udpechoserver.java

this gives details of the network programming

// Sample Java Program
// UDP Echo Server
//
// Based partially on the code in TCP/IP Sockets in Java, by Calvert and Donahoo
//
// Requires a single command line arg - the port number
//
// You can test this using nc (netcat) in UDP mode as the client:
// nc -u hostname port


import java.net.*;	// need this for InetAddress, Socket, ServerSocket
import java.io.*;	// need this for I/O stuff 

public class UDPEchoServer {

    // define a constant used as size of buffer
    static final int BUFSIZE=1024;

    // main starts things rolling

    // declare main as throwing a SocketException, this means we let
    // Java deal with this exception (it is thrown to the calling,
    // and in this case the calling is the JVM).


    static public void main(String args[]) throws SocketException {
	if (args.length != 1) {
	    throw new IllegalArgumentException("Must specify a port!");
	}

	int port = Integer.parseInt(args[0]);

	// Create UDP socket 
	DatagramSocket s = new DatagramSocket(port);

	// Create Datagrampacket we can use for receiving
	DatagramPacket dp = new DatagramPacket(new byte[BUFSIZE], BUFSIZE);

	try {
	    while (true) {
		// wait for incoming datagram, put into dp
		s.receive(dp);

		// print out client's address
		System.out.println("Message from " + dp.getAddress().getHostAddress());

		// Send it right back
		s.send(dp);
		dp.setLength(BUFSIZE);	// avoid shrinking the packet buffer
	    }
	} catch (IOException e) {
	    System.out.println("Fatal I/O Error !");
 	    System.exit(0);
	}

    }
}