JAVA基础知识之网络编程——-使用MutilcastSocket实现多点广播

IP多点广播原理

设置一组特殊网络地址作为多点广播地址，每一个多点广播地址都被看作一个组，当客户需要发送和接受信息时，加入到该组即可。

IP协议为多点广播提供了一批特殊的IP地址，范围是224.0.0.0 ~ 239.255.255.255

JAVA中使用MutilcastSocket加入一个广播组(joinGroup)，从而在组内发送和接受数据报DatagramPacket。

MutilcastSocket比起普通的UDP通信的DatagramSocket多了setTimeToLive(int ttl)，参数ttl决定广播组的范围，默认为1，表示局域网。

下面是一个简单例子，BroadcastSocket中用一个线程接收键盘输入（因为会阻塞）并使用MutilcastSocket发送数据，

用另一个线程从MutilcastSocket读取数据并打印出来，

package udp;

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

public class BroadcastSocket implements Runnable{
    private static final String BROADCAST_IP = "230.0.0.1";
    public static final int BROADCAST_PORT = 3000;
    private static final int DATA_LEN = 4096;
    private MulticastSocket socket = null;
    private InetAddress broadcastAddress = null;
    //用来接收键盘输入
    private Scanner scan = null;
    //用来接收网络字节
    byte[] inBuff = new byte[DATA_LEN];
    //用将inBuff封装进DatagramPacket
    private DatagramPacket inPacket = new DatagramPacket(inBuff, inBuff.length);
    //封装发送数据的DatagramPacket
    private DatagramPacket outPacket = null;
    public void init() throws IOException {
        try {
            Scanner scan = new Scanner(System.in);
            socket = new MulticastSocket(BROADCAST_PORT);
            broadcastAddress = InetAddress.getByName(BROADCAST_IP);
            socket.joinGroup(broadcastAddress);
            //设置本是否阻止MulticastSocket会发送给自身, true表示阻止 
            socket.setLoopbackMode(false);
            //初始化outPacket
            outPacket = new DatagramPacket(new byte[0], 0 , broadcastAddress, BROADCAST_PORT);
            new Thread(this).start();
            //读取键盘输入
            while (scan.hasNextLine()) {
                //需要先将键盘输入转为byte字节才能设置DatagramPacket的值
                byte[] buff = scan.nextLine().getBytes();
                outPacket.setData(buff);
                socket.send(outPacket);
            }
        } finally {
            socket.close();
        }
    }
    @Override
    public void run() {
        try {
            while (true) {
                socket.receive(inPacket);
                System.out.println("聊天信息："+new String(inBuff, 0, inPacket.getLength()));
            }
        } //捕获异常
        catch (IOException ex) {
            ex.printStackTrace();
            try {
                if (socket != null) {
                    socket.leaveGroup(broadcastAddress);
                    socket.close();
                }
                System.exit(1);
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
        
    }
    
    public static void main(String[] args) throws IOException {
        new BroadcastSocket().init();
    }
}

执行结果如下，