How can a persistenceContext link with multiple EntityManager

Question

Recently I have gone through PRO JPA2 book and find that "A single persistence context can be link with multiple EntityManager instance."

I have searched for the same but could not found satisfactory answer. Can anybody elaborate this with example?

Answer 1

It's difficult to know exactly what was meant without more context from the book. That said, if you're using container-managed JPA within a global transaction, then each injected EntityManager referring to the same persistence unit will be backed by the same persistence context. For example:

@Stateless
public class Bean {
    @PersistenceContext
    EntityManager em1;

    @EJB
    OtherBean otherBean;

    @TransactionAttribute(REQUIRED) // The type, but for illustration
    public void doWork() {
        // ... use em1
        otherBean.doMoreWork();
    }
}

@Stateless
public class OtherBean {
    @PersistenceContext
    EntityManager em2;

    public void doMoreWork() {
        // ... use em2, it shares a persistence context with em1
    }
}