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exception

我正在使用JPQL Native查询来连接表，查询结果存储在List 。

 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 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

我不熟悉JPQL Native查询，但您只需调试：

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

然后从那里工作。 如果它是一个向量，则迭代并找到向量中的内容。

Answer 2

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，向具有适当参数的实体添加构造函数。

 SELECT new org.somepackage.XEntity(xa, xb) FROM XEntity x