Will `CoroutineExceptionHandler` never handle `CancellationException` ? That seems to be the case (s...

# coroutines

[Martin Nordholts]

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”

[Martin Nordholts]

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]

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

[Martin Nordholts]

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

[Martin Nordholts]

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]

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