Is an "atomic" interrupt check possible in java?

Question

If using following "idiom" with interruption in Java, for example from this answer.

while (!Thread.currentThread().isInterrupted()) {
    try {
        Object value = queue.take();
        handle(value);
    } catch (InterruptedException e) {
        Thread.currentThread().interrupt();
    }
}

Where take is a blocking operation, can an interrupt not be ignored for the time being if an interrupt "arrives" between the check of Thread.currentThread().isInterrupted() and the call queue.take()? Is this not a "check-than-act" operation? If so, can it somehow be guaranteed that the loop is left in any case if the thread is interrupted?

It is possible to use poll with a timeout so that the loop is left after the timeout, but is it possible to check the interrupted status and act on it atomically?

How could an interrupt ever be missed by the above code? You realize that queue.take() will throw immediately if the thread is interrupted before the call to take(), right? — JB Nizet, Jun 10 '16 at 07:35
@JBNizet: I have checked the code, and you are right, methods throwing `InterruptedException` check for the interupted status first and throw. If you add an answer, I'll accept it. — user140547, Jun 10 '16 at 08:13

score 3 · Answer 1 · answered Jun 10 '16 at 07:33

3

I would swap the try/catch and while loop:

try {
  while (true) {
    Object value = queue.take();
    handle(value);
  }
} catch (InterruptedException e) {
  Thread.currentThread().interrupt();
}

The take() operation will throw an InterruptedException immediately if the thread is interrupted, and at the same time break out of the while loop.

answered Jun 10 '16 at 07:33

Andy Turner

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In the interrupt comes during handling, the exception may not be thrown. So - for sake of to be sure, I usually use while (!Thread.currentThread().isInterrupted()) – gusto2 Jun 10 '16 at 07:38
@GabrielVince Please explain the circumstances under "the exception may not be thrown". – Andy Turner Jun 10 '16 at 07:42
in case the "handle(..)" method is doing some computations, not necessarily waiting for I/O and the thread is interrupted, The exception is not thrown. I'd check it in the loop. I wouldn't assume the blocking I/O will throw an exception when the thread is entering with already interrupted state (though the comment from JB.Nizet states otherwise and I may be wrong here). – gusto2 Jun 10 '16 at 07:51
@GabrielVince The `handle()` function appears to be defined in the same class that you are editing. It will throw an `InterrutpedException` if you change it to check the flag and throw the exception. – Solomon Slow Jun 10 '16 at 13:57

score 0 · Answer 2 · answered Jun 10 '16 at 07:42

0

Only a call could clear then interrupted flag so nothing could happend between isInterrupted and queue.take().

answered Jun 10 '16 at 07:42

robinr

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score 0 · Answer 3 · answered Jun 10 '16 at 07:45

but is it possible to check the interrupted status and act on it atomically

Well - I don't know what you mean "atomically" here. May we assume you want something like onInterrupt(...)?

Interrupts are meant to "interrupt" the thread, so all default I/O operations throw an InterruptedException you can catch or check. It give the threads opportunity to stop gracefully closing / releasing any locked resources.

As event handling you may want to implement a Cancellable task, where you could handle your own cancel events (well, not the default JRE interrupts).

Is an "atomic" interrupt check possible in java?

3 Answers3

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