利用多线程、网络编程设计一个聊天程序。要求通过监听指定的端口号、目标IP地址和目标端口号，实现消息的发送和接收功能，并把聊天内容显示出来。 Java 实现

以下是一个简单的Java聊天程序的实现，它使用多线程和网络编程：

import java.io.*;
import java.net.*;

public class ChatServer implements Runnable {
    private ServerSocket serverSocket;
    private Socket clientSocket;
    private PrintWriter out;
    private BufferedReader in;
    private Thread thread;

    public ChatServer(int port) throws IOException {
        serverSocket = new ServerSocket(port);
        System.out.println("Chat server started on port " + port);
    }

    public void start() {
        if (thread == null) {
            thread = new Thread(this);
            thread.start();
        }
    }

    public void run() {
        try {
            clientSocket = serverSocket.accept();
            out = new PrintWriter(clientSocket.getOutputStream(), true);
            in = new BufferedReader(new InputStreamReader(clientSocket.getInputStream()));

            String inputLine;
            while ((inputLine = in.readLine()) != null) {
                System.out.println("Received message: " + inputLine);
                out.println(inputLine);
            }

            out.close();
            in.close();
            clientSocket.close();
            serverSocket.close();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    public static void main(String[] args) {
        int portNumber = 1234;
        try {
            ChatServer server = new ChatServer(portNumber);
            server.start();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

import java.io.*;
import java.net.*;

public class ChatClient {
    private Socket socket;
    private PrintWriter out;
    private BufferedReader in;

    public ChatClient(String address, int port) throws IOException {
        socket = new Socket(address, port);
        out = new PrintWriter(socket.getOutputStream(), true);
        in = new BufferedReader(new InputStreamReader(socket.getInputStream()));
    }

    public void sendMessage(String message) {
        out.println(message);
    }

    public String receiveMessage() throws IOException {
        return in.readLine();
    }

    public void close() throws IOException {
        out.close();
        in.close();
        socket.close();
    }

    public static void main(String[] args) {
        String serverAddress = "localhost";
        int portNumber = 1234;

        try {
            ChatClient client = new ChatClient(serverAddress, portNumber);
            BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));

            String inputLine;
            while ((inputLine = reader.readLine()) != null) {
                client.sendMessage(inputLine);
                System.out.println("Received message: " + client.receiveMessage());
            }

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

在这个聊天程序中，ChatServer类监听指定的端口号，接收来自客户端的消息，并将消息发送回客户端。ChatClient类连接到服务器并发送消息，然后接收服务器的响应并将其显示在控制台上。

要运行聊天程序，可以先启动ChatServer，然后启动一个或多个ChatClient实例。当客户端发送消息时，服务器将在控制台上显示接收到的消息。当服务器发送消息时，客户端将在控制台上显示接收到的消息。