Index: PortletWindowSessionNavigationalStates.java
===================================================================
--- PortletWindowSessionNavigationalStates.java	(revision 748068)
+++ PortletWindowSessionNavigationalStates.java	(working copy)
@@ -20,6 +20,7 @@
 import java.util.HashMap;
 import java.util.Iterator;
 import java.util.Map;
+import java.util.concurrent.ConcurrentHashMap;
 
 import javax.portlet.PortletMode;
 import javax.portlet.WindowState;
@@ -40,7 +41,7 @@
     {
         private static final long serialVersionUID = -2730733728229116932L;
         
-        public Map<String, PortletWindowBaseNavigationalState> windowStates = new HashMap<String, PortletWindowBaseNavigationalState>();
+        public Map<String, PortletWindowBaseNavigationalState> windowStates = new ConcurrentHashMap <String, PortletWindowBaseNavigationalState>();
         public String maximizedWindowId;
     }