The EJB 3 persistence model is at the core of J2EE 5, offering a simplier, light-weight model compared with earlier EJB specifications. The new model took into account experience with managed beans in prior J2EE releases, as well as practical advice from the Hibernate and JDO communities.

The most important difference from earlier EJB versions is that the EJB 3 persistence model can be used both within and without a container, and requires only a compatible persistence provider implementation. A J2EE server might offer such an implementation, and Hibernate 3 also happens to be a light-weight EJB 3 persistence provider.

In his recent EJB 3 tutorial, Jeff Brown shows how to persist a POJO, how to map relationships using annotations, and how to query an EJB 3 provider. While his examples use Hibernate, the only Hibernate-related information is in the persistence context descriptor, which is the only XML file required by the persistence model.

Jeff shows that a POJO must meet these four requirements to be persistable in the EJB 3 model:

• Must have a no-arg constructor
• Must not have any final methods
• Its class must not be final
• The class must be marked with @Entity, or have an associated XML descriptor

The annotation-based model alleviates the need for XML descriptor files. For instance, the article shows that by associating a few annotations with a POJO, an instance of this class can be persisted via any EJB 3 persistence provider:

@Entity
public class Person {

  private String firstName;

  private String lastName;

  private Integer id; // new field to represent identity

  public Person(String firstName, String lastName) {
    this.firstName = firstName;
    this.lastName = lastName;
  }

  public String getFirstName() {
    return firstName;
  }

  public void setFirstName(String firstName) {
    this.firstName = firstName;
  }

  public String getLastName() {
    return lastName;
  }

  public void setLastName(String lastName) {
    this.lastName = lastName;
  }

  @Id
  @GeneratedValue
  public Integer getId() {
    return id;
  }

  public void setId(Integer id) {
    this.id = id;
  }

  public String toString() {
    return lastName + ", " + firstName;
  }
}

Persisting an instance of this class is as simple as the following code snippet:

 EntityManagerFactory emf = 
      Persistence.createEntityManagerFactory("mycontext", 
            new HashMap());
    EntityManager em = emf.createEntityManager();
    EntityTransaction tx = em.getTransaction();
    tx.begin();
    em.persist(new Person("Ben", "Franklin"));
    tx.commit();
    em.close();

The persistence context descriptor can even define if the persistence manager should create a new table, if one for this entity does not already exist:

<?xml version="1.0" encoding="UTF-8"?>
<persistence>
 <persistence-unit name="mycontext">
    <provider>org.hibernate.ejb.HibernatePersistence</provider>
    <class>com.persistencedemo.Person</class>
    <properties>
        <property name="hibernate.connection.url" value="jdbc:mysql://localhost/demodb"/>
        <property name="hibernate.dialect" value="org.hibernate.dialect.MySQLDialect"/>
        <property name="hibernate.connection.driver_class" value="com.mysql.jdbc.Driver"/>
        <property name="hibernate.connection.password" value="javadude"/>
        <property name="hibernate.connection.username" value="javauser"/>
        <property name="hibernate.hbm2ddl.auto" value="update"/>
    </properties>
  </persistence-unit>
</persistence>

Here, the hibernate.hbm2ddl.auto property is set to update, indicating that a table should be updated to reflect the bean's properties. Again, this is the single XML file required by the persistence model.

What do you think of the EJB 3 persistence model?

  
EntityManagerFactory emf = 
   Persistence.createEntityManagerFactory(
       "mycontext", new HashMap());
EntityManager em = emf.createEntityManager();

 
EntityManager em = 
   EntityManager.createEntityManager(
      "mycontext", new HashMap());

    EntityManager em = 
        new EntityManager("mycontext", new HashMap());

 EntityManager em = new EntityManager("mycontext");