Support suspending handler methods in Spring MVC

This commit adds support for Kotlin Coroutines suspending functions to
Spring MVC, by converting those to a Mono that can then be handled by
the asynchronous request processing feature.

It also optimizes Coroutines detection with the introduction of an
optimized KotlinDetector.isSuspendingFunction() method that does not
require kotlin-reflect.

Closes gh-23611

spring-webmvc/src/main/java/org/springframework/web/servlet/handler/AbstractHandlerMethodMapping.java

@@ -38,7 +38,6 @@ import javax.servlet.http.HttpServletRequest;
 import org.springframework.aop.support.AopUtils;
 import org.springframework.beans.factory.BeanFactoryUtils;
 import org.springframework.beans.factory.InitializingBean;
-import org.springframework.core.KotlinDetector;
 import org.springframework.core.MethodIntrospector;
 import org.springframework.lang.Nullable;
 import org.springframework.util.Assert;
@@ -606,13 +605,6 @@ public abstract class AbstractHandlerMethodMapping<T> extends AbstractHandlerMap
 		}
 
 		public void register(T mapping, Object handler, Method method) {
-			// Assert that the handler method is not a suspending one.
-			if (KotlinDetector.isKotlinType(method.getDeclaringClass())) {
-				Class<?>[] types = method.getParameterTypes();
-				if ((types.length > 0) && "kotlin.coroutines.Continuation".equals(types[types.length - 1].getName())) {
-					throw new IllegalStateException("Unsupported suspending handler method detected: " + method);
-				}
-			}
 			this.readWriteLock.writeLock().lock();
 			try {
 				HandlerMethod handlerMethod = createHandlerMethod(handler, method);

spring-webmvc/src/main/java/org/springframework/web/servlet/mvc/method/annotation/ContinuationHandlerMethodArgumentResolver.java

@@ -0,0 +1,42 @@
+/*
+ * Copyright 2002-2020 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.web.servlet.mvc.method.annotation;
+
+import org.springframework.core.MethodParameter;
+import org.springframework.web.bind.support.WebDataBinderFactory;
+import org.springframework.web.context.request.NativeWebRequest;
+import org.springframework.web.method.support.HandlerMethodArgumentResolver;
+import org.springframework.web.method.support.ModelAndViewContainer;
+
+/**
+ * No-op resolver for method arguments of type {@link kotlin.coroutines.Continuation}.
+ *
+ * @author Sebastien Deleuze
+ * @since 5.3
+ */
+public class ContinuationHandlerMethodArgumentResolver implements HandlerMethodArgumentResolver {
+
+	@Override
+	public boolean supportsParameter(MethodParameter parameter) {
+		return "kotlin.coroutines.Continuation".equals(parameter.getParameterType().getName());
+	}
+
+	@Override
+	public Object resolveArgument(MethodParameter parameter, ModelAndViewContainer mavContainer, NativeWebRequest webRequest, WebDataBinderFactory binderFactory) throws Exception {
+		return null;
+	}
+}

spring-webmvc/src/main/java/org/springframework/web/servlet/mvc/method/annotation/RequestMappingHandlerAdapter.java

@@ -34,6 +34,7 @@ import org.springframework.beans.factory.BeanFactoryAware;
 import org.springframework.beans.factory.InitializingBean;
 import org.springframework.beans.factory.config.ConfigurableBeanFactory;
 import org.springframework.core.DefaultParameterNameDiscoverer;
+import org.springframework.core.KotlinDetector;
 import org.springframework.core.MethodIntrospector;
 import org.springframework.core.ParameterNameDiscoverer;
 import org.springframework.core.ReactiveAdapterRegistry;
@@ -670,6 +671,9 @@ public class RequestMappingHandlerAdapter extends AbstractHandlerMethodAdapter
 		resolvers.add(new ErrorsMethodArgumentResolver());
 		resolvers.add(new SessionStatusMethodArgumentResolver());
 		resolvers.add(new UriComponentsBuilderMethodArgumentResolver());
+		if (KotlinDetector.isKotlinPresent()) {
+			resolvers.add(new ContinuationHandlerMethodArgumentResolver());
+		}
 
 		// Custom arguments
 		if (getCustomArgumentResolvers() != null) {

spring-webmvc/src/main/java/org/springframework/web/servlet/mvc/method/annotation/ServletInvocableHandlerMethod.java

@@ -25,6 +25,7 @@ import java.util.concurrent.Callable;
 import javax.servlet.http.HttpServletRequest;
 import javax.servlet.http.HttpServletResponse;
 
+import org.springframework.core.KotlinDetector;
 import org.springframework.core.MethodParameter;
 import org.springframework.core.ResolvableType;
 import org.springframework.http.HttpHeaders;
@@ -271,6 +272,8 @@ public class ServletInvocableHandlerMethod extends InvocableHandlerMethod {
 			this.returnValue = returnValue;
 			this.returnType = (returnValue instanceof ReactiveTypeHandler.CollectedValuesList ?
 					((ReactiveTypeHandler.CollectedValuesList) returnValue).getReturnType() :
+					KotlinDetector.isSuspendingFunction(super.getMethod()) ?
+					ResolvableType.forMethodParameter(getReturnType()) :
 					ResolvableType.forType(super.getGenericParameterType()).getGeneric());
 		}