fix for WW-4573

NPE/ concurrent modification exception

using a CopyOnWriteArrayList. This is to keep Memory consistency on the
ValueStack.

xwork-core/src/main/java/com/opensymphony/xwork2/util/CompoundRoot.java

@@ -17,6 +17,7 @@ package com.opensymphony.xwork2.util;
 
 import java.util.ArrayList;
 import java.util.List;
+import java.util.concurrent.CopyOnWriteArrayList;
 
 
 /**
@@ -25,12 +26,14 @@ import java.util.List;
  * @author plightbo
  * @version $Revision$
  */
-public class CompoundRoot extends ArrayList {
+public class CompoundRoot extends CopyOnWriteArrayList<Object> {
+
+    private static final long serialVersionUID = 8563229069192473995L;
 
     public CompoundRoot() {
     }
 
-    public CompoundRoot(List list) {
+    public CompoundRoot(List<?> list) {
         super(list);
     }