Question

我正在尝试使用HQL查询打印已获取数据的详细信息，如下所示

List list = sess.createQuery("from PostMessages as pm "
                    + " left join pm.postImageses as pi "
                    + " left join pm.videosDescriptions as vd "
                    + "where pm.messageid=:msgId")
                       .setParameter("msgId", msgId).list();

            List<PostMessages> result = (List<PostMessages>) list;
            Iterator itr = result.iterator();
            while (itr.hasNext()) {
                PostMessages pm = (PostMessages) itr.next();
                System.out.print("msg: " + pm.getMessage());
                System.out.print(" type: " + pm.getMessageType());
                Set images = pm.getPostImageses();
                Iterator itr1 = images.iterator();
                while (itr1.hasNext()) {
                    PostImages pi = (PostImages) itr1.next();
                    System.out.print("id: " + pi.getImageId());
                    System.out.print("desc: " + pi.getImgDesc());
                }
                Set videos = pm.getVideosDescriptions();
                Iterator itr2 = videos.iterator();
                while (itr2.hasNext()) {
                    VideosDescription vd = (VideosDescription) itr1.next();
                    System.out.print("id: " + vd.getVideoId());
                    System.out.print("desc: " + vd.getDescription());
                }
            }

但这显示以下异常

java.lang.ClassCastException: [Ljava.lang.Object; cannot be cast to 
hibernetMappings.PostMessages
    at hibernetMappings.TestPost.main(TestPost.java:40)

如何解决此异常。

Answer 1

在查询中添加一个select子句：

select distinct pm from PostMessages as pm ...

请注意，左连接没用。如果目标是在单个查询中检索包含其图片和视频说明的帖子消息，则应使用left join fetch。

您还应该使用Iterator<PostMessages>，而不是原始迭代器。实体应该命名为PostMessage，而不是PostMessages：每个实例都是一条消息，而不是几条消息。 getImageses()应为getImages()。

Answer 2

尝试这样做。

  Iterator itr = result.iterator();

到

  Iterator<PostMessages> itr = result.iterator();

同时更改

List<PostMessages> list = sess.createQuery("from PostMessages as pm "
                    + " left join fetch pm.postImageses as pi "
                    + " left join fetch pm.videosDescriptions as vd "
                    + "where pm.messageid=:msgId")
                       .setParameter("msgId", msgId).list();

从hibernate获取数据时java.lang.ClassCastException

2 个答案: