code: public final Cache2kBuilder<K,V> addAsyncListener(CacheEntryOperationListener<K,V> listener) {
      config().getAsyncListeners().add(wrapCustomizationInstance(listener));
      return this;
    }
docs: A set of listeners. Listeners added in this collection will be
  executed in a asynchronous mode.
  
  @throws IllegalArgumentException if an identical listener is already added.
  @param listener The listener to add