public User getUser(long uid, long mid) {
   User user = userMapper.get(uid, mid);
   if (user == null) {
     return null;
   }
   fillUserExtInfo(user);
   return user;
 }
 private void checkMid(long uid, long mid) {
   if (userMapper.get(uid, mid) == null) {
     throw new IllegalArgumentException("user is not exist. uid=" + uid + ", mid=" + mid);
   }
 }