JPA with EJB with shared DAOs and service layers

Question

This is more of an architecture problem that I now have. I need help regarding best practices for integrating EJB and JPA into a project. I want to have EJBs that will run a service layer containing the business logic of my application. Right below that, I would like to have a DAO layer that my EJBs will have a descriptor using the DAO factory to separate the two layers as much as possible. Knowing this, I obviously cannot make my DAO as an EJB either because I don't want them to be entered automatically, since I want them to be created through the factory. This leads me to create the entitymanager manually using

Persistence.createEntityManagerFactory("PortalEJB").createEntityManager(); 

Now ... this call is in my abstract JPA DAO:

 public abstract class JPADataAccessorObject<K, E> implements DataAccessorObject<K, E> { protected Class<E> entityClass; protected EntityManager entityManager; protected JPADataAccessorObject(Class<E> pEntityClass) { this.entityManager = Persistence.createEntityManagerFactory("PortalEJB").createEntityManager(); this.entityClass = pEntityClass; } /* Other DAO functions (update, delete, create) */ } 

I think this is bad, right? All my specific aspects of this class will have a new copy of the persistence context, and I will get strange behavior. Moreover, when I do this, I think I myself must manage transactions at the service level. I was going to create Aspects for him, something like:

• Creating a transaction before any service level function / procedure
• (transaction rollback if any exception occurs)
• Complete transaction after any service level function / procedure

So here are my questions:

• How do I manage EntityManager?
• Do I have to have some kind of JPA utility class that will manage it so that one protection is protected against multithreading?
• If I make a terrible mistake, please provide the best practices.

Answer 1

Have you seen the post of Adam Bian JPA / EJB3 KILLED DAO and DAOS NOT DEAD - BUT THEY ARE REVERSED OR EXCLUDED ?

In the other hand, you can consider an abstract class for a service level:

 public abstract class AbstractFacade<E extends Serializable, PK extends Serializable> { private final transient Class<E> entityClass; public AbstractFacade(final Class<E> entityClass) { this.entityClass = entityClass; } protected abstract EntityManager getEntityManager(); public void create(final E entity) { final EntityManager entityManager = getEntityManager(); entityManager.persist(entity); } public final E find(final PK id) { return getEntityManager().find(entityClass, id); } // Other common operations } 

And specific service:

 @Stateless public class UserFacade extends AbstractFacade<User, String> { @PersistenceContext(unitName = "MyPU") private EntityManager em; @Override protected EntityManager getEntityManager() { return em; } public UserFacade() { super(User.class); } // Other methods of this service } 

Learn more in Java EE 6/7: The Lean Parts from JavaOne 2012 in San Francisco.