JAVA生成根据用户id唯一订单号，并发下唯一

/**
     * 生成订单号
     * yyyyMMddHHmmss 6位随机  4位用户id
     * @return
     */
    public static String getGenerateOrderNo(Long userId){
        String time = new SimpleDateFormat("yyyyMMddHHmmss").format(new Date());
        StringBuffer stringBuffer = new StringBuffer(userId.toString());
        //id长度大于4  取后四位
        if(stringBuffer.length()>4){
            stringBuffer = new StringBuffer(stringBuffer.substring(stringBuffer.length()-4,stringBuffer.length()));
        }
        //长度小于4  补全4位
        while (stringBuffer.length()<4){
            stringBuffer.insert(0,"0");
        }
        return time+RandomUtil.getNumber(6)+stringBuffer.toString();
    }