Exception starting filter struts2 java.lang.NullPointerException?

The error "Exception starting filter struts2 java.lang.NullPointerException" is shown when i 开发者_StackOverflowTry to run my application. I am programming in Eclipse 3.6 on Struts 2. part of my My Struts.xml looks like below:

   <struts>
    <!-- remove these constant elements in production -->
    <constant name="struts.enable.DynamicMethodInvocation" value="false" />
    <constant name="struts.devMode" value="true" />
    <package name="app02a" extends="struts-default">
        <action name="Product_input">
            <result>/jsp/Product.jsp</result>
        </action>
        <action name="Product_save" class="app02a.Product" method="execute">
            <result>/jsp/Details.jsp</result>
        </action>
    </package>

</struts>