How to create a thread safe EntityManagerFactory?

An easy way to "solve" this would be to use a helper class (a la HibernateUtil) and to initialize the EntityManagerFactory in a static initialization block. Something like this:

public class JpaUtil { 
    private static final EntityManagerFactory emf;

    static {
        try {
            factory = Persistence.createEntityManagerFactory("MyPu");
        } catch (Throwable ex) {
            logger.error("Initial SessionFactory creation failed", ex);
            throw new ExceptionInInitializerError(ex);
        }
    }

   ...

}

And the "problem" is gone.

I am not seeing any issues with the static block approach. Or you can do the same in the below manner which is a Singleton pattern with double-lock check

public class JPAHelper {

 private static JPAHelper myHelper = new JPAHelper();
 private static EntityManagerFactory myFactory = null;

 /**
  * Private constructor. Implementing synchronization with double-lock check
  */
 private JPAHelper() {

  if(myFactory == null) {
   synchronized (JPAHelper.class) {

    // This second check will be true only for the first thread entering the block incase 
    // of thread race
    if(myFactory == null) {
     myFactory = Persistence.createEntityManagerFactory("MyUnit");
    }
   }
  }
 }

 /**
  * Static Accessor Method
  * @return
  */
 public static JPAHelper getInstance() {
  if(myHelper == null) {
   myHelper = new JPAHelper();
  }
  return myHelper;
 }


 public EntityManagerFactory getJPAFactory() {
  return myFactory;
 }

And you will call

EntityManager myManager = JPAhelper.getInstance().getJPAFactory().createEntityManager();