SnowFlake雪花算法生成分布式id

author ：Twitter

/**
* 雪花算法SnowFlake
* Created on 2019/3/14.
*
* @author Twitter
*/
public class IdSnowFlake {

public static void main(String[] args) {
IdSnowFlake flake = new IdSnowFlake();
System.out.println(flake.nextId());
}

/**
* 起始的时间戳
*/
private final static long START_STMP = 1480166465631L;

/**
* 每一部分占用的位数,序列号占用的位数
*/
private final static long SEQUENCE_BIT = 12;
/**
* 机器标识占用的位数
*/
private final static long MACHINE_BIT = 5;

/**
* 数据中心占用的位数
*/
private final static long DATACENTER_BIT = 5;

/**
* 每一部分的最大值
*/
private final static long MAX_DATACENTER_NUM = -1L ^ (-1L << DATACENTER_BIT);
private final static long MAX_MACHINE_NUM = -1L ^ (-1L << MACHINE_BIT);
private final static long MAX_SEQUENCE = -1L ^ (-1L << SEQUENCE_BIT);

/**
* 每一部分向左的位移
*/
private final static long MACHINE_LEFT = SEQUENCE_BIT;
private final static long DATACENTER_LEFT = SEQUENCE_BIT + MACHINE_BIT;
private final static long TIMESTMP_LEFT = DATACENTER_LEFT + DATACENTER_BIT;

/**
* 数据中心
*/
private long datacenterId;
/**
* 机器标识
*/
private long machineId;

/**
* 序列号
*/
private long sequence = 0L;

/**
* 上一次时间戳
*/
private long lastStmp = -1L;

public IdSnowFlake(long datacenterId, long machineId) {
if (datacenterId > MAX_DATACENTER_NUM || datacenterId < 0) {
throw new IllegalArgumentException("datacenterId can't be greater than MAX_DATACENTER_NUM or less than 0");
}
if (machineId > MAX_MACHINE_NUM || machineId < 0) {
throw new IllegalArgumentException("machineId can't be greater than MAX_MACHINE_NUM or less than 0");
}
this.datacenterId = datacenterId;
this.machineId = machineId;
}

public IdSnowFlake(){

}

/**
* 产生下一个ID
*
* @return
*/
public synchronized long nextId() {
long currStmp = getNewstmp();
if (currStmp < lastStmp) {
throw new RuntimeException("Clock moved backwards.  Refusing to generate id");
}

if (currStmp == lastStmp) {
//相同毫秒内，序列号自增
sequence = (sequence + 1) & MAX_SEQUENCE;
//同一毫秒的序列数已经达到最大
if (sequence == 0L) {
currStmp = getNextMill();
}
} else {
//不同毫秒内，序列号置为0
sequence = 0L;
}

lastStmp = currStmp;

// 时间戳部分 + 数据中心部分 + 机器标识部分 + 序列号部分
return (currStmp - START_STMP) << TIMESTMP_LEFT
| datacenterId << DATACENTER_LEFT
| machineId << MACHINE_LEFT
| sequence;
}

private long getNextMill() {
long mill = getNewstmp();
while (mill <= lastStmp) {
mill = getNewstmp();
}
return mill;
}

private long getNewstmp() {
return System.currentTimeMillis();
}
}