Return 406 status code if welcome page is not accepted

Add `WelcomePageNotAcceptableHandlerMapping` which will return
an HTTP 406 status if a suitable welcome page is found but
cannot be accepted for the request.

An additional mapper is used so that we don't need to change
the order of the `WelcomePageHandlerMapping`. It's possible
that users may have additional root handler mappings
registered to run after the `WelcomePageHandlerMapping`
and we still need to respect those.

Fixes gh-35552

Author: philwebb

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/web/servlet/WebMvcAutoConfiguration.java

@@ -108,6 +108,7 @@
 import org.springframework.web.servlet.config.annotation.WebMvcConfigurationSupport;
 import org.springframework.web.servlet.config.annotation.WebMvcConfigurer;
 import org.springframework.web.servlet.handler.AbstractHandlerExceptionResolver;
+import org.springframework.web.servlet.handler.AbstractUrlHandlerMapping;
 import org.springframework.web.servlet.i18n.AcceptHeaderLocaleResolver;
 import org.springframework.web.servlet.i18n.FixedLocaleResolver;
 import org.springframework.web.servlet.mvc.method.annotation.ExceptionHandlerExceptionResolver;
@@ -437,12 +438,29 @@ protected RequestMappingHandlerAdapter createRequestMappingHandlerAdapter() {
 		@Bean
 		public WelcomePageHandlerMapping welcomePageHandlerMapping(ApplicationContext applicationContext,
 				FormattingConversionService mvcConversionService, ResourceUrlProvider mvcResourceUrlProvider) {
-			WelcomePageHandlerMapping welcomePageHandlerMapping = new WelcomePageHandlerMapping(
-					new TemplateAvailabilityProviders(applicationContext), applicationContext, getWelcomePage(),
-					this.mvcProperties.getStaticPathPattern());
-			welcomePageHandlerMapping.setInterceptors(getInterceptors(mvcConversionService, mvcResourceUrlProvider));
-			welcomePageHandlerMapping.setCorsConfigurations(getCorsConfigurations());
-			return welcomePageHandlerMapping;
+			return createWelcomePageHandlerMapping(applicationContext, mvcConversionService, mvcResourceUrlProvider,
+					WelcomePageHandlerMapping::new);
+		}
+
+		@Bean
+		public WelcomePageNotAcceptableHandlerMapping welcomePageNotAcceptableHandlerMapping(
+				ApplicationContext applicationContext, FormattingConversionService mvcConversionService,
+				ResourceUrlProvider mvcResourceUrlProvider) {
+			return createWelcomePageHandlerMapping(applicationContext, mvcConversionService, mvcResourceUrlProvider,
+					WelcomePageNotAcceptableHandlerMapping::new);
+		}
+
+		private <T extends AbstractUrlHandlerMapping> T createWelcomePageHandlerMapping(
+				ApplicationContext applicationContext, FormattingConversionService mvcConversionService,
+				ResourceUrlProvider mvcResourceUrlProvider, WelcomePageHandlerMappingFactory<T> factory) {
+			TemplateAvailabilityProviders templateAvailabilityProviders = new TemplateAvailabilityProviders(
+					applicationContext);
+			String staticPathPattern = this.mvcProperties.getStaticPathPattern();
+			T handlerMapping = factory.create(templateAvailabilityProviders, applicationContext, getIndexHtmlResource(),
+					staticPathPattern);
+			handlerMapping.setInterceptors(getInterceptors(mvcConversionService, mvcResourceUrlProvider));
+			handlerMapping.setCorsConfigurations(getCorsConfigurations());
+			return handlerMapping;
 		}
 
 		@Override
@@ -471,25 +489,25 @@ public FlashMapManager flashMapManager() {
 			return super.flashMapManager();
 		}
 
-		private Resource getWelcomePage() {
+		private Resource getIndexHtmlResource() {
 			for (String location : this.resourceProperties.getStaticLocations()) {
-				Resource indexHtml = getIndexHtml(location);
+				Resource indexHtml = getIndexHtmlResource(location);
 				if (indexHtml != null) {
 					return indexHtml;
 				}
 			}
 			ServletContext servletContext = getServletContext();
 			if (servletContext != null) {
-				return getIndexHtml(new ServletContextResource(servletContext, SERVLET_LOCATION));
+				return getIndexHtmlResource(new ServletContextResource(servletContext, SERVLET_LOCATION));
 			}
 			return null;
 		}
 
