Will `CoroutineExceptionHandler` never handle `CancellationException` That seems to be the case see example program in thread but I can t find a clear authoritative statement on that this is the case For example docs for <https kotlin github io kotlinx coroutines kotlinx coroutines core kotlinx coroutines coroutine exception handler |interface CoroutineExceptionHandler> states gt when no handler is installed <https kotlin github io kotlinx coroutines kotlinx coroutines core kotlinx coroutines cancellation exception index html|CancellationException> is ignored rather than <https kotlin github io kotlinx coroutines kotlinx coroutines core kotlinx coroutines cancellation exception index html|CancellationException> is always ignored with<https kotlin github io kotlinx coroutines kotlinx coroutines core kotlinx coroutines coroutine exception handler | CoroutineExceptionHandler>

Yes it will never handle it <https link medium com WPEB31h7F5|https link medium com WPEB31h7F5>

Thanks but in what way is that an authorative source thinking face gt Let s examine the rules That article seems to do what I already did deduce rules by experimentation

It is not an authorative source it s just me slightly smiling face but I looked at the official kotlin documentation and started experimenting from there

Will `CoroutineExceptionHandler` never handle `CancellationE kotlinlang #coroutines

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Title

Martin Nordholts

04/14/2020, 10:54 AM

Will

CoroutineExceptionHandler

never handle

CancellationException

? That seems to be the case (see example program in thread) but I can’t find a clear authoritative statement on that this is the case. For example, docs for interface CoroutineExceptionHandler states

when no handler is installed […] CancellationException […] is ignored

rather than “CancellationException is always ignored with CoroutineExceptionHandler”

➕ 1

fun main() = runBlocking {
    val scope =
        CoroutineScope(Job() + CoroutineExceptionHandler { _: CoroutineContext, throwable: Throwable ->
            println("handled  $throwable")
        })

    val job = scope.launch {
        try {
            throw CancellationException()
        } catch (e: CancellationException) {
            println("cancelled, rethrowing");
            throw e
        }
    }

    job.join()
    println("program completed")
}

but “handled …” is not printed (But if I change to

throw ArithmeticException()

then it is indeed printed)

streetsofboston

04/14/2020, 12:53 PM

Yes, it will never handle it. https://link.medium.com/WPEB31h7F5

Martin Nordholts

04/14/2020, 1:03 PM

Thanks, but in what way is that an authorative source? 🤔

Let’s examine the rules

That article seems to do what I already did; deduce rules by experimentation

I looked at the coroutine implementation and found this:

if (finalException != null) {
            val handled = cancelParent(finalException) || handleJobException(finalException)
            if (handled) (finalState as CompletedExceptionally).makeHandled()
        }

and inside cancelParent() we can find (among other things)

isCancellation = cause is CancellationException

and by following the code a bit more this is to me authorative enough…

streetsofboston

04/14/2020, 4:33 PM

It is not an authorative source (it’s just me 🙂 ), but I looked at the official kotlin documentation and started experimenting from there.

3 Views

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