DataTable rendered attribute

Question

I can't for the life of me figure out why once I include the rendered attribute my backing bean method gets call so many times .
view
<h:form>
     <h:commandButton action="#{contactController.addContact}">Add Contact</h:commandButton>
     <h:dataTable value="#{contactController.contacts}" var="contact" rendered="#{not empty contactController.contacts}">
     <h:column>
     <h:outputText value="#{contact.name}"/>
     </h:column>                         
     </h:dataTable>
</h:form>
bean
public class ContactController {
     private static List<Contact> contacts = new ArrayList<Contact>();
     public Object addContact() {
          System.out.println("in add contact ...");
          contacts.add(new Contact());
          return null;
     public List<Contact> getContacts() {
          System.out.println("in get contacts ...");
          return contacts;
The backing bean is in the request scope. If I press add contact I get the server output
in get contacts ...
in get contacts ...
in get contacts ...
in add contact ...
in get contacts ...
in get contacts ...
in get contacts ...
in get contacts ...
in get contacts ...
in get contacts ...
in get contacts ...
in get contacts ...
. If I remove the rendered attribute, the message "in get contacts" only appears twice as expected.
Thanks
-Serge-

Answer 1

This is speculation, but I imagine that each child of the dataTable component is trying to determine its rendered status by querying up the tree, resulting in the rendered property of the dataTable being resolved multiple times.
In general you should be aware that the results of an EL expression for a property on a component is not cached by the JSF implementation. You should also be aware that the property may need to be resolved multiple times over the lifetime of a component. If the result should be cached, you need to do the caching in your getter.