Table of Contents
List of Examples
class mapping elementid mapping elementproperty mapping elementorg.hibernate.SessionFactorypersistence.xmljavax.persistence.EntityManagerFactoryorg.hibernate.envers.AuditReaderTable of Contents
Working with both Object-Oriented software and Relational Databases can be cumbersome and time consuming. Development costs are significantly higher due to a paradigm mismatch between how data is represented in objects versus relational databases. Hibernate is an Object/Relational Mapping solution for Java environments. The term Object/Relational Mapping refers to the technique of mapping data between an object model representation to a relational data model representation. See http://en.wikipedia.org/wiki/Object-relational_mapping for a good high-level discussion.
You do not need a strong background in SQL to use Hibernate, but having a basic understanding of the concepts can help you understand Hibernate more fully and quickly. An understanding of data modeling principles is especially important. You might want to consider these resources as a good starting point:
Data Modeling Resources
Hibernate takes care of the mapping from Java classes to database tables, and from Java data types to SQL data types. In addition, it provides data query and retrieval facilities. It can significantly reduce development time otherwise spent with manual data handling in SQL and JDBC. Hibernate’s design goal is to relieve the developer from 95% of common data persistence-related programming tasks by eliminating the need for manual, hand-crafted data processing using SQL and JDBC. However, unlike many other persistence solutions, Hibernate does not hide the power of SQL from you and guarantees that your investment in relational technology and knowledge is as valid as always.
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 streamlines the common task of translating result sets from a tabular representation to a graph of objects.
Use Hibernate and report any bugs or issues you find. See http://hibernate.org/issuetracker.html for details.
Try your hand at fixing some bugs or implementing enhancements. Again, see http://hibernate.org/issuetracker.html.
Engage with the community using mailing lists, forums, IRC, or other ways listed at http://hibernate.org/community.html.
Help improve or translate this documentation. Contact us on the developer mailing list if you have interest.
Spread the word. Let the rest of your organization know about the benefits of Hibernate.
The referenced projects and code for the tutorials in this guide are available at files/hibernate-tutorials.zip.
Table of Contents
The Hibernate team provides release bundles hosted on the SourceForge File Release System, in
ZIP
and
TGZ
formats. Each release bundle containsJARs,
documentation, source code, and other goodness.
You can download releases of Hibernate, in your chosen format, from the list at http://sourceforge.net/projects/hibernate/files/hibernate4/.
The lib/required/ directory contains all the JARs Hibernate requires. All the
jars in this directory must also be included in your project's classpath.
The /lib/jpa/ directory contains the
hibernate-entitymanager jar and its dependencies beyond those
in lib/required/. This defines Hibernate support for
JPA.
The lib/envers directory contains the hibernate-envers
jar and its dependencies beyond those in lib/required/
The lib/optional directory contains the jars needed for optional features of
Hibernate.
The authoritative repository for Hibernate artifacts is the JBoss Maven repository. The team responsible for the JBoss Maven repository maintains a number of Wiki pages that contain important information.
Maven Repository Wiki Pages
http://community.jboss.org/docs/DOC-14900 - General information about the repository.
http://community.jboss.org/docs/DOC-15170 - Information about setting up the JBoss repositories in order to do development work on JBoss projects themselves.
http://community.jboss.org/docs/DOC-15169 - Information about setting up access to the repository to use JBoss projects as part of your own software.
Hibernate produces a number of artifacts (all under the org.hibernate groupId):
Hibernate Artifacts under groupId org.hibernate
The main artifact, needed to build applications using the native Hibernate APIs including
defining metadata in both annotations as well as Hibernate's own hbm.xml
format.
Represents Hibernate's implementation of JPA, as specified at http://jcp.org/en/jsr/detail?id=317.
This artifact depends on hibernate-core
An optional module that provides historical auditing of changes to your entities.
This artifact depends on both hibernate-core
and hibernate-entitymanager.
Provides integration between Hibernate and the C3P0 connection pool library. See http://sourceforge.net/projects/c3p0/ for information about C3P0.
This artifact depends on hibernate-core, but is generally included
in a project as a runtime dependency. It pulls in the C3P0
dependencies automatically.
Provides integration between Hibernate and the Proxool connection pool library. See http://proxool.sourceforge.net/ for more information about this library.
This artifact depends on hibernate-core, but is generally included
in a project as a runtime dependency. It pulls in the Proxool
dependencies automatically..
Privides integration between Hibernate and EhCache, as a second-level cache. See http://ehcache.sourceforge.net/ for more information aboutEhCache.
