Add WebFluxRegistrations for custom WebFlux beans

This commit adds a new `WebFluxRegistrations` interface that
allows developers to register custom instances of key WebFlux
infrastructure components, such as `RequestMappingHandlerMapping`
and `RequestMappingHandlerAdapter`.

Closes gh-13997

Author: artsiom

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

@@ -71,6 +71,8 @@
 import org.springframework.web.reactive.resource.VersionResourceResolver;
 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;
+import org.springframework.web.reactive.result.method.annotation.RequestMappingHandlerMapping;
 import org.springframework.web.reactive.result.view.ViewResolver;
 
 /**
@@ -226,8 +228,12 @@ public static class EnableWebFluxConfiguration
 
 		private final WebFluxProperties webFluxProperties;
 
-		public EnableWebFluxConfiguration(WebFluxProperties webFluxProperties) {
+		private final WebFluxRegistrations webFluxRegistrations;
+
+		public EnableWebFluxConfiguration(WebFluxProperties webFluxProperties,
+				ObjectProvider<WebFluxRegistrations> webFluxRegistrations) {
 			this.webFluxProperties = webFluxProperties;
+			this.webFluxRegistrations = webFluxRegistrations.getIfUnique();
 		}
 
 		@Bean
@@ -249,6 +255,24 @@ public Validator webFluxValidator() {
 			return ValidatorAdapter.get(getApplicationContext(), getValidator());
 		}
 
+		@Override
+		protected RequestMappingHandlerAdapter createRequestMappingHandlerAdapter() {
+			if (this.webFluxRegistrations != null && this.webFluxRegistrations
+					.getRequestMappingHandlerAdapter() != null) {
+				return this.webFluxRegistrations.getRequestMappingHandlerAdapter();
+			}
+			return super.createRequestMappingHandlerAdapter();
+		}
+
+		@Override
+		protected RequestMappingHandlerMapping createRequestMappingHandlerMapping() {
+			if (this.webFluxRegistrations != null && this.webFluxRegistrations
+					.getRequestMappingHandlerMapping() != null) {
+				return this.webFluxRegistrations.getRequestMappingHandlerMapping();
+			}
+			return super.createRequestMappingHandlerMapping();
+		}
+
 	}
 
 	@Configuration

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

@@ -0,0 +1,53 @@
+/*
+ * Copyright 2012-2018 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
+ *
+ *      http://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.reactive;
+
+import org.springframework.web.reactive.result.method.annotation.RequestMappingHandlerAdapter;
+import org.springframework.web.reactive.result.method.annotation.RequestMappingHandlerMapping;
+
+/**
+ * Interface to register key components of the {@link WebFluxAutoConfiguration} in place
+ * of the default ones provided by Spring WebFlux.
+ * <p>
+ * All custom instances are later processed by Boot and Spring WebFlux configurations. A
+ * single instance of this component should be registered, otherwise making it impossible
+ * to choose from redundant WebFlux components.
+ *
+ * @author Artsiom Yudovin
+ * @since 2.1.0
+ * @see org.springframework.boot.autoconfigure.web.reactive.WebFluxAutoConfiguration.EnableWebFluxConfiguration
+ */
+public interface WebFluxRegistrations {
+
+	/**
+	 * Return the custom {@link RequestMappingHandlerMapping} that should be used and
+	 * processed by the WebFlux configuration.
+	 * @return the custom {@link RequestMappingHandlerMapping} instance
+	 */
+	default RequestMappingHandlerMapping getRequestMappingHandlerMapping() {
+		return null;
+	}
+
+	/**
+	 * Return the custom {@link RequestMappingHandlerAdapter} that should be used and
+	 * processed by the WebFlux configuration.
+	 * @return the custom {@link RequestMappingHandlerAdapter} instance
+	 */
+	default RequestMappingHandlerAdapter getRequestMappingHandlerAdapter() {
+		return null;
+	}
+
+}

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

@@ -33,6 +33,7 @@
 import org.springframework.boot.web.reactive.filter.OrderedHiddenHttpMethodFilter;
 import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Configuration;
+import org.springframework.context.annotation.Import;
 import org.springframework.core.Ordered;
 import org.springframework.core.annotation.Order;
 import org.springframework.core.io.ClassPathResource;
@@ -370,6 +371,33 @@ public void hiddenHttpMethodFilterCanBeOverridden() {
 				});
 	}
 
+	@Test
+	public void customRequestMappingHandlerMapping() {
+		this.contextRunner.withUserConfiguration(CustomRequestMappingHandlerMapping.class)
+				.run((context) -> assertThat(context)
+						.getBean(RequestMappingHandlerMapping.class)
+						.isInstanceOf(MyRequestMappingHandlerMapping.class));
+	}
+
+	@Test
+	public void customRequestMappingHandlerAdapter() {
+		this.contextRunner.withUserConfiguration(CustomRequestMappingHandlerAdapter.class)
+				.run((context) -> assertThat(context)
+						.getBean(RequestMappingHandlerAdapter.class)
+						.isInstanceOf(MyRequestMappingHandlerAdapter.class));
+	}
+
+	@Test
+	public void multipleWebFluxRegistrations() {
+		this.contextRunner.withUserConfiguration(MultipleWebFluxRegistrations.class)
+				.run((context) -> {
+					assertThat(context.getBean(RequestMappingHandlerMapping.class))
+							.isNotInstanceOf(MyRequestMappingHandlerMapping.class);
+					assertThat(context.getBean(RequestMappingHandlerAdapter.class))
+							.isNotInstanceOf(MyRequestMappingHandlerAdapter.class);
+				});
+	}
+
 	@Configuration
 	protected static class CustomArgumentResolvers {
 
@@ -485,4 +513,55 @@ public HiddenHttpMethodFilter customHiddenHttpMethodFilter() {
 
 	}
 
+	@Configuration
+	static class CustomRequestMappingHandlerAdapter {
+
+		@Bean
+		public WebFluxRegistrations webMvcRegistrationsHandlerAdapter() {
+			return new WebFluxRegistrations() {
+
+				@Override
+				public RequestMappingHandlerAdapter getRequestMappingHandlerAdapter() {
+					return new WebFluxAutoConfigurationTests.MyRequestMappingHandlerAdapter();
+				}
+
+			};
+		}
+
+	}
+
+	private static class MyRequestMappingHandlerAdapter
+			extends RequestMappingHandlerAdapter {
+
+	}
+
+	@Configuration
+	@Import({ WebFluxAutoConfigurationTests.CustomRequestMappingHandlerMapping.class,
+			WebFluxAutoConfigurationTests.CustomRequestMappingHandlerAdapter.class })
+	static class MultipleWebFluxRegistrations {
+
+	}
+
+	@Configuration
+	static class CustomRequestMappingHandlerMapping {
+
+		@Bean
+		public WebFluxRegistrations webMvcRegistrationsHandlerMapping() {
+			return new WebFluxRegistrations() {
+
+				@Override
+				public RequestMappingHandlerMapping getRequestMappingHandlerMapping() {
+					return new MyRequestMappingHandlerMapping();
+				}
+
+			};
+		}
+
+	}
+
+	private static class MyRequestMappingHandlerMapping
+			extends RequestMappingHandlerMapping {
+
+	}
+
 }