java.net.SocketException有关问题

import java.awt.Button;import java.awt.Frame;import java.awt.TextArea;import java.awt.TextField;import java.awt.event.ActionEvent;import java.awt.event.ActionListener;import java.awt.event.WindowAdapter;import java.awt.event.WindowEvent;import java.net.DatagramPacket;import java.net.DatagramSocket;import java.net.InetAddress;class PC100 extends Frame implements Runnable,ActionListener{    TextField out=new TextField("send to pc2");    TextArea in=new TextArea();    Button b=new Button("send");        PC100(){        super("This is pc1");        setSize(200,200);        setVisible(true);        b.addActionListener(this);        add(out,"South");        add(in,"Center");        add(b,"North");        Thread t=new Thread(this);        t.start();                this.addWindowListener(new WindowAdapter(){            public void windowClosing(WindowEvent e){                System.exit(0);            }        });    }        @Override    public void run() {        // TODO Auto-generated method stub                DatagramPacket pack=null;        DatagramSocket mail=null;        byte data[]=new byte[8192];        try{            pack=new DatagramPacket(data,data.length);            mail=new DatagramSocket(9998);////////////////////////        }catch(Exception e){            e.printStackTrace();            System.out.println("run01 error!");            }        while(true){            if(mail==null)                break;            else                try{                    mail.receive(pack);   //////////异常。。。。。                    int length=pack.getLength();                    InetAddress add=pack.getAddress();                    int port=pack.getPort();                    String msg=new String(pack.getData(),0,length);                    in.append("length:"+length+"\n");                    in.append("From:"+add+"port:"+port+"\n");                    in.append("data:"+msg+"\n");                }catch(Exception e){                    e.printStackTrace();                    in.append("run22\n");break;                    }        }    }    @Override    public void actionPerformed(ActionEvent event) {        // TODO Auto-generated method stub                byte buffer[]=out.getText().trim().getBytes();        try{            InetAddress add=InetAddress.getByName("127.0.0.1");            //////////////////////            DatagramPacket data=new DatagramPacket(buffer,buffer.length,add,9998);            DatagramSocket mail=new DatagramSocket();            in.append("IP:"+data.getAddress()+"\n");            in.append("port:"+data.getPort()+"\n");            in.append("length:"+data.getLength()+"\n");            mail.send(data);        }catch(Exception e){System.out.println("send error!");}            }}public class PC1{    public static void main(String args[]){        PC100 pc10=new PC100();        pc10.validate();    }}

java.net.SocketException: Socket closed    at java.net.PlainDatagramSocketImpl.receive0(Native Method)    at java.net.PlainDatagramSocketImpl.receive(PlainDatagramSocketImpl.java:136)    at java.net.DatagramSocket.receive(DatagramSocket.java:712)    at PC100.run(PC1.java:56)    at java.lang.Thread.run(Thread.java:619)