[concurrency-interest] memoizer cache

[Bob Lee]

MapMaker is just a few classes if you want to copy it out (MapMaker,
CustomConcurrentHashMap, ComputingConcurrentHashMap, and Function I think).
We used to do that with Guice.

I'm not sure if it supports #4 yet. When an entry expires, all clients
typically wait for the updated value. You could certainly compute a value
and put it in the map though--other clients would still be able to retrieve
the original value during the computation.

Bob

On Fri, Oct 21, 2011 at 7:39 AM, bhm <bheem at sbcglobal.net> wrote:

> I'm trying to build a cache for configuration objects for my projects-
> Requirements:
>  1 only one instance per key should be created, either because its absent
>   for a given key or its expired (in that case it will be recreated).
>  2 most common path asking for a value for a key should not block or
> synchronize.
>  3 when a value doesnt exists for a key, one thread should create the
> instance
>   of value and put in the cache, other threads asking for same key should
> wait
>   till thats complete.
>  4 if a value is expired, one thread should (re)create the instance of
> value
>   and other threads asking for same key (when one thread is recreating it)
>   should use expired instance of value and should not block.
>
> My code is largely derived from Christian's reply
>
> http://cs.oswego.edu/pipermail/concurrency-interest/2010-November/007469.html
>
> Please comment or suggest on following, Thanks for you help.
>
>    final ConcurrentMap<K, Object> store = new ConcurrentHashMap<K,
> Object>();
>
>    V get(K key) {
>        Object ret = store.get(key);
>
>        if (ret == null) {
>            CountDownLatch latch = new CountDownLatch(1);
>            ret = store.putIfAbsent(key, latch);
>            if (null == ret) {
>                ret = new Entry<V>(factory.create(key));
>                store.replace(key, ret);
>                latch.countDown();
>            }
>        }
>
>        if (ret instanceof CountDownLatch) {
>            try {((CountDownLatch) ret).await();} // till new value is
> created
>            catch (InterruptedException e) {throw new RuntimeException(e);}
>            ret = store.get(key); // get new value
>        } else {
>            Entry<V> entry = (Entry<V>) ret;
>
>            if (evictionpolicy != null && evictionpolicy.isExpired(entry.v))
> {
>                final AtomicInteger sync = entry.sync;
>
>                if (sync.compareAndSet(0, 1)) { // otherwise retrun old
> value
>                    try {
>                        entry = (Entry<V>) store.get(key); // get again
>                        if (evictionpolicy.isExpired(entry.v)) { //
> double-check
>                            Entry<V> newval = new
> Entry<V>(factory.create(key));
>                            store.replace(key, newval);
>                            ret = newval;
>                        }
>                    }
>                    catch (Throwable catchall) {
>                      catchall.printStackTrace(System.err);//return old
> value
>                    }
>                    finally {sync.set(0);/*unlock*/}
>                }
>            }
>        }
>
>        return ((Entry<V>) ret).v;
>    }
>
>    class Entry<V> {
>        final AtomicInteger sync = new AtomicInteger(0);
>        final V v;
>        Entry(V v) {this.v=v;}
>    }
>
>
> form other discussions about similar topics in the list, I understand there
> are
> libraries to do that (MapMaker etc.), I wanted something specific, small,
> just
> to slove the given problem and avoid generalization.
> I couldnt get jcp's Memoizer to work when cache entries could expire. Its
> good
> for lazy one time load.
>
> Thanks again.
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