用java写一个多线程聊天程序

下面是一个简单的Java多线程聊天程序的示例：

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.net.ServerSocket;
import java.net.Socket;

public class ChatServer implements Runnable {
    private Socket socket;
    private static final int PORT = 8080;

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

    public void run() {
        try {
            BufferedReader reader = new BufferedReader(new InputStreamReader(socket.getInputStream()));
            PrintWriter writer = new PrintWriter(socket.getOutputStream(), true);

            writer.println("Welcome to the chat room.");

            while (true) {
                String message = reader.readLine();
                if (message.equals("bye")) {
                    break;
                }

                System.out.println("Received message: " + message);
                writer.println("You said: " + message);
            }

            reader.close();
            writer.close();
            socket.close();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    public static void main(String[] args) throws IOException {
        ServerSocket serverSocket = new ServerSocket(PORT);
        System.out.println("Chat server started on port " + PORT);

        while (true) {
            Socket socket = serverSocket.accept();
            System.out.println("New client connected.");

            Thread thread = new Thread(new ChatServer(socket));
            thread.start();
        }
    }
}

这个程序启动一个服务器，在接受到来自客户端的消息后，将消息原样返回给客户端。可以通过启动多个客户端连接到这个服务器，从而实现多人聊天功能。