Android系统MulticastSocket编程的有关问题

package xiaobenben.muticast;

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

import android.app.Activity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;

public class Muticast extends Activity {
    /** Called when the activity is first created. */
   private Button button_ok;
   private TextView textview;
   private Button button_begin;
   private EditText edittext;
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        button_ok = (Button)findViewById(R.id.button_ok);
        button_begin = (Button)findViewById(R.id.button_begin);
        textview = (TextView)findViewById(R.id.textview_text);
        edittext = (EditText)findViewById(R.id.editview);
        textview.setText("hi,xiaobenben!");
        button_begin.setOnClickListener(new Button.OnClickListener(){

            public void onClick(View v) {
                // TODO Auto-generated method stub
                try {
                    System.out.println("init() start");
                    new Multicast().init();
                } catch (IOException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }
            }
            
        });
    }
    
    class Multicast implements Runnable{
        private static final String BROADCAST_IP = "230.0.0.1";
        public static final int BROADCAST_PORT = 30000;
        private static final int DATA_LEN = 4096;
        private MulticastSocket socket = null;
        private InetAddress broadcastAddress = null;
        private Scanner scan = null;
        byte[] inBuff = new byte[DATA_LEN];
        private DatagramPacket inPacket = new DatagramPacket(inBuff , inBuff.length);
        private DatagramPacket outPacket = null;
        private String str;

        public void init()throws IOException
            {
            try
                {
                socket = new MulticastSocket(BROADCAST_PORT);
                broadcastAddress = InetAddress.getByName(BROADCAST_IP);
                socket.joinGroup(broadcastAddress);
                socket.setLoopbackMode(false);
                outPacket = new DatagramPacket(new byte[0] , 0 ,broadcastAddress , BROADCAST_PORT);
                new Thread(this).start();
                scan = new Scanner(edittext.getText().toString());
                while(scan.hasNextLine())
                    {
                    byte[] buff = scan.nextLine().getBytes();
                    outPacket.setData(buff);
                    socket.send(outPacket);
                    }
                }
            finally
            {
            socket.close();
            }
            }
        public void run(){
            try
            {
            while(true)
                {
                
                socket.receive(inPacket);
                
                System.out.println("聊天信息：" + new String(inBuff , 0 , inPacket.getLength()));
                textview.setText("聊天信息：" + new String(inBuff , 0 , inPacket.getLength()));
                System.out.println(inPacket.getAddress().getHostAddress());
                }
            }
        
        catch (IOException ex)
            {
            ex.printStackTrace();
            try
                {
                if (socket != null)
                    {
                    
                    socket.leaveGroup(broadcastAddress);
                    
                    socket.close();
                    }
                System.exit(1); 
                }
            catch (IOException e)
            {
            e.printStackTrace();
            }
            }
        }
    }
}