效率还可以，生成100万数据大概在1秒多左右，不想写说明了，直接粘贴~

public static void main(String[] args) {
    System.out.println("开始时间" + new SimpleDateFormat("YYYY-MM-dd HH:mm:ss:SSS").format(new Date()));
    List<String> result = new ArrayList<>();
    for (int i = 0; i < 1000000; i++){
        result.add(randomOrderCode());
    }
    System.out.println("结束时间" + new SimpleDateFormat("YYYY-MM-dd HH:mm:ss:SSS").format(new Date()));
    System.out.println("总生成数量：" + result.size());

    String code = randomOrderCode();
    System.out.println("订单号：" + code);
}

// 生成订单号方法
public static String randomOrderCode(){
    String fix = "DH"; // 前缀
    Integer len = 18; // 总长度，必须超过fix.length+dateran.length的长度
    // 获取时间字符串
    SimpleDateFormat dmDate = new SimpleDateFormat("yyyyMMddHHmmss");
    Date date = new Date();
    String dateran = dmDate.format(date);
    // 生成随机数
    Random ran = new Random();
    StringBuilder str = new StringBuilder();
    for (int i = 0; i < len - fix.length() - dateran.length(); i++) {
        str.append(ran.nextInt(10));
    }
    // 拼接字符串
    return fix + dateran + str;
}