Beyond the Basics - TCP/IP Sockets in Java Practical Guide for Programmers - page 82

RecvUDPMulticast.java

0 import java.net.*; // for MulticastSocket, DatagramPacket, and InetAddress
1 import java.io.*;

// for IOException

2
3 public class RecvUDPMulticast implements ItemQuoteTextConst {
4
5

public static void main(String[] args) throws Exception {

6
7

if (args.length != 2) // Test for correct # of args

8

throw new IllegalArgumentException("Parameter(s): <Multicast Addr> <Port>");

9
10

InetAddress address = InetAddress.getByName(args[0]); // Multicast address

11

if (!address.isMulticastAddress()) // Test if multicast address

12

throw new IllegalArgumentException("Not a multicast address");

13
14

int port = Integer.parseInt(args[1]); // Multicast port

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16

MulticastSocket sock = new MulticastSocket(port); // Multicast receiving socket

17

sock.joinGroup(address); // Join the multicast group

18
19 // Create and receive a datagram

20 DatagramPacket packet = new DatagramPacket(

21 new byte[MAX_WIRE_LENGTH], MAX_WIRE_LENGTH);

22 sock.receive(packet);

23
24 ItemQuoteDecoder decoder = new ItemQuoteDecoderText(); // Text decoding

25 ItemQuote quote = decoder.decode(packet);

26

System.out.println(quote);

27
28

sock.close();

29

}

30 }

RecvUDPMulticast.java

The only significant difference between our multicast and unicast receiver is that the
multicast receiver must join the multicast group by supplying the desired multicast address.
The topic's Web site also contains another example of a sender and receiver multicast pair.
MulticastImageSender.java transmits a set of images (JPEG or GIF) specified on the command
line, in three-second intervals. MulticastImageReceiver.java receives each image and displays
it in a window.
Multicast datagrams can, in fact, be sent from a DatagramSocket by simply using a
multicast address. You can test this by using SendUDP.java (see page 57) to send to the multicast
receiver. However, a MulticastSocket has a few capabilities that a DatagramSocket does not,