concurrency - Java long running task Thread interrupt vs cancel flag

Question

I have a long running task, something like:

public void myCancellableTask() {
    while ( someCondition ) {
       checkIfCancelRequested();
       doSomeWork();
    }
 }

The task can be cancelled (a cancel is requested and checkIfCancelRequested() checks the cancel flag). Generally when I write cancellable loops like this, I use a flag to indicate that a cancel has been requested. But, I know I could also use Thread.interrupt and check if the thread has been interrupted. I'm not sure which would be the preferred approach and why, thoughts?

thanks,

Jeff

Answer 1

One problem with using interrupt is that if you do not control all code being executed, you run the risk of the interrupt not working "properly" because of someone else's broken understanding of how to handle interrupts in their library. That is the API invisibly exports an API around its handling of interrupts which you become dependent on.

In your example, suppose doSomeWork was in a 3rd-party JAR and looks like:

public void doSomeWork() {
    try { 
        api.callAndWaitAnswer() ; 
    } 
    catch (InterruptedException e) { throw new AssertionError(); }
}

Now you have to handle an AssertionError (or whatever else the library you are using might throw). I've seen experienced developers throw all sorts of nonsense on receiving interrupts! On the other hand, maybe the method looked like this:

public void doSomeWork() {
    while (true) {
        try { 
            return api.callAndWaitAnswer() ; 
        } 
        catch (InterruptedException e) { /* retry! */ }
    }
}

This "improper handling" of interrupt causes your program to loop indefinitely. Again, don't dismiss this as ridiculous; there are a lot of broken interrupt handling mechanisms out there.

At least using your own flag will be completely invisible to any 3rd-party libraries.