Add AsyncHandlerMethodReturnValueHandler

Before this change HandlerMethodReturnValueHandler's were invoked in a
specific order (type-based, annotation-based, custom). However handlers
that deal with asynchronous return value handling need to always be
considered first. This affects custom handlers in particular since they
are normally ordered last.

This change introduces an AsyncHandlerMethodReturnValueHandler
sub-interface with a single method to determine if the return value is
asynchronous and if it is to look for a matching handler only among
those that are of type AsyncHandlerMethodReturnValueHandler.

Issue: SPR-13083

Author: rstoyanchev

spring-web/src/main/java/org/springframework/web/method/support/AsyncHandlerMethodReturnValueHandler.java

@@ -0,0 +1,50 @@
+/*
+ * 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.method.support;
+
+import org.springframework.core.MethodParameter;
+
+/**
+ * A {@link HandlerMethodReturnValueHandler} that handles return values that
+ * represent asynchronous computation. Such handlers need to be invoked with
+ * precedence over other handlers that might otherwise match the return value
+ * type -- e.g. a method that returns a Promise type that is also annotated with
+ * {@code @ResponseBody}.
+ *
+ * <p>In {@link #handleReturnValue}, implementations of this class should create
+ * a {@link org.springframework.web.context.request.async.DeferredResult} or
+ * adapt to it and then invoke {@code WebAsyncManager} to start async processing.
+ * For example:
+ * <pre>
+ * DeferredResult<?> deferredResult = (DeferredResult<?>) returnValue;
+ * WebAsyncUtils.getAsyncManager(webRequest).startDeferredResultProcessing(deferredResult, mavContainer);
+ * </pre>
+ * the return value to a DeferredResult
+ *
+ * @author Rossen Stoyanchev
+ * @since 4.2
+ */
+public interface AsyncHandlerMethodReturnValueHandler extends HandlerMethodReturnValueHandler {
+
+	/**
+	 * Whether the given return value represents asynchronous computation.
+	 * @param returnValue the return value
+	 * @param returnType the return type
+	 * @return {@code true} if the return value is asynchronous.
+	 */
+	boolean isAsyncReturnValue(Object returnValue, MethodParameter returnType);
+
+}

spring-web/src/main/java/org/springframework/web/method/support/HandlerMethodReturnValueHandlerComposite.java

@@ -34,7 +34,7 @@
  * @author Rossen Stoyanchev
  * @since 3.1
  */
-public class HandlerMethodReturnValueHandlerComposite implements HandlerMethodReturnValueHandler {
+public class HandlerMethodReturnValueHandlerComposite implements AsyncHandlerMethodReturnValueHandler {
 
 	protected final Log logger = LogFactory.getLog(getClass());
 
@@ -58,6 +58,15 @@ public boolean supportsReturnType(MethodParameter returnType) {
 		return getReturnValueHandler(returnType) != null;
 	}
 
+	private HandlerMethodReturnValueHandler getReturnValueHandler(MethodParameter returnType) {
+		for (HandlerMethodReturnValueHandler handler : this.returnValueHandlers) {
+			if (handler.supportsReturnType(returnType)) {
+				return handler;
+			}
+		}
+		return null;
+	}
+
 	/**
 	 * Iterate over registered {@link HandlerMethodReturnValueHandler}s and invoke the one that supports it.
 	 * @throws IllegalStateException if no suitable {@link HandlerMethodReturnValueHandler} is found.
@@ -66,32 +75,41 @@ public boolean supportsReturnType(MethodParameter returnType) {
 	public void handleReturnValue(Object returnValue, MethodParameter returnType,
 			ModelAndViewContainer mavContainer, NativeWebRequest webRequest) throws Exception {
 
-		HandlerMethodReturnValueHandler handler = getReturnValueHandler(returnType);
+		HandlerMethodReturnValueHandler handler = selectHandler(returnValue, returnType);
 		Assert.notNull(handler, "Unknown return value type [" + returnType.getParameterType().getName() + "]");
 		handler.handleReturnValue(returnValue, returnType, mavContainer, webRequest);
 	}
 
-	/**
-	 * Find a registered {@link HandlerMethodReturnValueHandler} that supports the given return type.
-	 */
-	private HandlerMethodReturnValueHandler getReturnValueHandler(MethodParameter returnType) {
-		for (HandlerMethodReturnValueHandler returnValueHandler : returnValueHandlers) {
-			if (logger.isTraceEnabled()) {
-				logger.trace("Testing if return value handler [" + returnValueHandler + "] supports [" +
-						returnType.getGenericParameterType() + "]");
+	private HandlerMethodReturnValueHandler selectHandler(Object value, MethodParameter returnType) {
+		boolean isAsyncValue = isAsyncReturnValue(value, returnType);
+		for (HandlerMethodReturnValueHandler handler : this.returnValueHandlers) {
+			if (isAsyncValue && !(handler instanceof AsyncHandlerMethodReturnValueHandler)) {
+				continue;
 			}
-			if (returnValueHandler.supportsReturnType(returnType)) {
-				return returnValueHandler;
+			if (handler.supportsReturnType(returnType)) {
+				return handler;
 			}
 		}
 		return null;
 	}
 
