Question

I have a problem with Observer-Pattern and deadlock using threads.

package observerDeadLock;

import java.util.Observable;

public class MyModel extends Observable {
Integer foo;

public MyModel() {
    foo = 0;
}

public void changeStatus(Integer newStatus) {
    foo = newStatus;
    notifyObservers(newStatus);     
}
}

---

package observerDeadLock;

public class Job extends Thread {
    public MyModel model;

    public Job(MyModel model) {
        super();

        this.model = model;
    }

    public void run() {
        prepareJob();
        runMyJob();
    }

    private void runMyJob() {
        // Some stuff
        Integer choice = 1;

        if (choice == 3) {
            return;
        }
        else if (choice == 2) {
            return;
        }
        else if (choice == 1) {                                     
            model.changeStatus(123);    // Set a particalar status that MyController receive as wrong!
                    // PROBLEM: The Controller listen the changeStatus(123) of notifyObserver of MyModel and call a join() because I want the thread join and quit()
            return; // <<- NEVER EXECUTED! join(timeout) isn't the solution IHMO...s
        }       

        return;
    }

    private void prepareJob() {
        // Do some stuff
    }

}

package observerDeadLock;

import java.util.Observable;
import java.util.Observer;

public class MyController implements Observer {
    private Job myJob;
    private MyModel model;

    public MyController() {

    }

    public void startJob() {
        model = new MyModel();
        model.addObserver(this);

        myJob = new Job(model);
        myJob.start();
    }

    @Override
    public void update(Observable o, Object arg) {
        if (o instanceof MyModel) {
            if (((Integer) arg) == 1) {     
                // do other stuff
            }
            else if (((Integer) arg) == 2) {        
                // do other stuff
            }
            else if (((Integer) arg) == 123) {      // 123 => Wrong state for myController, so must stop Job
                // Stop myJob!!!
                try {
                    //myJob.join(timeout); // isn' the solution IHMO

                    myJob.join();               // PROBLEM HERE!!! In job, the "return" statment is locked in changeStatus() -> myModel.notifyobserver() that lock here in JOIN();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }           
            }


        }
    }
}

What I can do to resolve that problem? I suppose that use Thread.join() than Thread.interrupt() is a bad practice…
Suggestion?

Thanks!

Answer 1

You’re apparently trying to join() the current thread (you can see this for yourself if you replace myJob.join() with System.out.println( Thread.currentThread() )), which isn’t a very good idea, as the thread will be stuck forever, or until someone else interrupts it from the outside.

Instead of join(), just call interrupt(), there’s nothing shameful about that. (Although whether it will have any effect depends on what else is in runMyJob())

Update: I can only assume that you edited out parts of your code for brevity, but if MyController really does nothing else if it encounters the exit value 123 than to stop the updating thread, then the whole construction is unnecessary. You should simply return from the runMyJob() method without setting anything and the thread will stop gracefully.