JPA 2.0: count for arbitrary CriteriaQuery?

Question

I am trying to implement the following convenience method:

/**
 * Counts the number of results of a search.
 * @param criteria The criteria for the query.
 * @return The number of results of the query.
 */
public <T> int findCountByCriteria(CriteriaQuery<?> criteria);

In Hibernate, this is done by

criteria.setProjection(Projections.rowCount());

What is the equivalent to the above in JPA? I found numerous simple count examples, but none of them made use of a CriteriaQuery whose row count should be determined.

Answer 1

Are you looking for something like this?

/**
 * Counts the number of results of a search.
 * 
 * @param criteria The criteria for the query.
 * @return The number of results of the query.
 */
public <T> Long findCountByCriteria(CriteriaQuery<?> criteria) {
    CriteriaBuilder builder = em.getCriteriaBuilder();

    CriteriaQuery<Long> countCriteria = builder.createQuery(Long.class);
    Root<?> entityRoot = countCriteria.from(criteria.getResultType());
    countCriteria.select(builder.count(entityRoot));
    countCriteria.where(criteria.getRestriction());

    return em.createQuery(countCriteria).getSingleResult();
}

That you could use like this:

// a search based on the Criteria API
CriteriaBuilder builder = em.getCriteriaBuilder();
CriteriaQuery<Person> criteria = builder.createQuery(Person.class);
Root<Person> personRoot = criteria.from(Person.class);
criteria.select(personRoot);
Predicate personRestriction = builder.and(
    builder.equal(personRoot.get(Person_.gender), Gender.MALE),
    builder.equal(personRoot.get(Person_.relationshipStatus), RelationshipStatus.SINGLE)
);
criteria.where(personRestriction);
//...

// and to get the result count of the above query
Long count = findCountByCriteria(criteria);

PS: I don't know if this is the right/best way to implement this, still learning the Criteria API...