SnowFlakeUtil.java
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package com.zteits.charge.util;
/**
* Copyright: Copyright (c) 2020 zteits
*
* @ClassName: IdTest.java
* @Description:
* @version: v1.0.0
* @author: nimz
* @date: 2020年12月02日 18:09
*/
public class SnowFlakeUtil {
private long workerId;
private long datacenterId;
private long sequence = 0L;
private long twepoch = 1288834974657L; // Thu, 04 Nov 2010 01:42:54 GMT 标记时间 用来计算偏移量，距离当前时间不同，得到的数据的位数也不同
private long workerIdBits = 5L; // 物理节点ID长度
private long datacenterIdBits = 4L; // 数据中心ID长度
private long maxWorkerId = -1L ^ (-1L << workerIdBits); // 最大支持机器节点数0~31，一共32个
private long maxDatacenterId = -1L ^ (-1L << datacenterIdBits); // 最大支持数据中心节点数0~31，一共32个
private long sequenceBits = 12L; // 序列号12位， 4095，同毫秒内生成不同id的最大个数
private long workerIdShift = sequenceBits; // 机器节点左移12位
private long datacenterIdShift = sequenceBits + workerIdBits; // 数据中心节点左移17位
private long timestampLeftShift = sequenceBits + workerIdBits + datacenterIdBits; // 时间毫秒数左移22位
private long sequenceMask = -1L ^ (-1L << sequenceBits); // 用于和当前时间戳做比较，以获取最新时间
private long lastTimestamp = -1L;
//成员类，SnowFlakeUtil的实例对象的保存域
private static class IdGenHolder {
private static final SnowFlakeUtil instance = new SnowFlakeUtil();
}
//外部调用获取SnowFlakeUtil的实例对象，确保不可变
public static SnowFlakeUtil get(){
return IdGenHolder.instance;
}
//初始化构造，无参构造有参函数，默认节点都是0
public SnowFlakeUtil() {
this(0L, 0L);
}
//设置机器节点和数据中心节点数，都是 0-31
public SnowFlakeUtil(long workerId, long datacenterId) {
if (workerId > maxWorkerId || workerId < 0) {
throw new IllegalArgumentException(String.format("worker Id can't be greater than %d or less than 0", maxWorkerId));
}
if (datacenterId > maxDatacenterId || datacenterId < 0) {
throw new IllegalArgumentException(String.format("datacenter Id can't be greater than %d or less than 0", maxDatacenterId));
}
this.workerId = workerId;
this.datacenterId = datacenterId;
}
//线程安全的id生成方法
@SuppressWarnings("all")
public synchronized long nextId() {
//获取当前毫秒数
long timestamp = timeGen();
//如果服务器时间有问题(时钟后退) 报错。
if (timestamp < lastTimestamp) {
throw new RuntimeException(String.format(
"Clock moved backwards. Refusing to generate id for %d milliseconds", lastTimestamp - timestamp));
}
//如果上次生成时间和当前时间相同,在同一毫秒内
if (lastTimestamp == timestamp) {
//sequence自增，因为sequence只有12bit，所以和sequenceMask相与一下，去掉高位
sequence = (sequence + 1) & sequenceMask;
//判断是否溢出,也就是每毫秒内超过4095，当为4096时，与sequenceMask相与，sequence就等于0
if (sequence == 0) {
//自旋等待到下一毫秒
timestamp = tilNextMillis(lastTimestamp);
}
} else {
//如果和上次生成时间不同,重置sequence，就是下一毫秒开始，sequence计数重新从0开始累加，每个毫秒时间内，都是从0开始计数，最大4095
sequence = 0L;
}
lastTimestamp = timestamp;
// 最后按照规则拼出ID 64位
// 000000000000000000000000000000000000000000 00000 00000 000000000000
//1位固定整数 time datacenterId workerId sequence
return ((timestamp - twepoch) << timestampLeftShift) | (datacenterId << datacenterIdShift)
| (workerId << workerIdShift) | sequence;
}
//比较当前时间和过去时间，防止时钟回退（机器问题），保证给的都是最新时间/最大时间
protected long tilNextMillis(long lastTimestamp) {
long timestamp = timeGen();
while (timestamp <= lastTimestamp) {
timestamp = timeGen();
}
return timestamp;
}
//获取当前的时间戳（毫秒）
protected long timeGen() {
return System.currentTimeMillis();
}
/**
* 获取全局唯一编码
*/
public static String getId(){
Long id = SnowFlakeUtil.get().nextId();
return id.toString();
}
public static void main(String[] args) {
System.out.println(SnowFlakeUtil.getId());
}
}