WeakHashMap, cleanup and API level guarantees

Question

When does WeakHashMap groom/cleanup the entries i.e. remove the WeakReferences from the Map? It appears it does it when the WeakHashMap is accessed. In other words, if for some reason the WeakHashMap is NOT accessed, the entries continue to stay as WeakReferences.
Is there an API level guarantee as to how the cleanup works? I do not see it documented in WeakHashMap API.
A hashtable-based Map implementation with weak keys. An entry in a WeakHashMap will automatically be removed when its key is no longer in ordinary use. More precisely, the presence of a mapping for a given key will not prevent the key from being discarded by the garbage collector, that is, made finalizable, finalized, and then reclaimed. When a key has been discarded its entry is effectively removed from the map, so this class behaves somewhat differently from other Map implementations.
Again, if there is any reference on how and when this -- An entry in a WeakHashMap will automatically be removed when its key is no longer in ordinary use. -- happens, it will be useful.
With regards,
M.P. Ardhanareeswaran

Answer 1

user514049 wrote:
This is more than casual curiosity.
WeakHashMap is actually working for me by "accident". In my case, I am storing some PhantomReferences in this Map as values (and not keys) for cleanup. There is a quiet period before the application is torn down during which there are no updates to the WeakHashMap. My application is able to retrieve the PhantomReferences at close time and clean them up.
It appears that it is "not advisable" to rely on this behavior.Not sure exactly what you mean here. Are the keys referenced anywhere else, or only in the map? If only in the map, then the entries can be removed from the map and the keys GCed at any time. If you need to know that you'll have access to some entry at some point in your code, then you'll have to make sure that there's a reference to the key outside of the map.
If "by accident" you mean that, so far, the entries are always there even without you maintaining other references to the keys, then, yes, that is just happenstance, and not to be relied on.
To quote the third sentence of WHM's javadoc (emphasis mine): "More precisely, the presence of a mapping for a given key will not prevent the key from being discarded by the garbage collector, that is, made finalizable, finalized, and then reclaimed."
Edited by: jverd on Nov 11, 2011 12:46 PM