code: public CustomizationCollection<CacheEntryOperationListener<K,V>> getAsyncListeners() {
      if (asyncListeners == null) {
        asyncListeners = new DefaultCustomizationCollection<CacheEntryOperationListener<K, V>>();
      }
      return asyncListeners;
    }
docs: A set of listeners. A listener can be added by adding it to the collection.
  Duplicate (in terms of equal objects) listeners will be ignored.
  
  @return Mutable collection of listeners