Java

Echo client

The client creates a DatagramSocket and uses it to connect() to the remote server. Data is sent in a DatagramPacket, with a payload in bytes. Lines read from stdin are converted to bytes (more detail in the next chapter), and are then send() to the server. A reply is read by receive(), its payload extracted and converted to a string.

If a read fails because a packet is lost, a timeout exception is triggered, and here just reports an error message.

The code is EchoClient.java

// adapted from "https://www.baeldung.com/udp-in-java

import java.net.*;
import java.io.IOException;
import java.io.*;
import java.nio.charset.StandardCharsets;


public class EchoClient {

    public static void main(String[] args){
	DatagramSocket socket = null;
	InetAddress address = null;
	int ONE_SECOND = 1000;
	int SERVER_PORT = 2000;
	
	byte[] buf;

        if (args.length != 1) {
            System.err.println("Usage: Client address");
            System.exit(1);
        }

        try {
            address = InetAddress.getByName(args[0]);
        } catch(UnknownHostException e) {
            e.printStackTrace();
            System.exit(2);
        }

        try {
            //socket = new DatagramSocket(SE
            socket = new DatagramSocket();
	    socket.connect(address, SERVER_PORT);
        } catch(IOException e) {
            e.printStackTrace();
            System.exit(3);
        }

	BufferedReader consoleReader =  
	    new BufferedReader(new InputStreamReader(System.in));
	
	String line = null;
	while (true) {
	    line = null;
	    try {
		System.out.print("Enter line:");
		line = consoleReader.readLine();
		System.out.println("Read '" + line + "'");
	    } catch(IOException e) {
		e.printStackTrace();
		System.exit(6);
	    }

	    if (line.equals("BYE"))
		break;

	    buf = line.getBytes(StandardCharsets.UTF_8);
	    DatagramPacket packet 
		= new DatagramPacket(buf, buf.length, address, 2000);
	    try {
		socket.send(packet);
	    } catch(IOException e) {
		System.err.println("Can't send to server");
		continue;
	    }
	    
	    try {
		socket.receive(packet);
	    } catch(SocketTimeoutException e) {
		System.err.println("Timeout read from server");
		continue;
	    } catch(IOException e) {
		System.err.println("Failed read from server");
		continue;
	    }
	    System.out.println(new String(packet.getData(), StandardCharsets.UTF_8));
	}
    }
}

Echo server

The following server is adapted from A Guide To UDP In Java

// adapted from "https://www.baeldung.com/udp-in-java

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

public class EchoServer {
 
    private DatagramSocket socket;
    private byte[] buf = new byte[2048];
 
    public EchoServer() {
	try {
	    socket = new DatagramSocket(2000);
	} catch(SocketException e) {
	    System.err.println("can't start server");
	    System.exit(1);
	}
	
	while (true) {
	    DatagramPacket packet 
		= new DatagramPacket(buf, buf.length);

	    try {
		socket.receive(packet);
	    } catch (IOException e) {
		// ignore
		continue;
	    }
	    
	    InetAddress address = packet.getAddress();
	    int port = packet.getPort();

	    // send using the length of data received
	    packet = new DatagramPacket(buf, packet.getLength(), address, port);

	    try {
		socket.send(packet);
	    } catch (IOException e) {
		// ignore
		continue;
	    }
	}
    }
 
    public static void main(String[] args) {
        new EchoServer();


    }
}

Resources

• A Guide To UDP In Java

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Copyright © Jan Newmarch, jan@newmarch.name

" Network Programming using Java, Go, Python, Rust, JavaScript and Julia" by Jan Newmarch is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License .
Based on a work at https://jan.newmarch.name/NetworkProgramming/ .

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