JPA从现有CriteriaQuery创建计数查询

Question

        CriteriaBuilder cb = em.getCriteriaBuilder();
        CriteriaQuery<A> cq = cb.createQuery(A.class);
        Root<A> root = cq.from(A.class);
        CriteriaQuery<Long> cq = cb.createQuery(A.class);
        Root<A> rootCount = countQuery.from(A.class);

        List<Predicate> predicates = new ArrayList<>();
        List<Predicate> predicatesCount = new ArrayList<>();
        
        Predicate p = cb.equal(root.get(A.ID), 1);
        predicates.add(p);
        Predicate p1 = cb.equal(rootCount.get(A.ID), 1);
        predicatesCount.add(p1);
        ...
        // execute both query to get result

感谢阅读！

Answer 1

下面的示例展示了如何设置标准构建器/谓词限制，然后重用该限制来执行计数查询。

CriteriaBuilder builder = entityManager.getCriteriaBuilder();
CriteriaQuery<EntityStub> criteriaQuery = builder.createQuery(EntityStub.class);
Root<EntityStub> entity_ = criteriaQuery.from(EntityStub.class);
entity_.alias("entitySub"); //assign alias to entity root

criteriaQuery.where(builder.equal(entity_.get("message"), "second"));

// Generic retrieve count
CriteriaQuery<Long> countQuery = builder.createQuery(Long.class);
Root<T> entity_ = countQuery.from(criteriaQuery.getResultType());
entity_.alias("entitySub"); //use the same alias in order to match the restrictions part and the selection part
countQuery.select(builder.count(entity_));
Predicate restriction = criteriaQuery.getRestriction();
if (restriction != null) {
  countQuery.where(restriction); // Copy restrictions
}
Long count = entityManager.createQuery(countQuery).getSingleResult();

看看这是否对您有帮助，注意根别名，在执行计数查询时，确保实体类类型是long.class

https://forum.hibernate.org/viewtopic.php?p=2471522p2471522