selectlist with actionsupport

1. <apex:selectlist id="slId" value="{!selectedValue}" multiselect="false" size="1">
2. <apex:selectoptions value="{!selectOptions}"></apex:selectoptions>
3. <apex:actionsupport event="onchange" action="{!doAction}" rerender="cat2List, cat3List, cat4List, proTable" status="stageStatus"></apex:actionsupport> 
4. </apex:selectlist> 
5. 
   
6. <apex:actionstatus id="stageStatus" starttext="Updating Next Criterion and Product list ...." ></apex:actionstatus>

1. public doAction() 
2. { 
3. 
   
4. 
   
5.  return null;
6. }