Phantom Referenced Objects

Question

Phantom References serve for post-mortem operations. The Java specification states that a phantom referenced object will not be deallocated until the phantom-re

Answer 1

Phantom references can be used to perform pre-garbage collection actions such as freeing resources. Instead, people usually use the finalize() method for this which is not a good idea. Finalizers have a horrible impact on the performance of the garbage collector and can break data integrity of your application if you're not very careful since the "finalizer" is invoked in a random thread, at a random time.

In the constructor of a phantom reference, you specify a ReferenceQueue where the phantom references are enqueued once the referenced objects becomes "phantom reachable". Phantom reachable means unreachable other than through the phantom reference. The initially confusing thing is that although the phantom reference continues to hold the referenced object in a private field (unlike soft or weak references), its getReference() method always returns null. This is so that you cannot make the object strongly reachable again.

From time to time, you can poll the ReferenceQueue and check if there are any new PhantomReferences whose referenced objects have become phantom reachable. In order to be able to to anything useful, one can for example derive a class from java.lang.ref.PhantomReference that references resources that should be freed before garbage collection. The referenced object is only garbage collected once the phantom reference becomes unreachable itself.

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