[concurrency-interest] We need to add blocking methods to CompletionStage!

Joe Bowbeer joe.bowbeer at gmail.com
Sun Sep 25 10:20:04 EDT 2016


This statement regarding what happens after cancel is called is correct:

"*After this method returns, subsequent calls to **isDone**() will always
return true*. Subsequent calls to isCancelled() will always return true if
this method returned true."

After cancel returns, the future is completed, hence isDone. If cancel
returns true, i.e. it was cancelled, then  isCancelled returns true. But,
for example if the future is already completed when cancel is called, then
cancel will return false and isCancelled will return false.

On Sep 25, 2016 6:49 AM, "David Holmes" <davidcholmes at aapt.net.au> wrote:

> I think that was meant to read “After this method returns _*true*_,
> subsequent calls …”
>
>
>
> David
>
>
>
> *From:* Concurrency-interest [mailto:concurrency-interest-
> bounces at cs.oswego.edu] *On Behalf Of *Viktor Klang
> *Sent:* Sunday, September 25, 2016 9:03 PM
> *To:* Martin Buchholz <martinrb at google.com>
> *Cc:* concurrency-interest <concurrency-interest at cs.oswego.edu>;
> core-libs-dev <core-libs-dev at openjdk.java.net>
> *Subject:* Re: [concurrency-interest] We need to add blocking methods to
> CompletionStage!
>
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> On Sun, Sep 25, 2016 at 12:34 PM, Viktor Klang <viktor.klang at gmail.com>
> wrote:
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> On Sat, Sep 24, 2016 at 10:41 PM, Martin Buchholz <martinrb at google.com>
> wrote:
>
> No one is suggesting we add cancel to CompletionStage - I agree that would
> break users, by making an immutable interface mutable.
>
>
>
> +10000
>
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> This also means that CompletionStage cannot extend Future.
>
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> +10000
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> I also would not want to have a toFuture method that would return a
> j.u.c.Future, because of misfit Future.cancel.
>
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> Would you mind elaborating here? According to the cancel method spec on
> Future it is completely fine for it to be a no-op which always returns
> false:
>
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> "This attempt will fail if the task has already completed, has already
> been cancelled, *or could not be cancelled for some other reason.*"
>
>
>
> Source: https://docs.oracle.com/javase/8/docs/api/java/util/
> concurrent/Future.html
>
>
>
> There's an interesting (read: weird) spec clause in cancel:
>
>
>
> "*After this method returns, subsequent calls to isDone() will always
> return true*. Subsequent calls to isCancelled() will always return true
> if this method returned true."
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>
> That clause is in contradiction with the previously quoted line.
>
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>   If we are adding cancel, then it will be to make a new interface, such
> as the suggested CancellableCompletionStage.
>
>
>
> I also agree that CompletionStage does *not* represent "a running task of
> sorts", j.u.c. Future specs are still confusing in that way due to
> FutureTask heritage.
>
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> +10000
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> On Thu, Sep 22, 2016 at 7:51 PM, James Roper <james at lightbend.com> wrote:
>
> For example, we often cache futures and return them from a libraries API,
> if a client could cancel a future, that would break everything else that
> received that future.
>
>
>
> On Fri, Sep 23, 2016 at 2:24 PM, Viktor Klang <viktor.klang at gmail.com>
> wrote:
>
> PPS: A misunderstanding is that CompletionStage represents a running task
> of sorts, one that can be cancelled etc. This is not the case.
>
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> --
>
> Cheers,
>
>>
>
>
>
>
> --
>
> Cheers,
>
>>
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