关于拦截器的有关问题 错误苦恼

@SuppressWarnings("serial")public class MyInterceptor extends AbstractInterceptor {    @SuppressWarnings("unchecked")    @Override    public String intercept(ActionInvocation arg0) throws Exception {//      HttpSession  hs =   ServletActionContext.getRequest().getSession();//        String username = (String) hs.getAttribute("username");//                              ActionContext ac  =  arg0.getInvocationContext();        Map session = ac.getSession();    System.out.println(session.size());                Userinfo userinfo = (Userinfo) session.get("user");                       if(userinfo==null){                return Action.LOGIN;                            }else{            return arg0.invoke();                            }                           }}

<interceptors>            <interceptor name="myInterceptor" class="cn.com.MyInterceptor"/>            <interceptor-stack name="myInterceptorStack">                <interceptor-ref name="defaultStack"/>                <interceptor-ref name="myInterceptor"/>            </interceptor-stack>        </interceptors>        <action name="list_*" class="cn.com.action.TheAction" method="{1}"> <!--拦截的TheAction中的方法-->            <interceptor-ref name="myInterceptorStack" />        </action>