After successfully implementing user authentication, the next step toward enabling real-time messaging is retrieving a list of online contacts. Without this list, users have no one to commnuicate with.

Upon successful login, the server spawns a dedicated thread to manage ongoing communication with that client, allowing it to handle additional login requests concurrently.

if (u.getOperation().equals("login")) {
    int userId = u.getAccount();
    boolean isAuthenticated = new UserDao().login(userId, u.getPassword());
    if (isAuthenticated) {
        System.out.println(MyData.getDate() + "'" + userId + "' connected!");
        m.setType(YQMessageType.SUCCESS);
        oos.writeObject(m);

        ServerConClientThread clientHandler = new ServerConClientThread(s);
        ManageServerConClient.addClientThread(userId, clientHandler);
        clientHandler.start();
    } else {
        m.setType(YQMessageType.FAIL);
        oos.writeObject(m);
    }
}

This dedicated thread listens for incoming messages from the client and processes them based on they type:

public class ServerConClientThread extends Thread {
    private Socket socket;

    public ServerConClientThread(Socket socket) {
        this.socket = socket;
    }

    public void run() {
        while (true) {
            try {
                ObjectInputStream input = new ObjectInputStream(socket.getInputStream());
                YQMessage msg = (YQMessage) input.readObject();

                if (msg.getType().equals(YQMessageType.COM_MES)) {
                    ServerConClientThread recipientHandler = 
                        ManageServerConClient.getClientThread(msg.getReceiver());
                    ObjectOutputStream output = 
                        new ObjectOutputStream(recipientHandler.socket.getOutputStream());
                    output.writeObject(msg);
                } 
                else if (msg.getType().equals(YQMessageType.GET_ONLINE_FRIENDS)) {
                    // Temporary implementation: returns all registered users as friends
                    String userList = new UserDao().getUser();
                    ServerConClientThread senderHandler = 
                        ManageServerConClient.getClientThread(msg.getSender());
                    ObjectOutputStream output = 
                        new ObjectOutputStream(senderHandler.socket.getOutputStream());
                    YQMessage response = new YQMessage();
                    response.setType(YQMessageType.RET_ONLINE_FRIENDS);
                    response.setContent(userList);
                    output.writeObject(response);
                }
            } catch (Exception e) {
                e.printStackTrace();
                try {
                    socket.close();
                } catch (IOException ignored) {}
                break;
            }
        }
    }
}

On the client side, after logging in successfully, a background thread maintains the connection to the server:

if (ms.getType().equals(YQMessageType.SUCCESS)) {
    ClientConServerThread connectionThread = new ClientConServerThread(context, s);
    connectionThread.start();
    ManageClientConServer.addClientConServerThread(user.getAccount(), connectionThread);
    b = true;
} else if (ms.getType().equals(YQMessageType.FAIL)) {
    b = false;
}

The client then immediately requests the on line friends list:

ObjectOutputStream out = new ObjectOutputStream(
    ManageClientConServer.getClientConServerThread(user.getAccount())
        .getSocket().getOutputStream()
);
YQMessage request = new YQMessage();
request.setType(YQMessageType.GET_ONLINE_FRIENDS);
request.setSender(user.getAccount());
out.writeObject(request);

Once the server responds with the list (currently all users due to simplified logic), the client populates a ListView using a standard adapter—implementation details of which are omitted here.