JPA: caching queries

Question

I\'m using JPA to load and persist entities in my Java EE-based web application. Hibernate is used as an implementation of JPA, but I don\'t use Hibernate-specific features and

Answer 1

JPA 2.1, section "3.1.1 EntityManager Interface":

The Query, TypedQuery, StoredProcedureQuery, CriteriaBuilder, Metamodel, and EntityTransaction objects obtained from an entity manager are valid while that entity manager is open.

The lesson to take home from this quote is that the enlisted query types can only be cached for as long as the entity manager remains open - which we have no saying about for container-managed entity managers.

Three solutions come to mind. 1) Named queries as others have pointed out. 2) Cache a CriteriaQuery instead and hopefully the provider can toss in some kind of optimizations out of it. 3) Use an application-managed entity manager (that remains open).

Cache a CriteriaQuery

@Stateless
public class OrderRepository
{
    @PersistenceUnit
    EntityManagerFactory emf;

    @PersistenceContext
    EntityManager em;

    private CriteriaQuery query;

    private Parameter param;

    @PostConstruct
    private void constructQuery() {
        CriteriaBuilder b = emf.getCriteriaBuilder();
        query = b.createQuery(Order.class);
        param = b.parameter(long.class);
        ...
    }

    public List findByCustomerKey(long key) {
        return em.createQuery(query)
                 .setParameter(param, key)
                 .getResultList();
    }
}

Use an application-managed entity manager

@Stateless
public class OrderRepository
{
    @PersistenceUnit
    EntityManagerFactory emf;

    private EntityManager em;

    private TypedQuery query;

    @PostConstruct
    private void initialize() {
        em = emf.createEntityManager();
        query = em.createQuery("SELECT o FROM Order o WHERE o.id = ?1", Order.class);
    }

    public List findByCustomerKey(long key) {
        try {
            return query.setParameter(1, key)
                        .getResultList();
        }
        finally {
            em.clear(); // returned entities are detached
        }
    }

    @PreDestroy
    private void closeEntityManager() {
        em.close();
    }
}