code: public  static String getOutTradeNo() {
  		SimpleDateFormat format = new SimpleDateFormat("MMddHHmmss", Locale.getDefault());
  		Date date = new Date();
  		String key = format.format(date);
  		key = key + System.currentTimeMillis();
  		key = key.substring(0, 15);
  		return key;
  	}
docs: 要求外部订单号必须唯一。
  @return {String}