Re: JCiP Memoizer

On 10/18/06, Tim Peierls <tim@xxxxxxxxxxx> wrote:
> It's only 1am here, but it looks funny to me, too. I remember that the
> intent was that a thread interrupted in the middle of a long computation
> should not leave the incomplete computation lying around in the cache. So
> any thread that detects a cancellation removes the task from the cache and
> tries again.
>
> But an interruption during ft.run() will not be seen as a cancellation of
> the FutureTask, so this doesn't work. The fix would probably involve
> checking for interruption after the call to ft.run().
>
>  I just read Joe's response, but I don't see how a Memoizer-created
> FutureTask, encapsulated as it is, could ever be cancelled. (Also, I don't
> think cache.remove(arg, f) ensures that only the task creator removes the
> task; it just ensures that the only the failed task is removed.)
>

(a bit later: early in the morning): Things are starting to become
more clear. I don't think there is any way a FutureTask can be
cancelled (cause I don't see it published). Also, I am not sure how
can you reach a CancellationException (for the same reasons).

Another question related to the same code would be: what if the
compute method must not perpetuate the InterruptedException? Wouldn't
this qualify for the cache.remove call? (because as far as I get it,
if the expensive computation resulted in an InterruptedException, then
you will never get the change to compute the real value.

Sorry if I ask wrong questions, but the code looks pretty interesting
(and I am doing an attempt to further detail it to my collegues, that
may need to use something similar).

./alex
--
.w( the_mindstorm )p.



> --tim
>
>
>
> On 10/18/06, Joe Bowbeer < joe.bowbeer@xxxxxxxxx> wrote:
> > On 10/17/06, Alexandru Popescu <
> the.mindstorm.mailinglist@xxxxxxxxx> wrote:
> > >
> > > I would start by saying that till now Chapter 5 is my favorite so far
> > > :">. Still, I have a small problem (indeed it is 5am) understanding
> > > the final version of Memoizer (page 108), or at least one line from
> > > it:
> > >
> > > public V compute(final A arg) throws InterruptedException {
> > >    while(true) {
> > >             // code
> > >     }
> > > }
> > >
> >
> > It's because of this line further down:
> >
> >             } catch (CancellationException e) {
> >                 cache.remove(arg, f);
> >
> > from http://jcip.net/listings/Memoizer.java
> >
> > Here's a partial explanation:
> >
> > The memoizing compute method is trying to reflect the exception from
> > the wrapped method back to the caller, but it also has to handle the
> > case where the task may have been cancelled (somehow).
> >
> > In this case, the compute method removes its task from the cache (iff
> > it created the task) and tries again.
> >
> > --Joe
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>
>