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

Question

我正在使用JPQL Native查询来连接表并存储查询结果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,它应该是String:

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

但我得到奇怪的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 :(

Answer 1

SELECT子句查询多个列或实体,结果聚合在getResultList()返回的java.util.List中的对象数组(Object [])中.

 //---

    Query query = manager.createQuery("SELECT v1.bitbit, v1.numnum, v1.someTime, t1.username, t1.anotherNum FROM MasatosanTest t1 JOIN MasatoView v1 ON v1.username = t1.username;");

    List results = query.getResultList( ); // Fetches list containing arrays of object
    Iterator it = results.iterator( );

    while (it.hasNext( )) {

       Object[] result = (Object[])it.next(); // Iterating through array object 

       Boolean first = (Boolean) result[0]; // Fetching the field from array

       /* Likewise for all the fields, casting accordingly to the sequence in SELECT query*/

    }

    //---

编辑: 为了避免显式转换,您可以使用构造函数表达式,使用适当的参数向实体添加构造函数.

SELECT new org.somepackage.XEntity(x.a, x.b) FROM XEntity x