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From: Tom Anderson <twic@urchin.earth.li>
Newsgroups: comp.lang.java.programmer
Subject: Re: Making one or more threads wait for another to produce a value or fail
Date: Wed, 1 Jun 2011 22:07:22 +0100
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On Tue, 31 May 2011, Peter Duniho wrote:

> On 5/31/11 8:46 AM, Tom Anderson wrote:
>> 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;
>
> Spurious wakeup, yes (why people continue to tolerate that in Java, I have no 
> idea…plenty of other APIs with concurrency support don't have that trouble).
>
> Happens-before/after, should come directly from a correct use of the wait() 
> and notifyAll() methods.

Or volatile. Working out when you can use each is one of the challenges.

> In particular, as long as you ensure that the writer doesn't get to 
> arrive at its "synchronized" statement in which it will call notify() 
> until after all the readers have arrived at their wait() statement, 
> you're assured all the readers will see the new value.

That is true, but that's a more stringent requirement than is needed: the 
happens-before relationship is between the writer's lock release, and the 
reader's lock acquisition. The reader's prospective wait must be inside a 
synchronized block, so even if a reader arrives at its wait statement long 
after the writer has called notify() (and unlocked) its lock acquisition 
will mean the writer's actions will have happened-before that moment.

I imagine you knew that, of course, and we're talking slightly at 
cross-purposes.

> Alternatively, maintain a separate "do I really need to wait?" flag that 
> is cleared when the delivered value is set, so that readers don't bother 
> waiting if the value's already been updated.

Yes. Which you need anyway, to deal with the spurious wakeups!

tom

-- 
That must be one of the best things you can possibly do with a piglet,
booze and a cannon. -- D
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