141 lines
5.3 KiB
Markdown
141 lines
5.3 KiB
Markdown
---
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title: "雪花算法"
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date: 2022-06-29T16:43:16+08:00
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tags: ["雪花算法"]
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categories: ["Learning", "StudyNotes"]
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---
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## 1. 历史
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[snowflake](https://github.com/twitter-archive/snowflake)是由 twitter 开源的分布式 id 生成算法，采 用 Scala 语言实现，是把一个 64 位的 long 型的 id，1 个 bit 是不用的，用其中的 41 bits 作为毫秒数，用 10 bits 作为工作机器 id，12 bits 作为序列号。
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小插曲：世界上没有两片相同的雪花，所以使用雪花来表示唯一
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## 2. 算法内容
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* 1:第一位不使用：为什么这里第一位不使用，因为对于long类型，如果第一位是`1` 则说明是负数
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* 2～42:表示时间戳，最多可以表示2^41-1次方的数值，可以是毫秒级。
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* 43～52:表示工作机器ID，最多支持2^10机器，也就是1024的机器。可以自己定义前几位为机房ID。
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* 53～64:表示自增ID，同一毫秒如果超过2^12次方的增长量，应该算非常大的了
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## 3. 代码实现
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```java
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public class SnowFlake {
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private final long workerId;
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private final long datacenterId;
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private long sequence;
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public SnowFlake(long workerId, long datacenterId, long sequence) {
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// sanity check for workerId
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// 这儿不就检查了一下，要求就是你传递进来的机房id和机器id不能超过32，不能小于0
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// 这个是二进制运算，就是 5 bit最多只能有31个数字，也就是说机器id最多只能是32以内
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// 这个是一个意思，就是 5 bit最多只能有31个数字，机房id最多只能是32以内
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long maxWorkerId = ~(-1L << workerIdBits);
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if (workerId > maxWorkerId || workerId < 0) {
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throw new IllegalArgumentException(
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String.format("worker Id can't be greater than %d or less than 0", maxWorkerId));
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}
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long maxDatacenterId = ~(-1L << datacenterIdBits);
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if (datacenterId > maxDatacenterId || datacenterId < 0) {
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throw new IllegalArgumentException(
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String.format("datacenter Id can't be greater than %d or less than 0", maxDatacenterId));
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}
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System.out.printf("worker starting. timestamp left shift %d, datacenter id bits %d, worker id bits %d, " +
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"sequence bits %d, workerid %d", timestampLeftShift, datacenterIdBits, workerIdBits,
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sequenceBits, workerId);
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this.workerId = workerId;
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this.datacenterId = datacenterId;
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this.sequence = sequence;
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}
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private final long workerIdBits = 5L;
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private final long datacenterIdBits = 5L;
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private final long sequenceBits = 12L;
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private final long timestampLeftShift = sequenceBits + workerIdBits + datacenterIdBits;
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private long lastTimestamp = -1L;
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public long getWorkerId() {
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return workerId;
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}
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public long getDatacenterId() {
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return datacenterId;
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}
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public long getTimestamp() {
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return System.currentTimeMillis();
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}
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public synchronized long nextId() {
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// 这儿就是获取当前时间戳，单位是毫秒
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long timestamp = timeGen();
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if (timestamp < lastTimestamp) {
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System.err.printf("clock is moving backwards. Rejecting requests until %d.", lastTimestamp);
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throw new RuntimeException(String.format(
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"Clock moved backwards. Refusing to generate id for %d milliseconds", lastTimestamp - timestamp));
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}
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if (lastTimestamp == timestamp) {
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// 这个意思是说一个毫秒内最多只能有4096个数字
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// 无论你传递多少进来，这个位运算保证始终就是在4096这个范围内，避免你自己传递个sequence超过了4096这个范围
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long sequenceMask = ~(-1L << sequenceBits);
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sequence = (sequence + 1) & sequenceMask;
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if (sequence == 0) {
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timestamp = tilNextMillis(lastTimestamp);
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}
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} else {
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sequence = 0;
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}
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// 这儿记录一下最近一次生成id的时间戳，单位是毫秒
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lastTimestamp = timestamp;
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// 这儿就是将时间戳左移，放到 41 bit那儿；
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// 将机房 id左移放到 5 bit那儿；
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// 将机器id左移放到5 bit那儿；将序号放最后12 bit；
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// 最后拼接起来成一个 64 bit的二进制数字，转换成 10 进制就是个 long 型
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long datacenterIdShift = sequenceBits + workerIdBits;
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long twepoch = 1288834974657L;
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return ((timestamp - twepoch) << timestampLeftShift) | (datacenterId << datacenterIdShift)
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| (workerId << sequenceBits) | sequence;
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}
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private long tilNextMillis(long lastTimestamp) {
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long timestamp = timeGen();
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while (timestamp <= lastTimestamp) {
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timestamp = timeGen();
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}
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return timestamp;
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}
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private long timeGen() {
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return System.currentTimeMillis();
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}
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// ---------------测试---------------
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public static void main(String[] args) {
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SnowFlake worker = new SnowFlake(1, 1, 1);
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for (int i = 0; i < 30; i++) {
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System.out.println(worker.nextId());
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}
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}
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}
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```
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## 参考地址：
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1. https://blog.csdn.net/qq_33797815/article/details/113178832 |