-		private Resource getIndexHtml(String location) {
-			return getIndexHtml(this.resourceLoader.getResource(location));
+		private Resource getIndexHtmlResource(String location) {
+			return getIndexHtmlResource(this.resourceLoader.getResource(location));
 		}
 
-		private Resource getIndexHtml(Resource location) {
+		private Resource getIndexHtmlResource(Resource location) {
 			try {
 				Resource resource = location.createRelative("index.html");
 				if (resource.exists() && (resource.getURL() != null)) {
@@ -603,6 +621,15 @@ ResourceChainResourceHandlerRegistrationCustomizer resourceHandlerRegistrationCu
 
 	}
 
+	@FunctionalInterface
+	interface WelcomePageHandlerMappingFactory<T extends AbstractUrlHandlerMapping> {
+
+		T create(TemplateAvailabilityProviders templateAvailabilityProviders, ApplicationContext applicationContext,
+				Resource indexHtmlResource, String staticPathPattern);
+
+	}
+
+	@FunctionalInterface
 	interface ResourceHandlerRegistrationCustomizer {
 
 		void customize(ResourceHandlerRegistration registration);

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/web/servlet/WelcomePage.java

@@ -0,0 +1,79 @@
+/*
+ * Copyright 2012-2023 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.servlet;
+
+import org.springframework.boot.autoconfigure.template.TemplateAvailabilityProviders;
+import org.springframework.context.ApplicationContext;
+import org.springframework.core.io.Resource;
+
+/**
+ * Details for a welcome page resolved from a resource or a template.
+ *
+ * @author Phillip Webb
+ */
+final class WelcomePage {
+
+	/**
+	 * Value used for an unresolved welcome page.
+	 */
+	static final WelcomePage UNRESOLVED = new WelcomePage(null, false);
+
+	private final String viewName;
+
+	private final boolean templated;
+
+	private WelcomePage(String viewName, boolean templated) {
+		this.viewName = viewName;
+		this.templated = templated;
+	}
+
+	/**
+	 * Return the view name of the welcome page.
+	 * @return the view name
+	 */
+	String getViewName() {
+		return this.viewName;
+	}
+
+	/**
+	 * Return if the welcome page is from a template.
+	 * @return if the welcome page is templated
+	 */
+	boolean isTemplated() {
+		return this.templated;
+	}
+
+	/**
+	 * Resolve the {@link WelcomePage} to use.
+	 * @param templateAvailabilityProviders the template availability providers
+	 * @param applicationContext the application context
+	 * @param indexHtmlResource the index HTML resource to use or {@code null}
+	 * @param staticPathPattern the static path pattern being used
+	 * @return a resolved {@link WelcomePage} instance or {@link #UNRESOLVED}
+	 */
+	static WelcomePage resolve(TemplateAvailabilityProviders templateAvailabilityProviders,
+			ApplicationContext applicationContext, Resource indexHtmlResource, String staticPathPattern) {
+		if (indexHtmlResource != null && "/**".equals(staticPathPattern)) {
+			return new WelcomePage("forward:index.html", false);
+		}
+		if (templateAvailabilityProviders.getProvider("index", applicationContext) != null) {
+			return new WelcomePage("index", true);
+		}
+		return UNRESOLVED;
+	}
+
+}

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/web/servlet/WelcomePageHandlerMapping.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2021 the original author or authors.
+ * Copyright 2012-2023 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.
@@ -27,19 +27,21 @@
 import org.springframework.boot.autoconfigure.template.TemplateAvailabilityProviders;
 import org.springframework.context.ApplicationContext;
 import org.springframework.core.io.Resource;
+import org.springframework.core.log.LogMessage;
 import org.springframework.http.HttpHeaders;
 import org.springframework.http.MediaType;
 import org.springframework.util.StringUtils;
 import org.springframework.web.servlet.handler.AbstractUrlHandlerMapping;
 import org.springframework.web.servlet.mvc.ParameterizableViewController;
 
