Java concurrent networking issues

Question

I am a Java newbie trying to learn network programming and concurrency, and I thought I'd try out writing a simple chat server where input from a client is echoed to all the clients. That's not happening. I added a couple print statements so that the program will announce that it is waiting for connections and each time it receives a connection. I am using Telnet locally to connect to the port on my machine.

The program announces success for the first and second concurrent connections but then does not announce success for subsequent connections until I close all connections. So, for example, I'll connect from five separate terminals, and the program will announce "Connection 1" and "Connection 2" but will not announce "Connection 3", 4, and 5 until I close all the terminals.

I'm looking for help figuring out where my errors lie as well as general advice for how to approach debugging a situation like this.

In a nutshell, my program has

1. A Main class, which starts the other three threads
2. A ClientListener class, which uses a SocketReader to listen for connections and stores the Sockets inputstreams and outputstreams in two Sets.
3. A MessageReader, which iterates over the inputstreams. If it finds a message, it puts it in a SynchronousQueue and waits for the
4. MessageWriter to remove it. The MessageWriter sends the message to all the outputstreams.

The code is below. Thanks for any help!

public class Main {

    public static void main(String[] args) {
        ClientListener clientListener = new ClientListener();
        Thread clientListenerThread = new Thread(clientListener);
        clientListenerThread.setPriority(Thread.MAX_PRIORITY);
        clientListenerThread.start();

        MessageReader messageReader = new MessageReader(clientListener);
        Thread messageReaderThread = new Thread(messageReader);
        messageReaderThread.setPriority(Thread.MIN_PRIORITY);
        messageReaderThread.start();

        MessageWriter messageWriter = new MessageWriter(messageReader, clientListener);
        Thread messageWriterThread = new Thread(messageWriter);
        messageWriterThread.setPriority(Thread.NORM_PRIORITY);
        messageWriterThread.start();
    }
}

public class ClientListener implements Runnable {
    private static final int DEFAULT_PORT = 5000;

    private Set<Scanner> clientIn = Collections.synchronizedSet(
            new LinkedHashSet<Scanner>());
    private Set<PrintWriter> clientOut = Collections.synchronizedSet(
            new LinkedHashSet<PrintWriter>());

    public Set<Scanner> getClientIn() {
        return clientIn;
    }

    public Set<PrintWriter> getClientOut() {
        return clientOut;
    }

    @Override
    public void run() {
        try {
            ServerSocket server = new ServerSocket(DEFAULT_PORT);
            System.out.println("Listening for connections...");
            int connectionNum = 0;

            while(true) {
                Socket socket = server.accept();
                connectionNum++;
                System.out.format("Connection %s%n", connectionNum);

                Scanner in = new Scanner(socket.getInputStream());
                PrintWriter out = new PrintWriter(socket.getOutputStream());
                clientIn.add(in);
                clientOut.add(out);
            }

        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}


public class MessageReader implements Runnable {
    private ClientListener clientListener;
    private BlockingQueue<String> messages = new SynchronousQueue<String>();

    public MessageReader(ClientListener clientListener) {
        this.clientListener = clientListener;
    }

    @Override
    public void run() {
        while(true) {
            Set<Scanner> clients = clientListener.getClientIn();
            synchronized (clients) {
                for(Scanner client: clients) {
                    if(client.hasNext()) {
                        try {
                            messages.put(client.next());
                        } catch (InterruptedException e) {
                            e.printStackTrace();
                        }
                    }
                }
            }
        }
    }

    public String getMessage() throws InterruptedException {
        return messages.take();
    }
}


public class MessageWriter implements Runnable {
    private ClientListener clientListener;
    private MessageReader messageReader;

    public MessageWriter(
            MessageReader messageReader, 
            ClientListener clientListener) {
        this.messageReader = messageReader;
        this.clientListener = clientListener;
    }

    @Override
    public void run() {
        try {

            while(true) {
                String message = messageReader.getMessage();

                Set<PrintWriter> clients = clientListener.getClientOut();
                synchronized (clients) {
                    for(PrintWriter client: clients) {
                        client.println(message);
                    }
                }
            }

        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

Answer 1

I'm not a threading expert, but in class MessageReader there is this line

if(client.hasNext())

Javadoc for Scanner.hasNext() say's "This method may block while waiting for input to scan. The scanner does not advance past any input."

If the scanner is still in wait the synchronized method never proceeds and block all other inputs. And as said in my earlier comment the line which says clientIn.add(in); in class ClientListener probably gets blocked given that its a synchronized Set, but since the print statment is written before it, it might give the impression that Connection 2 was succesfully established.