Connect to a daytime server using the UDP protocol : UDP « Network Protocol « Java

Connect to a daytime server using the UDP protocol


 * Copyright (c) 2004 David Flanagan.  All rights reserved.
 * This code is from the book Java Examples in a Nutshell, 3nd Edition.
 * It is provided AS-IS, WITHOUT ANY WARRANTY either expressed or implied.
 * You may study, use, and modify it for any non-commercial purpose,
 * including teaching and use in open-source projects.
 * You may distribute it non-commercially as long as you retain this notice.
 * For a commercial use license, or to purchase the book, 
 * please visit
//package je3.nio;

 * Connect to a daytime server using the UDP protocol. We use instead
 * of java.nio because DatagramChannel doesn't honor the setSoTimeout() method
 * on the underlying DatagramSocket
public class DaytimeClient {
  public static void main(String args[]) throws {
    // Figure out the host and port we're going to talk to
    String host = args[0];
    int port = 13;
    if (args.length > 1)
      port = Integer.parseInt(args[1]);

    // Create a socket to use
    DatagramSocket socket = new DatagramSocket();

    // Specify a 1-second timeout so that receive() does not block forever.

    // This buffer will hold the response. On overflow, extra bytes are
    // discarded: there is no possibility of a buffer overflow attack here.
    byte[] buffer = new byte[512];
    DatagramPacket packet = new DatagramPacket(buffer, buffer.length, new InetSocketAddress(host,

    // Try three times before giving up
    for (int i = 0; i < 3; i++) {
      try {
        // Send an empty datagram to the specified host (and port)
        packet.setLength(0); // make the packet empty
        socket.send(packet); // send it out

        // Wait for a response (or timeout after 1 second)
        packet.setLength(buffer.length); // make room for the response
        socket.receive(packet); // wait for the response

        // Decode and print the response
        System.out.print(new String(buffer, 0, packet.getLength(), "US-ASCII"));
        // We were successful so break out of the retry loop
      } catch (SocketTimeoutException e) {
        // If the receive call timed out, print error and retry
        System.out.println("No response");

    // We're done with the channel now


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