Question

Ok, I read bunch of articles/examples how to write Entity Manager Factory in singleton.

One of them easiest for me to understand a bit:

http://javanotepad.blogspot.com/2007/05/jpa-entitymanagerfactory-in-web.html

I learned that EntityManagerFactory (EMF) should only be created once preferably in application scope.

And also make sure to close the EMF once it’s used (?)

So I wrote EMF helper class for business methods to use:

public class EmProvider {

    private static final String DB_PU = "KogaAlphaPU";

    public static final boolean DEBUG = true;

    private static final EmProvider singleton = new EmProvider();

    private EntityManagerFactory emf;

    private EmProvider() {}

    public static EmProvider getInstance() {
        return singleton;
    }


    public EntityManagerFactory getEntityManagerFactory() {
        if(emf == null) {
            emf = Persistence.createEntityManagerFactory(DB_PU);
        }
        if(DEBUG) {
            System.out.println("factory created on: " + new Date());
        }
        return emf;
    }

    public void closeEmf() {
        if(emf.isOpen() || emf != null) {
            emf.close();
        }
        emf = null;
        if(DEBUG) {
            System.out.println("EMF closed at: " + new Date());
        }
    }

}//end class

And my method using EmProvider:

public String foo() {
    EntityManager em = null;
    List<Object[]> out = null;
    try {

        em = EmProvider.getInstance().getEntityManagerFactory().createEntityManager();
        Query query = em.createNativeQuery(JPQL_JOIN); //just some random query 
        out = query.getResultList();
    }
    catch(Exception e) {
        //handle error....
    }
    finally {
        if(em != null) {
             em.close(); //make sure to close EntityManager
        }
        //should I not close the EMF itself here?????
        EmProvider.getInstance().closeEmf();
    }

I made sure to close EntityManager (em) within method level as suggested. But when should EntityManagerFactory be closed then? And why EMF has to be singleton so bad??? I read about concurrency issues but as I am not experienced multi-thread-grammer, I can’t really be clear on this idea.

Answer 1

• EntityManagerFactory instances are
  heavyweight objects. Each factory
  might maintain a metadata cache,
  object state cache, EntityManager
  pool, connection pool, and more. If
  your application no longer needs an
  EntityManagerFactory, you should
  close it to free these resources.

• When an EntityManagerFactory closes,
  all EntityManagers from that factory,
  and by extension all entities managed
  by those EntityManagers, become
  invalid.

• It is much better to keep a factory
  open for a long period of time than
  to repeatedly create and close new
  factories. Thus, most applications
  will never close the factory, or only
  close it when the application is
  exiting.

• Only applications that require
  multiple factories with different
  configurations have an obvious reason
  to create and close multiple
  EntityManagerFactory instances.

• Only one EntityManagerFactory is
  permitted to be created for each
  deployed persistence unit
  configuration. Any number of
  EntityManager instances may be
  created from a given factory.

• More than one entity manager factory
  instance may be available
  simultaneously in the JVM. Methods of the EntityManagerFactory
  interface are threadsafe.