JPA: caching queries

Answer 1

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.

Answer 2

NamedQueries is the concept you're looking for.

Answer 3

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"

Answer 4

The 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.