Provides a Map where values are referenced either weakly or softly. This can be used to cache immutable objects without preventing them from being garbaged collected.
Concurrency and synchronisation of this class fallbacks to the Map implementation passed in as the cache parameter in the constructor. For example: use a Hashtable for synchronised access, a ConcurrentHashMap for concurrent access, and a HashMap for single threaded access (make sure to specify the argument indicating single thread use).
This implementation improves over org.apache.commons.collections.map.ReferenceMap in that the synchronisation and concurrency is determined through delegation to the map supplied in the constructor as described above.
Typically used by flyweight pattern implementations where the object itself cannot be used as the key in the cache due to object creation costs.
This is also somewhat an implementation of JSR166.
Why not use WeahHashMap<K,WeakReference<V>> ? When the value is garbage collected you still end up with a WeakReference holding a null and a Map.Entry<K,WeakReference<V>> in the jvm. So a ReferenceQueue is required to clear these things up too. This is all done for you (either in the foreground or background) by the ReferenceMap class.