Corrections to Chapter 13 of Java Network Programming, Multicast Sockets

p. 342, Example 13-3: In Java 1.1 the reuse of the same DatagramPacket object dp causes the UDPDiscardServer to act unexpectedly on at least some platforms. (Exact details appeuar to be dependent on native code.) dp is initialized to a length of 65,507 bytes but the DatagramSocket.receive() method resets this the length to reflect the number of data bytes received. When dp is reused, its length has been reset to a new and potentially smaller value. This seems to be something that changed from 1.0 to 1.1. However, the example can be written in a fashion that supports all cases in both Java 1.0 and 1.1 by moving the line that constructs the DatagramPacket into the while loop.

import sun.net.*;
import java.net.*;
import java.io.*;

public class MulticastSniffer {

  public static void main(String[] args) {
  
    InetAddress ia = null;
    byte[] buffer = new byte[65509];
    int port = 0;
  
    // read the address from the command line
    try {
      try {
        ia = InetAddress.getByName(args[0]);
      }
      catch (UnknownHostException e)  {
        ia = InetAddressFactory.newInetAddress(args[0]);
      }
      port = Integer.parseInt(args[1]);
    }  // end try
    catch (Exception e) {
      System.err.println(e);
      System.err.println("Usage: java MulticastSniffer MulticastAddress port");
      System.exit(1);
    }
  
    try {
      MulticastSocket ms = new MulticastSocket(port);
      ms.joinGroup(ia);
      while (true) {
        DatagramPacket dp = new DatagramPacket(buffer, buffer.length);
        ms.receive(dp);
        String s = new String(dp.getData(), 0, 0, dp.getLength());
        System.out.println(s);
      }
    }
    catch (SocketException se) {
      System.err.println(se);
    }
    catch (IOException ie) {
      System.err.println(ie);
    }  
  
  }

}

p. 344: Example 13-4, MulticastSender, is correct but it could be simpler. It does not need to specify a port because it is only sending, not receiving.