why throwExceptionIfNoHandlerFound not working in web.xml in spring mvc

    <servlet>
        <servlet-name>app1</servlet-name>
        <servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>

        <init-param>
            <param-name>contextClass</param-name>
            <param-value>
                org.springframework.web.context.support.AnnotationConfigWebApplicationContext
            </param-value>
        </init-param>

        <init-param>
            <param-name>contextConfigLocation</param-name>
            <param-value>
                com.web.springmvc
            </param-value>
        </init-param>

        <init-param>
            <param-name>throwExceptionIfNoHandlerFound</param-name>
            <param-value>true</param-value>
        </init-param>

        <load-on-startup>1</load-on-startup>
    </servlet>
    <servlet-mapping>
        <servlet-name>app1</servlet-name>
        <url-pattern>/</url-pattern>
    </servlet-mapping>

why throwExceptionIfNoHandlerFound not working and 404 page is returned although no handler was found for a request