When should EntityManagerFactory instance be created/opened?

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.