Question

I create a class that allows me to open a single instance of my Java program. It uses a daemon thread that open a ServerSocket. if the TCP Port was already taken throws an exception at instantiation time.

The code works normally under linux and windows.

Here is the code i am using:

public class SingleInstaceHandler extends Thread {

    private static final Logger log = Logger.getLogger(IEPLC_Tool.class);
    private boolean finished = false;

    @SuppressWarnings("unused")
    private ServerSocket serverSocket;

    /*
     * Constructor
     * Generate the server socket.
     * If the TCP door was busy throws IOException.
     */
    public SingleInstaceHandler() throws IOException {
        @SuppressWarnings("unused")
        ServerSocket serverSocket = new ServerSocket(44331);
        this.setDaemon(true);
        this.start();
        log.info("Server socket initialized"); //if commented out it works
    }

    public void run() {
        synchronized (this) {
            while (!finished) {
                try {
                    log.debug("Server socket goes to sleep");
                    this.wait();
                    log.debug("Server socket waken up");
                } catch (InterruptedException e) {
                    log.debug("ERROR while sending SocketThread2 in wait status");
                    e.printStackTrace();
                    System.exit(-1);
                }
                log.info("Server socket end");
            }
        }
    }

    public synchronized void shutdown() {
        log.debug("SingleInstaceHandler shutdown() caled");
        finished = true;
        notifyAll();
    }
}

Sometimes instead the port is not kept busy… any idea?

---

UPPENDED AFTER FURTHER TESTS:

running many other tests. it seams that if the port is taken by something like another SW instance new ServerSocket(44331); throws an exception but sometimes even if the port is not taken for some reason it can not get this resource. in this case no exception is launched and i can open as many instance as i want of my application. maybe i should do some other operation to force the thread to lock the port…

any idea?

Thanks,

Ste

Answer 1

I do feel a bit stupid in posting an answer to my question… well.. my code has a bug that took me long to figure out:

the problem is that in te constructor i do:

ServerSocket serverSocket = new ServerSocket(44331);

instead of:

this.serverSocket = new ServerSocket(44331);

I did not notice it before… basicaly the bug was that I was declaring a local socket within the constructor. When the constructur procedure was terminated the socket was released or not depending on the Garbadge Collector. Behavior was quite random. Funny part was that plenty of time calling/not calling the logger was enougth to make the Garbadge collector starting or not. It took me quite a while in noticing the mistake.

furthermore it’s better to put a :

this.serverSocket.close() after the wait.

Thans for helping anyway!

Cheers,

Ste