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

David M. Lloyd david.lloyd at redhat.com
Mon Apr 23 13:33:46 EDT 2012


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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>>>
>>>
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>>
>>
>> --
>> - DML
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>


-- 
- DML


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