Where to catch InterruptedException for Object.wait() with synchronized block?

Question

As I understand this is very common snippet for multithreading in Java.

boolean loaded = false;
Object lock = new Object();

public void block() {
    synchronized (lock) {
        while(!loaded)
            lock.wait(); // Unhandled exception type InterruptedException
    }
}

and in another thread

public void done () {
    synchronized (lock) {
        loaded = true;
        lock.notify();
    }
}

But I am not sure where I should put try and catch, I can surround whole synchronized block or only lock.wait()? What is rule of thumb and does it really matter?

For handling is it okay to call Thread.currentThread().interrupt() in this situation?

Solution 1

You might want to consider the article by Brian Goetz: Java theory and practice: Dealing with InterruptedException

In general, if you don't know what to do then don't do anything but let a caller code know that this method can be interrupted while waiting on a lock. You can do this by adding throws InterruptedException to a method signature.

If for some reason you are forced to add a try/catch block and handle the exception then Thread.currentThread().interrupt() will do but re-throwing it can be as suitable.

Solution 2

But I am not sure where I should put try and catch, I can surround whole synchronized block or only lock.wait()? What is rule of thumb and does it really matter?

lock.wait() is the only line that can throw an exception in your block method. So whether you put your try/catch around the wait or include the whole method body does not make a difference from an execution perspective (assuming you exit the loop in both cases of course - if you want to continue looping then the try/catch block should obviously be within the loop).

In other words it is only a matter of style - see this other discussion. I personally think this is more readable:

try {
    synchronized (lock) {
        while (!loaded) {
            lock.wait(); // Unhandled exception type InterruptedException
        }
    }
} catch (InterruptedException e) {
    //to ignore me or not to ignore me, that is the question
    Thread.currentThread().interrupt();
}

For handling is it okay to call Thread.currentThread().interrupt() in this situation?

In the first example below, it is okay to catch the InterruptedException and exit your loop without re-interrupting the thread, because in any case the run method will exit and the thread will die:

new Thread(new Runnable() {

    @Override
    public void run() {
        block();
    }
}).start();

In the second example below, you certainly should re-interrupt the thread to let the run method know whether the reason why block() exited is that loaded became true or that it got interrupted:

public void run() {
    while(!Thread.currentThread().isInterrupted()) {
        loaded = false;
        System.out.println("Launching data load");
        block(); //if false has become true, loop, if interrupted, exit
    }
}

If you don't know how the method will be used, good manners suggest that you should re-interrupt the thread.

Comments

[Nikolay Kuznetsov]

Yes, I have bookmarked this article before writing the question. But my main question is in bold.

[Boris Treukhov]

@NikolayKuznetsov it does not really matter, the changes in the program flow caused by exception have nothing to do with multithreading. What's important is that the interruption status should not be lost. But I'm not sure that putting checked exception on all methods in the stack is the best solution. P.S. Another thing that calling notify() instead of notifyAll() always requires some reasoning about safety.

[android developer]

I think this tutorial about notifying thread is nicer : tutorials.jenkov.com/java-concurrency/thread-signaling.html