@Override

    public void showCodeImg(HttpServletRequest request, HttpServletResponse response)  throws IOException {

        response.setHeader("Expires","-1");//控制缓存的失效日期

        response.setHeader("Cache-Control","no-cache");//必须先与服务器确认返回的响应是否被更改，然后才能使用该响应来满足后续对同一个网址的请求

        response.setHeader("Pragma","-1");

        CaptchaUtil util=CaptchaUtil.Instance();

        // 将验证码输入到session中，用来验证

        String code=util.getString();

        request.setCharacterEncoding("UTF-8");

        request.getSession().setAttribute(SecurityCodeConstant.CODE_NUM,code);

        // 输出到web页面

        ImageIO.write(util.getImage(), "jpg", response.getOutputStream());

    }