在JPA中使用TypedQuery而不是普通的Query [英] TypedQuery instead of normal Query in JPA

问题描述

是否可以将此查询编写为TypedQuery，并让两个Long运行到一个内部有两个公共Long字段的Object中。

Is it possible to write this Query as a TypedQuery and let the two Long's run into a Object with two public Long fields inside.

    Query q = em.createQuery(
            "SELECT c.id, COUNT(t.id) " +
            "FROM PubText t " +
            "JOIN t.comm c " +
            "WHERE c.element = ?1 " +
            "GROUP BY c.id");
    q.setParameter(1, e);
    List<?> rl = q.getResultList();
    Iterator<?> it = rl.iterator();
    HashMap<Long, Long> res = new HashMap<Long, Long>();
    while (it.hasNext()) {
        Object[] n = (Object[]) it.next();
        res.put((Long)n[0], (Long)n[1]);
    }
    return res;

推荐答案

JPA只为此提供了一个功能 - 构造函数表达式：

JPA has a feature just for this - constructor expressions:

Query q = entityManager.createQuery("SELECT NEW com.example.DTO( c.id, COUNT(t.id)) FROM ...");
List<DTO> dtos = q.getResultList();

您的DTO课程可以是POJO。它需要的是一个公共构造函数，接受2 Long s。请注意，您必须在 NEW 运算符后提供您班级的完全限定名称。

Your DTO class can be a POJO. All it will need is a public constructor accepting 2 Longs. Please note that you have to provide a fully qualified name of your class after the NEWoperator.

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