SpringMVC后端无限层级查询

有个需求，可以根据一个用户查询到他底下的所有下级，下级的下级，下级的下级的下级…，原理就是调用递归方法不断往下查询下属,代码很简单（其实如果可以By用户等级，代码可以更简单，就是取消queryUsersByLevel中的for循环，直接调用getAllUsers就OK了）.

    @RequestMapping(value = "queryUsersByLevel/{userId}", method = RequestMethod.GET, produces = {
        "application/json;charset=UTF-8" })
    @ResponseBody
    public List<User> queryUsersByLevel(@PathVariable("userId") String userId) {
        List<User> users = userService.queryUsersByBossNo(userId);
        if(!StringUtil.isCollectionEmpty(users)){
            for(User user:users){
                User u = getAllUsers(user);
                if(u != null)
                    user.sub_users = u.sub_users;
            }  
        }
        return users;
    }


    private User getAllUsers(User user) {
            List<User> users =  userService.queryUsersByBossNo(user.user_id);
            if(!StringUtil.isCollectionEmpty(users)){
                for(User u:users){
                    User u1 = getAllUsers(u);
                    if(u1 != null)
                        u.sub_users = u1.sub_users;
                }  
                user.sub_users = users;
                return user;
            }
        return null;
    }

实体类:

public class User{

    public List<User> sub_users;
    public String user_name;
    public String user_id;
    public String org_id;
    public String boss_no;
    public String boss_level;
}