Add ReactorNettyHttpClientMapper

This new customizer-like interface make it possible to configure the
Reactor Netty `HttpClient` that is going to be used by the
`WebClient.Builder` provided by Spring Boot.

Closes gh-21135

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/web/reactive/function/client/ClientHttpConnectorConfiguration.java

@@ -16,11 +16,10 @@
 
 package org.springframework.boot.autoconfigure.web.reactive.function.client;
 
-import java.util.function.Function;
-
 import org.eclipse.jetty.client.HttpClient;
 import org.eclipse.jetty.util.ssl.SslContextFactory;
 
+import org.springframework.beans.factory.ObjectProvider;
 import org.springframework.boot.autoconfigure.condition.ConditionalOnClass;
 import org.springframework.boot.autoconfigure.condition.ConditionalOnMissingBean;
 import org.springframework.context.annotation.Bean;
@@ -56,8 +55,12 @@ class ClientHttpConnectorConfiguration {
 
 		@Bean
 		@Lazy
-		public ReactorClientHttpConnector reactorClientHttpConnector(ReactorResourceFactory reactorResourceFactory) {
+		public ReactorClientHttpConnector reactorClientHttpConnector(ReactorResourceFactory reactorResourceFactory,
-			return new ReactorClientHttpConnector(reactorResourceFactory, Function.identity());
+				ObjectProvider<ReactorNettyHttpClientMapper> mapperProvider) {
+			ReactorNettyHttpClientMapper mapper = mapperProvider.orderedStream()
+					.reduce((before, after) -> (client) -> after.configure(before.configure(client)))
+					.orElse((client) -> client);
+			return new ReactorClientHttpConnector(reactorResourceFactory, mapper::configure);
 		}
 
 	}

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/web/reactive/function/client/ReactorNettyHttpClientMapper.java

@@ -0,0 +1,40 @@
+/*
+ * Copyright 2012-2020 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.boot.autoconfigure.web.reactive.function.client;
+
+import reactor.netty.http.client.HttpClient;
+
+import org.springframework.http.client.reactive.ReactorClientHttpConnector;
+
+/**
+ * Mapper that allows for custom modification of a {@link HttpClient} before it is used as
+ * the basis for a {@link ReactorClientHttpConnector}.
+ *
+ * @author Brian Clozel
+ * @since 2.3.0
+ */
+@FunctionalInterface
+public interface ReactorNettyHttpClientMapper {
+
+	/**
+	 * Configure the given {@link HttpClient} and return the newly created instance.
+	 * @param httpClient the client to configure
+	 * @return the new client instance
+	 */
+	HttpClient configure(HttpClient httpClient);
+
+}

spring-boot-project/spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/web/reactive/function/client/ClientHttpConnectorConfigurationTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2019 the original author or authors.
+ * Copyright 2012-2020 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -23,6 +23,9 @@ import org.eclipse.jetty.io.ByteBufferPool;
 import org.eclipse.jetty.util.thread.Scheduler;
 import org.junit.jupiter.api.Test;
 
+import org.springframework.boot.autoconfigure.AutoConfigurations;
+import org.springframework.boot.test.context.runner.ReactiveWebApplicationContextRunner;
+import org.springframework.context.annotation.Bean;
 import org.springframework.http.client.reactive.JettyClientHttpConnector;
 import org.springframework.http.client.reactive.JettyResourceFactory;
 import org.springframework.test.util.ReflectionTestUtils;
@@ -34,6 +37,7 @@ import static org.mockito.Mockito.mock;
  * Tests for {@link ClientHttpConnectorConfiguration}.
  *
  * @author Phillip Webb
+ * @author Brian Clozel
  */
 class ClientHttpConnectorConfigurationTests {
 
@@ -68,4 +72,24 @@ class ClientHttpConnectorConfigurationTests {
 		return ReflectionTestUtils.invokeMethod(jettyClient, "jettyClientHttpConnector", jettyResourceFactory);
 	}
 
+	@Test
+	void shouldApplyHttpClientMapper() {
+		new ReactiveWebApplicationContextRunner()
+				.withConfiguration(AutoConfigurations.of(ClientHttpConnectorConfiguration.ReactorNetty.class))
+				.withUserConfiguration(CustomHttpClientMapper.class)
+				.run((context) -> assertThat(CustomHttpClientMapper.called).isTrue());
+	}
+
+	static class CustomHttpClientMapper {
+
+		static boolean called = false;
+
+		@Bean
+		ReactorNettyHttpClientMapper clientMapper() {
+			called = true;
+			return (client) -> client.baseUrl("/test");
+		}
+
+	}
+
 }