Java 网络编程案例：使用 MulticastSocket 实现多点广播

Datagram只允许数据报发送给指定的目标地址，而MulticastSocket可以将数据报以广播方式发送到数量不等的多个客户端。

IP协议为多点广播提供了这批特殊的IP地址，这些IP地址的范围是224.0.0.0至239.255.255.255。

 MulticastSocket 常用的方法：

MulticastSocket(int port) ：创建多播套接字并将其绑定到特定端口。创建一个MulticastSocket对象后，还需要将该MulticastSocket加入到指定的多点广播地址，如果结束也需要脱离多点广播地址。
void joinGroup(InetAddress mcastaddr) ：加入多播组。 
void leaveGroup(InetAddress mcastaddr) ：离开多播组。 
void setLoopbackMode(boolean disable) ：启用/禁用多播数据报的本地回送。true 表示禁用 LoopbackMode。

　　案例：群聊

import java.io.IOException;
import java.net.DatagramPacket;
import java.net.InetAddress;
import java.net.MulticastSocket;
import java.util.Scanner;

public class Chat {
    private volatile static boolean exit = false;
    private static Scanner input = new Scanner(System.in);
    private static String username;
    
    public static void main(String[] args) throws IOException {
        MulticastSocket socket = new MulticastSocket(9999);
        InetAddress ip = InetAddress.getByName("230.0.0.1");
        socket.joinGroup(ip);
        socket.setLoopbackMode(false);
        
        System.out.print("请输入用户名：");
        username = input.nextLine();
        
        SendThread s = new SendThread(socket,ip);
        ReceiveThread r = new ReceiveThread(socket);
        
        s.start();
        r.start();
        
        try {
            s.join();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        
        socket.close();
        input.close();
    }
    static class SendThread extends Thread{
        private MulticastSocket socket;
        private InetAddress ip;
        
        public SendThread(MulticastSocket socket,InetAddress ip) {
            super();
            this.socket = socket;
            this.ip = ip;
        }
        public void run(){
            try {
                while (!exit) {
                    System.out.print("输入广播消息：");
                    String message = input.nextLine();
                    if ("bye".equals(message)) {
                        exit = true;
                        break;
                    }
    
                    byte[] data = (username+":"+message).getBytes();
                    DatagramPacket dp = new DatagramPacket(data, data.length, ip, 9999);
    
                    socket.send(dp);
                }
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
        
    }
    static class ReceiveThread extends Thread{
        private MulticastSocket socket;
        
        public ReceiveThread(MulticastSocket socket) {
            super();
            this.socket = socket;
        }
    
        public void run(){
            try {
                while (!exit) {
                    byte[] data = new byte[1024];
                    DatagramPacket dp = new DatagramPacket(data, data.length);
                    socket.receive(dp);
                    String str = new String(data, 0, dp.getLength());
                    System.out.println(str);
                }
            } catch (IOException e) {
                exit = false;
            }
        }
        
    }
}