JPA替代Hibernate投影。所有物

Question

乡亲们！

试图限制从数据库中获取的列的数量，但发现：Hibernate Criteria查询以获取特定列

Criteria cr = session.createCriteria(User.class)
    .setProjection(Projections.projectionList()
      .add(Projections.property("id"), "id")
      .add(Projections.property("Name"), "Name"))
    .setResultTransformer(Transformers.aliasToBean(User.class));

  List<User> list = cr.list();

当你使用Hibernate时，这非常好用，但我正试图用JPA做同样的事情（JPA提供者是Hibernate tho）。有可能用JPA来做吗？我是说用CriteriaBuilder限制列并映射到特定对象？

还有，我看到了这个：https://docs.jboss.org/hibernate/orm/5.0/userguide/html_single/chapters/query/criteria/Criteria.html

Hibernate offers an older, legacy org.hibernate.Criteria API which should be considered deprecated. No feature development will target those APIs. Eventually, Hibernate-specific criteria features will be ported as extensions to the JPA javax.persistence.criteria.CriteriaQuery. For details on the org.hibernate.Criteria API, see Legacy Hibernate Criteria Queries.

那么，似乎可以通过JPA的hibernate扩展来实现这一点？

Answer 1

在JPA中没有替代语法，Hibernate是JPA的超集，它提供了超出JPA规范的功能。在JPA中，您必须显式地使用选择语句（在您的情况下是多选择语句）单独进行预测。

条件查询

 // Create instance of CriteriaBuilder, where em is EntityManager
CriteriaBuilder cb = em.getCriteriaBuilder();      

  // Use CriteriaBuilder interface to create an instance 
  // of CriteriaQuery. For multiselect result set is Object [].
CriteriaQuery<Object[]> c = cb.createQuery(Object[].class);

   // Establish the root of the query by invoking from() to get back a Root object. 
Root<User> user = c.from(User.class); 

  // Establish the SELECT clause of the query by passing the root into the multiselect() method
criteriaQuery.multiselect(user.get("property1"), user.get("property2"));

另一种选择是构造函数调用：

CriteriaQuery<User> c = cb.createQuery(User.class);
Root<User> user = c.from(User.class);
c.select(cb.construct(User.class,
                      user.get("property1"),
                      user.get("property2")));

TupleQuery

    CriteriaQuery<Tuple> c = cb.createTupleQuery();
    Root<User> user = c.from(User.class);
    c.select(cb.tuple(user.get("property1"), user.get("property2")));
    Query query = entityManager.createQuery(c);
    List<Tuple> results = query.getResultList();

还有array（）方法（它返回Object[]）。

    c.select(cb.array(user.get("property1"), user.get("property2")));