LB40 LB40 - 4 months ago 9
Java Question

Boundary between EJB 3.0 and JPA?

that may seem obvious but I've seen contradictory statements : Is JPA part of EJB 3.0 ?
I'm no specialist and It's quite confusing for me.

If so, JPA manipulates Entity Beans ? These entity beans being the interface between the persistence layer and the business layer implementing the logic with stateless beans ?

The underlying question for me is how to implement a "search for user based on various criteria" function, where the "search" request -its string representation- should be built ?
I mean, if JPA is not part of EJB, my beans shouldn't be aware of the data model, right ?

Tell me if I'm completely wrong but I'm kind of lost between all the J* and the JSR XXX... :-)..

thanks

Answer

Is JPA part of EJB 3.0 ?

Yes and no... Yes because every application server claiming to implement EJB 3.0 spec must also provide JPA implementation. No because JPA can be easily outside of EJB, in standalone applications or Spring-managed ones.

JPA manipulates Entity Beans ?

Entity beans was a scary idea in pre-3.0 EJBs. JPA uses the term entities to distinguish itself from the disgraceful history. But yes, JPA entities are a way to map database tables to plain Java objects. In principle changes made to object are propagated to database and vice-versa (oversimplification).

As I said, JPA does not have any dependency on EJB (considered as stateless and stateful session beans) and the other way around. But there is nothing preventing you from using JPA in EJB.

In your scenario you'll have a stateless EJB constructing the query and interacting with the database via JPA. Technically speaking you will call methods on EntityManager injected to your bean:

@Stateless
public class SearchService {

    @PersistenceContext
    private EntityManager em;

    public List<User> findUsersBornAfter(Date date) {
        return em.
            createQuery("SELECT u FROM User u WHERE u.birthDate > :birthDate ORDER BY name").
            setParameter("birthDate", date).
            getResultList();
    }
}

As you can see the business layer is aware of the data model (obviously), but there is no dependency on EJB/business services as far as entities are concerned. In this example JPQL (query) is formed in the service layer and User is a JPA entity. Calling getResultList() causes the JPA provider to translate JPQL to SQL, run the query and transalte the results back to User object instances.

Is the border between EJB and JPA clear now?