UDP协议实现群聊

代码：

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import java.net.*;
import java.io.IOException;
import java.lang.String;

public class liaotian extends JFrame{
   private static final int DEFAULT_PORT=8899;
   private JLabel stateLB;
   private JTextArea centerTextArea;
   private JPanel southPanel;
   private JTextArea inputTextArea;
   private JPanel bottomPanel;
   private JTextField ipTextField;
   private JTextField remotePortTF;
   private JButton sendBT;
   private JButton clearBT;
   private DatagramSocket datagramSoket;
   private void setUpUI(){
       setTitle("聊天");
       setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
       setSize(400,400);
       setResizable(false);//窗口大小不可改变
       setLocationRelativeTo(null);//设置窗口相对于指定组件的位置
       stateLB=new JLabel("weijianting");
       stateLB.setHorizontalAlignment(JLabel.RIGHT);
       centerTextArea=new JTextArea();
       centerTextArea.setEditable(false);
       centerTextArea.setBackground(new Color(211,211,211));
       southPanel=new JPanel(new BorderLayout());
       inputTextArea=new JTextArea(5,20);
       bottomPanel=new JPanel(new FlowLayout(FlowLayout.CENTER,5,5));
       ipTextField=new JTextField("127.0.0.1",8);
       remotePortTF=new JTextField(String.valueOf(DEFAULT_PORT),3);
       sendBT=new JButton("发送");
       clearBT=new JButton("清屏");
       bottomPanel.add(ipTextField);
       bottomPanel.add(remotePortTF);
       bottomPanel.add(sendBT);
       bottomPanel.add(clearBT);
       southPanel.add(new JScrollPane(inputTextArea),BorderLayout.CENTER);
       southPanel.add(bottomPanel,BorderLayout.SOUTH);
       add(stateLB,BorderLayout.NORTH);
       add(new JScrollPane(centerTextArea),BorderLayout.CENTER);
       add(southPanel,BorderLayout.SOUTH);
       setVisible(true);
   }
private void setListener(){
   sendBT.addActionListener(new ActionListener(){
       public void actionPerformed(ActionEvent e){
           final String ipAddress=ipTextField.getText();
           final String remotePort=remotePortTF.getText();
           if(ipAddress==null||ipAddress.trim().equals("")||remotePort==null||remotePort.trim().equals("")){
               JOptionPane.showMessageDialog(liaotian.this,"请输入IP地址和端口号");
               return;
           }
           if(datagramSoket==null||datagramSoket.isClosed()){
               JOptionPane.showMessageDialog(liaotian.this,"监听未成功");
               return;
           }
           String sendContent=inputTextArea.getText();
           byte[] buf=sendContent.getBytes();
           try{
               centerTextArea.append("我对"+ipAddress+":"+remotePort+"说:\n"+inputTextArea.getText()+"\n\n");
               centerTextArea.setCaretPosition(centerTextArea.getText().length());
               datagramSoket.send(new DatagramPacket(buf,buf.length,InetAddress.getByName(ipAddress),Integer.parseInt(remotePort)));
               inputTextArea.setText("");
           }catch(IOException e1){
               JOptionPane.showMessageDialog(liaotian.this, "出错了，发送不成功");
               e1.printStackTrace();
           }
       };
   });
   clearBT.addActionListener(new ActionListener(){
       public void actionPerformed(ActionEvent e){
           centerTextArea.setText("");
       }
   });
}
private void initSocket(){
   int port=DEFAULT_PORT;
   while(true){
       try{
           if(datagramSoket!=null&&!datagramSoket.isConnected()){
               datagramSoket.close();
           }
           try{
               port=Integer.parseInt(JOptionPane.showInputDialog(this,"请输入端口号","端口号",JOptionPane.QUESTION_MESSAGE));
               if(port<1||port>65535){
                   throw new RuntimeException("端口号超出范围");
               }
           }catch(Exception e){
               JOptionPane.showMessageDialog(null,"你输入的端口不正确，请输入1~65535之间的数");
               continue;
           }
           datagramSoket=new DatagramSocket(port);
           startListen();
           stateLB.setText("已在"+port+"端口监听");
           break;
       }catch(SocketException e){
           JOptionPane.showMessageDialog(this, "端口号被占用，请重新设置端口");
           stateLB.setText("当前未启动监听");
       }
   }
}
private void startListen(){
   new Thread(){
       private DatagramPacket p;
       public void run(){
           byte[] buf=new byte[1024];
           p=new DatagramPacket(buf,buf.length);
           while(!datagramSoket.isConnected()){
               try{
                   datagramSoket.receive(p);
                   centerTextArea.append(p.getAddress().getHostAddress()+":"+((InetSocketAddress)p.getSocketAddress()).getPort()+"对我说：\n"+new String(p.getData(),0,p.getLength())+"\n\n");
                   centerTextArea.setCaretPosition(centerTextArea.getText().length());
               }catch(IOException e){
                   e.printStackTrace();
               }
           }
       }
   }.start();
}
       public static void main(String[] args) {
           liaotian a=new liaotian();
a.setUpUI();
           a.initSocket();
           a.setListener();
       }

}