WebAsyncManager defensively ignores attribute type mismatch

Issue: SPR-15709
(cherry picked from commit c4694c3)

Author: jhoeller

spring-web/src/main/java/org/springframework/web/context/request/async/WebAsyncUtils.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2015 the original author or authors.
+ * Copyright 2002-2017 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.
@@ -44,7 +44,11 @@ public abstract class WebAsyncUtils {
 	 * found, create and associate it with the request.
 	 */
 	public static WebAsyncManager getAsyncManager(ServletRequest servletRequest) {
-		WebAsyncManager asyncManager = (WebAsyncManager) servletRequest.getAttribute(WEB_ASYNC_MANAGER_ATTRIBUTE);
+		WebAsyncManager asyncManager = null;
+		Object asyncManagerAttr = servletRequest.getAttribute(WEB_ASYNC_MANAGER_ATTRIBUTE);
+		if (asyncManagerAttr instanceof WebAsyncManager) {
+			asyncManager = (WebAsyncManager) asyncManagerAttr;
+		}
 		if (asyncManager == null) {
 			asyncManager = new WebAsyncManager();
 			servletRequest.setAttribute(WEB_ASYNC_MANAGER_ATTRIBUTE, asyncManager);
@@ -58,7 +62,11 @@ public static WebAsyncManager getAsyncManager(ServletRequest servletRequest) {
 	 */
 	public static WebAsyncManager getAsyncManager(WebRequest webRequest) {
 		int scope = RequestAttributes.SCOPE_REQUEST;
-		WebAsyncManager asyncManager = (WebAsyncManager) webRequest.getAttribute(WEB_ASYNC_MANAGER_ATTRIBUTE, scope);
+		WebAsyncManager asyncManager = null;
+		Object asyncManagerAttr = webRequest.getAttribute(WEB_ASYNC_MANAGER_ATTRIBUTE, scope);
+		if (asyncManagerAttr instanceof WebAsyncManager) {
+			asyncManager = (WebAsyncManager) asyncManagerAttr;
+		}
 		if (asyncManager == null) {
 			asyncManager = new WebAsyncManager();
 			webRequest.setAttribute(WEB_ASYNC_MANAGER_ATTRIBUTE, asyncManager, scope);