Portlet AbstractController consistently uses session mutex if "synchronizeOnSession" is active

org.springframework.web.portlet/src/main/java/org/springframework/web/portlet/mvc/AbstractController.java

 /*
- * Copyright 2002-2009 the original author or authors.
+ * Copyright 2002-2010 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.
@@ -188,7 +188,8 @@ public abstract class AbstractController extends PortletContentGenerator impleme
 		if (this.synchronizeOnSession) {
 			PortletSession session = request.getPortletSession(false);
 			if (session != null) {
-				synchronized (session) {
+				Object mutex = PortletUtils.getSessionMutex(session);
+				synchronized (mutex) {
 					handleActionRequestInternal(request, response);
 					return;
 				}