Datagram Communication

Part:   1  2   3 

(Continued from previous part...)

The following diagram shows the steps involved in sending a datagram from one program to another program using the methods provided by DatagramSocket and DatagramPacket classes:

       Sender Program              Receiver Program
       Internet Address #a         Internet Address #b
Step   Available Port #p           Available Port #q

1                                  ds = new DatagramSocket(#b+#q)
2                                  dp = new DatagramPacket()
3                                  ds.receive(dp)
4      ds = new DatagramSocket()   (waiting)
5      dp = new DataPacket()       (waiting)
6      dp.setAddress()             (waiting)
7      dp.setPort()                (waiting)
8      dp.setData()                (waiting)
9      ds.send(dp)                 (receiving datagram)
10                                 (dp is ready)

Datagram Socket CLient And Server Programs

The following program, called DatagramServer, is a simple datagram socket application, which acts a server listening on given port number for datagram packets to arrive:

/**
 * DatagramServer.java
 * Copyright (c) 2002 by Dr. Herong Yang
 */
import java.io.*;
import java.net.*;
public class DatagramServer {
   public static void main(String[] args) {
      byte[] buf = new byte[1024];
      DatagramPacket dp = new DatagramPacket(buf, buf.length);
      try {
         // binding to the default address and port 7777
         DatagramSocket ds = new DatagramSocket(7777);
         printDatagramSocketInfo(ds);
         while (true) {
            ds.receive(dp);
            printDatagramSocketInfo(ds);
            printDatagramPacketInfo(dp);
            // no need to call buf = dp.getData();
            // reversing the characters, assuming one-type characters
            int n = dp.getLength();
            for (int i=0; i<n/2; i++) {
               byte t = buf[i];
               buf[i] = buf[n-1-i];
               buf[n-1-i] = t;
            }
            ds.send(dp);            
         }
      } catch (IOException e) {
         System.err.println(e.toString());
      }
   }
   private static void printDatagramSocketInfo(DatagramSocket s) {
      System.out.println("Datagram Socket Info:");
      if (s.isConnected()) {
         System.out.println("   Connected to a remote system.");
         System.out.println("   Remote address = "
            +s.getInetAddress().toString());
         System.out.println("   Remote port = "
            +s.getPort());
      } else {
         System.out.println("   Not connected to a remote system.");
      }
      System.out.println("   Local socket address = "
         +s.getLocalSocketAddress().toString());
      System.out.println("   Local address = "
         +s.getLocalAddress().toString());
      System.out.println("   Local port = "
         +s.getLocalPort());
   }
   private static void printDatagramPacketInfo(DatagramPacket d) {
      System.out.println("Datagram Packet Info:");
      System.out.println("   Remote socket address = "
         +d.getSocketAddress().toString());
      System.out.println("   Remote address = "
         +d.getAddress().toString());
      System.out.println("   Remote port = "
         +d.getPort());
      System.out.println("   Data length = "
         +d.getLength());
   } 
}

Whenever DatagramServer receives a datagram packet, it takes the data, reverses the data, and sends the data back to where it comes from.

Run DatagramServer in a command window, you will see the following output:

Datagram Socket Info:
   Not connected to a remote system.
   Local socket address = 0.0.0.0/0.0.0.0:7777
   Local address = 0.0.0.0/0.0.0.0
   Local port = 7777

The server is ready to receive packets at port 7777.

The following program, called DatagraClient, acts as a client program to DatagramServer. It repeatedly sends a datagram packet to DatagramServer, and waits a return datagram packet:

(Continued on next part...)

Part:   1  2   3