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 from an object model representation to a relational data model representation (and visa versa). See http://en.wikipedia.org/wiki/Object-relational_mapping for a good high-level discussion.
While having a strong background in SQL is not required to use Hibernate, having a basic understanding of the concepts can greatly help you understand Hibernate more fully and quickly. Probably the single best background is an understanding of data modeling principles. You might want to consider these resources as a good starting point:
Hibernate not only takes care of the mapping from Java classes to database tables (and from Java data types to SQL data types), but also provides data query and retrieval facilities. It can 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 will help with the common task of result set translation 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.
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 information.
You can download releases of Hibernate, in your chosen format, from the list at http://sourceforge.net/projects/hibernate/files/hibernate3/.
hibernate3.jar
is an aggregation of all the Hibernate Core classes.
This must be included in your project's classpath.
The
lib/required/
directory contains JARs Hibernate requires. All the jars in this directory must also be included in
your project's classpath as well.
The
slf4j
JAR has additional
requirements for it to function properly. The exact
requirements depend on your logging back-end. See
slf4j site
for details.
The
/lib/jpa/
directory contains the
JPA
API JAR. This JAR needs to be in your project's classpath if
you want to use the JPA APIs or JPA annotations.
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, which contains all the Hibernate classes, in
packageorg.hibernate. You need these to
build applications using the native Hibernate APIs. It includes
capabilities for using native Hibernate mapping in
hbm.xml
files, as well as annotations.
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.
Provides integration between Hibernate and JBossCache, as a second-level cache. See http://jboss.org/jbosscache for information about JBossCache.
This artifact depends on hibernate-core, but is generally included
in a project as a runtime dependency. It pulls in the JBossCache
dependencies automatically.
Provides integration between Hibernate and OSCache as a second-level cache. See http://www.opensymphony.com/oscache/ for information about OSCache.
This artifact depends on hibernate-core, but is generally included
in a project as a runtime dependency. It pulls in the OSCache
dependencies automatically.
Provides integration between Hibernate and SwarmCache, as a second-level cache. See http://swarmcache.sourceforge.net/ for more information about SwarmCache.
This artifact depends on hibernate-core, but is generally included
in a project as a runtime dependency. It pulls in the SwarmCache
dependencies automatically.
This tutorial is located within the download bundle under basic and illustrates
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 these are all specific to running H2 in its in-memory mode.
connection.pool_size is used to configure Hibernate's built-in connection pool
how many connections to pool.
The built-in Hibernate connection pool is in no way intended for production use. It lacks several features found on any decent connection pool. See the section discussion in Hibernate Developer Guide for further information.
The dialect property specifies the particular SQL variant Hibernate with which
Hibernate will converse.
In most cases, Hibernate is able to properly determine which dialect to use which is invaluable if your application targets multiple databases. This is discussed in detail in the Hibernate Developer Guide
The hbm2ddl.auto property turns on 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 says to attempt to locate that mapping as a
classpath resource (via 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 find out 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 you want to define 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 strictly necessary 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, the Hibernate team strongly
recommends 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. In this
example a simple incrementing count is used.
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 andtitle. 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 makes this mapping type determination using reflection when the mapping files are processed. This process can take 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 example code in these tutorials is done as JUnit tests mainly for ease of use. However it is
nice in that setUp and tearDown roughly illustrate
how a org.hibernate.SessionFactory would be 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();
}
The org.hibernate.cfg.Configuration class is the first thing to notice. In this
tutorial everything is simply configured via the hibernate.cfg.xml file
discussed inSection 2.1, “The Hibernate configuration file”.
The org.hibernate.cfg.Configuration is then used to create the
org.hibernate.SessionFactory which is a thread-safe object that is
instantiated once to serve the entire application.
The org.hibernate.SessionFactory acts as a factory for
org.hibernate.Session instances as can be seen in the
testBasicUsage method. 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" via the save method. At that
point, Hibernate 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 then illustrates use of the Hibernate Query Language (HQL) to
load all existing Event objects from the database. Hibernate will generate the
appropriate SELECT SQL, send it to the database and populate
Event objects with the result set data.
This tutorial is located within the download bundle under basic and illustrates
using annotations to provide mapping information
using the native Hibernate APIs
The contents are exactly the same as in Section 2.1, “The Hibernate configuration file”.
The single difference is 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 is still following JavaBean conventions. In fact the class itself is exactly the same as we saw
in Section 2.2, “The entity Java class”, the only difference being the use of
annotations to provide the metadata instead of 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's function is essentially the same as the class
mapping element discussed in Section 2.3, “The mapping file”.
Additionally the @javax.persistence.Table annotation is
used to explicitly specify the table name (the default table name would have been
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 defining 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;
}
Just as discussed 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”.
Try the following exercises:
With help of the Developer Guide, add an association to
the Event entity to model a message thread.
With help of the Developer Guide, add a callback to
receive notifications when an Event is created, updated or deleted. Try
the same with an event listener.
This tutorial is located within the download bundle under entitymanager and illustrates
using annotations to provide mapping information
using JPA
The previous tutorials used the Hibernate-specific
hibernate.cfg.xmlpersistence.xml. How this bootstrapping works 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. This name is how applications reference the configuration
while obtaining an javax.persistence.EntityManagerFactory reference.
The settings defined in the properties element were already discussed in
Section 2.1, “The Hibernate configuration file”. Here the javax.persistence-prefixed
varieties are used when possible. For the remaining Hibernate-specific configuration setting names notice
that they are now prefixed with hibernate..
Additionally, the class element functions the same as discussed in
Section 3.1, “The Hibernate configuration file”.
The entity is exactly the same as that from the annotations tutorial. See 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 the use of org.hibernate.tutorial.jpa as the
persistence unit name, which matches from 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 pretty similar to Example 2.5, “Saving entities”. Here
we use an javax.persistence.EntityManager as opposed to a
org.hibernate.Session. 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”.
This tutorial is located within the download bundle under envers and illustrates
configuring Envers
using the Envers APIs to look back through history
This file was discussed in the JPA tutorial, and is largely the same here. The major difference is the set of properties defining listeners. Essentially this enables Envers to recieve notfications from Hibernate processing of certain events such as an entity being saved or updated.
Again, the entity is largely the same as seen in the JPA tutorial. The major
difference to notice 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 entites and then uses the Envers API to pull back the initial revision (version) as well as the updated revision.
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 );
...
}
First an org.hibernate.envers.AuditReader is obtained
from the org.hibernate.envers.AuditReaderFactory wrapping the
javax.persistence.EntityManager.
Then the find method is used to retrieve 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".
Copyright © 2004 Red Hat, Inc.