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     Next     DatagramServer.java     DatagramClient.java      All lectures
                    DatagramSocket Class    DatagramPacket Class    InetAddress Class

Datagram Server

The example span this slide and the next shows how a client assembles a datagram packet, sends it to a server, how the server sends a response to a client. Any datagram packet received by the server is considered a request for the current date and time. The response is encoded as a human readable ascii string, inserted into a packet and sent to the initiator.

Start the server by clicking the "Start Server" button. Get a date by clicking the "Get Next Date" button in the client.

import java.awt.*;
import java.io.*;
import java.net.*;
import java.util.*;
import java.awt.event.*;
import java.applet.*;
import javax.swing.*;

class DatagramServerThread extends Thread {
   DatagramServer server;
   DatagramSocket socket = null;
   TextArea textin, textout;
   TextField status;

   DatagramServerThread(DatagramServer server) {
      this.server = server;
      try {
         socket = new DatagramSocket(8189);
         server.status.setText("Serving port: " + socket.getLocalPort());
      catch (java.net.SocketException e) {
         server.status.setText("Could not create datagram socket.");

   public void run() {
      if (socket == null) return;

      while (true) {
         try {
            byte[] buf = new byte[256];
            DatagramPacket packet;
            InetAddress address;
            int port;
            String dString = null;

            // receive request
            packet = new DatagramPacket(buf, 256);
            address = packet.getAddress();
            port = packet.getPort();
            server.textin.append("From "+String.valueOf(address)+", "+String.valueOf(port)+"\n");

            // send response
            while (true) {
               try { Thread.sleep(1000); } catch (Exception e) { }
               dString = new Date().toString();
               buf = dString.getBytes();
               packet = new DatagramPacket(buf, buf.length, address, port);
         catch (Exception e) {
            server.status.setText("Exception:  " + String.valueOf(e));
   protected void finalize() {
      if (socket != null) {
         socket = null;
         server.status.setText("Closing datagram socket.");

public class DatagramServer extends Applet implements ActionListener {
   JTextArea textin, textout;
   JTextField status;
   JButton sb;
   public void init () {
      setLayout(new BorderLayout());
      Panel p = new Panel();
      p.setLayout(new GridLayout(2,1,10,10));
      Panel q = new Panel();
      q.setLayout(new BorderLayout());
      q.add("North", new JLabel("Receiving"));
      q.add ("Center", new JScrollPane(textin = new JTextArea(10,30)));
      q = new Panel();
      q.setLayout(new BorderLayout());
      q.add("North", new JLabel("Sending"));
      q.add("Center", new JScrollPane(textout = new JTextArea(10,30)));
      add("Center", p);
      p = new Panel();
      p.setLayout(new BorderLayout());
      p.add("Center", status = new JTextField(20));
      p.add("East", sb = new JButton("Start Server"));
      add("South", p);

   public void start () {
   public void actionPerformed (ActionEvent evt) {
      if (evt.getSource() == sb) {
         new DatagramServerThread(this).start();