This artifact depends on hibernate-core, but is generally included
in a project as a runtime dependency. It pulls in the Ehcache
dependencies automatically.
Provides integration between Hibernate and Infinispan, as a second-level cache. See http://jboss.org/infinispan for more information about Infinispan.
This artifact depends on hibernate-core, but is generally included
in a project as a runtime dependency. It pulls in the Infinispan
dependencies automatically.
Table of Contents
This tutorial is located within the download bundle under basic/.
Objectives
using Hibernate mapping files (hbm.xml) to provide mapping information
using the native Hibernate APIs
The resource file hibernate.cfg.xml defines Hibernate configuration
information.
The connection.driver_class, connection.url,
connection.username and connection.password
property elements define JDBC connection information. These tutorials utilize the H2
in-memory database, So the values of these properties are all specific to running H2 in its in-memory mode.
connection.pool_size is used to configure the number of connections in Hibernate's
built-in connection pool.
The built-in Hibernate connection pool is in no way intended for production use. It lacks several features found on production-ready connection pools. See the section discussion in Hibernate Developer Guide for further information.
The dialect property specifies the particular SQL variant with which Hibernate will
converse.
In most cases, Hibernate is able to properly determine which dialect to use. This is particularly useful if your application targets multiple databases. This is discussed in detail in the Hibernate Developer Guide
The hbm2ddl.auto property enables automatic generation of database schemas directly into
the database.
Finally, add the mapping file(s) for persistent classes to the configuration. The resource
attribute of the mapping element causes Hibernate to attempt to locate that mapping as a
classpath resource, using a java.lang.ClassLoader lookup.
The entity class for this tutorial is org.hibernate.tutorial.hbm.Event.
Notes About the Entity
This class uses standard JavaBean naming conventions for property getter and setter methods, as well as private visibility for the fields. Although this is the recommended design, it is not required.
The no-argument constructor, which is also a JavaBean convention, is a
requirement for all persistent classes. Hibernate needs to create objects for you, using Java
Reflection. The constructor can be private. However, package or public visibility is required for runtime
proxy generation and efficient data retrieval without bytecode instrumentation.
The hbm.xml mapping file for this tutorial is the classpath resource
org/hibernate/tutorial/hbm/Event.hbm.xml as we saw in
Section 2.1, “The Hibernate configuration file”
Hibernate uses the mapping metadata to determine how to load and store objects of the persistent class. The Hibernate mapping file is one choice for providing Hibernate with this metadata.
Functions of the class mapping element
The name attribute (combined here with the package attribute from
the containing hibernate-mapping element) names the FQN of the class to be
defined as an entity.
The table attribute names the database table which contains the data for this
entity.
Instances of the Event class are now mapped to rows in the
EVENTS table.
Hibernate uses the property named by the id element to uniquely identify rows in the
table.
It is not required for the id element to map to the table's actual primary key
column(s), but it is the normal convention. Tables mapped in Hibernate do not even need to define
primary keys. However, it is strongly recommend that all schemas define proper referential
integrity. Therefore id and primary key are used interchangeably
throughout Hibernate documentation.
The id element here identifies the EVENT_ID column as
the primary key of the EVENTS table. It also identifies the
id property of the Event class as the property containing the
identifier value.
The generator element nested inside the id element informs Hibernate
about which strategy is used to generated primary key values for this entity. This example uses a simple
incrementing count.
Example 2.3. The property mapping element
<property name="date" type="timestamp" column="EVENT_DATE"/> <property name="title"/>
The two property elements declare the remaining two properties of the
Event class: date and title. The
date property mapping includes the column attribute, but the
title does not. In the absence of a column attribute, Hibernate
uses the property name as the column name. This is appropriate for title, but since
date is a reserved keyword in most databases, you need to specify a non-reserved
word for the column name.
The title mapping also lacks a type attribute. The types
declared and used in the mapping files are neither Java data types nor SQL database types. Instead,
they are Hibernate mapping types. Hibernate mapping types are
converters which translate between Java and SQL data types. Hibernate attempts to determine the correct
conversion and mapping type autonomously if the type attribute is not present in the
mapping, by using Java reflection to determine the Java type of the declared property and using a
default mapping type for that Java type.
In some cases this automatic detection might not chose the default you expect or need, as seen with the
date property. Hibernate cannot know if the property, which is of type
java.util.Date, should map to a SQL DATE,
TIME, or TIMESTAMP datatype.
Full date and time information is preserved by mapping the property to a timestamp converter,
which identifies an instance of the class org.hibernate.type.TimestampType.
