EntityManager usage pattern

[Dain Sundstrom]

Based on the feed back I got on the "Flush all caches" thread, I  
wanted to clarify the preferred usage pattern for EntityManager.  Abe  
White said, "You must use a new EntityManager for each test" and I'm  
not sure if that was advice for my specific use case, or if in  
general, I should create a new entity manager instance for each  
transaction and close it when the transaction terminates.  Basically,  
something like this:

public void doWork() {
  beginTx()
  try {
    em = emf.createEntityManager()
    doSomething()
  } finally {
    completeTx()
    em.close()
  }
}
-dain