Java, EJB, Lock - synchronization for private fields?

Question

I've got a follow-up question to: EJB 3.1 container managed concurrency vs. synchronized

Is a Lock only preventing concurrent access to data within the persistence context, etc. or is it also providing synchronization for private fields?

Answer 1

It's easiest to imagine that there is a per-bean java.util.concurrent.ReadWriteLock on which the container is calling lock() and unlock() around the method call on either the readLock() or writeLock(), depending on the configured @Lock for the method. So, instance variables are protected.

However, be aware that a container-managed @PersistenceContext field is not "shared" state because the container actually injects a proxy object. Each method call on that proxy EntityManager will delegate to a per-transaction EntityManager. Since transactions are not shared across threads, it is thread-safe even if the bean is using @ConcurrencyManagement(BEAN) with no other synchronization.