Is there a practical use for weak references? [duplicate]

Answer 1

I use WeakSet to encode links in a graph. If a node is deleted, the links automatically disappear.

Answer 2

We use it for that reason - in our example, we have a variety of listeners that must register with a service. The service keeps weak references to the listeners, while the instantiated classes keep strong references. If the classes at any time get GC'ed, the weak reference is all that remains of the listeners, which will then be GC'ed as well. It makes keeping track of the intermediary classes much easier.

Answer 3

If a WeakReference is the only reference to an object, and you want the object to hang around, you should probably be using a SoftReference instead.

WeakReferences are best used in cases where there will be other references to the object, but you can't (or don't want to have to) detect when those other references are no longer used. Then, the other reference will prevent the object from being garbage collected, and the WeakReference will just be another way of getting to the same object.

Two common use cases are:

1. For holding additional (often expensively calculated but reproducible) information about specific objects that you cannot modify directly, and whose lifecycle you have little control over. WeakHashMap is a perfect way of holding these references: the key in the WeakHashMap is only weakly held, and so when the key is garbage collected, the value can be removed from the Map too, and hence be garbage collected.
2. For implementing some kind of eventing or notification system, where "listeners" are registered with some kind of coordinator, so they can be informed when something occurs – but where you don't want to prevent these listeners from being garbage collected when they come to the end of their life. A WeakReference will point to the object while it is still alive, but point to "null" once the original object has been garbage collected.