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Re: Making one or more threads wait for another to produce a value or fail

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On Tue, 31 May 2011, Peter Duniho wrote:

> On 5/31/11 7:00 AM, Tom Anderson wrote:
>
>> A Future<Verdict> looks ideal - it provides synchronisation, and a 
>> value, and provides the same value to all requesters once it's 
>> delivered, and also handles failure and cancellation. But i don't see 
>> an easy way to make one for a simple value. There is FutureTask, but 
>> that seems more geared to wrapping Callable and Runnable.
>> 
>> Any suggestions?
>
> Personally, I'd just use the Object.wait() and Object.notifyAll() methods.

That probably is good enough. There are some subtleties: one has to be 
aware of the possibility of spurious wakeup, and guard the wait() with a 
suitable test; one has to think a bit about ensuring a happens-before 
relationship between the setting of the result or exception variables and 
their reading; there may be other things so subtle i haven't thought of 
them. The nice thing about classes in java.util.concurrent is that they 
come with an implicit contract that Doug Lea has already thought about the 
subtleties for you. I love it when he does that.

> I also don't see why FutureTask<Verdict> doesn't work for you (Future<V> 
> is just an interface…FutureTask<V> implements that interface);

FutureTask<Verdict> demands a Callable<Verdict>, which i'm not in a 
position to supply. I am currently trying an approach using a do-nothing 
Callable, but i am not happy about it.

> just because you have more processing to do after delivering the value, 
> that doesn't necessarily mean that processing has to occur in the same 
> thread, does it?

In my case, Penelope is actually a framework thread running an event loop. 
When the verdict arrives, it will be delivered to an event listener i 
supply, but there is no way to get from there to returning a value from a 
Callable.

Well, i suppose the listener could post the result to a BlockingQueue, 
from which it would be read by a thread in a Callable, which could then 
return it to its executor to fulfil a Future. But that seems a little 
baroque.

> (Alternatively, you could provide a custom implementation of Future<V> 
> that doesn't wrap a Runnable like FutureTask<V>, letting your thread 
> continue even after delivering the Future<V>…but such an implementation 
> would be more complicated than just using the wait()/notifyAll() 
> pattern,

Not *that* much more complicated, i don't think. Both varieties of get map 
on to calls to their corresponding variety of wait in a loop guarded by a 
call to isDone, and isDone maps on to a check on whether the verdict is 
delivered. cancel/isCancelled would require code over and above the 
simplest wait/notify implentation, but not a lot.

The thing that bugs me is that if this is so simple, and as generally 
useful as i think it is, why isn't it in the JDK already?

> so probably not worth the effort unless you expected to reuse the 
> implementation a lot).

Perhaps. Although implementing Future might be useful for documentation 
purposes, because it immediately makes it clear what many of the methods 
do.

> It would be helpful if you could elaborate on why neither of those 
> simple, straightforward approaches satisfy your goals.

I hope i've explained myself a bit better now. I think writing my own 
implementation of Future, as you suggest (well, as you imply - what you 
actually suggest is not writing my own implementation of Future), may be a 
good idea.

tom

-- 
Don't get me wrong, I'm a nutt case and proud of it, but I am also
safe. -- graham yukabuk.com

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Making one or more threads wait for another to produce a value or fail Tom Anderson <twic@urchin.earth.li> - 2011-05-31 15:00 +0100
  Re: Making one or more threads wait for another to produce a value or fail Peter Duniho <NpOeStPeAdM@NnOwSlPiAnMk.com> - 2011-05-31 07:14 -0700
    Re: Making one or more threads wait for another to produce a value or fail Tom Anderson <twic@urchin.earth.li> - 2011-05-31 16:46 +0100
      Re: Making one or more threads wait for another to produce a value or fail Deeyana <d.awlberg@hotmail.invalid> - 2011-05-31 19:23 +0000
        Re: Making one or more threads wait for another to produce a value or fail Tom Anderson <twic@urchin.earth.li> - 2011-06-01 22:12 +0100
      Re: Making one or more threads wait for another to produce a value or fail Peter Duniho <NpOeStPeAdM@NnOwSlPiAnMk.com> - 2011-05-31 20:45 -0700
        Re: Making one or more threads wait for another to produce a value or fail Patricia Shanahan <pats@acm.org> - 2011-05-31 21:37 -0700
          Re: Making one or more threads wait for another to produce a value or fail Peter Duniho <NpOeStPeAdM@NnOwSlPiAnMk.com> - 2011-06-01 07:18 -0700
            Re: Making one or more threads wait for another to produce a value or fail Patricia Shanahan <pats@acm.org> - 2011-06-01 09:01 -0700
        Re: Making one or more threads wait for another to produce a value or fail Lawrence D'Oliveiro <ldo@geek-central.gen.new_zealand> - 2011-06-01 16:40 +1200
          Re: Making one or more threads wait for another to produce a value or fail "John B. Matthews" <nospam@nospam.invalid> - 2011-06-01 11:22 -0400
            Re: Making one or more threads wait for another to produce a value or fail Lawrence D'Oliveiro <ldo@geek-central.gen.new_zealand> - 2011-06-02 10:15 +1200
              Re: Making one or more threads wait for another to produce a value or fail "John B. Matthews" <nospam@nospam.invalid> - 2011-06-01 21:38 -0400
        Re: Making one or more threads wait for another to produce a value or fail Tom Anderson <twic@urchin.earth.li> - 2011-06-01 22:07 +0100
          Re: Making one or more threads wait for another to produce a value or fail Peter Duniho <NpOeStPeAdM@NnOwSlPiAnMk.com> - 2011-06-01 22:46 -0700
  Re: Making one or more threads wait for another to produce a value or fail markspace <-@.> - 2011-05-31 08:26 -0700
    Re: Making one or more threads wait for another to produce a value or fail Tom Anderson <twic@urchin.earth.li> - 2011-05-31 19:13 +0100
  Re: Making one or more threads wait for another to produce a value or fail Patricia Shanahan <pats@acm.org> - 2011-05-31 09:06 -0700
    Re: Making one or more threads wait for another to produce a value or fail Paul Cager <paul.cager@googlemail.com> - 2011-05-31 10:08 -0700
      Re: Making one or more threads wait for another to produce a value or fail Patricia Shanahan <pats@acm.org> - 2011-05-31 10:39 -0700
        Re: Making one or more threads wait for another to produce a value or fail Paul Cager <paul.cager@googlemail.com> - 2011-05-31 16:24 -0700
  Re: Making one or more threads wait for another to produce a value or fail Lawrence D'Oliveiro <ldo@geek-central.gen.new_zealand> - 2011-06-01 12:33 +1200
    Re: Making one or more threads wait for another to produce a value or fail "John B. Matthews" <nospam@nospam.invalid> - 2011-06-01 00:38 -0400
  Re: Making one or more threads wait for another to produce a value or fail markspace <-@.> - 2011-06-02 14:10 -0700
    Re: Making one or more threads wait for another to produce a value or fail markspace <-@.> - 2011-06-02 14:25 -0700
    Re: Making one or more threads wait for another to produce a value or fail Patricia Shanahan <pats@acm.org> - 2011-06-02 14:43 -0700

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