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);
}

如上生成并且存储验证码的代码，在登录页面 url 不输工程名的时候（如 http://127.0.0.1:7878/），会报异常 Cannot create a session after the response has been committed

因此需要把 session.setAttribute(RandomValidateCode.RANDOMCODEKEY, randomString); 在 response 写成验证码图片前执行