Sending and receiving objects using sockets and threads not working properly

Question

I am currently creating a service allowing to send objects from a client to a server and vice-versa, but experiencing an issue that I unfortunately cannot explain and fix.

First of all, here are the useful classes (I haven't put all methods such as getters and setters in this post).

/**
 * This launcher creates a NetworkInterface, waits for a connection, sends a message to the connected client and waits for an incoming message
 *
 */
public class ServerLauncher {

    public static void main(String[] args) {
        try {
            NetworkSystem n = new NetworkSystem(4096);
            n.startServerManager();

            while (n.getCommunications().isEmpty()) {
                // this line is unexpectedly magic
                System.out.println("Waiting for a new connection...");
            }

            do {
                n.getCommunications().get(0).send(new String("Hello, are you available?")); 
            } while (n.getCommunications().get(0).getReceiveManager().getReadObjects().isEmpty());

            System.out.println(n.getCommunications().get(0).getReceiveManager().getReadObjects().get(0));

        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }
}

/**
 * This launcher creates a NetworkSystem, connects to the server, waits for an incoming message and anwers back
 *
 */
public class ClientLauncher {

    public static void main(String[] args) {
        try {
            NetworkSystem n = new NetworkSystem(8192);
            n.instanciateCommunication(new Socket(InetAddress.getLocalHost(), 4096));

            while (n.getCommunications().get(0).getReceiveManager().getReadObjects().isEmpty()) {
                // this line is unexpectedly magic
                System.out.println("Waiting for an object...");
            }

            System.out.println(n.getCommunications().get(0).getReceiveManager().getReadObjects().get(0));
            n.getCommunications().get(0).getSendManager().send(new String("No, I am not! We will talk later..."));
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }
}

/**
 * This class handles every incoming messages.
 */
public class ReceiveManager implements Runnable {

    private ObjectInputStream inputStream;
    private CommunicationManager communicationManager;
    private List readObjects;
    private boolean receive;

    public ReceiveManager(CommunicationManager communicationManager) throws IOException {
        this.communicationManager = communicationManager;
        this.inputStream = new ObjectInputStream(this.communicationManager.getSocket().getInputStream());
        this.readObjects = new ArrayList();
        this.receive = true;
    }

    @Override
    public void run() {
        Object object = null;

        try {
            while ((object = this.inputStream.readObject()) != null && this.hasToReceive()) {
                this.readObjects.add(object);
            }
        } catch (ClassNotFoundException | IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
            this.setContinueToReceive(false);
        }
    }

    private boolean hasToReceive() {
        return this.receive;
    }

    public void setContinueToReceive(boolean value) {
        this.receive = value;
    }
}

/**
 * This class allows the user to send messages
 */
public class SendManager {

    private ObjectOutputStream outputStream;
    private CommunicationManager communicationManager;

    public SendManager(CommunicationManager communicationManager) throws IOException {
        this.communicationManager = communicationManager;
        this.outputStream = new ObjectOutputStream(this.communicationManager.getSocket().getOutputStream());
    }

    public void send(Object object) throws IOException {
        this.outputStream.writeObject(object);
        this.outputStream.flush();
    }
}

So basically, as you may have noticed in the ServerLauncher and the ClientLauncher, there are two "magic" instructions. When those two lines are commented and I run the server then the client, nothing happens. The server and the client are simply running and never stop. However, when I uncomment these two magic lines, every works like a charm: messages are properly sent and received.

Would you guys know the reason of this unexpected behaviour ?

Oh yeah, I forgot, if you guys want me to upload everything to test the project or whatever, just tell me :-)

Answer 1

You're starving the CPU with those spin loops. You should sleep or wait while the queues are empty, or better still just take()from blocking queues.

NB Your loop condition isn't correct:

1. readObject() doesn't return null at end of stream. It throws EOFException.
2. You should also test hasToReceive() before calling readObject() rather than afterwards. Otherwise you always do an extra read.