java群聊功能是怎么实现的？

Java群聊功能可以通过socket编程实现。Socket编程是一种应用程序编程接口(API)，用于在不同计算机之间进行网络通信。

在实现Java群聊功能时，可以使用多线程技术来处理来自客户端的请求。具体实现方式为：

1. 服务器端启动一个ServerSocket，监听来自客户端的连接请求。

2. 当一个客户端连接到服务器时，服务器通过accept()方法接受该连接，并为该客户端启动一个新线程。

3. 服务器端通过该线程向所有已连接的客户端发送消息。

4. 客户端通过Socket连接到服务器，并在发送消息时向服务器发送消息。

5. 服务器将收到的消息广播到所有连接的客户端。

以下是一个简单的Java群聊功能示例代码：

服务器端代码：

import java.io.IOException;
import java.net.ServerSocket;
import java.net.Socket;
import java.util.ArrayList;
import java.util.List;

public class ChatServer {
    private static final int PORT = 8888;
    private List<ClientThread> clients = new ArrayList<>();

    public void start() {
        try {
            ServerSocket server = new ServerSocket(PORT);
            System.out.println("Server started...");

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

                ClientThread client = new ClientThread(socket);
                clients.add(client);
                client.start();
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    private class ClientThread extends Thread {
        private Socket socket;

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

        public void run() {
            try {
                while (true) {
                    String message = Utils.receiveMessage(socket);
                    System.out.println("Received message from " + socket + ": " + message);

                    for (ClientThread client : clients) {
                        if (client != this) {
                            Utils.sendMessage(client.socket, message);
                        }
                    }
                }
            } catch (IOException e) {
                System.err.println("Error handling client: " + socket);
                clients.remove(this);
            }
        }
    }

    public static void main(String[] args) {
        ChatServer server = new ChatServer();
        server.start();
    }
}

客户端代码：

import java.io.IOException;
import java.net.Socket;
import java.util.Scanner;

public class ChatClient {
    private static final String SERVER_IP = "localhost";
    private static final int SERVER_PORT = 8888;

    public static void main(String[] args) {
        try {
            Socket socket = new Socket(SERVER_IP, SERVER_PORT);
            System.out.println("Connected to server: " + socket);

            Thread inputThread = new Thread(new InputThread(socket));
            inputThread.start();

            Thread outputThread = new Thread(new OutputThread(socket));
            outputThread.start();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    private static class InputThread implements Runnable {
        private Socket socket;

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

        public void run() {
            try {
                while (true) {
                    String message = Utils.receiveMessage(socket);
                    System.out.println("Received message: " + message);
                }
            } catch (IOException e) {
                System.err.println("Error reading message from server");
            }
        }
    }

    private static class OutputThread implements Runnable {
        private Socket socket;

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

        public void run() {
            try {
                Scanner scanner = new Scanner(System.in);

                while (true) {
                    String message = scanner.nextLine();
                    Utils.sendMessage(socket, message);
                }
            } catch (IOException e) {
                System.err.println("Error sending message to server");
            }
        }
    }
}

其中，Utils是一个工具类，用于提供发送和接收消息的方法：

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

public class Utils {
    public static void sendMessage(Socket socket, String message) throws IOException {
        PrintWriter out = new PrintWriter(socket.getOutputStream(), true);
        out.println(message);
    }

    public static String receiveMessage(Socket socket) throws IOException {
        BufferedReader in = new BufferedReader(new InputStreamReader(socket.getInputStream()));
        return in.readLine();
    }
}