4. Datagrams

4. Datagrams
	Chapter 11. Distributed Programming

The UDP protocol provides a mode of network communication whereby applications send packets of data, called datagrams to one another:

4.1. What Is a Datagram?

4.1.1. Definition

A datagram is an independent, self-contained message sent over the network whose arrival, arrival time, and content are not guaranteed.

4.1.2. Java classes

DatagramPacket class;
DatagramSocket class.

4.2. Writing a Datagram Client and Server

4.2.1. The Quote server

It waits for Datagram requests for quotes ans servers them in Datagram packets. Three classes implement the application:

QuoteServer
QuoteServerThread.
QuoteClient.

4.2.2. The QuoteServer Class

Just calls the server thread:

class QuoteServer {
    public static void main(String[] args) {
        new QuoteServerThread().start();
    }
}

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4.2.3. The QuoteServerThread Class

The QuoteServerThread constructor:

QuoteServerThread() {
    super("QuoteServer");
    try {
        socket = new DatagramSocket();
        System.out.println("QuoteServer listening on port: " + socket.getLocalPort());
    } catch (java.net.SocketException e) {
        System.err.println("Could not create datagram socket.");
    }
    this.openInputFile();
}

The run() method's infinite loop:

Section that receives requests:

packet = new DatagramPacket(buf, 256);
socket.receive(packet);
address = packet.getAddress();
port = packet.getPort();

Section that construct the responses:

if (qfs == null)
    dString = new Date().toString();
else
    dString = getNextQuote();
dString.getBytes(0, dString.length(), buf, 0);
packet = new DatagramPacket(buf, buf.length, address, port);
socket.send(packet);

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4.2.4. The QuoteClient Class

The main() method for the client application:

int port;
InetAddress address;
DatagramSocket socket = null;
DatagramPacket packet;
byte[] sendBuf = new byte[256];

if (args.length != 2) {
     System.out.println("Usage: java DatagramClient <hostname> <port#>");
     return;
}

The client program sends a request to the server:

address = InetAddress.getByName(args[0]);
port = Integer.parseInt(args[1]);
packet = new DatagramPacket(sendBuf, 256, address, port);
socket.send(packet);
System.out.println("Client sent request packet.");

The client gets a response from the server:

packet = new DatagramPacket(sendBuf, 256);
socket.receive(packet);
String received = new String(packet.getData(), 0);
System.out.println("Client received packet: " + received);

4.2.5. Run the Client

Quote of the Moment: Life is wonderful. Without it we'd all be dead.

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3. Sockets and Streams		Chapter 12. Applets