Java中List<T> 集合转换成json，如何不改变T对象的属性顺序？

Question

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);

现在要将list转换成json，要求解析之后的key顺序和user的顺序一样，如果user的某个属性是null，那么就解析成空串，请问怎么解决？