概述

分布式系统中，有一些需要使用全局唯一ID的场景，这种时候为了防止ID冲突可以使用36位的UUID，但是UUID有一些缺点，首先他相对比较长，另外UUID一般是无序的。

有些时候我们希望能使用一种简单一些的ID，并且希望ID能够按照时间有序生成。

而twitter的snowflake解决了这种需求，最初Twitter把存储系统从MySQL迁移到Cassandra，因为Cassandra没有顺序ID生成机制，所以开发了这样一套全局唯一ID生成服务。

结构

snowflake的结构如下(每部分用-分开):

0 - 0000000000 0000000000 0000000000 0000000000 0 - 00000 - 00000 - 000000000000

第一位为未使用，接下来的41位为毫秒级时间(41位的长度可以使用69年)，然后是5位datacenterId和5位workerId(10位的长度最多支持部署1024个节点） ，最后12位是毫秒内的计数（12位的计数顺序号支持每个节点每毫秒产生4096个ID序号）

一共加起来刚好64位，为一个Long型。(转换成字符串长度为18)

snowflake生成的ID整体上按照时间自增排序，并且整个分布式系统内不会产生ID碰撞（由datacenter和workerId作区分），并且效率较高。据说：snowflake每秒能够产生26万个ID。

4. private final long twepoch = 1288834974657L;

5. private final long workerIdBits = 5L;

6. private final long datacenterIdBits = 5L;

7. private final long maxWorkerId = -1L ^ (-1L << workerIdBits);

8. private final long maxDatacenterId = -1L ^ (-1L << datacenterIdBits);

9. private final long sequenceBits = 12L;

10. private final long workerIdShift = sequenceBits;

11. private final long datacenterIdShift = sequenceBits + workerIdBits;

12. private final long timestampLeftShift = sequenceBits + workerIdBits + datacenterIdBits;

13. private final long sequenceMask = -1L ^ (-1L << sequenceBits);

16. private long datacenterId;

17. private long sequence = 0L;

18. private long lastTimestamp = -1L;

20. public IdWorker(long workerId, long datacenterId) {

21. if (workerId > maxWorkerId || workerId < 0) {

22. throw new IllegalArgumentException(String.format("worker Id can't be greater than %d or less than 0", maxWorkerId));

24. if (datacenterId > maxDatacenterId || datacenterId < 0) {

25. throw new IllegalArgumentException(String.format("datacenter Id can't be greater than %d or less than 0", maxDatacenterId));

27. this.workerId = workerId;

28. this.datacenterId = datacenterId;

31. public synchronized long nextId() {

32. long timestamp = timeGen();

33. if (timestamp < lastTimestamp) {

34. throw new RuntimeException(String.format("Clock moved backwards. Refusing to generate id for %d milliseconds", lastTimestamp - timestamp));

36. if (lastTimestamp == timestamp) {

37. sequence = (sequence + 1) & sequenceMask;

39. timestamp = tilNextMillis(lastTimestamp);

45. lastTimestamp = timestamp;

47. return ((timestamp - twepoch) << timestampLeftShift) | (datacenterId << datacenterIdShift) | (workerId << workerIdShift) | sequence;

50. protected long tilNextMillis(long lastTimestamp) {

51. long timestamp = timeGen();

52. while (timestamp <= lastTimestamp) {

58. protected long timeGen() {

59. return System.currentTimeMillis();

62. public static void main(String[] args) {

63. IdWorker idWorker = new IdWorker(0, 0);

64. for (int i = 0; i < 1000; i++) {

65. long id = idWorker.nextId();