Question

Like a TCP server, a UDP server's job is to set up a communication endpoint and passively waits for the client to initiate the communication; however, since UDP is connectionless, UDP communication is initiated by a datagram from the client, without going through a connection setup as in TCP.

Write a program to demonstrate the typical UDP server goes through the following four steps:

1. Construct an instance of DatagramSocket, specifying the local port and, optionally, the local address.

2. Receive an instance of DatagramPacket using the receive() method of DatagramSocket. When receive() returns, the datagram contains the client's address so we know where to send the reply.

3. Communicate by sending and receiving DatagramPackets using the send() and receive() methods of DatagramSocket.

4. When finished, deallocate the socket using the close() method of DatagramSocket.

Note: The program is very simple: it loops forever, receiving datagrams and then sending the same datagrams back to the client. Actually, your server only receives and sends back the first 255 (ECHOMAX) characters of the datagram; any excess is silently discarded by the socket implementation

Answer 1

Step 1 : Save the following code in ServerUDP.java

import java.net.DatagramPacket;
import java.net.DatagramSocket;
import java.net.InetAddress;

public class ServerUDP {
   // for receiving data
   byte[] buffer;
   public ServerUDP(int port) {
       DatagramSocket socket = null;
       System.out.println("Server running at port : " + port);
       try {
           // Create server socket
           socket = new DatagramSocket(port);          
           while (true) {
               buffer = new byte[512];
               DatagramPacket request = new DatagramPacket(buffer, buffer.length);
               // Receive data
               socket.receive(request);              
               String reqMsg = new String(buffer, 0, request.getLength());
               System.out.println("Received : " + reqMsg);
               InetAddress clientAddress = request.getAddress();
               int clientPort = request.getPort();              
               DatagramPacket packet = new DatagramPacket(reqMsg.getBytes(),
                       reqMsg.length(), clientAddress, clientPort);
               // Send the data back to client
               socket.send(packet);
               System.out.println("Sending reply : " + reqMsg);
           }
       } catch (Exception ex) {
           ex.printStackTrace();
           socket.close();
       }      
   }

   public static void main(String[] args) {
       new ServerUDP(5000);
   }
}

Step 2 : Save the following code in ClientUDP.java

import java.net.DatagramPacket;
import java.net.DatagramSocket;
import java.net.InetAddress;

public class ClientUDP {
   public ClientUDP(String hostName, int port) {
       DatagramSocket socket = null;
       byte[] message = "Hello World!".getBytes();
       try {
           // Create client socket
           socket = new DatagramSocket();
           InetAddress address = InetAddress.getByName(hostName);
           DatagramPacket request = new DatagramPacket(
                   message, message.length, address, port);
           // Send data to server
           socket.send(request);
           System.out.println("Sending : " + new String(message));
           byte[] buffer = new byte[512];
           DatagramPacket response = new DatagramPacket(buffer, buffer.length);
           // Receive data
           socket.receive(response);
           String respMsg = new String(buffer, 0, response.getLength());
           System.out.println("Received : " + respMsg);
       } catch (Exception ex) {
           System.out.println("Error: " + ex.getMessage());
           ex.printStackTrace();
       } finally {
           try {
               socket.close();
           } catch (Exception ex) {}
       }
   }

   public static void main(String[] args) {
       new ClientUDP("localhost", 5000);
   }
}

Step 3 : Build the project and run ServerUDP.class file.

Following is my screen shot of the output from server

Step 4 : Build the project and run ClientUDP.class file.

Following is my screen shot of the output from client