[concurrency-interest] How to implement a self populating memoizer cache?

fabo wang wangfabo1986 at gmail.com
Wed Nov 3 21:10:20 EDT 2010


Chapter 5,section 6.the book 《java concurrency in practic》

2010/11/4 Kevin Bourrillion <kevinb at google.com>

> Actually, I think he's looking at the GWT version of MapMaker, which is
> Java code that is meant to be converted to Javascript and run --
> single-threaded! -- in the browser.  Confusing, I know.
>
>
>
> On Wed, Nov 3, 2010 at 1:59 PM, Charles Fry <fry at google.com> wrote:
>
>> It looks like you're looking at an old version of MapMaker. Check out the
>> version in the guava project:
>>
>>
>> http://code.google.com/p/guava-libraries/source/browse/trunk/src/com/google/common/collect/ComputingConcurrentHashMap.java#76
>>
>> Charles
>>
>> On Wed, Nov 3, 2010 at 16:47, Nader Aeinehchi <nader at aeinehchi.com>wrote:
>>
>>>  Dear Kevin,
>>>
>>> If I understand you correctly, MapMaker.ExpiringComputingMap (which I
>>> believe you are referring to) is thread safe.  Furthermore, it guarantees
>>> that threads will wait until the first thread computes for a given key.
>>>
>>> Following is an excerpt of MapMaker.ExpiringComputingMap.  Here, if the
>>> result is null, the compute method is called and the calculated value is
>>> put.  How is it guaranteed that another thread does not concurrently perform
>>> the latter (calculation and put operations)?  Is not this a
>>> read-modify-write scheme which according to JCiP book is a compound (not
>>> atomic) operation?
>>>
>>>
>>> In advance, thank you very much.
>>>
>>>
>>>     @Override
>>>     public V get(Object k) {
>>>       // from CustomConcurrentHashMap
>>>       V result = super.get(k);
>>>       if (result == null && computer != null) {
>>>         /*
>>>          * This cast isn't safe, but we can rely on the fact that K is
>>> almost
>>>          * always passed to Map.get(), and tools like IDEs and Findbugs
>>> can
>>>          * catch situations where this isn't the case.
>>>          *
>>>          * The alternative is to add an overloaded method, but the
>>> chances of
>>>          * a user calling get() instead of the new API and the risks
>>> inherent
>>>          * in adding a new API outweigh this little hole.
>>>          */
>>>         @SuppressWarnings("unchecked")
>>>         K key = (K) k;
>>>         result = compute(key);
>>>       }
>>>       return result;
>>>     }
>>>
>>>     private V compute(K key) {
>>>       // from MapMaker
>>>       V value;
>>>       try {
>>>         value = computer.apply(key);
>>>       } catch (Throwable t) {
>>>         throw new ComputationException(t);
>>>       }
>>>
>>>       if (value == null) {
>>>         String message = computer + " returned null for key " + key +
>>> ".";
>>>         throw new NullPointerException(message);
>>>       }
>>>       put(key, value);
>>>       return value;
>>>
>>>     }
>>>
>>>
>>>
>>> On 11/03/2010 03:15 PM, Kevin Bourrillion wrote:
>>>
>>> On Wed, Nov 3, 2010 at 5:31 AM, Nader Aeinehchi <nader at aeinehchi.com>wrote:
>>>
>>>  1. Please forgive me for my ignorance, but ExpiringComputingMap does
>>>> not guarantee that a computable to be calculated only once for concurrent
>>>> threads?
>>>>
>>>
>>>  "Map.get(java.lang.Object)<http://java.sun.com/javase/6/docs/api/java/util/Map.html?is-external=true#get%28java.lang.Object%29>
>>>  either returns an already-computed value for the given key, atomically
>>> computes it using the supplied function, or, if another thread is currently
>>> computing the value for this key, simply waits for that thread to finish and
>>> returns its computed value. Note that the function may be executed
>>> concurrently by multiple threads, but only for distinct keys."
>>>
>>>
>>>  --
>>> Kevin Bourrillion @ Google
>>> http://guava-libraries.googlecode.com
>>>
>>>
>>> _______________________________________________
>>> Concurrency-interest mailing list
>>> Concurrency-interest at cs.oswego.edu
>>> http://cs.oswego.edu/mailman/listinfo/concurrency-interest
>>>
>>>
>>
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>
>
> --
> Kevin Bourrillion @ Google
> http://guava-libraries.googlecode.com
>
>
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>


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