RestoringWeakReference contains a scala.ref.WeakReference that, after it has been nulled out, uses a restorer function to restore the value. This can be used for data that can afford to be evicted by the garbage collector, but will be needed later. One good example is Lift form callbacks, which may need the value of an object, but where you don't necessarily want to be retaining the object indefinitely while someone is on a page in the face of GC contention.

You can use RestoringWeakReference in a couple of basic ways:

val ref = RestoringWeakReference(() => User.find(id))

In this situation, the RestoringWeakReference will immediately call the passed function to provide the starting value for the reference, and then the same function will be used if the reference is evicted.

val ref = RestoringWeakReference(starter, () => User.find(id))

Here, starter is an Option[User] and User.find returns an Option[User]. The first parameter will be used to initialize the weak reference, while the function will be used to restore the value if the reference is evicted.

val baseRef = new WeakReference(starter)
val ref = new RestoringWeakReference(baseRef, () => User.find(id))

If you already have a WeakReference instance, you can instantiate RestoringWeakReference directly and pass that reference as the starting value, as well.

In all these cases, you use ref.value to get a hold of the value. That function will return the value if it is available, and, if not, it will restore the WeakReference and then return the value.