In JPA 2, using a CriteriaQuery, how to count results

Question

I am rather new to JPA 2 and it's CriteriaBuilder / CriteriaQuery API:

http://java.sun.com/javaee/6/docs/api/javax/persistence/criteria/CriteriaQuery.html

http://java.sun.com/javaee/6/docs/tutorial/doc/gjivm.html

I would like to count the results of a CriteriaQuery without actually retrieving them. Is that possible, I did not find any such method, the only way would be to do this:

    CriteriaBuilder cb = entityManager.getCriteriaBuilder();

    CriteriaQuery<MyEntity> cq = cb
            .createQuery(MyEntityclass);

    // initialize predicates here

    return entityManager.createQuery(cq).getResultList().size();

And that can't be the proper way to do it...

Is there a solution?

Answer 1

CriteriaBuilder cb = em.getCriteriaBuilder();
CriteriaQuery<Long> cq = cb.createQuery(Long.class);
cq.select(cb.count(cq.from(MyEntity.class)));

return em.createQuery(cq).getSingleResult();

Answer 2

A query of type MyEntity is going to return, MyEntity! You want a query for a Long, no?

CriteriaBuilder qb = entityManager.getCriteriaBuilder();
CriteriaQuery<Long> cq = qb.createQuery(Long.class);
cq.select(qb.count(cq.from(MyEntity.class));
cq.where(//your stuff);
return entityManager.createQuery(cq).getSingleResult();

Obviously you will want to build up your expression with whatever restrictions and groupings etc you skipped in the example.