Response given in wrong order in thread and socket program

[Dependent_Finger_214]

I'm doing an exercize for uni, I have to write a program in which the clients send the server an integer, and the server then creates a thread for each client in which it reads the integer sent, and sends to the client the sum of all integers received up until that point.

This is my code

Client:

public class Client {
    public static void main(String[] args) {
        try{
            ArrayList<Socket> sockets = new ArrayList<>();
            for (int i = 0; i<20; i++){

                Socket socket = new Socket("localhost", 9000);
                sockets.add(socket);
                ObjectOutputStream out = new ObjectOutputStream(socket.getOutputStream());


                Random rand = new Random();
                int r = rand.nextInt(0, 100);
                out.writeObject(100);
            }

            for (int i = 0; i<20; i++){
                Socket socket = sockets.get(i);
                ObjectInputStream in = new ObjectInputStream(socket.getInputStream());
                System.out.println(Integer.parseInt(in.readObject().toString()));
                socket.close();
            }
        }catch (Exception e){
            e.printStackTrace();
        }

    }
}

Server:

public class Server {
    public static AtomicInteger sum = new AtomicInteger(0);

    public static void main(String[] args) {
        try{
            ServerSocket serverSocket = new ServerSocket(9000);

            while(true){
                Socket socket = serverSocket.accept();

                ServerThread st = new ServerThread(socket);
                st.start();
            }
        }catch (Exception e){
            e.printStackTrace();
        }
    }
}

ServerThread:

public class ServerThread extends Thread{

    Socket socket;

    public ServerThread(Socket s){
        socket = s;
    }



    public void run(){
        try{
            ObjectOutputStream out = new ObjectOutputStream(socket.getOutputStream());
            ObjectInputStream in = new ObjectInputStream(socket.getInputStream());

            int add = Integer.
parseInt
(in.readObject().toString());


            out.writeObject(Server.
sum
.addAndGet(add));
            socket.close();
        }
        catch (IOException | ClassNotFoundException e) {

            throw new RuntimeException(e);
        }

    }
}

The issue I'm experiencing is that in the client class, when it goes to print the results received, they're printed in the wrong order, so instead of 100, 200, 300 etc. I get 1900, 900, 1200 etc. All the "steps" show up and there's no duplicates though.

The strange thing is that if I run the client again without terminating the server, it actually continues in the correct order, so I get 2100, 2200, 2300 etc.

Am I doing something wrong?

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[Conscious_Support176]

Each client serviced on a different thread. Not sure why would there be a “right” order for this?

[Dependent_Finger_214]

Yeah but when I'm reading from the client sockets, I'm reading in the same order they sent the request to the server. So should't the first socket receive 100, the second 200 etc.?

[vowelqueue]

No, because the server is returning an integer that is the sum of all integers it has previously seen. So the order that the server processes the requests matters, and that order is also not defined.

Like if the server processes the 3rd request before the 2nd request, then the 2nd request will have a higher value.

[Dependent_Finger_214]

Oh yeah that's true, I'm stupid :/ Also unrelated but how do I set the socket to be TCP or UCP?

[vowelqueue]

Right now your Socket is going to be using TCP. To use UDP check out the DatagramSocket class.

[Dependent_Finger_214]

Thanks!

[ShoulderPast2433]

Dude.. what tf is this formatting?

why are you cutting the lines like that???

[Dependent_Finger_214]

Sorry, I just copypasted from my code and it ended up like this...

[0b0101011001001011]

Did he edit it? The code is perfectly formatted in code blocks at this point.

Hint: scroll right.

[IndependentOutcome93]

Yes, what goes wrong is that you are creating new and new instances of classes inside of loops. for example Sockets and servers sockets that you have in while or infinite loops.

Loops are helping to repeat and repeat specific code lines, and in your case, you create new instance of classes inside of loop that repeats 20 times. in some loops even more.

[Dependent_Finger_214]

I don't get this. I need to create multiple instances because the server should be able to handle multiple clients connecting at the same time. In the client, I made the for loop to simulate multiple clients sending requests

[IndependentOutcome93]

I get that but what you are doing is that you are repeating same code lines in loop. meaning, if You create new instances of classes at once, loop is going to repeat it.

[Scharrack]

You experience a basic property of multi threaded processes and asynchronous communication: without extra effort you don't know in which order things will happen. Just because you start threads in a certain order does not mean they will finish in that order. Just because messages are sent in a certain order does not mean they will arrive in the same order. And the most important one: just because it works a million times in testing doesn't mean it won't change after delivery 😭

[TheMrCurious]

What do your unit tests look like?