JAVA ServerSocket how to synchronize multiple incoming connections

Question

I have a server application that may receives incoming connections from multiple clients at same time. It is used to receive data from the internet and send it to a POS Printer locally. Here is the code:

public static void main(String args[]) {
    RFServer2 server = new RFServer2();
    while (true) {
        server.run();
    }
}

public RFServer2() {
}

public synchronized void run() {

    try {
        while (printing)
            wait();

        printing = true;
        // 1. creating a server socket
        providerSocket = new ServerSocket(43520);

        // 2. Wait for connection
        System.out.println("Waiting connection...");
        connection = providerSocket.accept();
        System.out.println("Connection received from " + connection.getInetAddress().getHostName());
        connection.setSoTimeout(8000);

        // 3. get Input and Output streams
        out = new ObjectOutputStream(connection.getOutputStream());
        out.flush();
        try {
            in = new ObjectInputStream(connection.getInputStream());
        } catch (Exception e) {
            throw new Exception("Weird connection received, could not read, aborting...");
        }
        // sendMessage("connected");

        // 4. The two parts communicate via the input and output streams

        try {
            message = (String) in.readObject();

            if (message.contains(".")) {
                Socket printerSocket = new Socket(message, 9100);
                printerSocket.setSoTimeout(3000);
                printer = printerSocket.getOutputStream();
                printer.flush();
                sendMessage("connected");

                while (!message.contentEquals("done")) {

                    try {
                        message = (String) in.readObject();
                        if (!message.contentEquals("done")) {
                            printer.write(message.getBytes("UTF-8"));
                            printer.flush();
                        }
                    } catch (Exception e) {
                        System.err.println("Failed to print.");
                        e.printStackTrace();
                        break;
                    }
                }
                this.message = "";
catch (Exception e) {
        System.err.println(e.getMessage());
    } finally {
        // 5: Closing connection
        try {
            this.message = "";
            if (printer != null) {
                System.out.println("Closing connection to the printer...");
                printer.close();
            }
            if (in != null) {
                System.out.println("Closing input...");
                in.close();
            }
            if (out != null) {
                System.out.println("Closing output...");
                out.close();
            }
            System.out.println(new Date() + " - letting server listening...");

            if (providerSocket != null)
                providerSocket.close();

            printing = false;
            notify();
        } catch (Exception ioException) {
            ioException.printStackTrace();
        }
    }
}

All right, if I send multiple connections from the same client (eg. my machine), the application handles the threads with the wait() and notify(), synchronizing them. But if I send 2 connections at same time from 2 different computers, My server got stuck at "Connection received from host", and the two clients drop due to timeout.

All I need is to solve this little problem...I've read those threads, but was unsuccessful to solve my problem

JAVA threads (different stacks) synchronization

Java thread simple queue

Stopping a ServerSocket accept() loop thread

Answer 1

First you program is running in single thread.

For what I can see in your code, your problem is you reset you server for each request/client you attend.

You should use something like this:

        ServerSocket serverSocket = new ServerSocket(port);
        while (true) {
            Socket client = serverSocket.accept();

            //Do your work

            client.close();
        }

In your case I think you will not need multi threading, but if u want to use, delegate client socket to another thread an accept another request...

If you want to use threads, I suggest you to create a thread pool, and create a mutex for exclusive access to your printer. Some useful links: thread pool and mutex