Interesting problem. I've heard this referred to as the "Multiton" pattern.
Dhanji.
On Fri, Oct 15, 2010 at 3:27 AM, Fred Faber <ffa...@faiser.com> wrote:
> Basically you have a factory that should return a singleton instance for
> each unique string value passed into it?
>
> FactoryProvider won't do this for you out of the box, as it will create a
> new instance per invocation (as you've illustrated).
>
> What I would do in this case is to decorate your factory with an
> implementation that controls singletonness:
>
> interface CacheFactory {
> // This can be named anything you want, not just "create"
> NamedCache create(String name);
> }
>
> class CachingCacheFactory implements CacheFactory {
>
> private final CacheFactory underlyingFactory;
>
> //
> http://guava-libraries.googlecode.com/svn/trunk/javadoc/com/google/common/collect/MapMaker.html
> private final Map<String, NamedCache> caches = new MapMaker()
> .makeComputingMap(new Function<String, NamedCache>(){
> @Override public NamedCache apply(String name) {
> return underlyingFactory.create(name);
> }
> });
>
> @Inject
> CachingCacheFactory {
> @AlwaysCreatesNewInstances CacheFactory cacheFactory) {
> this.underlyingFactory = cacheFactory;
> }
>
> @Override
> NamedCache create(String name) {
> return caches.get(name);
> }
> }
>
>
> * * *
>
> public class YourModule extends AbstractModule {
>
> @Override protected void configure() {
> // as an alternative to binding this as a singleton, you could
> inject the map of caches
> // as a singleton instance. 6 or 1/2 dozen, unless you want to
> unittest your
> // implementation, in which case you'll want to inject the singleton
> map
> bind(CacheFactory.class)
> .to(CachingCacheFactory.class)
> .in(Singleton.class);
>
> //
> http://google-guice.googlecode.com/svn/trunk/javadoc/com/google/inject/assistedinject/FactoryModuleBuilder.html
> // this binds a "CacheFactory" instance, annotated with
> @AlwaysCreatesNewInstances, to
> // a default factory implementation which will return instances of
> NamedCacheLocal when
> // its create() method is invoked
> install(new FactoryModuleBuilder()
> .implement(NamedCache, NamedCacheLocal)
> .build(Key.get(NamedCache.class,
> AlwaysCreatesNewInstances.class));
> }
> }
>
> That should do the trick.
>
> -Fred
>
>
> On Thu, Oct 14, 2010 at 4:25 AM, Sondre Bjornebekk <
> sondre.bjorneb...@gmail.com> wrote:
>
>> Hi,
>>
>> Trying to use assisted inject and find it pretty neat, the minor
>> detail is that it does not quite work for me yet :-)
>>
>> I have created a Map wrapper to ease switching of caches (typically to
>> something like Coherence) for my application. I want the Map to be
>> returned "in a Singleton scope, based on the value of the (assisted)
>> parameter". I am doing:
>>
>> bind(CacheFactory.class).toProvider(
>> FactoryProvider.newFactory(CacheFactory.class,
>> NamedCacheLocal.class) );
>>
>> And I have:
>> /**
>> * A factory for named caches, that later might hide its
>> implementation using a cluster wide storage.
>> */
>>
>> public interface CacheFactory {
>> //had to be named create to work with guice assisted inject, so
>> renamed from getNamedCache
>> public NamedCache create(String name);
>> }
>>
>> public interface NamedCache<K,V> extends Map<K,V> {
>> public int getHitPercent();
>> public String getHitPercentString();
>> ...
>> }
>>
>> /**
>> * A named cache with a local map as implementation for storage.
>> */
>>
>> public class NamedCacheLocal<K,V> implements NamedCache<K,V> {
>> ...
>>
>> /**
>> * Constructs a NamedCacheLocal with its dependencies.
>> */
>> @Inject
>> public NamedCacheLocal(@Assisted String cacheName) {
>> ...
>> }
>>
>> I have this (failing) unit test that checks that the same cache is
>> used for the same name:
>>
>> System.out.println("doing put / get test");
>> NamedCache<String,String> namedCache =
>> namedCacheFactory.create("myCache");
>> namedCache.put("fisk","fesk");
>> for(int i=0;i<10; i++){
>> Object o = namedCache.get("non-hit");
>> namedCache.put("test"+i,"testval"+i);
>> }
>> Object hit = namedCache.get("fisk");
>> System.out.println("cache " + namedCache + " delivered value "
>> + hit + " to a hit percent of " + namedCache.getHitPercentString());
>> assertTrue(namedCache.getHitPercent() > 8 &&
>> namedCache.getHitPercent() < 10);
>> NamedCache<String,String> namedCacheWithSameName =
>> namedCacheFactory.create("myCache");
>> for(int i=0;i<10; i++){
>> String val = namedCacheWithSameName.get("test"+i);
>> System.out.println("got val from cache: " + val);
>> }
>> //should have used the same reference, thus hit pct now higher
>> System.out.println("done with cache " +
>> namedCacheWithSameName);
>> assertTrue(namedCache.getHitPercent() > 10);
>> assertTrue(namedCache.equals(namedCacheWithSameName));
>>
>>
>> So is the thing I put in quotation marks in the intro ("in a Singleton
>> scope, based on the value of the (assisted) parameter") even possible
>> just by Guice config, or do I have to go two steps back and one
>> forward and just implement the CacheFactory myself with this logic?
>> The first thing I tried was
>>
>> bind( CacheFactory.class ).
>> toProvider( FactoryProvider.newFactory(CacheFactory.class,
>> NamedCacheLocal.class) ) .
>> in ( Singleton.class );
>>
>> But that (of course) just gives me a singleton CacheFactory, not the
>> NamedCacheLocal "singleton" based on the assisted factory parameter.
>>
>> Cheers,
>>
>> Sondre
>>
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