ensureLoginAction();
Topic topic = topicService.findTopic(topicId);
notNull(topic, "主题不存在");
Reply reply = new Reply();
reply.topicId = topicId;
reply.content = range(notNull(trim(content)), 1, 4000, "回复不能为空且最长4000个字");
reply.authorId = loginUser.id;
reply.postDate = new Timestamp(System.currentTimeMillis());
topicService.insertReply(topicId, reply);