Add customizer variant of `requestFactorySettings`

Add `requestFactorySettings` that accepts a `UnaryOperator` so that
the existing `requestFactorySettings` can be customized rather than
replaced.

Closes gh-43258

spring-boot-project/spring-boot-test/src/main/java/org/springframework/boot/test/web/client/TestRestTemplate.java

@@ -25,6 +25,7 @@ import java.time.Duration;
 import java.util.Map;
 import java.util.Set;
 import java.util.concurrent.TimeUnit;
+import java.util.function.UnaryOperator;
 
 import javax.net.ssl.SSLContext;
 
@@ -974,6 +975,22 @@ public class TestRestTemplate {
 				this.restTemplate.getUriTemplateHandler());
 	}
 
+	/**
+	 * Creates a new {@code TestRestTemplate} with the same configuration as this one,
+	 * except that it will customize the {@link ClientHttpRequestFactorySettings}. The
+	 * request factory used is a new instance of the underlying {@link RestTemplate}'s
+	 * request factory type (when possible).
+	 * @param requestFactorySettingsCustomizer a {@link UnaryOperator} to update the
+	 * settings
+	 * @return the new template
+	 * @since 3.4.1
+	 */
+	public TestRestTemplate withRequestFactorySettings(
+			UnaryOperator<ClientHttpRequestFactorySettings> requestFactorySettingsCustomizer) {
+		return new TestRestTemplate(this.builder.requestFactorySettings(requestFactorySettingsCustomizer),
+				this.restTemplate.getUriTemplateHandler());
+	}
+
 	@SuppressWarnings({ "rawtypes", "unchecked" })
 	private RequestEntity<?> createRequestEntityWithRootAppliedUri(RequestEntity<?> requestEntity) {
 		return new RequestEntity(requestEntity.getBody(), requestEntity.getHeaders(), requestEntity.getMethod(),

spring-boot-project/spring-boot-test/src/test/java/org/springframework/boot/test/web/client/TestRestTemplateTests.java

@@ -168,7 +168,7 @@ class TestRestTemplateTests {
 	}
 
 	@Test
-	void withSettingsUpdatesRedirectsForHttpComponents() {
+	void withRequestFactorySettingsRedirectsForHttpComponents() {
 		TestRestTemplate template = new TestRestTemplate();
 		assertThat(getRequestConfig(template).isRedirectsEnabled()).isFalse();
 		assertThat(getRequestConfig(template
@@ -177,7 +177,7 @@ class TestRestTemplateTests {
 	}
 
 	@Test
-	void withSettingsUpdatesRedirectsForJdk() {
+	void withRequestFactorySettingsRedirectsForJdk() {
 		TestRestTemplate template = new TestRestTemplate(
 				new RestTemplateBuilder().requestFactoryBuilder(ClientHttpRequestFactoryBuilder.jdk()));
 		assertThat(getJdkHttpClient(template).followRedirects()).isEqualTo(Redirect.NORMAL);
@@ -186,6 +186,16 @@ class TestRestTemplateTests {
 			.followRedirects()).isEqualTo(Redirect.NEVER);
 	}
 
+	@Test
+	void withRequestFactorySettingsUpdateRedirectsForJdk() {
+		TestRestTemplate template = new TestRestTemplate(
+				new RestTemplateBuilder().requestFactoryBuilder(ClientHttpRequestFactoryBuilder.jdk()));
+		assertThat(getJdkHttpClient(template).followRedirects()).isEqualTo(Redirect.NORMAL);
+		assertThat(getJdkHttpClient(
+				template.withRequestFactorySettings((settings) -> settings.withRedirects(Redirects.DONT_FOLLOW)))
+			.followRedirects()).isEqualTo(Redirect.NEVER);
+	}
+
 	private RequestConfig getRequestConfig(RestTemplateBuilder builder, HttpClientOption... httpClientOptions) {
 		builder = (builder != null) ? builder : new RestTemplateBuilder();
 		TestRestTemplate template = new TestRestTemplate(builder, null, null, httpClientOptions);

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/web/client/RestTemplateBuilder.java

@@ -29,6 +29,7 @@ import java.util.Map;
 import java.util.Set;
 import java.util.function.Function;
 import java.util.function.Supplier;
+import java.util.function.UnaryOperator;
 
 import org.springframework.beans.BeanUtils;
 import org.springframework.boot.http.client.ClientHttpRequestFactoryBuilder;
@@ -451,6 +452,22 @@ public class RestTemplateBuilder {
 				this.requestCustomizers);
 	}
 
+	/**
+	 * Update the {@link ClientHttpRequestFactorySettings} using the given customizer.
+	 * @param requestFactorySettingsCustomizer a {@link UnaryOperator} to update request
+	 * factory settings
+	 * @return a new builder instance
+	 * @since 3.4.1
+	 */
+	public RestTemplateBuilder requestFactorySettings(
+			UnaryOperator<ClientHttpRequestFactorySettings> requestFactorySettingsCustomizer) {
+		Assert.notNull(requestFactorySettingsCustomizer, "ClientHttpRequestFactorySettingsCustomizer must not be null");
+		return new RestTemplateBuilder(requestFactorySettingsCustomizer.apply(this.requestFactorySettings),
+				this.detectRequestFactory, this.rootUri, this.messageConverters, this.interceptors,
+				this.requestFactoryBuilder, this.uriTemplateHandler, this.errorHandler, this.basicAuthentication,
+				this.defaultHeaders, this.customizers, this.requestCustomizers);
+	}
+
 	/**
 	 * Sets the connection timeout on the underlying {@link ClientHttpRequestFactory}.
 	 * @param connectTimeout the connection timeout