更新了主线程ArrayList，但是在发送给客户端时已更正

Question

这是我的服务器代码。我有一个全局ArrayList变量（用户），当用户登录时，它将推送到“用户”，然后将其发送给所有连接的客户端。

问题：我在发送“用户”之前先对其进行了检查，但是当客户端收到该消息时，“用户”不会更新。另外，当新客户端连接到服务器时，它将获得更新的“用户”。谢谢

加：使用ArrayList的情况我使用修改过的String，客户端已更新

public class ServerController {
    private ServerSocket server;
    private int port = ServerConfig.getInstance().PORT;


    private HashSet<ObjectOutputStream> players = new HashSet<ObjectOutputStream>();
    private volatile ArrayList<User> users = new ArrayList<User>();

    //open connection and do a few stuffs


    private class Handler extends Thread {
        private ObjectOutputStream oos;
        private ObjectInputStream ois;
        private Socket client;

        public Handler(Socket s) {
            client = s;
        }

        public void listening() {
            try {
                ois = new ObjectInputStream(client.getInputStream());
                oos = new ObjectOutputStream(client.getOutputStream());
                players.add(oos);

                for (;;) {
                    //receive packet from client and navigate 
                }
        }



        //Client wants to Sign In
        private void handleSignIn(DataPackage dp) {
            System.out.println("sign in");
            User u = (User)dp.getData();
            try {
                DataPackage re = LoginController.getInstance().getUserDAO(u);
                if (re != null) {
                    oos.writeObject(re);
                    addUser(u);
                    handleUpdateTable();
                }
                else {
                    oos.writeObject(new DataPackage(null, ActionType.SIGN_IN));
                }
            } catch (IOException ex) {
                ex.printStackTrace();
            }
        }


        //updating table when someone change 
        private void handleUpdateTable() {
            System.out.println("update table");
            System.out.println(users.size());
            try {
                DataPackage dp = new DataPackage(users, ActionType.UPDATE);
                for (ObjectOutputStream oos : players)
                    oos.writeObject(dp);
            } catch (IOException ex) {
                System.err.println(ex);
            }
        }

        //removing signed out user from users 
        private void handleSignOut(DataPackage dp) {
            User u = (User)dp.getData();
            removeUser(u.getUsername());
            handleUpdateTable();
        }

        @Override
        public void run() {
            listening();
        }

        private void addUser(User u) {
            synchronized (users) {
                users.add(u);
            }
        }

        private void removeUser(String username) {
            Iterator<User> iter = users.iterator();

            while (iter.hasNext()) {
                User u = iter.next();
                if (u.getUsername().equals(username))
                    iter.remove();
            }
        }

    }


}