With application-managed entity managers, on the other hand, the persistence context is not propagated to application components, and the life cycle of EntityManager instances is managed by the application.
Application-managed entity managers are used when applications need to access a persistence context that is not propagated with the JTA transaction across EntityManager instances in a particular persistence unit. In this case, each EntityManager creates a new, isolated persistence context. The EntityManager, and its associated persistence context, is created and destroyed explicitly by the application.
Applications create EntityManager instances in this case by using the createEntityManager method of javax.persistence.EntityManagerFactory.
To obtain an EntityManager instance, you first must obtain an EntityManagerFactory instance by injecting it into the application component by means of the javax.persistence.PersistenceUnit annotation:
@PersistenceUnit EntityManagerFactory emf;
Then, obtain an EntityManager from the EntityManagerFactory instance:
EntityManager em = emf.createEntityManager();