JPA本机查询连接返回对象但是取消引用会抛出类强制转换异常 [英] JPA native query join returns object but dereference throws class cast exception

问题描述

我正在使用JPQL Native查询来连接表，查询结果存储在 List< Object []> 。

I'm using JPQL Native query to join table and query result is stored in List<Object[]>.

public String getJoinJpqlNativeQuery() {



            String final SQL_JOIN = 
               "SELECT v1.bitbit, v1.numnum, v1.someTime, t1.username, 
                t1.anotherNum FROM MasatosanTest t1 
                JOIN MasatoView v1 ON v1.username = t1.username;"

            System.out.println("get join jpql native query is being called 
        ============================");

            EntityManager em = null;
            List<Object[]> out = null;
            try {
                em = EmProvider.getDefaultManager();
                Query query = em.createNativeQuery(SQL_JOIN);
                out = query.getResultList();

                System.out.println("return object ==========>" + out);

                System.out.println(out.get(0));

                String one = out.get(0).toString(); //LINE 77 where ClassCastException
                System.out.println(one);
            }
            catch(Exception e) {
            }
            finally {
                if(em != null) { em.close; }
            }
}

问题是

System.out.println（return object ==========>+ out）; 输出：

return object ==========>
[[true, 0, 2010-12-21 15:32:53.0, masatosan, 0.020], 
[false, 0, 2010-12-21 15:32:53.0, koga, 0.213]]

System.out.println（out.get（0））输出：

[true, 0, 2010-12-21 15:32:53.0, masatosan, 0.020]

所以我假设我可以指定out.get（0）的返回值，这应该是字符串：

So I assumed that I can assign return value of out.get(0) which should be String:

String one = out.get(0).toString();

但我得到奇怪的ClassCastException。

But I get weird ClassCastException.

java.lang.ClassCastException: java.util.Vector cannot be cast to 
[Ljava.lang.Object;
        at local.test.jaxrs.MasatosanTestResource.getJoinJpqlNativeQuery
(MasatosanTestResource.java:77)

那真正发生了什么？甚至 Object [] foo = out.get（0）; 会抛出ClassCastException：（

So what's really going on? Even Object[] foo = out.get(0); would throw an ClassCastException :(

推荐答案

我不熟悉JPQL Native查询，但只需调试：

I'm not familiar with JPQL Native query, but you simply debug with:

Object o = out.get（0）;
System.out.println（o.getClass（））;

Object o = out.get(0); System.out.println(o.getClass());

然后从那里开始工作。如果它是一个向量，则迭代并找到向量中的内容。

Then work from there. If it's a vector, iterate through and find what's in the vector.

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