Support CompletableFuture as alternative to DeferredResult in MVC

Issue: SPR-12597

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

@@ -0,0 +1,76 @@
+/*
+ * Copyright 2002-2015 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.web.servlet.mvc.method.annotation;
+
+import java.util.concurrent.CompletionStage;
+import java.util.function.Consumer;
+import java.util.function.Function;
+
+import org.springframework.core.MethodParameter;
+import org.springframework.lang.UsesJava8;
+import org.springframework.web.context.request.NativeWebRequest;
+import org.springframework.web.context.request.async.DeferredResult;
+import org.springframework.web.context.request.async.WebAsyncUtils;
+import org.springframework.web.method.support.HandlerMethodReturnValueHandler;
+import org.springframework.web.method.support.ModelAndViewContainer;
+
+/**
+ * Handles return values of type {@link CompletionStage} (implemented by
+ * {@link java.util.concurrent.CompletableFuture} for example).
+ *
+ * @author Sebastien Deleuze
+ * @since 4.2
+ */
+@UsesJava8
+public class CompletionStageReturnValueHandler implements HandlerMethodReturnValueHandler {
+
+	@Override
+	public boolean supportsReturnType(MethodParameter returnType) {
+		return CompletionStage.class.isAssignableFrom(returnType.getParameterType());
+	}
+
+	@Override
+	public void handleReturnValue(Object returnValue, MethodParameter returnType,
+			ModelAndViewContainer mavContainer, NativeWebRequest webRequest) throws Exception {
+
+		if (returnValue == null) {
+			mavContainer.setRequestHandled(true);
+			return;
+		}
+
+		final DeferredResult<Object> deferredResult = new DeferredResult<Object>();
+		WebAsyncUtils.getAsyncManager(webRequest).startDeferredResultProcessing(deferredResult, mavContainer);
+
+		@SuppressWarnings("unchecked")
+		CompletionStage<Object> future = (CompletionStage<Object>) returnValue;
+		future.thenAccept(new Consumer<Object>() {
+			@Override
+			public void accept(Object result) {
+				deferredResult.setResult(result);
+			}
+		});
+		future.exceptionally(new Function<Throwable, Object>() {
+			@Override
+			public Object apply(Throwable ex) {
+				deferredResult.setErrorResult(ex);
+				return null;
+			}
+		});
+
+	}
+
+}

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

@@ -47,6 +47,7 @@ import org.springframework.http.converter.support.AllEncompassingFormHttpMessage
 import org.springframework.http.converter.xml.SourceHttpMessageConverter;
 import org.springframework.ui.ModelMap;
 import org.springframework.util.Assert;
+import org.springframework.util.ClassUtils;
 import org.springframework.util.CollectionUtils;
 import org.springframework.util.ReflectionUtils.MethodFilter;
 import org.springframework.web.accept.ContentNegotiationManager;
@@ -115,6 +116,10 @@ import org.springframework.web.util.WebUtils;
 public class RequestMappingHandlerAdapter extends AbstractHandlerMethodAdapter
 		implements BeanFactoryAware, InitializingBean {
 
+	private static final boolean completionStagePresent = ClassUtils.isPresent("java.util.concurrent.CompletionStage",
+			RequestMappingHandlerAdapter.class.getClassLoader());
+
+
 	private List<HandlerMethodArgumentResolver> customArgumentResolvers;
 
 	private HandlerMethodArgumentResolverComposite argumentResolvers;
@@ -653,6 +658,9 @@ public class RequestMappingHandlerAdapter extends AbstractHandlerMethodAdapter
 		handlers.add(new DeferredResultMethodReturnValueHandler());
 		handlers.add(new AsyncTaskMethodReturnValueHandler(this.beanFactory));
 		handlers.add(new ListenableFutureReturnValueHandler());
+		if (completionStagePresent) {
+			handlers.add(new CompletionStageReturnValueHandler());
+		}
 
 		// Annotation-based return value types
 		handlers.add(new ModelAttributeMethodProcessor(false));