package com.whale.tiger.utils;
/**
* 全局唯一ID生成器 在開發中，有時非常需要一個全局唯一的ID值，不管是業務需求，還是爲了以後可能的分表需求，全局唯一值都非常有用，
* 本篇大象就來講講這個實現並對ID生成器性能進行一下測試。
* 這個全局唯一ID生成器，其實是Twitter公開的一個算法，源碼是用Scala寫的，被國內的開源愛好者改寫成了Java版本。
* 獲取18位數字
*/
public class IdGen {
private long workerId;
private long datacenterId;
private long sequence = 0L;
private long twepoch = 1288834974657L; // Thu, 04 Nov 2010 01:42:54 GMT
private long workerIdBits = 5L; // 節點ID長度
private long datacenterIdBits = 5L; // 數據中心ID長度
private long maxWorkerId = -1L ^ (-1L << workerIdBits); // 最大支持機器節點數0~31，一共32個
private long maxDatacenterId = -1L ^ (-1L << datacenterIdBits); // 最大支持數據中心節點數0~31，一共32個
private long sequenceBits = 12L; // 序列號12位
private long workerIdShift = sequenceBits; // 機器節點左移12位
private long datacenterIdShift = sequenceBits + workerIdBits; // 數據中心節點左移17位
private long timestampLeftShift = sequenceBits + workerIdBits + datacenterIdBits; // 時間毫秒數左移22位
private long sequenceMask = -1L ^ (-1L << sequenceBits); // 4095
private long lastTimestamp = -1L;
private static class IdGenHolder {
private static final IdGen instance = new IdGen();
}
public static IdGen get() {
return IdGenHolder.instance;
}
public IdGen() {
this(0L, 0L);
}
public IdGen(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;
}
public synchronized long nextId() {
long timestamp = timeGen(); // 獲取當前毫秒數
// 如果服務器時間有問題(時鐘後退) 報錯。
if (timestamp < lastTimestamp) {
throw new RuntimeException(String.format(
"Clock moved backwards. Refusing to generate id for %d milliseconds", lastTimestamp - timestamp));
}
// 如果上次生成時間和當前時間相同,在同一毫秒內
if (lastTimestamp == timestamp) {
// sequence自增，因爲sequence只有12bit，所以和sequenceMask相與一下，去掉高位
sequence = (sequence + 1) & sequenceMask;
// 判斷是否溢出,也就是每毫秒內超過4095，當爲4096時，與sequenceMask相與，sequence就等於0
if (sequence == 0) {
timestamp = tilNextMillis(lastTimestamp); // 自旋等待到下一毫秒
}
} else {
sequence = 0L; // 如果和上次生成時間不同,重置sequence，就是下一毫秒開始，sequence計數重新從0開始累加
}
lastTimestamp = timestamp;
// 最後按照規則拼出ID。
// 000000000000000000000000000000000000000000 00000 00000 000000000000
// time datacenterId workerId sequence
return ((timestamp - twepoch) << timestampLeftShift) | (datacenterId << datacenterIdShift)
| (workerId << workerIdShift) | sequence;
}
protected long tilNextMillis(long lastTimestamp) {
long timestamp = timeGen();
while (timestamp <= lastTimestamp) {
timestamp = timeGen();
}
return timestamp;
}
protected long timeGen() {
return System.currentTimeMillis();
}
public static void main(String[] args) {
System.out.println(IdGen.get().nextId());
}
}