Programming Tutorials

UDP Datagram sample program in Java

By: Jagan  

For most of your internetworking needs, you will be happy with TCP/IP-style networking. It provides a serialized, predictable, reliable stream of packet data. This is not without its cost, however. TCP includes many complicated algorithms for dealing with congestion control on crowded networks, as well as pessimistic expectations about packet loss. This leads to a somewhat inefficient way to transport data. Datagrams provide an alternative.

Datagrams are bundles of information passed between machines. They are somewhat like a hard throw from a well-trained but blindfolded catcher to the third baseman. Once the datagram has been released to its intended target, there is no assurance that it will arrive or even that someone will be there to catch it. Likewise, when the datagram is received, there is no assurance that it hasn't been damaged in transit or that whoever sent it is still there to receive a response.

Java implements datagrams on top of the UDP protocol by using two classes: The DatagramPacket object is the data container, while the DatagramSocket is the mechanism used to send or receive the DatagramPackets.

The following example implements a very simple networked communications client and server. Messages are typed into the window at the server and written across the network to the client side, where they are displayed.

// Demonstrate Datagrams.
import java.net.*;
class WriteServer {
public static int serverPort = 666;
public static int clientPort = 999;
public static int buffer_size = 1024;
public static DatagramSocket ds;
public static byte buffer[] = new byte[buffer_size];
public static void TheServer() throws Exception {
int pos=0;
while (true) {
int c = System.in.read();
switch (c) {
case -1:
System.out.println("Server Quits.");
return;
case '\\r':
break;
case '\\n':
ds.send(new DatagramPacket(buffer,pos,
InetAddress.getLocalHost(),clientPort));
pos=0;
break;
default:
buffer[pos++] = (byte) c;
}
}
}
public static void TheClient() throws Exception {
while(true) {
DatagramPacket p = new DatagramPacket(buffer,
buffer.length);
ds.receive(p);
System.out.println(new String(p.getData(), 0,
p.getLength()));
}
}
public static void main(String args[]) throws Exception {
if(args.length == 1) {
ds = new DatagramSocket(serverPort);
TheServer();
} else {
ds = new DatagramSocket(clientPort);
TheClient();
}
}
}

This sample program is restricted by the DatagramSocket constructor to running between two ports on the local machine. To use the program, run

java WriteServer

in one window; this will be the client. Then run

java WriteServer 1

This will be the server. Anything that is typed in the server window will be sent to the client window after a newline is received.
Note This example requires that your computer be connected to the Internet.

This tutorial is an extract from the "The Complete Reference Part 2 by Herbert Schildt".

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