@RequestMapping("/captcha.jpg")
    public void captcha(HttpServletResponse response) throws ServletException, IOException {
        response.setHeader("Cache-Control", "no-store, no-cache");
        response.setContentType("image/jpeg");

        // 生成文字验证码
        String text = producer.createText();
        // 生成图片验证码
        BufferedImage image = producer.createImage(text);
        // 保存到shiro session
        ShiroUtils.setSessionAttribute(Constants.KAPTCHA_SESSION_KEY, text);

        ServletOutputStream out = response.getOutputStream();
        ImageIO.write(image, "jpg", out);
    }