 /**
- * An {@link AbstractUrlHandlerMapping} for an application's welcome page. Supports both
- * static and templated files. If both a static and templated index page are available,
- * the static page is preferred.
+ * An {@link AbstractUrlHandlerMapping} for an application's HTML welcome page. Supports
+ * both static and templated files. If both a static and templated index page are
+ * available, the static page is preferred.
  *
  * @author Andy Wilkinson
  * @author Bruce Brouwer
+ * @see WelcomePageNotAcceptableHandlerMapping
  */
 final class WelcomePageHandlerMapping extends AbstractUrlHandlerMapping {
 
@@ -48,37 +50,31 @@ final class WelcomePageHandlerMapping extends AbstractUrlHandlerMapping {
 	private static final List<MediaType> MEDIA_TYPES_ALL = Collections.singletonList(MediaType.ALL);
 
 	WelcomePageHandlerMapping(TemplateAvailabilityProviders templateAvailabilityProviders,
-			ApplicationContext applicationContext, Resource welcomePage, String staticPathPattern) {
-		if (welcomePage != null && "/**".equals(staticPathPattern)) {
-			logger.info("Adding welcome page: " + welcomePage);
-			setRootViewName("forward:index.html");
-		}
-		else if (welcomeTemplateExists(templateAvailabilityProviders, applicationContext)) {
-			logger.info("Adding welcome page template: index");
-			setRootViewName("index");
-		}
-	}
-
-	private boolean welcomeTemplateExists(TemplateAvailabilityProviders templateAvailabilityProviders,
-			ApplicationContext applicationContext) {
-		return templateAvailabilityProviders.getProvider("index", applicationContext) != null;
-	}
-
-	private void setRootViewName(String viewName) {
-		ParameterizableViewController controller = new ParameterizableViewController();
-		controller.setViewName(viewName);
-		setRootHandler(controller);
+			ApplicationContext applicationContext, Resource indexHtmlResource, String staticPathPattern) {
 		setOrder(2);
+		WelcomePage welcomePage = WelcomePage.resolve(templateAvailabilityProviders, applicationContext,
+				indexHtmlResource, staticPathPattern);
+		if (welcomePage != WelcomePage.UNRESOLVED) {
+			logger.info(LogMessage.of(() -> (!welcomePage.isTemplated()) ? "Adding welcome page: " + indexHtmlResource
+					: "Adding welcome page template: index"));
+			ParameterizableViewController controller = new ParameterizableViewController();
+			controller.setViewName(welcomePage.getViewName());
+			setRootHandler(controller);
+		}
 	}
 
 	@Override
 	public Object getHandlerInternal(HttpServletRequest request) throws Exception {
+		return (!isHtmlTextAccepted(request)) ? null : super.getHandlerInternal(request);
+	}
+
+	private boolean isHtmlTextAccepted(HttpServletRequest request) {
 		for (MediaType mediaType : getAcceptedMediaTypes(request)) {
 			if (mediaType.includes(MediaType.TEXT_HTML)) {
-				return super.getHandlerInternal(request);
+				return true;
 			}
 		}
-		return null;
+		return false;
 	}
 
 	private List<MediaType> getAcceptedMediaTypes(HttpServletRequest request) {

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/web/servlet/WelcomePageNotAcceptableHandlerMapping.java

