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

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

CriteriaQuery javadoc

CriteriaQuery in the Java EE 6 tutorial

I would like to count the results of a CriteriaQuery without actually retrieving them. Is t开发者_开发技巧hat 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?

---

A query of type MyEntity is going to return MyEntity. You want a query for a Long.

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.

---

I've sorted this out using the cb.createQuery() (without the result type parameter):

public class Blah() {

    CriteriaBuilder criteriaBuilder = entityManager.getCriteriaBuilder();
    CriteriaQuery query = criteriaBuilder.createQuery();
    Root<Entity> root;
    Predicate whereClause;
    EntityManager entityManager;
    Class<Entity> domainClass;

    ... Methods to create where clause ...

    public Blah(EntityManager entityManager, Class<Entity> domainClass) {
        this.entityManager = entityManager;
        this.domainClass = domainClass;
        criteriaBuilder = entityManager.getCriteriaBuilder();
        query = criteriaBuilder.createQuery();
        whereClause = criteriaBuilder.equal(criteriaBuilder.literal(1), 1);
        root = query.from(domainClass);
    }

    public CriteriaQuery<Entity> getQuery() {
        query.select(root);
        query.where(whereClause);
        return query;
    }

    public CriteriaQuery<Long> getQueryForCount() {
        query.select(criteriaBuilder.count(root));
        query.where(whereClause);
        return query;
    }

    public List<Entity> list() {
        TypedQuery<Entity> q = this.entityManager.createQuery(this.getQuery());
        return q.getResultList();
    }

    public Long count() {
        TypedQuery<Long> q = this.entityManager.createQuery(this.getQueryForCount());
        return q.getSingleResult();
    }
}

Hope it helps :)

---

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

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

---

As others answers are correct, but too simple, so for completeness I'm presenting below code snippet to perform SELECT COUNT on a sophisticated JPA Criteria query (with multiple joins, fetches, conditions).

It is slightly modified this answer.

public <T> long count(final CriteriaBuilder cb, final CriteriaQuery<T> selectQuery,
        Root<T> root) {
    CriteriaQuery<Long> query = createCountQuery(cb, selectQuery, root);
    return this.entityManager.createQuery(query).getSingleResult();
}

private <T> CriteriaQuery<Long> createCountQuery(final CriteriaBuilder cb,
        final CriteriaQuery<T> criteria, final Root<T> root) {

    final CriteriaQuery<Long> countQuery = cb.createQuery(Long.class);
    final Root<T> countRoot = countQuery.from(criteria.getResultType());

    doJoins(root.getJoins(), countRoot);
    doJoinsOnFetches(root.getFetches(), countRoot);

    countQuery.select(cb.count(countRoot));
    countQuery.where(criteria.getRestriction());

    countRoot.alias(root.getAlias());

    return countQuery.distinct(criteria.isDistinct());
}

@SuppressWarnings("unchecked")
private void doJoinsOnFetches(Set<? extends Fetch<?, ?>> joins, Root<?> root) {
    doJoins((Set<? extends Join<?, ?>>) joins, root);
}

private void doJoins(Set<? extends Join<?, ?>> joins, Root<?> root) {
    for (Join<?, ?> join : joins) {
        Join<?, ?> joined = root.join(join.getAttribute().getName(), join.getJoinType());
        joined.alias(join.getAlias());
        doJoins(join.getJoins(), joined);
    }
}

private void doJoins(Set<? extends Join<?, ?>> joins, Join<?, ?> root) {
    for (Join<?, ?> join : joins) {
        Join<?, ?> joined = root.join(join.getAttribute().getName(), join.getJoinType());
        joined.alias(join.getAlias());
        doJoins(join.getJoins(), joined);
    }
}

Hope it saves somebody's time.

Because IMHO JPA Criteria API is not intuitive nor quite readable.

---

It is a bit tricky, depending on the JPA 2 implementation you use, this one works for EclipseLink 2.4.1, but doesn't for Hibernate, here a generic CriteriaQuery count for EclipseLink:

public static Long count(final EntityManager em, final CriteriaQuery<?> criteria)
  {
    final CriteriaBuilder builder=em.getCriteriaBuilder();
    final CriteriaQuery<Long> countCriteria=builder.createQuery(Long.class);
    countCriteria.select(builder.count(criteria.getRoots().iterator().next()));
    final Predicate
            groupRestriction=criteria.getGroupRestriction(),
            fromRestriction=criteria.getRestriction();
    if(groupRestriction != null){
      countCriteria.having(groupRestriction);
    }
    if(fromRestriction != null){
      countCriteria.where(fromRestriction);
    }
    countCriteria.groupBy(criteria.getGroupList());
    countCriteria.distinct(criteria.isDistinct());
    return em.createQuery(countCriteria).getSingleResult();
  }

The other day I migrated from EclipseLink to Hibernate and had to change my count function to the following, so feel free to use either as this is a hard problem to solve, it might not work for your case, it has been in use since Hibernate 4.x, notice that I don't try to guess which is the root, instead I pass it from the query so problem solved, too many ambiguous corner cases to try to guess:

  public static <T> long count(EntityManager em,Root<T> root,CriteriaQuery<T> criteria)
  {
    final CriteriaBuilder builder=em.getCriteriaBuilder();
    final CriteriaQuery<Long> countCriteria=builder.createQuery(Long.class);

    countCriteria.select(builder.count(root));

    for(Root<?> fromRoot : criteria.getRoots()){
      countCriteria.getRoots().add(fromRoot);
    }

    final Predicate whereRestriction=criteria.getRestriction();
    if(whereRestriction!=null){
      countCriteria.where(whereRestriction);
    }

    final Predicate groupRestriction=criteria.getGroupRestriction();
    if(groupRestriction!=null){
      countCriteria.having(groupRestriction);
    }

    countCriteria.groupBy(criteria.getGroupList());
    countCriteria.distinct(criteria.isDistinct());
    return em.createQuery(countCriteria).getSingleResult();
  }

---

You can also use Projections:

ProjectionList projection = Projections.projectionList();
projection.add(Projections.rowCount());
criteria.setProjection(projection);

Long totalRows = (Long) criteria.list().get(0);