fireObservedChange does not work with List?

[Vincent]

I am using fireObservedChange to prevent the other user to access the
same page at the same time , i.e. concurrency problem, I am apply this
for the attributes but it seems does not work with the list , does
anyone got the idea? Or am I miss something there?

<code>
private LIst list;

public List getList() {
        return list;
}

public void setList(List list) {
        this.list = list;
        Tapestry.fireObservedChange(this , "list" , list);
}
</code>