code: public void addListener(PreInvalidationListener listener) {
      	if (preInvalidationListenerCount == 1 && tc.isDebugEnabled())
      		Tr.debug(tc, "Over-writing current PreInvalidationListener with new one");
          currentPreInvalidationListener = listener;
          preInvalidationListenerCount = 1;
      }
docs: This adds a new listener to the PreInvalidation listener.
  @param listener The listener to be added.