Can anyone tell me what the expected behaviour of ...

# coroutines

[vngantk]

Can anyone tell me what the expected behaviour of the following code is:

fun main() = runBlocking {
    val job = Job()
    launch {
        delay(1000)
        job.completeExceptionally(IllegalStateException("Failed"))
    }
    try {
        job.join()
        println("foo")
    } catch (e: Exception) {
        println("bar")
        e.printStackTrace()
    }
}

[Dico]

It prints foo

[vngantk]

Why? I expect that the IllegalStateException would be caught and print "bar" and the stack trace.

[Dico]

I'm pretty sure join just waits until the job completes, if it's not already completed.

[Dico]

The exception never gets thrown

[Dico]

await from deferred will throw the exception though

[vngantk]

So, the behaviour is not orthogonal

[vngantk]

What if I do this:

fun main() = runBlocking {
    val job = launch {
        delay(1000)
        throw IllegalStateException("Failed")
    }
    try {
        job.join()
        println("foo")
    } catch (e: Exception) {
        println("bar")
        e.printStackTrace()
    }
}

[vngantk]

I have checked that for this case, the exception is thrown.

[vngantk]

The Job() method returns a CompletableJob object which implements the Job interface, and so the behaviour of the join() method of a CompletableJob should be the same as we would expect from a normal Job object.

[vngantk]

I am asking if this behaviour of the join() method of CompletableJob is implemented incorrectly or it is the correct behaviour intended by its author?

[Dico]

Are you sure it's not just the default exception handler printing the stack trace?

[Dico]

Oh, I read your issue

[Dico]

As per documentation, join doesn't throw

[Dico]

It just awaits completion for any reason.

[Dico]

await is a different function. Its behaviour is not tied.

[bdawg.io]

This snippet on the other-hand will print bar

fun main() = runBlocking {
    val deferred = async {
        delay(1000)
        throw IllegalStateException("Failed")
    }
    try {
        deferred.await()
        println("foo")
    } catch (e: Exception) {
        println("bar")
        e.printStackTrace()
    }
}