Creating an Server-Client Application using the DatagramPacket and DatagramSocket classes

To create an application that uses UDP to establish the connection between a client and server, we need to perform the following steps:

• Create a server program
• Create a client program
• Execute the client and server program

Let’s perform the steps in the following subsections:

Creating the Server Program

Let’s create the server class, named UDPServerEx which takes messages from a user and sends the messages (datagrams) to the clients. Listing 1 shows the code of the UDPServerEx.java file: 

Filename: UDPServerEx.java

Java

// A server that sends messages to the client

import java.net.*;

class UDPServerEx {

    public static DatagramSocket mySocket;
    public static byte myBuffer[] = new byte[2000];

    public static void serverMethod() throws Exception
    {
        int position = 0;
        while (true) {
            int charData = System.in.read();
            switch (charData) {
            case -1:
                System.out.println(
                    "The execution of "
                    + "the server has been terminated");
                return;
            case '\r':
                break;
            case '\n':
                mySocket.send(
                    new DatagramPacket(
                        myBuffer,
                        position,
                        InetAddress.getLocalHost(),
                        777));
                position = 0;
                break;
            default:
                myBuffer[position++]
                    = (byte)charData;
            }
        }
    }
    public static void main(String args[]) throws Exception
    {
        System.out.println("Please enter some text here");
        mySocket = new DatagramSocket(888);
        serverMethod();
    }
}

To compile the UDPServerEx.java file:

D:\UDPExample>javac UDPServerEx.java

Note: The path may vary according to where you save file.

Creating the ClientProgram

Let’s create a client class, named UDPClient, which accepts the messages sent from the server, UDPServerEx class. The client then displays the messages received in the Command Prompt. Listing 2 shows the code of the UDPClient.java file: 

Filename: UDPClient.java 

Java

// UDPClient that receives and
// displays messages sent from the server

import java.net.*;
class UDPClient {

    public static DatagramSocket mySocket;
    public static byte myBuffer[] = new byte[2000];

    public static void clientMethod() throws Exception
    {
        while (true) {
            DatagramPacket dataPacket
                = new DatagramPacket(myBuffer,
                                     myBuffer.length);
            mySocket.receive(dataPacket);
            System.out.println("Message Received :");
            System.out.println(
                new String(
                    dataPacket.getData(),
                    0,
                    dataPacket.getLength()));
        }
    }
    public static void main(String args[]) throws Exception
    {
        System.out.println(
            "You need to press CTRL+C"
            + " in order to quit.");
        mySocket = new DatagramSocket(777);
        clientMethod();
    }
}

Use the following command to compile the UDPClient.java file:

D:\UDPExample>javac UDPClient.java

Output

Note: To execute the UDPServerEx and UDPClient classes, run the UDPServerEx.java and UDPClient.java in two separate Command Prompt windows. Remember, the UDPServerEx class is executed before the UDPClient class. Figure 1 shows the output of the UDP Server java and UDPClient.java files:

Showing the Output of the UDPServerEx and UDPClient Classes