public interface Interceptor extends Serializable {
 
  void destroy();
 
  void init();
 
  String intercept(ActionInvocation invocation) throws Exception;
}

public String intercept(ActionInvocation invocation) throws Exception {
    
    System.out.println("在action执行之前");
    ActionContext actionContext=invocation.getInvocationContext();
    Map<String,Object> session=actionContext.getSession();
    
    Object currentUser=session.get("currentUser");
    String result=null;
    if(currentUser!=null){
      result=invocation.invoke();
      
    }else{
      
      HttpServletRequest request=(HttpServletRequest)invocation.getInvocationContext().get(ServletActionContext.HTTP_REQUEST);
      request.setAttribute("error", "请先登录");
      result="error";
    }
    System.out.println("result+"+result);
    System.out.println("在action执行之后");
    return result;
  }

<interceptors>
       <interceptor name="myInterceptor"
       class="com.fangchao.interceptor.MyInterceptor"></interceptor>
       <interceptor name="loginInterceptor"
       class="com.fangchao.interceptor.LoginInterceptor"></interceptor>
       
       <interceptor-stack name="myStack">
         <interceptor-ref name="loginInterceptor"></interceptor-ref>
         <interceptor-ref name="defaultStack"></interceptor-ref>  
       </interceptor-stack>
     </interceptors>

<interceptor-ref name="validation">
 <param name="excludeMethods">myValidationExcudeMethod</param>
</interceptor-ref>
<interceptor-ref name="workflow">
 <param name="excludeMethods">myWorkflowExcludeMethod</param>
</interceptor-ref>

<interceptor-ref name="defaultStack">
  <param name="validation.excludeMethods">myValidationExcludeMethod</param>
  <param name="workflow.excludeMethods">myWorkflowExcludeMethod</param>
</interceptor-ref>