Hibernate determines the mapping type using reflection when the mapping files are processed. This process adds overhead in terms of time and resources. If startup performance is important, consider explicitly defining the type to use.
The org.hibernate.tutorial.hbm.NativeApiIllustrationTest class illustrates using
the Hibernate native API.
The examples in these tutorials are presented as JUnit tests, for ease of use. One benefit of this
approach is that setUp and tearDown roughly illustrate
how a org.hibernate.SessionFactory is created at the start-up of an
application and closed at the end of the application lifecycle.
Example 2.4. Obtaining the org.hibernate.SessionFactory
protected void setUp() throws Exception {
// A SessionFactory is set up once for an application
sessionFactory = new Configuration()
.configure() // configures settings from hibernate.cfg.xml
.buildSessionFactory();
}
Procedure 2.1. Tutorial Workflow
The configuration is loaded.
The org.hibernate.cfg.Configuration class is the first thing to notice. In this
tutorial, all configuration details are located in the hibernate.cfg.xml file
discussed in Section 2.1, “The Hibernate configuration file”.
The org.hibernate.SessionFactory is created.
The org.hibernate.cfg.Configuration then creates the
org.hibernate.SessionFactory which is a thread-safe object that is
instantiated once to serve the entire application.
SessionFactory creates Session instances.
The org.hibernate.SessionFactory acts as a factory for
org.hibernate.Session instances as can be seen in the
testBasicUsage method.
Sessions perform work.
A org.hibernate.Session should be thought of as a corollary to a "unit of
work".
Example 2.5. Saving entities
Session session = sessionFactory.openSession(); session.beginTransaction(); session.save( new Event( "Our very first event!", new Date() ) ); session.save( new Event( "A follow up event", new Date() ) ); session.getTransaction().commit(); session.close();
testBasicUsage first creates some new Event objects and
hands them over to Hibernate for management, using the save method. Hibernate now
takes responsibility to perform an INSERT on the database.
Example 2.6. Obtaining a list of entities
session = sessionFactory.openSession();
session.beginTransaction();
List result = session.createQuery( "from Event" ).list();
for ( Event event : (List<Event>) result ) {
System.out.println( "Event (" + event.getDate() + ") : " + event.getTitle() );
}
session.getTransaction().commit();
session.close();
testBasicUsage illustrates use of the Hibernate Query Language
(HQL) to load all existing Event objects from the database and generate the
appropriate SELECT SQL, send it to the database and populate Event
objects with the result set data.
Table of Contents
This tutorial is located within the download bundle under basic.
Objectives
Use annotations to provide mapping information
Use the native Hibernate APIs
The contents are identical to Section 2.1, “The Hibernate configuration file”, with one important
difference. The mapping element at the very end naming the annotated entity class using
the class attribute.
The entity class in this tutorial is org.hibernate.tutorial.annotations.Event which
follows JavaBean conventions. In fact the class itself is identical to the one in Section 2.2, “The entity Java class”, except that annotations are used to provide the metadata,
rather than a separate hbm.xml file.
Example 3.1. Identifying the class as an entity
@Entity
@Table( name = "EVENTS" )
public class Event {
...
}
The @javax.persistence.Entity annotation is used to mark a class as an entity.
It functions the same as the class mapping element discussed in Section 2.3, “The mapping file”. Additionally the
@javax.persistence.Table annotation explicitly specifies the table
name. Without this specification, the default table name would be EVENT).
Example 3.2. Identifying the identifier property
@Id
@GeneratedValue(generator="increment")
@GenericGenerator(name="increment", strategy = "increment")
public Long getId() {
return id;
}
@javax.persistence.Id marks the property which defines the
entity's identifier. @javax.persistence.GeneratedValue and
@org.hibernate.annotations.GenericGenerator work in tandem
to indicate that Hibernate should use Hibernate's increment generation
strategy for this entity's identifier values.
Example 3.3. Identifying basic properties
public String getTitle() {
return title;
}
@Temporal(TemporalType.TIMESTAMP)
@Column(name = "EVENT_DATE")
public Date getDate() {
return date;
}
As in Section 2.3, “The mapping file”, the date property needs
special handling to account for its special naming and its SQL type.
org.hibernate.tutorial.annotations.AnnotationsIllustrationTest is essentially the
same as org.hibernate.tutorial.hbm.NativeApiIllustrationTest discussed in
Section 2.4, “Example code”.
Practice Exercises
Add an association to the Event entity to model a message thread. Use the
Developer Guide as a guide.
Add a callback to receive notifications when an Event is created, updated or
deleted. Try the same with an event listener. Use the Developer
Guide as a guide.
