JPA flush vs commit

Question

in JPA, if we call EntityTransaction.commit(), does it automatically call EntityManager.flush()? or should we call them both? what is the difference? because i have problem

Answer 1

If you have a @Version annotated column in your entity and call entityManager.flush(), then you will either (immediately!) get an OptimisticLockException, or the database will lock this row (or table). In the later case you can still call setRollbackOnly(), and the lock will later be released without a DB change.

Or from a different perspective, with flush() you can create a (pessimistic) lock on that database row. The others will still see the old entry, but if they try to update they will be blocked, until the lock is released.

All this is also true for CMT (container managed transactions). Instead of waiting for the moment, where the service method is finished and the CMT commit is performed, you can call flush() (even several times) in your service method and handle the OptimisticLockException(s) immediately.