code: public CustomizationCollection<CacheEntryOperationListener<K,V>> getListeners() {
      if (listeners == null) {
        listeners = new DefaultCustomizationCollection<CacheEntryOperationListener<K, V>>();
      }
      return listeners;
    }
docs: A set of listeners. Listeners added in this collection 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.
  
  <p>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