spring mvc fastjson整合

spring mvc 与fastjson怎么弄

mvc:annotation-driven
mvc:message-converters
 class="com.alibaba.fastjson.support.spring.FastJsonHttpMessageConverter" />
/mvc:message-converters
/mvc:annotation-driven
这是配置
@RequestMapping("/neworders")
@ResponseBody
public String querynew(HttpServletRequest req, HttpServletResponse res) {
    String pageNum = req.getParameter("");
    if (StringUtils.isEmpty(pageNum.trim())) {
        json = "页码不能为空";
        return json;
    }
    PageBean page = new PageBean();
    page.setCurrentPageNo(Integer.parseInt(pageNum));
    return "";
}

这是controller
为什么pageNum没值的情况下为什么没有返回json