WeakHashMap


              WeakHashMap is one of the lesser known member of Map family. It is a special kind of hashtable based Map implementation which stores weak references to the keys. That means keys of this kind of data structure will be stored in a WeakReference . So Entry of the Map will be reclaimed by the garbage collector when its key object is no longer used. 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.

             In HashMap , if reference of a key object no longer exist , corresponding Entry ( key – value pair ) will not be removed from Map . Garbage collector can not collect live objects . As long as Map object is live , all buckets or Entry are live . So they can not be collected by garbage collector .

            WeakHashMap uses a weak references of object to hold a key . Entry inner class of WeakHashMap extends WeakReference to implement weak reference of key object .

private static class Entry<K, V>
extends WeakReference<Object>
implements Map.Entry<K, V>
{
V value;
int hash;
Entry<K, V> next;

....

}


            Garbage collector treats this kind of object n special way . Normally garbage collector reclaims an object if garbage collector finds that particular object has no reference to it . But if Object is only reachable by weak reference , garbage collector still reclaimes the same , but places the weak reference that led to into queue . It periodically goes on checking the queue for newly arrieved weak references . Arrival of new weak references in queue signifies that the key was no longer used by anyone and has been collected . The WeakHashMap then removes the stale entry .
WeakHashMap has a private method expungeStaleEntries() . It is used to remove stale entries from Map . This method is used internally from getTable() , size() , resize() methods .

private void expungeStaleEntries()
{
Reference localReference;
while ((localReference = queue.poll()) != null) {
synchronized (queue)
{
Entry localEntry1 = (Entry)localReference;
int i = indexFor(hash, table.length);
Object localObject1 = table[i];
Entry localEntry2;
for (Object localObject2 = localObject1; localObject2 != null; localObject2 = localEntry2)
{
localEntry2 = next;
if (localObject2 == localEntry1)
{
if (localObject1 == localEntry1) {
table[i] = localEntry2;
} else {
next = localEntry2;
}
value = null;
size -= 1;
break;
}
localObject1 = localObject2;
}
}
}
}

This method keep polling queue for new entry and removes the associated entry if new object found .


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