Question

I was reading some java thread interruption and I don’t understand some stuff. Hope someone will explain me. So, it’s done the following code

public Integer getInteger(BlockingQueue<Integer> queue) {
    boolean interrupted = false;
    try {
        while (true) {
            try {
                return queue.take();
            } catch (InterruptedException e) {
                interrupted = true;
                // fall through and retry
            }
        }
    } finally {
        if (interrupted)
            Thread.currentThread().interrupt();
    }
}

The explanation is as follows :

Activities that do not support cancellation but still call
interruptible blocking methods will have to call them in a loop,
retrying when interruption is detected. In this case, they should save
the interruption status locally and restore it just before returning,
as shown in listing. rather than immediately upon catching
InterruptedException. Setting the interrupted status too early could
result in an infinite loop, because most interruptible blocking
methods check the interrupted status on entry and throw
InterruptedException immediately if it is set. (Interruptible methods
usually poll for interruption before blocking or doing any significant
work, so as to be as responsive to interruption as possible.)

I don’t understand why we should save the interrupted status locally.
I would be glad to hear some explanation.

Answer 1

By the design the method could not ever throw the InterruptedException. So it means we always expect take a value from the queue. But somebody could want the thread to be interrupted, that’s why we have to save-restore the interrupted state after we have finally took a value from the queue.

So, the thread get finished only after taking a value from the queue.

UPDATE: Look into the take() method implementation. It has the following as the first statements:

public final void acquireInterruptibly(int arg) throws InterruptedException {
    if (Thread.interrupted())
        throw new InterruptedException();
...
}