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，