Question

I want to make a method that safely stops a thread running in a loop, allowing for the last loop to finish before returning control to the stopper.

Right now, no matter what I try, I freeze. Possibly out of deadlocks or whatnot; Java is not my usual environment, hence why this may be yet-another wait/notify question.

boolean isRunning = true;

@Override
public void run() {
    super.run();

    while (isRunning) {
        // Do work...
    }

    synchronized(this) {
        this.notify();
    }
}

public void stopSafely() {
    isRunning = false;

    try {
        synchronized(this) {
            this.wait();
        }
    } catch (InterruptedException ex) {
        // Handle...
    }
}

The problem with this approach (apart from the fact that I synchronize on this, but it’s for the sake of example simplicity), is that if notify gets called before wait, the caller will freeze.

I’m sure that playing with the blocks I surround in synchronized could fix the problem, but I can’t seem to get the right combination.

Any idea?

Answer 1

Just go for the real simple solution:

private volatile boolean isRunning = true;

@Override
public void run() {
    while (isRunning) {
        // Do work...
    }
}

public void stopThread() {
    isRunning = false;
}

that’s basically what Thread.interrupted() does internally, so you can just use this as well:

@Override
public void run() {
    while (Thread.interrupted()) {
        // Do work...
    }
}

in which case you have to call interrupt() on the thread.