code: public final Cache2kBuilder<K, V> addListener(CacheEntryOperationListener<K,V> listener) {
      config().getListeners().add(wrapCustomizationInstance(listener));
      return this;
    }
docs: Add a listener. The listeners  will be executed in a synchronous mode, meaning,
  further processing for an entry will stall until a registered listener is executed.
  The expiry will be always executed asynchronously.
  
  @throws IllegalArgumentException if an identical listener is already added.
  @param listener The listener to add