/** 
 * This program is an example from the book "Internet 
 * programming with Java" by Svetlin Nakov. It is freeware. 
 * For more information: http://www.nakov.com/books/inetjava/ 
 */ 
import java.net.*; 
import java.util.Date; 
 
public class UDPDateServer { 
    public static final int LISTENING_UDP_PORT = 12345; 
    public static final String DATE_REQUEST = "GET DATE"; 
    public static final int RECEIVE_BUFFER_SIZE = 256; 
 
    public static void main(String[] args) throws Exception { 
        // Create UDP socket 
        DatagramSocket datagramSocket = 
            new DatagramSocket(LISTENING_UDP_PORT); 
        System.out.println("UDP Date Server is listening " + 
            "on port " + LISTENING_UDP_PORT); 
 
        while (true) { 
            // Receive UDP client request 
            byte[] receiveBuf = new byte[RECEIVE_BUFFER_SIZE]; 
            DatagramPacket packetIn = new 
                DatagramPacket(receiveBuf, receiveBuf.length); 
            datagramSocket.receive(packetIn); 
            String request = 
                new String(receiveBuf, 0, packetIn.getLength()); 
 
            // Send response to the client 
            if (request.equalsIgnoreCase(DATE_REQUEST)) { 
                String response = new Date().toString(); 
                byte[] responseBuf = response.getBytes(); 
                InetAddress senderIP = packetIn.getAddress(); 
                int senderPort = packetIn.getPort(); 
                DatagramPacket packetOut = new DatagramPacket( 
                    responseBuf, responseBuf.length, 
                    senderIP, senderPort); 
                datagramSocket.send(packetOut); 
            } 
        } 
    } 
}