@@ -0,0 +1,58 @@
+/*
+ * Copyright 2012-2023 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.servlet;
+
+import javax.servlet.http.HttpServletRequest;
+import javax.servlet.http.HttpServletResponse;
+
+import org.springframework.boot.autoconfigure.template.TemplateAvailabilityProviders;
+import org.springframework.context.ApplicationContext;
+import org.springframework.core.io.Resource;
+import org.springframework.http.HttpStatus;
+import org.springframework.web.servlet.ModelAndView;
+import org.springframework.web.servlet.handler.AbstractUrlHandlerMapping;
+import org.springframework.web.servlet.mvc.Controller;
+
+/**
+ * An {@link AbstractUrlHandlerMapping} for an application's welcome page that was
+ * ultimately not accepted.
+ *
+ * @author Phillip Webb
+ */
+class WelcomePageNotAcceptableHandlerMapping extends AbstractUrlHandlerMapping {
+
+	WelcomePageNotAcceptableHandlerMapping(TemplateAvailabilityProviders templateAvailabilityProviders,
+			ApplicationContext applicationContext, Resource indexHtmlResource, String staticPathPattern) {
+		setOrder(LOWEST_PRECEDENCE - 10); // Before ResourceHandlerRegistry
+		WelcomePage welcomePage = WelcomePage.resolve(templateAvailabilityProviders, applicationContext,
+				indexHtmlResource, staticPathPattern);
+		if (welcomePage != WelcomePage.UNRESOLVED) {
+			setRootHandler((Controller) this::handleRequest);
+		}
+	}
+
+	private ModelAndView handleRequest(HttpServletRequest request, HttpServletResponse response) {
+		response.setStatus(HttpStatus.NOT_ACCEPTABLE.value());
+		return null;
+	}
+
+	@Override
+	protected Object getHandlerInternal(HttpServletRequest request) throws Exception {
+		return super.getHandlerInternal(request);
+	}
+
+}

spring-boot-project/spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/web/servlet/WebMvcAutoConfigurationTests.java

@@ -169,7 +169,7 @@ void handlerAdaptersCreated() {
 
 	@Test
 	void handlerMappingsCreated() {
-		this.contextRunner.run((context) -> assertThat(context).getBeans(HandlerMapping.class).hasSize(5));
+		this.contextRunner.run((context) -> assertThat(context).getBeans(HandlerMapping.class).hasSize(6));
 	}
 
 	@Test
@@ -687,8 +687,8 @@ void welcomePageHandlerMappingIsAutoConfigured() {
 		this.contextRunner.withPropertyValues("spring.web.resources.static-locations:classpath:/welcome-page/")
 			.run((context) -> {
 				assertThat(context).hasSingleBean(WelcomePageHandlerMapping.class);
-				WelcomePageHandlerMapping bean = context.getBean(WelcomePageHandlerMapping.class);
-				assertThat(bean.getRootHandler()).isNotNull();
+				assertThat(context.getBean(WelcomePageHandlerMapping.class).getRootHandler()).isNotNull();
+				assertThat(context.getBean(WelcomePageNotAcceptableHandlerMapping.class).getRootHandler()).isNotNull();
 			});
 	}
 

spring-boot-project/spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/web/servlet/WelcomePageHandlerMappingTests.java

@@ -116,7 +116,6 @@ void handlesRequestWithEmptyAcceptHeader() {
 				.perform(get("/").header(HttpHeaders.ACCEPT, ""))
 				.andExpect(status().isOk())
 				.andExpect(forwardedUrl("index.html")));
-
 	}
 
 	@Test

spring-boot-project/spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/web/servlet/WelcomePageIntegrationTests.java

@@ -29,6 +29,7 @@
 import org.springframework.boot.test.web.server.LocalServerPort;
 import org.springframework.context.annotation.Configuration;
 import org.springframework.context.annotation.Import;
+import org.springframework.http.HttpStatus;
 import org.springframework.http.MediaType;
 import org.springframework.http.RequestEntity;
 import org.springframework.http.ResponseEntity;
@@ -57,6 +58,16 @@ void contentStrategyWithWelcomePage() throws Exception {
 			.build();
 		ResponseEntity<String> content = this.template.exchange(entity, String.class);
 		assertThat(content.getBody()).contains("/custom-");
+		assertThat(content.getStatusCode()).isEqualTo(HttpStatus.OK);
+	}
+
+	@Test
+	void notAcceptableWelcomePage() throws Exception {
+		RequestEntity<?> entity = RequestEntity.get(new URI("http://localhost:" + this.port + "/"))
+			.header("Accept", "spring/boot")
+			.build();
+		ResponseEntity<String> content = this.template.exchange(entity, String.class);
+		assertThat(content.getStatusCode()).isEqualTo(HttpStatus.NOT_ACCEPTABLE);
 	}
 
 	@Configuration