Continuing from the post Mind thread-safety when injecting EntityManager, here is a second part: container-managed EntityManager vs application-managed EntityManager.

There are important differences between the injected EntityManager, and the EntityManager created from an injected EntityManagerFactory. Basically, injected EntityManager is container-managed, meaning all of its lifecycle is controlled by the container (web container or EJB container). Application code cannot close it, or otherwise interfere with its life.

In addition, for a container-managed EntityManager, its associated PersistenceContext is automatically propagated along with the underlying JTA, from servlet A to servlet B, from servlet to EJB, from EJB a to EJB B, and so on. As such, EntityManager of the same configuration injected into various classes can share the same PersistenceContext in a call stack.

On the other hand, EntityManager created from EntityManagerFactory is application-managed EntityManager. Application code is responsible for managing its whole lifecycle. And there is no PersistenceContext propagation for application-managed EntityManager.

Are all EntityManager's obtained from EntityManagerFactory application-managed EntityManager? Yes.

Is it possible to get a container-managed EntityManager from EntityManagerFactory? No.

You may have read about the method EntityManagerFactory.getEntityManager(), which returns a container-managed EntityManager. This method was considered and included in the early draft version of Java Persistence API, but was eventually removed from its final release.