How to get the value of in Bean

Question

I'm trying to get value of selectOneMenu from primefaces to Bean. it returns the old value not the new.

ManagedBean

public void doSetPrivilege( AjaxBehaviorEvent event)
 {
        SelectOneMenu selectOneMenu =  (SelectOneMenu) event.getSource();
        String value = selectOneMenu.getValue().toString();
        System.out.println(value);
 }

I get property privilegeApp from database

public Privilege getPrivilegeApp() {

            TreeTable treeTable = new TreeTable();
            treeTable = (TreeTable) FacesContext.getCurrentInstance().getViewRoot().findComponent(":idform:apps");

            data = treeTable.getRowNode();

            if (data.getType().equals("type1")) {
                Application app = applicationService.loadApplicationByDesignation(data.getData().toString());

                privilegeApp = privilegeService.getPrivilegeApplicationUtilisateur(getUtilisateur().getIdUtilisateur(),
                        app.getIdApplication());
            } 

           return privilegeApp;
        }

xhtml

  <p:treeTable id="apps" value="#{treeTableTest3.root}" var="app" selectionMode="single" selection="#{treeTableTest3.selectedNode}" >
   <p:column headerText="Designation" >
    <h:outputText value="#{app}" />
   </p:column>
     <p:column headerText="Etat" >  
    <center> 
            <p:selectOneMenu value="#{treeTableTest3.privilegeApp.etat}" id="etat" style="width:100px; size:30px; float:center;">
            <f:selectItem itemLabel="::Etat::" itemValue="#{null}"/>
            <f:selectItems value="#{treeTableTest3.etats}" 
             var="etat"
             itemValue="#{etat}" >
            </f:selectItems>
            <p:ajax event="change" listener="#{treeTableTest3.doSetPrivilege}" process="@all"/>
            </p:selectOneMenu>
            </center>
        </p:column> 

   </p:treeTable>

Thank's for your help