Programming Tutorials

UDP Datagram sample program in Java

By: Jagan in Java Tutorials on 2022-09-15  

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.

Here's an example program that demonstrates how to use UDP DatagramSocket in Java:

import java.net.*;

public class UDPServer {
    public static void main(String[] args) throws Exception {
        DatagramSocket serverSocket = new DatagramSocket(9876);

        byte[] receiveData = new byte[1024];
        byte[] sendData = new byte[1024];

        while (true) {
            DatagramPacket receivePacket = new DatagramPacket(receiveData, receiveData.length);
            serverSocket.receive(receivePacket);

            String sentence = new String(receivePacket.getData());
            InetAddress IPAddress = receivePacket.getAddress();
            int port = receivePacket.getPort();

            String capitalizedSentence = sentence.toUpperCase();
            sendData = capitalizedSentence.getBytes();

            DatagramPacket sendPacket = new DatagramPacket(sendData, sendData.length, IPAddress, port);
            serverSocket.send(sendPacket);
        }
    }
}

And here's an example client program:

import java.io.*;
import java.net.*;

public class UDPClient {
    public static void main(String[] args) throws Exception {
        BufferedReader inFromUser = new BufferedReader(new InputStreamReader(System.in));
        DatagramSocket clientSocket = new DatagramSocket();

        InetAddress IPAddress = InetAddress.getByName("localhost");

        byte[] sendData = new byte[1024];
        byte[] receiveData = new byte[1024];

        String sentence = inFromUser.readLine();
        sendData = sentence.getBytes();

        DatagramPacket sendPacket = new DatagramPacket(sendData, sendData.length, IPAddress, 9876);
        clientSocket.send(sendPacket);

        DatagramPacket receivePacket = new DatagramPacket(receiveData, receiveData.length);
        clientSocket.receive(receivePacket);

        String modifiedSentence = new String(receivePacket.getData());
        System.out.println("FROM SERVER: " + modifiedSentence);

        clientSocket.close();
    }
}

In this example, the server waits for incoming datagrams on port 9876. When a datagram is received, it converts the data to uppercase and sends it back to the client. The client sends a sentence to the server and waits for the response. Once the response is received, it prints it to the console.