EntityManagerFactory

From CauchoWiki

The EntityManagerFactory produces EntityManger instances for applications.

In many applications, it is more convenient to get the persistence context EntityManager directly using @PersistenceContext or JNDI, rather than using an EntityManagerFactory.

[edit] API

   package javax.persistence;
 
   public interface EntityManagerFactory {
       public EntityManager createEntityManager();
       public EntityManager createEntityManager(PersistenceContextType type);
       public EntityManager getEntityManager();
   
       public boolean isOpen();
       public void close();
   }

• createEntityManager() creates a new EntityManager with a TRANSACTIONAL PersistenceContextType.

• createEntityManager(type) creates a new EntityManager with an application-specified PersistenceContextType.

• getEntityManager() returns the ThreadLocal EntityManager for the persistence unit, i.e. the persistence context. Using @PersistenceUnit in combination with getEntityManager() is equivalent to use @PersistenceContext.

• isOpen() checks if the EntityManagerFactory is valid.

• close() closes the EntityManagerFactory.

[edit] @PersistenceUnit injection

The EntityManagerFactory for a persistence unit can be assigned using EJB injection with the @PersistenceUnit annotation. The configuration for a servlet might look like:

   public class MyServlet extends GenericServlet {
 
       @PersistenceUnit
       EntityManagerFactory _emf;

       public void service(ServletRequest req, ServletResponse res)
       {
           EntityManager em = _emf.createEntityManager();
           ...
       }
   }

@PersistenceUnit(unitName="xxx") can be used if the environment has more than one persistence unit.

@PersistenceUnit can be used for session beans, message-driver beans, servlets, filters, and JSP pages.

In many cases, the @PersistenceContext is more appropriate, since it injects a JTA-aware EntityManager directly.

[edit] Obtaining an EntityManagerFactory using JNDI

In Amber, the EntityManagerFactory for a persistence unit is stored as "java:comp/env/persistence/PersistenceUnit/xxx", where "xxx" is the name of the persistence unit.

(The CMP 2.1 implementation uses a name of "resin-ejb".)