问题
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 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?
回答1:
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.
回答2:
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 :)
回答3:
CriteriaBuilder cb = em.getCriteriaBuilder();
CriteriaQuery<Long> cq = cb.createQuery(Long.class);
cq.select(cb.count(cq.from(MyEntity.class)));
return em.createQuery(cq).getSingleResult();
回答4:
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.
回答5:
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();
}
回答6:
You can also use Projections:
ProjectionList projection = Projections.projectionList();
projection.add(Projections.rowCount());
criteria.setProjection(projection);
Long totalRows = (Long) criteria.list().get(0);
回答7:
With Spring Data Jpa, we can use this method:
/*
* (non-Javadoc)
* @see org.springframework.data.jpa.repository.JpaSpecificationExecutor#count(org.springframework.data.jpa.domain.Specification)
*/
@Override
public long count(@Nullable Specification<T> spec) {
return executeCountQuery(getCountQuery(spec, getDomainClass()));
}
来源：https://stackoverflow.com/questions/2883887/in-jpa-2-using-a-criteriaquery-how-to-count-results