例如：List<User> list = new ArrayList<User>();User user1 = new User("userId","name","age","sex");User user2 = new User("userId1","name1","age1","sex1");User user3 = new User("userId2","name2","age2","sex2");list.add(user1);list.add(user2);list.add(user3);現在要將list轉換成json，要求解析之后的key順序和user的順序一樣，如果user的某個屬性是null，那么就解析成空串，請問怎么解決？