ClassNotFoundException with "use"

<zk>
    <window id="root">    
        <tabbox use="com.MyTabbox" id="tabbox" width="100%" >  
            <tabs>  
                <tab label="Preload"/>  
                <tab label="OnDemand 1"/>  
                <tab label="OnDemand 2"/>  
            </tabs>  
            <tabpanels>  
                <tabpanel>  
                    This panel is pre-loaded.  
                </tabpanel>  
                <tabpanel id="second"> 
                    <include id="secondContent" src=""/>
                </tabpanel>  
                <tabpanel id="third"> 
                    <include id="thirdContent" src=""/>
                </tabpanel>
            </tabpanels> 
        </tabbox> 
    </window>
</zk>

package com;

import org.zkoss.zk.ui.Desktop;
import org.zkoss.zk.ui.Path;
import org.zkoss.zul.Include;
import org.zkoss.zul.Tabpanel;

public class MyTabbox extends org.zkoss.zul.Tabbox {
    
private int i=0;
    
    public void onSelect() {
        
        i++;
        
        Tabpanel item = getSelectedPanel();
        
        Desktop desktop = this.getDesktop();
        
        desktop.setAttribute("time",i);
        
        if(item != null && item.getId().equals("second")) {
            Include inc = (Include)Path.getComponent("/root/secondContent");
            inc.setSrc("secondTab.zul");
        }
        if(item != null && item.getId().equals("third")) {
            Include inc = (Include)Path.getComponent("/root/thirdContent");
            inc.invalidate();
            inc.setSrc("thirdTab.zul");
        }
    }
    
}

<?xml version="1.0" encoding="UTF-8"?>
<web-app id="WebApp_ID" version="2.4" xmlns="http://java.sun.com/xml/ns/j2ee" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee http://java.sun.com/xml/ns/j2ee/web-app_2_4.xsd">
	<display-name>testing with the load on demand</display-name>
	<listener>
		<description>
		Used to cleanup when a session is destroyed</description>
		<display-name>
		ZK Session Cleaner</display-name>
		<listener-class>org.zkoss.zk.ui.http.HttpSessionListener</listener-class>
	</listener>
	<servlet>
		<description>
		The servlet loads the DSP pages.</description>
		<servlet-name>dspLoader</servlet-name>
		<servlet-class>
		org.zkoss.web.servlet.dsp.InterpreterServlet</servlet-class>
	</servlet>
	<servlet>
		<description>
		ZK loader for ZUML pages</description>
		<servlet-name>zkLoader</servlet-name>
		<servlet-class>
		org.zkoss.zk.ui.http.DHtmlLayoutServlet</servlet-class>
		<init-param>
			<param-name>update-uri</param-name>
			<param-value>/zkau</param-value>
		</init-param>
		<load-on-startup>1</load-on-startup>
	</servlet>
	<servlet>
		<description>
		The asynchronous update engine for ZK</description>
		<servlet-name>auEngine</servlet-name>
		<servlet-class>
		org.zkoss.zk.au.http.DHtmlUpdateServlet</servlet-class>
	</servlet>
	<servlet-mapping>
		<servlet-name>dspLoader</servlet-name>
		<url-pattern>*.dsp</url-pattern>
	</servlet-mapping>
	<servlet-mapping>
		<servlet-name>zkLoader</servlet-name>
		<url-pattern>*.zul</url-pattern>
	</servlet-mapping>
	<servlet-mapping>
		<servlet-name>zkLoader</servlet-name>
		<url-pattern>*.zhtml</url-pattern>
	</servlet-mapping>
	<servlet-mapping>
		<servlet-name>auEngine</servlet-name>
		<url-pattern>/zkau/*</url-pattern>
	</servlet-mapping>
	<welcome-file-list>
		<welcome-file>index.html</welcome-file>
		<welcome-file>index.htm</welcome-file>
		<welcome-file>index.jsp</welcome-file>
		<welcome-file>default.html</welcome-file>
		<welcome-file>default.htm</welcome-file>
		<welcome-file>default.jsp</welcome-file>
		<welcome-file>index.zul</welcome-file>
	</welcome-file-list>
</web-app>