code: public CaffeineCache<K, V> doWithAsync(final AsyncCallback<K, V> configurer) {
      requireNonNull(configurer, "Configurer callback is required.");
      this.callback = configurer::apply;
      return this;
    }
docs: Configure a cache builder and creates a new {@link AsyncLoadingCache}.
  
  @param configurer Configurer callback.
  @return This instance.