Behaviour of fork() after scope closing (was "When should I use onFork?")

[David Alayachew]

Thanks for the context Alan and Viktor. I guess since Virtual Threads kick
off tasks so fast (due to their throughput), I never had a chance to
observe this behaviour.

But I think I get it -- once the scope closes, all future calls to fork()
fail, so onFork() simply doesn't get called.

That is a very important detail though. Hope you don't mind, but let me
change subject to make sure I understand this behaviour.

For example, let's say I use the anySuccessfulOrThrow Joiner. Then,
theoretically speaking, if I have enough tasks such that by the time the
scope closes, I am still feeding tasks into fork(), then my scope will
fail? Am I understanding that correctly?

On Sun, Aug 24, 2025, 7:21 AM Viktor Klang <viktor.klang at oracle.com> wrote:

> Not sure this applies to your specific situation, David, but for the other
> readers of this conversation I also want to raise the point that
> implementing a Joiner must be done in a thread-safe manner.
>
> Cheers,
> √
>
>
> *Viktor Klang*
> Software Architect, Java Platform Group
> Oracle
> ------------------------------
> *From:* loom-dev <loom-dev-retn at openjdk.org> on behalf of Alan Bateman <
> alan.bateman at oracle.com>
> *Sent:* Sunday, 24 August 2025 08:19
> *To:* David Alayachew <davidalayachew at gmail.com>
> *Cc:* loom-dev <loom-dev at openjdk.org>
> *Subject:* Re: When should I use onFork?
>
>
>
> On 23/08/2025 20:27, David Alayachew wrote:
> > Woah, very good to know. And thus, even when the scope is closed,
> > onFork will still be called is what you are saying?
> >
> > Thanks for this. Very useful.
> >
> No, fork is specified to throw IllegalStateException if closed. My
> comment is pointing out that if you cancel the scope then the result
> from any subtasks that are still executing does not contribute to the
> outcome. Once you cancel then onComplete for these "still running (and
> interrupted)" subtasks won't be called. To put it another way, if the
> outcome from join is something derived from all subtasks that were
> forked then you'll need to implement onFork. It may be more common that
> the outcome is derived from the result/exception from a subset of the
> subtasks, in which case it will be the onComplete method that you
> implement.
>
> -Alan
>
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