code: public RestTemplateBuilder requestFactory(
  			Class<? extends ClientHttpRequestFactory> requestFactory) {
  		Assert.notNull(requestFactory, "RequestFactory must not be null");
  		return requestFactory(() -> createRequestFactory(requestFactory));
  	}
docs: Set the {@link ClientHttpRequestFactory} class that should be used with the
  {@link RestTemplate}.
  @param requestFactory the request factory to use
  @return a new builder instance