Question

I am implementing a really basic server-client model in Java, by using UDP sockets and I have come across a really strange issue.

All I want to do is let the user (client) send a message to the server and then the server will print it.

I have an example but I am missing something since I have the following issue:

If the client sends the message "a" to the server it gets received correctly.
If the client sends the message "bbb" to the server it gets received correctly.
If the client sends the message "c" to the server, then the server will print "cbb" as the received message.

It seems as if the server does clean some kind of buffer when it gets a new message.

This is the code I am using:

Server

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



public class UDPServer {
public static void main(String args[]) throws Exception {
    byte[] receive_data = new byte[256];
    int recv_port;

    DatagramSocket server_socket = new DatagramSocket(5000);

    System.out.println("Server - Initialized server. Waiting for client on port 5000");

    while (true) {
        // System.out.println("Server - Listening for connections...");
        DatagramPacket receive_packet = new DatagramPacket(receive_data, receive_data.length);
        
        server_socket.receive(receive_packet);
        
        String data = new String(receive_packet.getData());

        InetAddress IPAddress = receive_packet.getAddress();

        recv_port = receive_packet.getPort();

        if (data.equals("q") || data.equals("Q")) {
            System.out.println("Server - Exiting !");
            break;
        } else {
            System.out.println("Server - Client from IP " + IPAddress + " @ port " + recv_port + " said : " + data + " (length: " + receive_packet.getLength() + ")");
        }
    }
}
}

Client

public class UDPClient {
public static void main(String args[]) throws Exception {
    byte[] send_data = new byte[256];

    BufferedReader infromuser = new BufferedReader(new InputStreamReader(System.in));

    DatagramSocket client_socket = new DatagramSocket();

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

    System.out.println("Client - Initialized the client...");

    while (true) {
        System.out.print("Client - Type Something (q or Q to quit): ");

        String data = infromuser.readLine();

        if (data.equals("q") || data.equals("Q")) {
            System.out.println("Client - Exited !");
            DatagramPacket send_packet = new DatagramPacket(send_data, send_data.length, IPAddress, 5000);
            System.out.println("Client - Sending data : <" + data + ">");
            client_socket.send(send_packet);
            break;
        } else {
            send_data = data.getBytes();
            DatagramPacket send_packet = new DatagramPacket(send_data, send_data.length, IPAddress, 5000);
            System.out.println("Client - Sending data : <" + data + ">");
            client_socket.send(send_packet);
        }
    }

    client_socket.close();
}
}

I suppose that the mistake is something trivial, but my skills in network programming are limited, therefore I don’t know what exactly it is.

Just to make clear, I am running both the server and the client at the same machine (mac) on different terminals, just in case it affects the situation in anyway.

Any help would be greatly appreciated.

EDIT

…And I come back to answer my own question.
The problem was that I was not defining the amount of data that the server socket should expect to read.
Therefore when I change

String data = new String(receive_packet.getData());

with

String data = new String(receive_packet.getData(), 0, receive_packet.getLength());

everything worked smoothly.

Just for future reference and for people who might come across the same problem 🙂

Answer 1

On your first call this is what receive_data looks like:

 --------------
|"a"|    |    |
 --------------

On your second call:

 --------------
|"b"|"b"| "b" |             notice that the "a" in data_receive was overwritten
 --------------

On your third call, you only send a single letter,
so the only part of the array that gets overwritten is the first element:

 --------------
|"c"|"b"| "b" | 
 --------------

This is happening because there is still data left in the receive_data array in between messages to the server, a simple way around this would to just initialize a new array inside of you receive loop. That way every time you receive a message you will have a fresh array waiting for you.

while (true) 
{
    byte[] receive_data = new byte[256];
    ......
}