8.9. Closing  public boolean isClosed (); public void close ();
When a PersistenceManager is no longer needed, you should call
its close method. Closing a PersistenceManager releases any
resources it is using. The persistent instances managed by the
PersistenceManager become invalid, as do any Query and Extent
objects it created. Calling any method other than isClosed on a
closed PersistenceManager results in a JDOUserException.