+	@Override
+	public boolean isAsyncReturnValue(Object value, MethodParameter returnType) {
+		for (HandlerMethodReturnValueHandler handler : this.returnValueHandlers) {
+			if (handler instanceof AsyncHandlerMethodReturnValueHandler) {
+				if (((AsyncHandlerMethodReturnValueHandler) handler).isAsyncReturnValue(value, returnType)) {
+					return true;
+				}
+			}
+		}
+		return false;
+	}
+
 	/**
 	 * Add the given {@link HandlerMethodReturnValueHandler}.
 	 */
 	public HandlerMethodReturnValueHandlerComposite addHandler(HandlerMethodReturnValueHandler handler) {
-		returnValueHandlers.add(handler);
+		this.returnValueHandlers.add(handler);
 		return this;
 	}
 

spring-web/src/test/java/org/springframework/web/method/support/HandlerMethodReturnValueHandlerCompositeTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2012 the original author or authors.
+ * 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.
@@ -22,76 +22,114 @@
 import org.springframework.core.MethodParameter;
 
 import static org.junit.Assert.*;
+import static org.mockito.Mockito.*;
+
+import java.lang.annotation.Documented;
+import java.lang.annotation.ElementType;
+import java.lang.annotation.Retention;
+import java.lang.annotation.RetentionPolicy;
+import java.lang.annotation.Target;
 
 /**
  * Test fixture with {@link HandlerMethodReturnValueHandlerComposite}.
- *
  * @author Rossen Stoyanchev
  */
+@SuppressWarnings("unused")
 public class HandlerMethodReturnValueHandlerCompositeTests {
 
 	private HandlerMethodReturnValueHandlerComposite handlers;
 
+	private HandlerMethodReturnValueHandler integerHandler;
+
 	ModelAndViewContainer mavContainer;
 
-	private MethodParameter paramInt;
+	private MethodParameter integerType;
+
+	private MethodParameter stringType;
 
-	private MethodParameter paramStr;
 
 	@Before
 	public void setUp() throws Exception {
-		handlers = new HandlerMethodReturnValueHandlerComposite();
+
+		this.integerType = new MethodParameter(getClass().getDeclaredMethod("handleInteger"), -1);
+		this.stringType = new MethodParameter(getClass().getDeclaredMethod("handleString"), -1);
+
+		this.integerHandler = mock(HandlerMethodReturnValueHandler.class);
+		when(this.integerHandler.supportsReturnType(this.integerType)).thenReturn(true);
+
+		this.handlers = new HandlerMethodReturnValueHandlerComposite();
+		this.handlers.addHandler(this.integerHandler);
+
 		mavContainer = new ModelAndViewContainer();
-		paramInt = new MethodParameter(getClass().getDeclaredMethod("handleInteger"), -1);
-		paramStr = new MethodParameter(getClass().getDeclaredMethod("handleString"), -1);
 	}
 
 	@Test
 	public void supportsReturnType() throws Exception {
-		registerHandler(Integer.class);
-
-		assertTrue(this.handlers.supportsReturnType(paramInt));
-		assertFalse(this.handlers.supportsReturnType(paramStr));
+		assertTrue(this.handlers.supportsReturnType(this.integerType));
+		assertFalse(this.handlers.supportsReturnType(this.stringType));
 	}
 
 	@Test
 	public void handleReturnValue() throws Exception {
-		StubReturnValueHandler handler = registerHandler(Integer.class);
-		this.handlers.handleReturnValue(Integer.valueOf(55), paramInt, mavContainer, null);
+		this.handlers.handleReturnValue(55, this.integerType, this.mavContainer, null);
+		verify(this.integerHandler).handleReturnValue(55, this.integerType, this.mavContainer, null);
+	}
+
+	@Test
+	public void handleReturnValueWithMultipleHandlers() throws Exception {
+		HandlerMethodReturnValueHandler anotherIntegerHandler = mock(HandlerMethodReturnValueHandler.class);
+		when(anotherIntegerHandler.supportsReturnType(this.integerType)).thenReturn(true);
 
