JPA: caching queries

I'm using JP开发者_JAVA技巧A 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 only work with pure JPA.

Here is some DAO class, notice getOrders method:

class OrderDao {
  EntityManager em;

  List getOrders(Long customerId) {
    Query q = em.createQuery(
      "SELECT o FROM Order o WHERE o.customerId = :customerId");
    q.setParameter("customerId", customerId);
    return q.getResultList();
  }
}

Method is pretty simple but it has a big drawback. Each time the method is called following actions are performed somewhere within JPA implementation:

1. JPQL expression is parsed and compiled to SQL.
2. Either Statement or PreparedStatement instance is created and initialized.
3. Statement instance is filled with parameters and executed.

I believe that steps 1 and 2 of above should be implemented once per application lifetime. But how to make it? In other words, I need that Query instances to be cached.

Of course I can implement such a cache on my side. But wait, I am using modern powerful ORM's! Didn't they already made this for me?

Notice that I'm not mentioning something like Hibernate query cache which caches result of queries. Here I'd like to execute my queries a bit more quickly.

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Use statically defined named queries. They are more efficient because the JPA persistence provider can translate the JP QL string to SQL once at application startup time, as opposed to every time the query is executed, and are recommended in particular for queries that are executed frequently.

A named query is defined using the @NamedQuery annotation that is typically used on the entity class of the result. In your case, on the Order entity:

@Entity
@NamedQueries({
    @NamedQuery(name="Order.findAll",
                query="SELECT o FROM Order o"),
    @NamedQuery(name="Order.findByPrimaryKey",
                query="SELECT o FROM Order o WHERE o.id = :id"),
    @NamedQuery(name="Order.findByCustomerId",
                query="SELECT o FROM Order o WHERE o.customerId = :customerId")
})
public class Order implements Serializable {
    ...
}

It is also recommended to prefix named queries with the entity name (to have some kind of name space and avoid collisions).

And then in the DAO:

class OrderDao {
    EntityManager em;

    List getOrders(Long customerId) {
        return em.createNamedQuery("Order.findByCustomerId")
                 .setParameter("customerId", customerId);
                 .getResultList();
    }
}

PS: I reused the query you suggested as example but it's somehow weird to have the customerId on the Order, I would expect a Customer instead.

References

• JPA 1.0 Specification
  • Section 3.6.4 "Named Queries"

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There is a query plan cache in Hibernate. So the HQL is not parsed every time the DAO is called (so #1 really occurs only once in your application life-time). It's QueryPlanCache. It's not heavily documented, as it "just works". But you can find more info here.

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NamedQueries is the concept you're looking for.

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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<Order> query;

    private Parameter<Long> param;

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

    public List<Order> 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<Order> query;

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

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

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

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What you want is a NamedQuery. On your Order entity you put:

@NamedQueries({
    @NamedQuery( name = "getOrderByCustomerId", query = "SELECT o FROM Order o WHERE o.customerId = :customerId")
})

Then in your DAO use em.createNamedQuery("getOrderByCustomerId") instead of recreating the query.

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You can't prepare queries that are not named. That is the main reason you should try to have named queries rather than simple queries inside your code. Also, named queries can be cached while simple queries inside your java code cannot. Of course this is an optional feature and is enabled using hints on your named query.