Other Kinds of Java Networking

P2P Programming in Java

The simplest form of P2P programming in Java employs UDP multicasting, which is datagram packets with a MulticastSocket object. A MulticastSocket requires a multicast IP address (a class D IP address) and a port number. Class D IP addresses fall in the range 224.0.0.0 to 239.255.255.255, though certain addresses are reserved.

A peer wishing to subscribe to a multicast group must create a MulticastSocket representing the group and use joinGroup( ) to begin receiving communication. A peer leaves a group by calling leaveGroup( ) or by terminating.

Currently, applets aren't allowed to use multicast sockets.

The application described here takes a (welcome) break from accessing/modifying high score lists, which doesn't make for a particularly suitable P2P example. Instead, a MultiTimeServer transmits a packet to a multicast group every second, containing the current time and date. MultiTimeClient objects wait for packets to appear in the group and to print them to standard output. The situation is shown in Figure 29-16.

Figure 29-16. UDP multicasting clients and server

The use of the words "client" and "server" are a little misleading since all the objects involved in the group can potentially send and receive messages. It's my choice to restrict the server to sending and the clients to reading.

The code for the multicasting client and server can be found in the NetBasics/Multicast/ directory.

The multicasting time server

The MultiTimeServer object creates a multicast socket for a group at IP address 228.5.6.7, port 6789, and enters a loop which sends a packet out every second:

     public class MultiTimeServer     {       private static final String MHOST = "228.5.6.7";       private static final int PORT = 6789;       public static void main(String args[]) throws Exception       {          InetAddress address = InetAddress.getByName(MHOST);          MulticastSocket msock = new MulticastSocket(PORT);          msock.joinGroup(address);          DatagramPacket packet;          System.out.print("Ticking");          while(true){            Thread.sleep(1000);   // one-second delay            System.out.print(".");            String str = (new Date( )).toString( );            packet = new DatagramPacket(str.getBytes( ), str.length( ), address, PORT);            msock.send(packet);          }       }     } // end of MultiTimeServer class

The server is started this way:

     >java MultiTimeServer     Ticking.......

The multicasting time client

The client creates a multicast socket for the same group and enters an infinite loop waiting for packets to appear:

     public class MultiTimeClient     {       private static final String MHOST = "228.5.6.7";       private static final int PORT = 6789;       public static void main(String args[]) throws IOException       {          InetAddress address = InetAddress.getByName(MHOST);          MulticastSocket msock = new MulticastSocket(PORT);          msock.joinGroup(address);          byte[] buf = new byte[1024];          DatagramPacket packet = new DatagramPacket(buf, buf.length);          String dateStr;          while(true){            msock.receive(packet);            dateStr = new String( packet.getData( ) ).trim( );            System.out.println(packet.getAddress( ) + " : " + dateStr);          }        }     }

A client's execution is:

     >java MultiTimeClient     /172.30.3.176 : Mon Jan 24 16:09:48 ICT 2005     /172.30.3.176 : Mon Jan 24 16:09:49 ICT 2005     /172.30.3.176 : Mon Jan 24 16:09:50 ICT 2005     /172.30.3.176 : Mon Jan 24 16:09:51 ICT 2005     /172.30.3.176 : Mon Jan 24 16:09:52 ICT 2005

Chapter 30 contains a UDP multicasting version of a chat application.