Programming Tutorials

Tutorial Using the Java Persistence API (JPA) in Hibernate

By: Felix  

This tutorial is located within the download bundle under entitymanager and illustrates using annotations to provide mapping information using JPA

persistence.xml

Instead of using the Hibernate-specific hibernate.cfg.xml configuration file, JPA, defines a different bootstrap process that uses its own configuration file named persistence.xml. How this bootstrapping works is defined by the JPA specification. In Javaâ„¢ 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.

<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.

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..

The annotated entity Java class

This tutorial uses the JPA APIs instead of the Hibernate APIs.

Example: 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: 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();

Here we use an javax.persistence.EntityManager as opposed to a org.hibernate.Session. JPA calls this operation persist instead of save.

Example: 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();


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