Hi Tim,

According to my understanding, unfortunately, Guava MapMaker has a limitation which makes it unusable in this instance:

Note: by default, the returned map uses equality comparisons (the equals method) to determine equality for keys or values. However, if weakKeys() or softKeys() was specified, the map uses identity (==) comparisons instead for keys.

Not sure why they have that limitation. I would want to use .equals for soft keys.

Thanks!

On Sun, Jun 26, 2011 at 6:29 PM, Tim Peierls <tim <at> peierls.net> wrote:

In this case, consider using Guava MapMaker. The example in the javadoc is close to what you're looking for.

--tim

On Sun, Jun 26, 2011 at 6:30 AM, Sanchay Harneja <sanchay.h <at> gmail.com> wrote:

Suppose I have a pure method f, of 2 arguments, a1, a2. Now, it takes a long time to compute f(a1,a2), so I want to memoize it, but at the same time want to make it memory sensitive, i.e., remove memoized entries to reclaim memory if required. So keys as SoftReferences come to mind. Another requirement is that this memoized cache should be thread safe, many threads will be writing and reading from it. With all this in mind, I have the following questions:

1.  Is extra166y.CustomConcurrentHashMap stable enough to use in production? Or shall I go with org.apache.commons.collections.map.ReferenceMap (and use Collections.SynchronizedMap)? Or shall I consider writing my own implementation.

2. Assuming CustomConcurrentHashMap, I have the following pattern in mind,

class Context{

  a1;

  a2;

  // equal and hashcode based on both a1,a2

}

CustomConcurrentHashMap<Context,V> cache = new CustomConcurrentHashMap<Context,V>(CustomConcurrentHashMap.Strength.SOFT, CustomConcurrentHashMap.Equivalence.EQUALS, CustomConcurrentHashMap.Strength.STRONG, CustomConcurrentHashMap.Equivalence.EQUALS, 0);

V f(a1,a2){

  Context ctx = new Context(a1,a2); //.......... 1.

  V ret = cache.get(ctx);                  //.......... 2.

  if (ret != null)                                 //.......... 3.

    return ret;                                   //.......... 4.

  // actually compute f(a1,a2);          //........... 5.

  ret.putIfAbsent(ctx, val);                //........... 6.

  return val;                                    //........... 7.

}

I think this pattern does the job, what do you guys suggest ? Is there a more idiomatic way to doing memoization in Java ? Line 2. above relies on the assumption that f(a1,a2) will never be null, which is fine, but can this assumption be somehow avoided? Also at line 6. above I don't strictly need putIfAbsent, should I just go for put (if it is cheaper) ?

3. Suppose thread t1 is busy computing f(5,10), meanwhile another thread t2 calls f(5,10), what can be done so that t2 waits for t1's computation to finish and then just takes the value from the cache ? Note it is possible that by the time t2 checks again, (5,10) is garbage collected and some t3 starts computing (5,10), so it would require a loop.

Thanks!

Sanchay

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