Chapter 4. Tutorial Using the Java Persistence API (JPA)

This tutorial is located within the download bundle under entitymanager.

Objectives

Use annotations to provide mapping information.
Use JPA.

4.1. `persistence.xml`

The previous tutorials used the Hibernate-specific hibernate.cfg.xml configuration file. JPA, 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”.

4.2. The annotated entity Java class

The entity is exactly the same as in Section 3.2, “The annotated entity Java class”

4.3. Example code

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”.

4.4. Take it further!

Practice Exercises

Develop an EJB Session bean to investigate implications of using a container-managed persistence context (javax.persistence.EntityManager). Try both stateless and stateful use-cases.