在简单的聊天程序中，服务器发送字符串的Arraylist，但客户端接收旧值

我想用服务器和多个客户端创建一个简单的游戏。服务器将具有多个Hashmap和Arraylist。服务器将广播这些值给客户端，然后客户端可以逐个修改它们并发送回服务器，然后服务器会将更新后的值广播给所有客户端。

首先，我创建了服务器-客户端聊天应用程序。当客户端向服务器发送String消息时，Server会将该String消息添加到其Arraylist中，并将该arraylist广播给所有客户端。我使用了线程，以便多个客户端可以同时发送消息，但是我还没有应用线程安全。

让我们来解决问题。客户端第一次将String发送到服务器时，服务器会很好地打印它，添加到它的arraylist中，然后将其广播给所有客户端，所有客户端也可以看到。但是下一次客户端发送String消息时，服务器接受它，将其添加到arraylist并广播它，但是这一次所有客户端都获得旧的arraylist（仅添加了一个String的列表被首先添加）。我在广播之前已经打印了arraylist，它显示了修改后的值，但是在客户端，它仅显示了一个条目的列表。

服务器代码的一部分

public class ServerGUI extends javax.swing.JFrame {

public static final int SERVER_PORT = 4000;
private ServerSocket ss;
ArrayList<String> al;
ArrayList<ClientHandler> clients;

public ServerGUI() {
    initComponents();
    setVisible(true);
    al = new ArrayList<>();
    clients = new ArrayList<>();
    initNet();
}

private void initNet() {
    Socket ds = null;
    try {
        ss = new ServerSocket(SERVER_PORT, 1);
        while (true) {

            ds = ss.accept();

            clients.add(new ClientHandler(ds));
        }
    } catch (Exception e) {

        System.out.println("shutting down server......");
    }
}

class ClientHandler extends Thread {

    private Socket ds;
    private ObjectOutputStream out;
    private ObjectInputStream in;

    public ClientHandler(Socket ds) throws Exception {
        this.ds = ds;
        out = new ObjectOutputStream(ds.getOutputStream());
        in = new ObjectInputStream(ds.getInputStream());
        start();
    }

    public ObjectOutputStream getOut() {
        return out;
    }

    public void run() {
        try {
            while (true) {
                acceptData(in);
                broadcastData();
            }
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            System.out.println("Finally called. socket closed");
            if (ds != null) {
                try {
                    ds.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }
    }
}

private void acceptData(ObjectInputStream in) throws Exception {
    System.out.println("acceptData called by " + Thread.currentThread().getName());
    String s = (String) in.readObject();
    al.add(s);
    jta.setText(al.toString());
}

private void broadcastData() throws Exception {
    System.out.println("broadcast called by " + Thread.currentThread().getName());
    System.out.println("al is : \n" + al);

    for (ClientHandler clnt : clients) {
        clnt.getOut().writeObject(al);
        clnt.getOut().flush();
    }
}

客户代码的一部分

public class ClientGUI extends javax.swing.JFrame {

public static final int SERVER_PORT = 4000;
public static final String SERVER_IP = "127.0.0.1";
private Socket s1;
private ObjectOutputStream out;
private ObjectInputStream in;
private ArrayList<String> al;

public ClientGUI() {
    initComponents();
    setVisible(true);
    initNet();
}

private void initNet() {
    try {
        s1 = new Socket(SERVER_IP, SERVER_PORT);
        out = new ObjectOutputStream(s1.getOutputStream());            
        in = new ObjectInputStream(s1.getInputStream());
        System.out.println("connected to server");

        new ReadData();

    } catch (Exception e) {
        e.printStackTrace();
    }
}

class ReadData extends Thread {

    public ReadData() {
        start();
    }

    public void run() {
        System.out.println("client thread started");
        try {
            while (true) {
                al = (ArrayList<String>) in.readObject();
                System.out.println("client read completed, al is "+al);

                jta.setText(al.toString());                    
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}
private void textFieldActionPerformed(java.awt.event.ActionEvent evt) {                                    
    try {
        out.writeObject(jtf.getText());
        out.flush();
    } catch (Exception e) {
        e.printStackTrace();
    }
}