Will GC collect an object referred to by a SoftReference and a WeakReference?

Question

I have a cache built from a Map to SoftReferences. When they are added they get put into another queue to be lazily compressed down via gzip or some such. My idea is this: I want to have WeakReferences to the objects in the compress queue, so that when the compressor task gets to the object, if it is already gone we needn't bother compressing it - and also that the compressor's queue doesn't keep objects alive that would otherwise be GC'd.

So if there is exactly one SoftReference and one WeakReference, does the semantic of SoftReference apply still?

Answer 1

Yes the semantic of SoftReferences still applies: SoftReferences are stronger than WeakReferences.

WeakReferences are basically treated as non existing for the GC. So an object that is only weakly reachable may be GCed immediately. Objects only reachable by a SoftReferences as the strongest type, however, are only considered for GCing if demands on memory needs to be fullfilled.

So if there are both soft and weak references, the semantic of SoftReference is applied.

Weak reference objects, which do not prevent their referents from being made finalizable, finalized, and then reclaimed.

http://download.oracle.com/javase/6/docs/api/java/lang/ref/WeakReference.html

Soft reference objects, which are cleared at the discretion of the garbage collector in response to memory demand. Soft references are most often used to implement memory-sensitive caches.

http://download.oracle.com/javase/6/docs/api/java/lang/ref/SoftReference.html

Answer 2

yes, there is no problem to GC the object that has as many soft/weak references as you want, until it has almost one strong reference.