请教一个js访問Action的問趧

public ActionForward execute(ActionMapping mapping, ActionForm form,
            HttpServletRequest request, HttpServletResponse response) {
        try {
            response.setContentType("text/html");
            PrintWriter out = response.getWriter();
            MessageForm messageForm = (MessageForm)form;

            UserBean userBean = (UserBean)request.getSession().getAttribute("userBean");//得到用户信息的JavaBean

            List list = ims.ifMessage(Integer.parseInt(userBean.getUser_dwid()));//这个方法为显示状态为未读的信息列表；

            //如果有信息，则输出信息的id号；否则则输出0
            if(list.size()>0){
                DccMessage dccMessage = (DccMessage)list.get(0);
                out.print(dccMessage.getId());
            }else{
                out.print("0");
            }
        } catch (Exception e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
        return null;

<script type="text/javascript">
setInterval("showMessage()", 30000);//每隔30秒执行一次

//定义Ajax
function showMessage(){
new Ajax.Request("/DCCCZXT/message/ifMessageAction.do",{method:'get',onComplete:po});
}

//如果接收到Action里面out出来的值为非0的情况下，则打开该新消息的窗口
function po(res){
var a=res.responseText
if(a!=0){
    window.open("/DCCCZXT/message/openMessageAction.do?id="+res.responseText,"","width=320,height=400");
}
}

</script>