Java Program for Datagram Server and Client

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What is datagram in java program?

Datagram are bundles of information shared between the machines in network.

How to implement datagram in java program?

Java implements datagram on top of UDP protocol using two classes.

DatagramPacket
DatagramSocket

What is DatagramPacket?

DatagramPacket specifies the buffer that will receive data and size of the packet.

What is DatagramSocket?

DatagramSocket: speciies the target address and port to determine where the data in the packet will be sent.

How to implement Datagram Server and Client in Java program?

import java.net.*;                                                              
                                                                                
class DatagramTest {                                                            
                                                                                
 public static int serverPort = 887;                                            
 public static int clientPort = 888;                                            
                                                                                
 public static DatagramSocket ds;                                               
 public static byte buffer[] = new byte[1024];                                  
                                                                                
 public static void serverMethod() throws Exception {                           
  int seek = 0;                                                                 
  while(true) {                                                                 
   int ch = System.in.read();                                                   
   if (ch == -1) {                                                              
    System.out.println("Server exists!");                                       
    return;                                                                     
   }                                                                            
   else if (ch == '\n') {                                                       
    ds.send(new DatagramPacket(buffer, seek, InetAddress.getLocalHost(), clientPort));
    seek = 0;                                                                   
    break;                                                                      
   }                                                                            
   else {                                                                       
    buffer[seek++] = (byte) ch;                                                 
   }                                                                            
  }                                                                             
 }                                                                              
                                                                                
 public static void clientMethod() throws Exception {                           
  while(true) {                                                                 
   DatagramPacket dp = new DatagramPacket(buffer, buffer.length);               
   ds.receive(dp);                                                              
   System.out.println(new String(dp.getData(), 0, dp.getLength()));             
  }                                                                             
 }                                                                              
                                                                                
 public static void main(String args[]) throws Exception {                      
  if(args[0].equals( "server")) {                                                       
   ds = new DatagramSocket(serverPort);                                         
   serverMethod();                                                              
  } else {                                                                      
    ds = new DatagramSocket(clientPort);                                        
    clientMethod();                                                             
  }                                                                             
 }                                                                              
}

Output:

//Client
step1: $ java DatagramTest client 
step4: hello world!

//Server
step2: $ java DatagramTest server
//once pressed enter key, data will be sent to client
step3: hello world!

If DatagramTest.java program throws exception like this, change user as administrator or root on the machine.

$ java DatagramTest
Exception in thread "main" java.net.BindException: Permission denied (Bind failed)
	at java.net.PlainDatagramSocketImpl.bind0(Native Method)
	at java.net.AbstractPlainDatagramSocketImpl.bind(AbstractPlainDatagramSocketImpl.java:93)
	at java.net.DatagramSocket.bind(DatagramSocket.java:392)
	at java.net.DatagramSocket.(DatagramSocket.java:242)
	at java.net.DatagramSocket.(DatagramSocket.java:299)
	at java.net.DatagramSocket.(DatagramSocket.java:271)
	at DatagramTest.main(DatagramTest.java:43)

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