Fix issue with async return value type determination

Before this change, the type of asynchronously produced return values
(e.g. Callable, DeferredResult, ListenableFuture) could not be
properly determined with an actual resulting value of null. Or even
with an actual value returned, the generic type could not be properly
determined. This change fixes both of those issues.

Issue: SPR-12287

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2012 the original author or authors.
+ * Copyright 2002-2014 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.
@@ -19,8 +19,11 @@ package org.springframework.web.servlet.mvc.method.annotation;
 import java.io.IOException;
 import java.lang.annotation.Annotation;
 import java.lang.reflect.Method;
+import java.lang.reflect.Type;
 import java.util.concurrent.Callable;
 
+import org.springframework.core.MethodParameter;
+import org.springframework.core.ResolvableType;
 import org.springframework.http.HttpStatus;
 import org.springframework.util.ClassUtils;
 import org.springframework.util.StringUtils;
@@ -51,6 +54,9 @@ import org.springframework.web.util.NestedServletException;
  */
 public class ServletInvocableHandlerMethod extends InvocableHandlerMethod {
 
+	private static final Method CALLABLE_METHOD = ClassUtils.getMethod(Callable.class, "call");
+
+
 	private HttpStatus responseStatus;
 
 	private String responseReason;
@@ -174,46 +180,46 @@ public class ServletInvocableHandlerMethod extends InvocableHandlerMethod {
 	}
 
 	/**
-	 * Create a ServletInvocableHandlerMethod that will return the given value from an
-	 * async operation instead of invoking the controller method again. The async result
-	 * value is then either processed as if the controller method returned it or an
-	 * exception is raised if the async result value itself is an Exception.
+	 * Create a nested ServletInvocableHandlerMethod sub-class that returns the
+	 * the given value (or raises an Exception if the value is one) rather than
+	 * actually invoking the controller method. This is useful when processing
+	 * async return values (e.g. Callable, DeferredResult, ListenableFuture).
 	 */
-	ServletInvocableHandlerMethod wrapConcurrentResult(final Object result) {
-
-		return new CallableHandlerMethod(new Callable<Object>() {
-
-			@Override
-			public Object call() throws Exception {
-				if (result instanceof Exception) {
-					throw (Exception) result;
-				}
-				else if (result instanceof Throwable) {
-					throw new NestedServletException("Async processing failed", (Throwable) result);
-				}
-				return result;
-			}
-		});
+	ServletInvocableHandlerMethod wrapConcurrentResult(Object result) {
+		return new ConcurrentResultHandlerMethod(result, new ConcurrentResultMethodParameter(result));
 	}
 
 
 	/**
-	 * A sub-class of {@link HandlerMethod} that invokes the given {@link Callable}
-	 * instead of the target controller method. This is useful for resuming processing
-	 * with the result of an async operation. The goal is to process the value returned
-	 * from the Callable as if it was returned by the target controller method, i.e.
-	 * taking into consideration both method and type-level controller annotations (e.g.
-	 * {@code @ResponseBody}, {@code @ResponseStatus}, etc).
+	 * A nested sub-class of {@code ServletInvocableHandlerMethod} that uses a
+	 * simple {@link Callable} instead of the original controller as the handler in
+	 * order to return the fixed (concurrent) result value given to it. Effectively
+	 * "resumes" processing with the asynchronously produced return value.
 	 */
-	private class CallableHandlerMethod extends ServletInvocableHandlerMethod {
+	private class ConcurrentResultHandlerMethod extends ServletInvocableHandlerMethod {
 
-		public CallableHandlerMethod(Callable<?> callable) {
-			super(callable, ClassUtils.getMethod(callable.getClass(), "call"));
-			this.setHandlerMethodReturnValueHandlers(ServletInvocableHandlerMethod.this.returnValueHandlers);
+		private final MethodParameter returnType;
+
+
+		public ConcurrentResultHandlerMethod(final Object result, ConcurrentResultMethodParameter returnType) {
+			super(new Callable<Object>() {
+				@Override
+				public Object call() throws Exception {
+					if (result instanceof Exception) {
+						throw (Exception) result;
+					}
+					else if (result instanceof Throwable) {
+						throw new NestedServletException("Async processing failed", (Throwable) result);
+					}
+					return result;
+				}
+			}, CALLABLE_METHOD);
+			setHandlerMethodReturnValueHandlers(ServletInvocableHandlerMethod.this.returnValueHandlers);
+			this.returnType = returnType;
 		}
 
 		/**
-		 * Bridge to type-level annotations of the target controller method.
+		 * Bridge to actual controller type-level annotations.
 		 */
 		@Override
 		public Class<?> getBeanType() {
@@ -221,7 +227,16 @@ public class ServletInvocableHandlerMethod extends InvocableHandlerMethod {
 		}
 
 		/**
-		 * Bridge to method-level annotations of the target controller method.
+		 * Bridge to actual return value or generic type within the declared
+		 * async return type, e.g. Foo instead of {@code DeferredResult<Foo>}.
+		 */
+		@Override
+		public MethodParameter getReturnValueType(Object returnValue) {
+			return this.returnType;
+		}
+
+		/**
+		 * Bridge to controller method-level annotations.
 		 */
 		@Override
 		public <A extends Annotation> A getMethodAnnotation(Class<A> annotationType) {
@@ -229,4 +244,33 @@ public class ServletInvocableHandlerMethod extends InvocableHandlerMethod {
 		}
 	}
 
+	/**
+	 * MethodParameter sub-class based on the actual return value type or if
+	 * that's null falling back on the generic type within the declared async
+	 * return type, e.g. Foo instead of {@code DeferredResult<Foo>}.
+	 */
+	private class ConcurrentResultMethodParameter extends HandlerMethodParameter {
+
+		private final Object returnValue;
+
+		private final ResolvableType returnType;
+
+
+		public ConcurrentResultMethodParameter(Object returnValue) {
+			super(-1);
+			this.returnValue = returnValue;
+			this.returnType = ResolvableType.forType(super.getGenericParameterType()).getGeneric(0);
+		}
+
+		@Override
+		public Class<?> getParameterType() {
+			return (returnValue != null ? returnValue.getClass() : this.returnType.getRawClass());
+		}
+
+		@Override
+		public Type getGenericParameterType() {
+			return this.returnType.getType();
+		}
+	}
+
 }