@ApiOperation(value = "获取验证码", produces = "image/gif")
@GetMapping(value = "/captcha", produces = "image/gif")
public void captcha(HttpServletRequest request, HttpServletResponse response) throws Exception {
    // 设置请求头为输出图片类型
    response.setContentType("image/gif");
    response.setHeader("Cache-Control", "no-store, no-cache, must-revalidate");
    response.setHeader("Cache-Control", "post-check=0, pre-check=0");
    response.setHeader("Pragma", "no-cache");
    response.setDateHeader("Expires", 0);
    Captcha captcha = (Captcha) captchaCodeService.generateCaptcha();
    logger.info("生成了验证码：" + captcha.text());
    // 验证码存入session
    request.getSession()
           .setAttribute("captcha",
                         captcha.text()
                                .toLowerCase());
    captcha.out(response.getOutputStream());
}