server.java

import java.io.IOException;
import java.net.*;

public class Rfc865UdpServer {
    public static void main(String[] args) {
        //
        // open udp socket at well know port
        //

        DatagramSocket socket = null;
        try {
            socket = new DatagramSocket(17);
        } catch (SocketException e) {
            System.out.println(e.getMessage());
        }

        int byteLength = 512;
        byte[] receiveData = new byte[byteLength];

        while (true) {
            try {
                //
                // listen for UDP request from client
                //
                System.out.println("Waiting to receive request");
                DatagramPacket request = new DatagramPacket(receiveData, receiveData.length);
                try {

                    socket.receive(request);
                } catch (NullPointerException nullEx) {
                    nullEx.getMessage();
                }
				String receiveMsg = new String(receiveData);
				System.out.println(receiveMsg);


                //
                // Send UDP reply to client
                //
                InetAddress clientAddress = request.getAddress();
                int clientPort = request.getPort();

                String msg = "Message received! waiting for another message";

                byte[] replyMsg = new byte[512];

                replyMsg = msg.getBytes();

                DatagramPacket reply = new DatagramPacket(replyMsg,replyMsg.length,clientAddress,clientPort);
                socket.send(reply);
                System.out.println("Reply send");


            } catch (IOException e) {
                e.getMessage();
            }
		}
    }
}