-		assertEquals(Integer.valueOf(55), handler.getReturnValue());
+		this.handlers.handleReturnValue(55, this.integerType, this.mavContainer, null);
+
+		verify(this.integerHandler).handleReturnValue(55, this.integerType, this.mavContainer, null);
+		verifyNoMoreInteractions(anotherIntegerHandler);
 	}
 
+	// SPR-13083
+
 	@Test
-	public void handleReturnValueMultipleHandlers() throws Exception {
-		StubReturnValueHandler h1 = registerHandler(Integer.class);
-		StubReturnValueHandler h2 = registerHandler(Integer.class);
-		this.handlers.handleReturnValue(Integer.valueOf(55), paramInt, mavContainer, null);
+	public void handleReturnValueWithAsyncHandler() throws Exception {
+
+		Promise<Integer> promise = new Promise<>();
+		MethodParameter promiseType = new MethodParameter(getClass().getDeclaredMethod("handlePromise"), -1);
+
+		HandlerMethodReturnValueHandler responseBodyHandler = mock(HandlerMethodReturnValueHandler.class);
+		when(responseBodyHandler.supportsReturnType(promiseType)).thenReturn(true);
+		this.handlers.addHandler(responseBodyHandler);
+
+		AsyncHandlerMethodReturnValueHandler promiseHandler = mock(AsyncHandlerMethodReturnValueHandler.class);
+		when(promiseHandler.supportsReturnType(promiseType)).thenReturn(true);
+		when(promiseHandler.isAsyncReturnValue(promise, promiseType)).thenReturn(true);
+		this.handlers.addHandler(promiseHandler);
+
+		this.handlers.handleReturnValue(promise, promiseType, this.mavContainer, null);
 
-		assertEquals("Didn't use the 1st registered handler", Integer.valueOf(55), h1.getReturnValue());
-		assertNull("Shouldn't have use the 2nd registered handler", h2.getReturnValue());
+		verify(promiseHandler).isAsyncReturnValue(promise, promiseType);
+		verify(promiseHandler).supportsReturnType(promiseType);
+		verify(promiseHandler).handleReturnValue(promise, promiseType, this.mavContainer, null);
+		verifyNoMoreInteractions(promiseHandler);
+		verifyNoMoreInteractions(responseBodyHandler);
 	}
 
 	@Test(expected=IllegalArgumentException.class)
 	public void noSuitableReturnValueHandler() throws Exception {
-		registerHandler(Integer.class);
-		this.handlers.handleReturnValue("value", paramStr, null, null);
+		this.handlers.handleReturnValue("value", this.stringType, null, null);
 	}
 
-	private StubReturnValueHandler registerHandler(Class<?> returnType) {
-		StubReturnValueHandler handler = new StubReturnValueHandler(returnType);
-		handlers.addHandler(handler);
-		return handler;
-	}
 
-	@SuppressWarnings("unused")
 	private Integer handleInteger() {
 		return null;
 	}
 
-	@SuppressWarnings("unused")
 	private String handleString() {
 		return null;
 	}
 
+	private Promise<Integer> handlePromise() {
+		return null;
+	}
+
+	private static class Promise<T> {}
+
 }

spring-web/src/test/java/org/springframework/web/method/support/StubReturnValueHandler.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2014 the original author or authors.
+ * 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.
@@ -21,7 +21,7 @@
 import org.springframework.web.context.request.NativeWebRequest;
 import org.springframework.web.context.request.async.WebAsyncTask;
 import org.springframework.web.context.request.async.WebAsyncUtils;
-import org.springframework.web.method.support.HandlerMethodReturnValueHandler;
+import org.springframework.web.method.support.AsyncHandlerMethodReturnValueHandler;
 import org.springframework.web.method.support.ModelAndViewContainer;
 
 /**
@@ -30,7 +30,7 @@
  * @author Rossen Stoyanchev
  * @since 3.2
  */
-public class AsyncTaskMethodReturnValueHandler implements HandlerMethodReturnValueHandler {
+public class AsyncTaskMethodReturnValueHandler implements AsyncHandlerMethodReturnValueHandler {
 
 	private final BeanFactory beanFactory;
 
@@ -45,6 +45,11 @@ public boolean supportsReturnType(MethodParameter returnType) {
 		return WebAsyncTask.class.isAssignableFrom(returnType.getParameterType());
 	}
 
+	@Override
+	public boolean isAsyncReturnValue(Object returnValue, MethodParameter returnType) {
+		return (returnValue != null && returnValue instanceof WebAsyncTask);
+	}
+
 	@Override
 	public void handleReturnValue(Object returnValue, MethodParameter returnType,
 			ModelAndViewContainer mavContainer, NativeWebRequest webRequest) throws Exception {