我现在正在学习<<java网络编程》，书上有个例子，是做2个线程，一个发送udp包，另一个接收udp包，收发均使用同一个socket,但我一运行就提示：java.net.PortUnreachableException: ICMP Port Unreachable
发送线程类：
package test;

import java.net.*;
import java.io.*;

public class SenderThread extends  Thread{
    private InetAddress server;
    private DatagramSocket socket;
    private boolean stopped=false;
    private int port;

    public SenderThread(InetAddress server, int port) throws SocketException{
        this.server = server;
        this.port = port;
        this.socket=new DatagramSocket();
        this.socket.connect(server,port);
    }

    public void halt(){
        this.stopped=true;
    }

    public DatagramSocket getSocket(){
        return this.socket;
    }

    public void run() {

        BufferedReader userInput=new BufferedReader(
                new InputStreamReader(System.in)
        );

        try {
            while(true){
                if (stopped){
                    return;
                }
                String theLine=userInput.readLine();
                if (theLine.equals(".")){
                        break;
                }
                byte[] data=theLine.getBytes();
                DatagramPacket output=new DatagramPacket(data,data.length,server,port);
                socket.send(output);
                Thread.yield();
            }
        } catch (Exception e) {
            e.printStackTrace();  //To change body of catch statement use File | Settings | File Templates.
        }
    }
}

接收线程类：
package test;

import java.net.*;
import java.io.*;

public class ReceiverThread extends Thread{
    DatagramSocket socket;
    private boolean stopped=false;

    public ReceiverThread(DatagramSocket socket) {
        this.socket = socket;
    }

    public void halt(){
        this.stopped=true;
    }

    public void run() {
        byte[] buffer=new byte[3000];
        while (true){
            if (stopped){
                return;
            }
            DatagramPacket dp=new DatagramPacket(buffer,buffer.length);
            try {
                socket.receive(dp);//异常出现在这行。！！！！！！！！！！！！！！！！
                String s=new String(dp.getData(),0,dp.getLength());
                System.out.println(s);
                Thread.yield();
            } catch (Exception e) {
                e.printStackTrace();  //To change body of catch statement use File | Settings | File Templates.
            }
        }
    }
}

调用者：
package test;

import java.net.*;
import java.io.*;

public class UDPEchoClient {
    public final static int DEFAULT_PORT=7;

    public static void main(String[] args) {
        try {
            String hostname="localhost";
            int port;

            try {
                port=Integer.parseInt(args[1]);
            } catch (Exception e) {
                e.printStackTrace();  //To change body of catch statement use File | Settings | File Templates.
                port=DEFAULT_PORT;
            }


            if (args.length>0){
                hostname=args[0];
            }

            InetAddress ia=InetAddress.getByName(hostname);
            SenderThread sender=new SenderThread(ia,port);
            sender.start();

            Thread receiver=new ReceiverThread(sender.getSocket());
            receiver.start();
        } catch (UnknownHostException e) {
            e.printStackTrace();  //To change body of catch statement use File | Settings | File Templates.
        } catch (Exception e) {
            e.printStackTrace();  //To change body of catch statement use File | Settings | File Templates.
        }
    }
}