code: public void addStaleCacheObserver (String cache, StaleCacheObserver observer)
      {
          ObserverList<StaleCacheObserver> list = _cacheobs.get(cache);
          if (list == null) {
              list = ObserverList.newFastUnsafe();
              _cacheobs.put(cache, list);
          }
          list.add(observer);
      }
docs: Registers a stale cache observer.