HttpSession 存储验证码失败

try {
    RandomValidateCode randomValidateCode = new RandomValidateCode();
    String randomString = randomValidateCode.getRandcode(req, resp);//输出图片方法
    
    // Cannot create a session after the response has been committed
    HttpSession session = req.getSession();
    session.setAttribute(RandomValidateCode.RANDOMCODEKEY, randomString);
} catch (Exception e) {
    logger.error(e.getMessage(), e);
}