Are WeakHashMap cleared during a full GC?

Question

I encountered some troubles with WeakHashMap.

Consider this sample code:

List list = new ArrayList();

Map

        

Answer 1

Bit of digging reveals that this is explicitly covered in the JLS, section 12.6.1:

Optimizing transformations of a program can be designed that reduce the number of objects that are reachable to be less than those which would naively be considered reachable. For example, a compiler or code generator may choose to set a variable or parameter that will no longer be used to null to cause the storage for such an object to be potentially reclaimable sooner.

(Bolding is my addition.)

http://java.sun.com/docs/books/jls/third_edition/html/execution.html#12.6.1

So in essence, the JIT is allowed to remove strong references whenever it wants if it can work out that they'll never be used again - which is exactly what's happening here.

This is a great question though and makes for a great puzzler that can easily show just because an object appears to have a strong reference in scope, doesn't necessarily mean it hasn't been garbage collected. Following on from this it means you explicitly can't guarantee anything about when a finalizer will run, this may even be in the case where it seems like the object is still in scope!

Eg:

List list = new ArrayList();

Object thing = new Object() {
    protected void finalize() {
        System.out.println("here");
    }
};
WeakReference