PhantomReference

PhantomReference is a concept in computer science related to memory management, particularly in garbage collection. It describes a situation where an object is still referenced by a weak reference, but all strong references to it have been cleared. In typical garbage collection scenarios, an object is reclaimed when there are no strong references pointing to it. However, a weak reference does not prevent an object from being garbage collected.

When an object is about to be deallocated because all strong references are gone, but it is still pointed to by one or more weak references, those weak references become "phantom references." These phantom references are then enqueued into a reference queue. This allows a program to be notified when an object is about to be garbage collected, even if it cannot directly access the object itself through the phantom reference.

The primary use case for phantom references is to perform cleanup actions that must occur after an object has been determined to be unreachable by any strong references, but before its memory is actually reclaimed. This can be useful for releasing non-memory resources associated with the object, such as file handles or network connections, ensuring that these resources are properly managed. The Java `java.lang.ref.PhantomReference` class is a concrete implementation of this concept, allowing developers to register a cleanup action with a `ReferenceQueue`.