Java Networking Datagrams Tutorial

Client server communication with Datagrams 

 

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Generally, Datagrams are transmits  packets of information between machines. Once the datagram has released to its intended target, there is no assurance that it will arrive without loss of data. Java implements datagrams on top of UDP protocol by using 2 classes. They are..

1.DatagramSocket

constructors
 
DatagramSocket()throws SocketException
DatagramSocket(int port)throws SocketException
DatagramSocket(int port,InetAddrss ipaddress)throws SocketException
DatagramSocket(SocketAddress address)throws SocketException

SocketAddress

It is an abstract class implemented by concrete class InetSocketAddress. It encapsulates
an IP address with a port no.


methods
 

void send(DatagramPacket packet) throws IOException
void receive(DatagramPacket packet) throws IOException

2.DatagramPacket

 
its object is the data container.

DatagramPacket(byte data[], int size)
DatagramPacket(byte data[], int offset,int size)
DatagramPacket(byte data[], int size,InetAddress ipAddress,int port)
DatagramPacket(byte data[], int offset, int size,InetAddress ipAddress,int port)

methods
 

InetAddress getAddress()
byte[] getData()
setData(byte[])
int getLength()
getPort()
setPort()

 

The following ex. implements a very simple networked communication client and server.  
Message are typed into the window at the server and written across the network to the
client side, where they are display.
 

 

// 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, In one window run the program as shown below

    c:\>java WriteServer

 this will be the client. Then run in another window..

    c:\>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.

 

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