Improving the speed of Thread interrupt checking

[Jeroen Frijters]

Charles Oliver Nutter wrote:
> You need CAS because one form of the interrupt check clears it and
> another does not. So the get + check + set of interrupt status needs to
> be atomic, or another thread could jump in and change it during that
> process.

I believe Thread.interrupted() and Thread.isInterrupted() can both be implemented without a lock or CAS.

Here are correct implementations:

    private volatile boolean interruptPending;

    public boolean isInterrupted() {
        return interruptPending;
    }

    public static boolean interrupted() {
        Thread current = currentThread();
        if (!current.interruptPending) {
            return false;
        }
        current.interruptPending = false;
        return true;
    }

Any interrupts that happen before we clear the flag are duplicates that we can ignore and any that happen after are new ones that will be returned by a subsequent call. The key insight is that the interruptPending flag can be set by any thread, but it can only be cleared by the thread it applies to.

Regards,
Jeroen