Table of Contents
This tutorial is located within the download bundle under entitymanager.
Objectives
Use annotations to provide mapping information.
Use JPA.
The previous tutorials used the Hibernate-specific
hibernate.cfg.xmlJPA, however, defines a different bootstrap process that uses its own configuration
file named persistence.xml. This bootstrapping process is defined by the
JPA specification. In Java™ SE environments the persistence
provider (Hibernate in this case) is required to locate all JPA configuration files
by classpath lookup of the META-INF/persistence.xml resource name.
Example 4.1. persistence.xml
<persistence xmlns="http://java.sun.com/xml/ns/persistence"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://java.sun.com/xml/ns/persistence http://java.sun.com/xml/ns/persistence/persistence_2_0.xsd"
version="2.0">
<persistence-unit name="org.hibernate.tutorial.jpa">
...
</persistence-unit>
</persistence>
persistence.xml files should provide a unique name for each persistence
unit. Applications use this name to reference the configuration when obtaining an
javax.persistence.EntityManagerFactory reference.
The settings defined in the properties element are discussed in Section 2.1, “The Hibernate configuration file”. Here the javax.persistence-prefixed
varieties are used when possible. Notice that the remaining Hibernate-specific configuration setting names
are now prefixed with hibernate..
Additionally, the class element functions the same as in Section 3.1, “The Hibernate configuration file”.
The entity is exactly the same as in Section 3.2, “The annotated entity Java class”
The previous tutorials used the Hibernate APIs. This tutorial uses the JPA APIs.
Example 4.2. Obtaining the javax.persistence.EntityManagerFactory
protected void setUp() throws Exception {
entityManagerFactory = Persistence.createEntityManagerFactory( "org.hibernate.tutorial.jpa" );
}
Notice again that the persistence unit name is org.hibernate.tutorial.jpa, which matches
Example 4.1, “persistence.xml”
Example 4.3. Saving (persisting) entities
EntityManager entityManager = entityManagerFactory.createEntityManager(); entityManager.getTransaction().begin(); entityManager.persist( new Event( "Our very first event!", new Date() ) ); entityManager.persist( new Event( "A follow up event", new Date() ) ); entityManager.getTransaction().commit(); entityManager.close();
The code is similar to Example 2.5, “Saving entities”. An
javax.persistence.EntityManager interface is used instead of a
org.hibernate.Session interface. JPA calls this
operation persist instead of save.
Example 4.4. Obtaining a list of entities
entityManager = entityManagerFactory.createEntityManager();
entityManager.getTransaction().begin();
List<Event> result = entityManager.createQuery( "from Event", Event.class ).getResultList();
for ( Event event : result ) {
System.out.println( "Event (" + event.getDate() + ") : " + event.getTitle() );
}
entityManager.getTransaction().commit();
entityManager.close();
Again, the code is pretty similar to Example 2.6, “Obtaining a list of entities”.
Table of Contents
This tutorial is located within the download bundle under envers.
Objectives
Configure Envers.
Use the Envers APIs to view and analyze historical data.
This file was discussed in the JPA tutorial in Section 4.1, “persistence.xml”, and is essentially the same here.
Again, the entity is largely the same as in Section 4.2, “The annotated entity Java class”. The major
difference is the addition of the @org.hibernate.envers.Audited annotation, which
tells Envers to automatically track changes to this entity.
Again, this tutorial makes use of the JPA APIs. However, the code also makes a change to one
of the entities, then uses the Envers API to pull back the initial revision as well as the updated
revision. A revision refers to a version of an entity.
Example 5.1. Using the org.hibernate.envers.AuditReader
public void testBasicUsage() {
...
AuditReader reader = AuditReaderFactory.get( entityManager );
Event firstRevision = reader.find( Event.class, 2L, 1 );
...
Event secondRevision = reader.find( Event.class, 2L, 2 );
...
}
Procedure 5.1. Description of Example
An org.hibernate.envers.AuditReader is obtained from the
org.hibernate.envers.AuditReaderFactory which wraps the
javax.persistence.EntityManager.
Next, the find method retrieves specific revisions of the entity. The first call
reads find revision number 1 of Event with id 2. The second call reads find
revision number 2 of Event with id 2.
Practice Exercises
Provide a custom revision entity to additionally capture who made the changes.
Write a query to retrieve only historical data which meets some criteria. Use the Envers User Guide to see how Envers queries are constructed.
Experiment with auditing entities which have many-to-one, many-to-many relations as well as collections. Try retrieving historical versions (revisions) of such entities and navigating the object tree.