by "?". Never use string manipulation to bind a non-
constant value in a query! Even better, consider using named parameters in queries.
Don't manage your own JDBC connections.
Hibernate lets the application manage JDBC connections. This approach should be considered a last-resort.
If you can't use the built-in connections providers, consider providing your own implementation of
org.hibernate.connection.ConnectionProvider.
Consider using a custom type.
Suppose you have a Java type, say from some library, that needs to be persisted but doesn't provide the ac-
cessors 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 (eg. mass update / delete) might bene-
fit from direct JDBC. But please, wait until you know something is a bottleneck. And don't assume that dir-
ect JDBC is necessarily faster. If need to use direct JDBC, it might be worth opening a Hibernate Session
and using that SQL connection. That way you can still use the same transaction strategy and underlying
connection provider.
Understand Session flushing.
From time to time the Session synchronizes its persistent state with the database. Performance will be af-
fected if this process occurs too often. You may sometimes minimize unnecessary flushing by disabling
automatic flushing or even by changing the order of queries and other operations within a particular trans-
action.
In a three tiered architecture, consider using saveOrUpdate().
When using a servlet / session bean architecture, you could 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.update()
or Session.saveOrUpdate() to update the persistent state of an object.
Hibernate 3.0alpha 144
In a two tiered architecture, consider using session disconnection.
Database Transactions have to be as short as possible for best scalability. However, it is often neccessary to
implement long running Application Transactions, a single unit-of-work from the point of view of a user.
This Application Transaction might span several client requests and response cycles. Either use Detached
Objects or, in two tiered architectures, simply disconnect the Hibernate Session from the JDBC connection
and reconnect it for each subsequent request. Never use a single Session for more than one Application
Transaction usecase, otherwise, you will run into stale data.
Don't treat exceptions as recoverable.
This is more of a necessary practice than a "best" practice. When an exception occurs, roll back the Trans-
action and close the Session. If you don't, Hibernate can't guarantee that in-memory state accurately rep-
resents persistent state. As a special case of this, do not use Session.load() to determine if an instance
with the given identifier exists on the database; use find() instead.
Prefer lazy fetching for associations.
Use eager (outer-join) fetching sparingly. Use proxies and/or lazy collections for most associations to
classes that are not cached at the JVM-level. For associations to cached classes, where there is a high prob-
ability of a cache hit, explicitly disable eager fetching using outer-join="false". When an outer-join
fetch is appropriate to a particular use case, use a query with a left join.
Consider abstracting your business logic from Hibernate.
Hide (Hibernate) data-access code behind an interface. Combine the DAO and Thread Local Session pat-
terns. You can even have some classes persisted by handcoded JDBC, associated to Hibernate via a User-
Type. (This advice is intended for "sufficiently large" applications; it is not appropriate for an application
with five tables!)
Implement equals() and hashCode() using a unique business key.
If you compare objects outside of the Session scope, you have to implement equals() and hashCode(). In-
side the Session scope, Java object identity is guaranteed. If you implement these methods, never ever use
the database identifier! A transient object doesn't have an identifier value and Hibernate would assign a
value when the object is saved. If the object is in a Set while being saved, the hash code changes, breaking
the contract. To implement equals() and hashCode(), use a unique business key, that is, compare a unique
combination of class properties. Remember that this key has to be stable and unique only while the object is
in a Set, not for the whole lifetime (not as stable as a database primary key). Never use collections in the
equals() comparison (lazy loading) and be careful with other associated classes that might be proxied.
Don't use exotic association mappings.
Good usecases for a real many-to-many associations are rare. Most of the time you need additional inform-
ation stored in the "link table". In this case, it is much better to use two one-to-many associations to an in-
termediate link class. In fact, we think that most associations are one-to-many and many-to-one, you should
be careful when using any other association style and ask yourself if it is really neccessary.
Best Practices
Hibernate 3.0alpha 145
