code: private void autoRemoveExpiredCaptcha() {
  		timer = new Timer("CaptchaCache", true);
  		timer.schedule(
  			new TimerTask() {
  				public void run() {
  					for (Entry<String, Captcha> e : map.entrySet()) {
  						if (e.getValue().isExpired()) {
  							map.remove(e.getKey());
  						}
  					}
  				}
  			},
  			interval,
  			interval
  		);
  	}
docs: 定期移除过期的验证码