Polish RestClient request factories

This commit changes the default request factory from the
SimpleClientHttpRequestFactory to the JdkClientHttpRequestFactory if
available. It also adds detection logic for OkHttp and Jetty.

Author: poutsma

framework-docs/modules/ROOT/pages/integration/rest-clients.adoc

@@ -13,7 +13,7 @@ The Spring Framework provides the following choices for making calls to REST end
 == `RestClient`
 
 Reference documentation is forthcoming.
-For now, please refer to the https://docs.spring.io/spring-framework/docs/6.1.0-M1/javadoc-api/org/springframework/web/client/RestClient.html[API documentation].
+For now, please refer to the https://docs.spring.io/spring-framework/docs/6.1.0-M2/javadoc-api/org/springframework/web/client/RestClient.html[API documentation].
 
 
 [[rest-webclient]]

spring-web/src/main/java/org/springframework/web/client/DefaultRestClientBuilder.java

@@ -30,6 +30,9 @@
 import org.springframework.http.client.ClientHttpRequestInitializer;
 import org.springframework.http.client.ClientHttpRequestInterceptor;
 import org.springframework.http.client.HttpComponentsClientHttpRequestFactory;
+import org.springframework.http.client.JdkClientHttpRequestFactory;
+import org.springframework.http.client.JettyClientHttpRequestFactory;
+import org.springframework.http.client.OkHttp3ClientHttpRequestFactory;
 import org.springframework.http.client.SimpleClientHttpRequestFactory;
 import org.springframework.http.converter.ByteArrayHttpMessageConverter;
 import org.springframework.http.converter.HttpMessageConverter;
@@ -57,8 +60,18 @@
  */
 final class DefaultRestClientBuilder implements RestClient.Builder {
 
+	// request factories
+
 	private static final boolean httpComponentsClientPresent;
 
+	private static final boolean okHttpClientPresent;
+
+	private static final boolean jettyClientPresent;
+
+	private static final boolean jdkClientPresent;
+
+	// message factories
+
 	private static final boolean jackson2Present;
 
 	private static final boolean gsonPresent;
@@ -74,7 +87,12 @@ final class DefaultRestClientBuilder implements RestClient.Builder {
 
 	static {
 		ClassLoader loader = DefaultRestClientBuilder.class.getClassLoader();
+
 		httpComponentsClientPresent = ClassUtils.isPresent("org.apache.hc.client5.http.classic.HttpClient", loader);
+		okHttpClientPresent = ClassUtils.isPresent("okhttp3.OkHttpClient", loader);
+		jettyClientPresent = ClassUtils.isPresent("org.eclipse.jetty.client.HttpClient", loader);
+		jdkClientPresent = ClassUtils.isPresent("java.net.http.HttpClient", loader);
+
 		jackson2Present = ClassUtils.isPresent("com.fasterxml.jackson.databind.ObjectMapper", loader) &&
 				ClassUtils.isPresent("com.fasterxml.jackson.core.JsonGenerator", loader);
 		gsonPresent = ClassUtils.isPresent("com.google.gson.Gson", loader);
@@ -345,6 +363,16 @@ private ClientHttpRequestFactory initRequestFactory() {
 		else if (httpComponentsClientPresent) {
 			return new HttpComponentsClientHttpRequestFactory();
 		}
+		else if (okHttpClientPresent) {
+			return new OkHttp3ClientHttpRequestFactory();
+		}
+		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();
 		}

spring-web/src/main/java/org/springframework/web/client/RestClient.java

@@ -346,6 +346,15 @@ Builder defaultStatusHandler(Predicate<HttpStatusCode> statusPredicate,
 		 * Configure the {@link ClientHttpRequestFactory} to use. This is useful
 		 * for plugging in and/or customizing options of the underlying HTTP
 		 * client library (e.g. SSL).
+		 * <p>If no request factory is specified, {@code RestClient} uses
+		 * {@linkplain org.springframework.http.client.HttpComponentsClientHttpRequestFactory Apache Http Client},
+		 * {@linkplain org.springframework.http.client.OkHttp3ClientHttpRequestFactory OkHttp 3}, or
+		 * {@linkplain org.springframework.http.client.JettyClientHttpRequestFactory Jetty Http Client}
+		 * if available on the classpath, and defaults to the
+		 * {@linkplain org.springframework.http.client.JdkClientHttpRequestFactory JDK HttpClient}
+		 * if the {@code java.net.http} module is loaded, or to a
+		 * {@linkplain org.springframework.http.client.SimpleClientHttpRequestFactory simple default}
+		 * otherwise.
 		 * @param requestFactory the request factory to use
 		 * @return this builder
 		 */