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 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.

EDIT:

I unfortunately found out that @Pascal’s answer is not the correct one. The problem is very subtle and only shows up when you use joins:

// Same query, but readable:
// SELECT *
// FROM Brain b
// WHERE b.iq = 170

CriteriaQuery<Person> query = cb.createQuery(Person.class);
Root<Person> root = query.from(Person.class);
Join<Object, Object> brainJoin = root.join("brain");
Predicate iqPredicate = cb.equal(brainJoin.<Integer>get("iq"), 170);
query.select(root).where(iqPredicate);

When calling findCountByCriteria(query), it dies with the following exception:

org.hibernate.hql.ast.QuerySyntaxException: Invalid path: 'generatedAlias1.iq' [select count(generatedAlias0) from xxx.tests.person.dom.Person as generatedAlias0 where generatedAlias1.iq=170]

Is there any other way to provide such a CountByCriteria method?

Answer 1

I wrote a utility class, JDAL JpaUtils to do it:

• count results: Long count = JpaUtils.count(em, criteriaQuery);
• copy CriteriaQueries: JpaUtils.copyCriteria(em, criteriaQueryFrom, criteriaQueryTo);
• get count criteria: CriteriaQuery<Long> countCriteria = JpaUtils.countCriteria(em, criteria)

and so on…

If you are interested in the source code, see JpaUtils.java