Fix welcome page support in WebFlux

Prior to this commit, the welcome page support implemented in gh-9785
would override existing index views in both annotation and functional
variants.

This comes from the fact that the feature was implemented as a
`RouterFunction` configured in the main `RouterFunctionMapping` bean.
Due to ordering issues between mappings, this would override existing
application mappings in some cases.

This commit ensures that the welcome page `RouterFunction` is
contributed to the context in its own handler mapping, ordered after the
application ones.

Fixes gh-21909

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/web/reactive/WebFluxAutoConfiguration.java

@@ -63,6 +63,7 @@ import org.springframework.web.reactive.config.WebFluxConfigurationSupport;
 import org.springframework.web.reactive.config.WebFluxConfigurer;
 import org.springframework.web.reactive.function.server.RouterFunction;
 import org.springframework.web.reactive.function.server.ServerResponse;
+import org.springframework.web.reactive.function.server.support.RouterFunctionMapping;
 import org.springframework.web.reactive.result.method.HandlerMethodArgumentResolver;
 import org.springframework.web.reactive.result.method.annotation.ArgumentResolverConfigurer;
 import org.springframework.web.reactive.result.method.annotation.RequestMappingHandlerAdapter;
@@ -101,12 +102,18 @@ public class WebFluxAutoConfiguration {
 	public static class WelcomePageConfiguration {
 
 		@Bean
-		public RouterFunction<ServerResponse> welcomePageRouterFunction(ApplicationContext applicationContext,
+		public RouterFunctionMapping welcomePageRouterFunctionMapping(ApplicationContext applicationContext,
 				WebFluxProperties webFluxProperties, ResourceProperties resourceProperties) {
 			WelcomePageRouterFunctionFactory factory = new WelcomePageRouterFunctionFactory(
 					new TemplateAvailabilityProviders(applicationContext), applicationContext,
 					resourceProperties.getStaticLocations(), webFluxProperties.getStaticPathPattern());
-			return factory.createRouterFunction();
+			RouterFunction<ServerResponse> routerFunction = factory.createRouterFunction();
+			if (routerFunction != null) {
+				RouterFunctionMapping routerFunctionMapping = new RouterFunctionMapping(routerFunction);
+				routerFunctionMapping.setOrder(1);
+				return routerFunctionMapping;
+			}
+			return null;
 		}
 
 	}

spring-boot-project/spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/web/reactive/WebFluxAutoConfigurationTests.java

@@ -62,6 +62,7 @@ import org.springframework.web.reactive.HandlerMapping;
 import org.springframework.web.reactive.accept.RequestedContentTypeResolver;
 import org.springframework.web.reactive.config.WebFluxConfigurationSupport;
 import org.springframework.web.reactive.config.WebFluxConfigurer;
+import org.springframework.web.reactive.function.server.support.RouterFunctionMapping;
 import org.springframework.web.reactive.handler.SimpleUrlHandlerMapping;
 import org.springframework.web.reactive.resource.CachingResourceResolver;
 import org.springframework.web.reactive.resource.CachingResourceTransformer;
@@ -108,6 +109,7 @@ class WebFluxAutoConfigurationTests {
 			assertThat(context).getBeans(RequestMappingHandlerMapping.class).hasSize(1);
 			assertThat(context).getBeans(RequestMappingHandlerAdapter.class).hasSize(1);
 			assertThat(context).getBeans(RequestedContentTypeResolver.class).hasSize(1);
+			assertThat(context).getBeans(RouterFunctionMapping.class).hasSize(1);
 			assertThat(context.getBean("resourceHandlerMapping", HandlerMapping.class)).isNotNull();
 		});
 	}
@@ -440,6 +442,15 @@ class WebFluxAutoConfigurationTests {
 				});
 	}
 
+	@Test
+	void welcomePageHandlerMapping() {
+		this.contextRunner.withPropertyValues("spring.resources.static-locations=classpath:/welcome-page/").run((context) -> {
+			assertThat(context).getBeans(RouterFunctionMapping.class).hasSize(2);
+			assertThat(context.getBean("welcomePageRouterFunctionMapping", HandlerMapping.class)).isNotNull()
+					.extracting("order").isEqualTo(1);
+		});
+	}
+
 	private Map<PathPattern, Object> getHandlerMap(ApplicationContext context) {
 		HandlerMapping mapping = context.getBean("resourceHandlerMapping", HandlerMapping.class);
 		if (mapping instanceof SimpleUrlHandlerMapping) {