Java CDI @PersistenceContext and thread safety

Question

Is an EntityManager @Inject[ed] as follows in muliple classes threadsafe?

@PersistenceContext(unitName=\"blah\")
private EntityManager em;

Answer 1

I feel I need to go deeper into this because my first answer was not absolutely true.

I will refer to JSR-220 (EJB 3.0). In section 5.2 Obtaining an EntityManager you may find:

An entity manager may not be shared among multiple concurrently executing threads. Entity managers may only be accessed in a single-threaded manner.

Well that's it. You may stop reading here and never use EntityManager in singleton beans unless properly synchronized.

But I believe there is a confusion in the spec. There are actually two different EntityManager implementations. The first is one is provider implementation (saying Hibernate) which is not obliged to be threadsafe.

On the other hand there is a container implementation of EntityManager. Which is also not supposed to be threadsafe according to the above. But container's implementation acts as a proxy and delegates all calls to the real provider's EntityManager.

So further in the spec in 5.9 Runtime Contracts between the Container and Persistence Provider:

For the management of a transaction-scoped persistence context, if there is no EntityManager already associated with the JTA transaction: The container creates a new entity manager by calling EntityManagerFactory.createEntityManager when the first invocation of an entity manager with Persistence- ContextType.TRANSACTION occurs within the scope of a business method executing in the JTA transaction.

This means in turn that there will be a different EntityManager instance for each transaction started. The code that creates an EntityManager is safe according to 5.3:

Methods of the EntityManagerFactory interface are threadsafe.

But what if there is an EntityManager associated with JTA transaction? The code that binds an EntityManager associated with current JTA transaction may be not threadsafe according to the spec.

But I can't really think of an application server implementation that works correctly with EntityManager injected into stateless beans and not correctly within singletons.

So my conclusions are:

1. If you want to follow the JSR-220 strictly then never use EntityManager in singletons until synchronizing the access to it.
2. I personally will continue to use EntityManager in singleton because my application server implementation works perfectly with it. You may want to check your implementation before doing so.