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

Question

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

Answer 1

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

private  CriteriaQuery createCountQuery(final CriteriaBuilder cb,
        final CriteriaQuery criteria, final Root root) {

    final CriteriaQuery countQuery = cb.createQuery(Long.class);
    final Root 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> joins, Root root) {
    doJoins((Set>) joins, root);
}

private void doJoins(Set> 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> 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.