I can not figure out why ClassNotFoundException happened when i learn ServletContextListener

1. platform:eclipse_ee_mars + tomcat8.5
2. SimpleServletContext is mainly inspect the attributes.

3. Here is my SimpleServletConetxtListenerCode

   package zhang.listenerandfilter;
   import java.util.Date;
   import javax.servlet.ServletContext;
   import javax.servlet.ServletContextAttributeEvent;
   import javax.servlet.ServletContextAttributeListener;
   import javax.servlet.ServletContextEvent;
   import javax.servlet.ServletContextListener;
   public abstract class SimpleServletContextListener 
   implements ServletContextListener , ServletContextAttributeListener{
   @Override
   public void contextInitialized(ServletContextEvent sce) {
       System.out.println("Start Time:" + new Date());
   }
   @Override
   public void contextDestroyed(ServletContextEvent arg0) {
       System.out.println("Stop Time:" + new Date());
   
   }
   @Override
   public void attributeAdded(ServletContextAttributeEvent scae) {
       System.out.println("Added an Attribute:" + scae.getName() + "=" + scae.getValue());
   }
   
   @Override
   public void attributeRemoved(ServletContextAttributeEvent scae) {
       System.out.println("Removed an Attribute:" + scae.getName() + "=" + scae.getValue());
   }
   
   @Override
   public void attributeReplaced(ServletContextAttributeEvent scae) {
       ServletContext sc = scae.getServletContext();
       System.out.println("Replaced an Attribute(Old):" + scae.getName() + "=" + scae.getValue());
       System.out.println("(New):" + scae.getName() + "=" + sc.getAttribute(scae.getName()));
   }
   
   

   }

here is my web.xml

<listener>
    <listener-class>zhang.listenerandfilter.SimpleServletContextlistener</listener-class>
</listener>