Fix startup failure when WebFlux auto-config is skipped

Prior to this commit, the `HttpHandlerAutoConfiguration` would rely on
the `WebFluxProperties` bean being present - this is most of the time
true when the appplication is using the WebFlux auto-configuration.

If the application is overriding the WebFlux auto-configuration and
providing its own setup, the properties bean is not present and we
should skip its usage.

Fixes gh-20891

Author: bclozel

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

@@ -19,6 +19,7 @@
 import java.util.Collections;
 import java.util.Map;
 
+import org.springframework.beans.factory.ObjectProvider;
 import org.springframework.boot.autoconfigure.AutoConfigureAfter;
 import org.springframework.boot.autoconfigure.AutoConfigureOrder;
 import org.springframework.boot.autoconfigure.EnableAutoConfiguration;
@@ -60,9 +61,10 @@ public AnnotationConfig(ApplicationContext applicationContext) {
 		}
 
 		@Bean
-		public HttpHandler httpHandler(WebFluxProperties properties) {
+		public HttpHandler httpHandler(ObjectProvider<WebFluxProperties> propsProvider) {
 			HttpHandler httpHandler = WebHttpHandlerBuilder.applicationContext(this.applicationContext).build();
-			if (StringUtils.hasText(properties.getBasePath())) {
+			WebFluxProperties properties = propsProvider.getIfAvailable();
+			if (properties != null && StringUtils.hasText(properties.getBasePath())) {
 				Map<String, HttpHandler> handlersMap = Collections.singletonMap(properties.getBasePath(), httpHandler);
 				return new ContextPathCompositeHandler(handlersMap);
 			}

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

@@ -25,8 +25,10 @@
 import org.springframework.context.annotation.Configuration;
 import org.springframework.http.server.reactive.ContextPathCompositeHandler;
 import org.springframework.http.server.reactive.HttpHandler;
+import org.springframework.web.reactive.DispatcherHandler;
 import org.springframework.web.reactive.function.server.RouterFunction;
 import org.springframework.web.reactive.function.server.ServerResponse;
+import org.springframework.web.server.WebHandler;
 
 import static org.assertj.core.api.Assertions.assertThat;
 import static org.springframework.web.reactive.function.server.RequestPredicates.GET;
@@ -58,6 +60,12 @@ void shouldConfigureHttpHandlerAnnotation() {
 				.run((context) -> assertThat(context).hasSingleBean(HttpHandler.class));
 	}
 
+	@Test
+	void shouldConfigureHttpHandlerWithoutWebFluxAutoConfiguration() {
+		this.contextRunner.withUserConfiguration(CustomWebHandler.class)
+				.run((context) -> assertThat(context).hasSingleBean(HttpHandler.class));
+	}
+
 	@Test
 	void shouldConfigureBasePathCompositeHandler() {
 		this.contextRunner.withConfiguration(AutoConfigurations.of(WebFluxAutoConfiguration.class))
@@ -85,4 +93,14 @@ RouterFunction<ServerResponse> routerFunction() {
 
 	}
 
+	@Configuration(proxyBeanMethods = false)
+	static class CustomWebHandler {
+
+		@Bean
+		WebHandler webHandler() {
+			return new DispatcherHandler();
+		}
+
+	}
+
 }