}

  private static final class FinalizableReference
      extends FinalizablePhantomReference<ClosingFuture<?>> {
    static final FinalizableReferenceQueue referenceQueue = new FinalizableReferenceQueue();
    static final Set<FinalizableReference> references = newConcurrentHashSet();

    static void register(ClosingFuture<?> future) {
      references.add(new FinalizableReference(future));
    }