[concurrency-interest] Proper workaround for FutureTask.set()/get() race (CR 7132378)

Kirk Pepperdine kirk at kodewerk.com
Mon Apr 23 15:51:18 EDT 2012


what makes this case exceptional?

On 2012-04-23, at 7:33 PM, David M. Lloyd wrote:

> If you call putIfAbsent but the key is already mapped, then the put fails and the old value is returned instead.
> 
> On 04/23/2012 12:31 PM, Kirk Pepperdine wrote:
>> define failure for putIfAbsent???
>> 
>> 
>> On 2012-04-23, at 7:10 PM, David M. Lloyd wrote:
>> 
>>> I disagree.  May times using set() on an object like this is the point of coordination.  Would you throw an exception if putIfAbsent() failed?
>>> 
>>> On 04/23/2012 11:37 AM, Nathan Reynolds wrote:
>>>> Consider throwing an exception instead of returning false. The exception
>>>> will force the caller to deal with the already set condition. Returning
>>>> false is a bug waiting to happen.
>>>> 
>>>> Calling set() twice usually indicates a larger bug in the caller code.
>>>> Why would the algorithm call it twice? Is there a race between 2 threads
>>>> in the caller code? Is one part of the caller code not aware of what the
>>>> other part of code did?
>>>> 
>>>> I raise this concern because FindBugs flags issues if the caller doesn't
>>>> deal with the result of the java.io.File and java.util.concurrent.Lock
>>>> APIs. For many that don't use FindBugs, they will have a lot of bugs.
>>>> 
>>>> Nathan Reynolds
>>>> <http://psr.us.oracle.com/wiki/index.php/User:Nathan_Reynolds>  |
>>>> Consulting Member of Technical Staff | 602.333.9091
>>>> Oracle PSR Engineering<http://psr.us.oracle.com/>  | Server Technology
>>>> 
>>>> On 4/21/2012 1:33 PM, Joe Bowbeer wrote:
>>>>> On Sat, Apr 21, 2012 at 12:08 PM, Aleksey Shipilev wrote:
>>>>> 
>>>>>    Yes. SettableFuture-like class is something missing from concurrent
>>>>>    classes I'm redoing over and over again in most of the projects.
>>>>>    Implementing it directly on top of AQS might provide some benefits
>>>>>    comparing to extending from FutureTask? Oh wait, it smells like
>>>>>    another
>>>>>    API enhancement proposal? :)
>>>>> 
>>>>> 
>>>>> Alex,
>>>>> 
>>>>> Bill Pugh once suggested a separate concurrency abstraction: an
>>>>> externally settable FutureValue<V>, which supports the following methods:
>>>>> 
>>>>> V get() - Waits if necessary for the value to be set, and then returns
>>>>> the value.
>>>>> boolean isDone() - Returns true if the value is set
>>>>> boolean set(V v) - Sets the value if it was not already set.
>>>>> Returns true if the value was set by this call, false if it
>>>>> was set by another call.
>>>>> 
>>>>> Having set(v) return a boolean seems like a good idea. What do you think?
>>>>> 
>>>>> To this I would also add the other Future methods such as cancel(),
>>>>> plus setException(). In other words, I envision FutureValue as a
>>>>> Future with two additional methods: boolean set(v) and boolean
>>>>> setException(e).
>>>>> 
>>>>> Are there any other Future enhancements that you think are sorely
>>>>> needed in j.u.c.?
>>>>> 
>>>>> --Joe
>>>>> 
>>>>> 
>>>>> 
>>>>> _______________________________________________
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>>>>> Concurrency-interest at cs.oswego.edu
>>>>> http://cs.oswego.edu/mailman/listinfo/concurrency-interest
>>>> 
>>>> 
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>>> 
>>> --
>>> - DML
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>> 
> 
> 
> -- 
> - DML




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