Very simple, just define a packaging system for your servlet  classes. Some of application servers don't like a servlet without packaging.

    <servlet>
        <servlet-name>webTierEntryPoint</servlet-name>
        <display-name>HTML Client Front Controller Servlet</display-name>
        <description>no description</description>
        <!-- here is the point: it cannot be just a MainSerlvet without any package -->
        <servlet-class>com.nirvana.components.waf.controller.web.MainServlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>webTierEntryPoint</servlet-name>
        <url-pattern>*.do</url-pattern>
    </servlet-mapping>