EntityManager.close() should throw an ISE if called on already closed EntityManager
Calling EntityManager.close() on an already closed entity manager, should throw an IllegalStateException. From JPA2.2 spec:
* Close the factory, releasing any resources that it holds.
* After a factory instance has been closed, all methods invoked
* on it will throw the IllegalStateException, except for isOpen,
* which will return false. Once an EntityManagerFactory has
* been closed, all its entity managers are considered to be
* in the closed state.
* @throws IllegalStateException if the entity manager factory
* has been closed
public void close();