Is EntityManager really thread-safe?

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No, an EntityManager is NOT thread safe. Adam Bien though is also correct. You are just not looking at the question correctly. The question he is answering isn’t if an EntityManager is thread safe, he is stating that using container managed EntityManger in a stateless session bean is safe, which it is. He then explains the reasoning and wording of the spec that allows the container to work its magic – “each instance sees only a serialized sequence of method calls”. That allows container injection to have different EntityManager contexts per method invocation, similar to how each invocation can be tied to their own transaction and isolated resources.

Injection is really just injecting an EntityManager proxy that gives the container control over the lifecycle of the JPA EntityManagers underneath, allowing it to be tied to the thread and the transaction.

So an EntityManager is NOT thread safe, but the container injected EntityManager proxies are required to be safe to use within stateless session beans.

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