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Trail: Custom Networking
Lesson: All About Datagrams

## Broadcasting to Multiple Recipients

> In addition to `DatagramSocket`, which lets programs send packets to one another, java.net includes a class called `MulticastSocket`. This kind of socket is used on the client-side to listen for packets that the server broadcasts to multiple clients.

> Let's rewrite the quote server so that it broadcasts `DatagramPacket`s to multiple recipients. Instead of sending quotes to a specific client that makes a request, the new server now needs to broadcast quotes at a regular interval. The client needs to be modified so that it passively listens for quotes and does so on a `MulticastSocket`.

> This example is comprised of three classes which are modifications of the three classes from the previous example: `MulticastServer`, `MulticastServerThread`, and `MulticastClient`. This discussion highlights the interesting parts of these classes.

> Here is the new version of the server's main program. The differences between this code and the previous version, `QuoteServer`, are shown in bold:

> >     import java.io.*;
> >     public class MulticastServer {
> >         public static void main(String[] args) throws IOException {
> >             new MulticastServerThread().start();
> >         }
> >     }

> Basically, the server got a new name and creates a `MulticastServerThread` instead of a `QuoteServerThread`. Now let's look at the `MulticastServerThread` which contains the heart of the server. Here's its class declaration:

> >     public class MulticastServerThread extends QuoteServerThread {
> >         ...
> >     }

> We've made this class a subclass of `QuoteServerThread` so that it can use the constructor, and inherit some member variable and the `getNextQuote` method. Recall that `QuoteServerThread` creates a `DatagramSocket` bound to port 4445 and opens the quote file. The `DatagramSocket`'s port number doesn't actually matter in this example because the client never send anything to the server.

> The only method explicitly implemented in `MulticastServerThread` is its `run` method. The differences between this `run` method and the one in `QuoteServerThread` are shown in bold:

> >     public void run() {
> >         while (moreQuotes) {
> >             try {
> >                 byte[] buf new byte[256];
> >                 // don't wait for request...just send a quote

> >                 String dString = null;
> >                 if (in == null)
> >                     dString = new Date().toString();
> >                 else
> >                     dString = getNextQuote();
> >                 buf = dString.getBytes();

> >                 InetAddress group = InetAddress.getByName(
> >                                                 "230.0.0.1");
> >                 DatagramPacket packet;
> >                 packet = new DatagramPacket(buf, buf.length,
> >                                             group, 4446);
> >                 socket.send(packet);

> >                 try {
> >                     sleep((long)Math.random() * FIVE_SECONDS);
> >                 } catch (InterruptedException e) { }
> >             } catch (IOException e) {
> >                 e.printStackTrace();
> >                 moreQuotes = false;
> >             }
> >         }
> >         socket.close();
> >     }

> The interesting change is how the `DatagramPacket` is constructed, in particular, the `InetAddress` and port used to construct the `DatagramPacket`. Recall that the previous example retrieved the `InetAddress` and port number from the packet sent to the server from the client. This was because the server needed to reply directly to the client. Now, the server needs to address multiple clients. So this time both the `InetAddress` and the port number are hard-coded.

> The hard-coded port number is 4446 (the client must have a `MulticastSocket` bound to this port). The hard-coded `InetAddress` of the `DatagramPacket` is "230.0.0.1" and is a group identifier (rather than the Internet address of the machine on which a single client is running). This particular address was arbitrarily chosen from the reserved for this purpose.

> Created in this way, the `DatagramPacket` is destined for all clients listening to port number 4446 who are member of the "230.0.0.1" group.

> To listen to port number 4446, the new client program just created its `MulticastSocket` with that port number. To become a member of the "230.0.0.1" group, the client calls the `MulticastSocket`'s `joinGroup` method with the `InetAddress` that identifies the group. Now, the client is set up to receive `DatagramPacket`s destined for the port and group specified. Here's the relevant code from the new client program (which was also rewritten to passively receive quotes rather than actively request them). The bold statements are the ones that interact with the `MulticastSocket`:

> >     MulticastSocket socket = new MulticastSocket(4446);
> >     InetAddress group = InetAddress.getByName("230.0.0.1");
> >     socket.joinGroup(group);

> >     DatagramPacket packet;
> >     for (int i = 0; i < 5; i++) {
> >         byte[] buf = new byte[256];
> >         packet = new DatagramPacket(buf, buf.length);
> >         socket.receive(packet);

> >         String received = new String(packet.getData());
> >         System.out.println("Quote of the Moment: " + received);
> >     }
> >     socket.leaveGroup(group);
> >     socket.close();

> Notice that the server uses a `DatagramSocket` to broadcast packet received by the client over a `MulticastSocket`. Alternatively, it could have used a `MulticastSocket`. The socket used by the server to send the `DatagramPacket` is not important. What's important when broadcasting packets is the addressing information contained in the `DatagramPacket`, and the socket used by the client to listen for it

> > Try this: Run the `MulticastServer` and several clients. Watch how the clients all get the same quotes.

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