Add ClientHttpRequestFactoryBuilderCustomizer support

Add `ClientHttpRequestFactoryBuilderCustomizer` interface that can be
used to customize the `ClientHttpRequestFactoryBuilder`.

Closes gh-44987

Author: philwebb

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/http/client/ClientHttpRequestFactoryBuilderCustomizer.java

@@ -0,0 +1,38 @@
+/*
+ * Copyright 2012-2025 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.http.client;
+
+import org.springframework.boot.http.client.ClientHttpRequestFactoryBuilder;
+
+/**
+ * Customizer that can be used to modify the auto-configured
+ * {@link ClientHttpRequestFactoryBuilder} when its type matches.
+ *
+ * @param <B> the builder type
+ * @author Phillip Webb
+ * @since 3.5.0
+ */
+public interface ClientHttpRequestFactoryBuilderCustomizer<B extends ClientHttpRequestFactoryBuilder<?>> {
+
+	/**
+	 * Customize the given builder.
+	 * @param builder the builder to customize
+	 * @return the customized builder
+	 */
+	B customize(B builder);
+
+}

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/http/client/HttpClientAutoConfiguration.java

@@ -16,6 +16,8 @@
 
 package org.springframework.boot.autoconfigure.http.client;
 
+import java.util.List;
+
 import org.springframework.beans.factory.ObjectProvider;
 import org.springframework.boot.autoconfigure.AutoConfiguration;
 import org.springframework.boot.autoconfigure.EnableAutoConfiguration;
@@ -29,6 +31,7 @@
 import org.springframework.boot.http.client.HttpClientSettings;
 import org.springframework.boot.http.client.HttpRedirects;
 import org.springframework.boot.ssl.SslBundles;
+import org.springframework.boot.util.LambdaSafe;
 import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Conditional;
 import org.springframework.http.client.ClientHttpRequestFactory;
@@ -48,8 +51,19 @@ public class HttpClientAutoConfiguration {
 
 	@Bean
 	@ConditionalOnMissingBean
-	ClientHttpRequestFactoryBuilder<?> clientHttpRequestFactoryBuilder(HttpClientProperties httpClientProperties) {
-		return httpClientProperties.factoryBuilder();
+	ClientHttpRequestFactoryBuilder<?> clientHttpRequestFactoryBuilder(HttpClientProperties httpClientProperties,
+			ObjectProvider<ClientHttpRequestFactoryBuilderCustomizer<?>> clientHttpRequestFactoryBuilderCustomizers) {
+		ClientHttpRequestFactoryBuilder<?> builder = httpClientProperties.factoryBuilder();
+		return customize(builder, clientHttpRequestFactoryBuilderCustomizers.orderedStream().toList());
+	}
+
+	@SuppressWarnings("unchecked")
+	private ClientHttpRequestFactoryBuilder<?> customize(ClientHttpRequestFactoryBuilder<?> builder,
+			List<ClientHttpRequestFactoryBuilderCustomizer<?>> customizers) {
+		ClientHttpRequestFactoryBuilder<?>[] builderReference = { builder };
+		LambdaSafe.callbacks(ClientHttpRequestFactoryBuilderCustomizer.class, customizers, builderReference[0])
+			.invoke((customizer) -> builderReference[0] = customizer.customize(builderReference[0]));
+		return builderReference[0];
 	}
 
 	@Bean

spring-boot-project/spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/http/client/HttpClientAutoConfigurationTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2024 the original author or authors.
+ * Copyright 2012-2025 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.
@@ -27,9 +27,14 @@
 import org.springframework.boot.http.client.ClientHttpRequestFactoryBuilder;
 import org.springframework.boot.http.client.ClientHttpRequestFactorySettings;
 import org.springframework.boot.http.client.ClientHttpRequestFactorySettings.Redirects;
+import org.springframework.boot.http.client.HttpComponentsClientHttpRequestFactoryBuilder;
+import org.springframework.boot.http.client.JettyClientHttpRequestFactoryBuilder;
 import org.springframework.boot.http.client.SimpleClientHttpRequestFactoryBuilder;
 import org.springframework.boot.test.context.runner.ApplicationContextRunner;
 import org.springframework.boot.test.context.runner.ReactiveWebApplicationContextRunner;
+import org.springframework.context.annotation.Bean;
+import org.springframework.context.annotation.Configuration;
+import org.springframework.http.client.ClientHttpRequestFactory;
 
 import static org.assertj.core.api.Assertions.assertThat;
 
@@ -89,4 +94,30 @@ void whenReactiveWebApplicationBeansAreNotConfigured() {
 				.doesNotHaveBean(ClientHttpRequestFactorySettings.class));
 	}
 
+	@Test
+	void clientHttpRequestFactoryBuilderCustomizersAreApplied() {
+		this.contextRunner.withUserConfiguration(ClientHttpRequestFactoryBuilderCustomizersConfiguration.class)
+			.run((context) -> {
+				ClientHttpRequestFactory factory = context.getBean(ClientHttpRequestFactoryBuilder.class).build();
+				assertThat(factory).extracting("connectTimeout").isEqualTo(5L);
+			});
+	}
+
+	@Configuration(proxyBeanMethods = false)
+	static class ClientHttpRequestFactoryBuilderCustomizersConfiguration {
+
+		@Bean
+		ClientHttpRequestFactoryBuilderCustomizer<HttpComponentsClientHttpRequestFactoryBuilder> httpComponentsCustomizer() {
+			return (builder) -> builder.withCustomizer((factory) -> factory.setConnectTimeout(5));
+		}
+
+		@Bean
+		ClientHttpRequestFactoryBuilderCustomizer<JettyClientHttpRequestFactoryBuilder> jettyCustomizer() {
+			return (builder) -> {
+				throw new IllegalStateException();
+			};
+		}
+
+	}
+
 }

spring-boot-project/spring-boot-docs/src/docs/antora/modules/reference/pages/io/rest-client.adoc

@@ -272,7 +272,7 @@ spring:
       redirects: dont-follow
 ----
 
-For more complex customizations, you can declare your own javadoc:org.springframework.boot.http.client.ClientHttpRequestFactoryBuilder[] bean which will cause auto-configuration to back off.
+For more complex customizations, you can use javadoc:org.springframework.boot.autoconfigure.http.client.ClientHttpRequestFactoryBuilderCustomizer[] or declare your own javadoc:org.springframework.boot.http.client.ClientHttpRequestFactoryBuilder[] bean which will cause auto-configuration to back off.
 This can be useful when you need to customize some of the internals of the underlying HTTP library.
 
 For example, the following will use a JDK client configured with a specific javadoc:java.net.ProxySelector[]: