Inconsitent way of choosing request factory implementation for RestClient

[matsior]

Used Spring Boot version: 3.3.2

There is inconsistency with creating org.springframework.web.client.RestClient.
When using org.springframework.web.client.RestClient#builder() method there is possibility that org.springframework.http.client.JdkClientHttpRequestFactory will be used if it's present. Code fragment from org.springframework.web.client.DefaultRestClientBuilder#build method:

@Override
	public RestClient build() {
		ClientHttpRequestFactory requestFactory = initRequestFactory();
		UriBuilderFactory uriBuilderFactory = initUriBuilderFactory();
		HttpHeaders defaultHeaders = copyDefaultHeaders();
		List<HttpMessageConverter<?>> messageConverters = (this.messageConverters != null ?
				this.messageConverters : initMessageConverters());
		return new DefaultRestClient(requestFactory,
				this.interceptors, this.initializers, uriBuilderFactory,
				defaultHeaders,
				this.defaultRequest,
				this.statusHandlers,
				messageConverters,
				this.observationRegistry,
				this.observationConvention,
				new DefaultRestClientBuilder(this)
				);
	}

	private ClientHttpRequestFactory initRequestFactory() {
		if (this.requestFactory != null) {
			return this.requestFactory;
		}
		else if (httpComponentsClientPresent) {
			return new HttpComponentsClientHttpRequestFactory();
		}
		else if (jettyClientPresent) {
			return new JettyClientHttpRequestFactory();
		}
		else if (jdkClientPresent) {
			// java.net.http module might not be loaded, so we can't default to the JDK HttpClient
			return new JdkClientHttpRequestFactory();
		}
		else {
			return new SimpleClientHttpRequestFactory();
		}
	}

So the priority of request factory looks following: HttpComponentsClientHttpRequestFactory > JettyClientHttpRequestFactory > JdkClientHttpRequestFactory > SimpleClientHttpRequestFactory.

On the other side using builder from org.springframework.boot.autoconfigure.web.client.RestClientAutoConfiguration looks different. Code snippet from org.springframework.boot.autoconfigure.web.client.RestClientAutoConfiguration#restClientBuilder

  @Bean
  @Scope("prototype")
  @ConditionalOnMissingBean
  RestClient.Builder restClientBuilder(RestClientBuilderConfigurer restClientBuilderConfigurer) {
    RestClient.Builder builder = RestClient.builder().requestFactory(ClientHttpRequestFactories.get(ClientHttpRequestFactorySettings.DEFAULTS));
    return restClientBuilderConfigurer.configure(builder);
  }

and org.springframework.boot.web.client.ClientHttpRequestFactories#get(org.springframework.boot.web.client.ClientHttpRequestFactorySettings)

  public static ClientHttpRequestFactory get(ClientHttpRequestFactorySettings settings) {
    Assert.notNull(settings, "Settings must not be null");
    if (APACHE_HTTP_CLIENT_PRESENT) {
      return ClientHttpRequestFactories.HttpComponents.get(settings);
    } else if (JETTY_CLIENT_PRESENT) {
      return ClientHttpRequestFactories.Jetty.get(settings);
    } else {
      return (ClientHttpRequestFactory)(OKHTTP_CLIENT_PRESENT ? ClientHttpRequestFactories.OkHttp.get(settings) : ClientHttpRequestFactories.Simple.get(settings));
    }
  }

In this case priority of request factory is different: HttpComponentsClientHttpRequestFactory > JettyClientHttpRequestFactory > OkHttp3ClientHttpRequestFactory > SimpleClientHttpRequestFactory.

I think it's confusing. In first example there is attempt to use JdkClientHttpRequestFactory, in second there is not. On the other side there is attempt to use OkHttp3ClientHttpRequestFactory in second example, while in first not. Are there any reasons why there are differences in choosing request factory implementation? If not, my proposition is to make it consistent for both cases.

[wilkinsona]

Duplicates #38856.