雪花算法的本质为生成一个64位长度的具有自增性的分布式全局唯一id。在64bits中，会对不同段的位进行划分。可分为：
依赖服务器时间。若服务器时钟回拨，可能会导致生成的id重复。可在代码中新增lastTimeMillis字段，在获取nextId时根据系统当前时间进行判断解决。
但若不进行持久化处理，服务重启后发生时钟回拨依旧会出现重复问题。
/**
* Created by QQ.Cong on 2022-07-22 / 9:48
*
* @author: CongQingquan
* @Description: Snowflake util
*/
public class SnowflakeUtils {
// ============================== Basic field ==============================//
// Datacenter id
private long datacenterId;
// Worker id
private long workerId;
// Increment sequence
private long sequence;
// ============================== Bits ==============================//
// Bits of datacenter id
private long datacenterIdBits;
// Bits of worker id
private long workerIdBits;
// Bits of sequence
private long sequenceBits;
// ============================== Largest ==============================//
// Largest datacenter id
private long largestDatacenterId;
// Largest worker id
private long largestWorkerId;
// Largest sequence
private long largestSequence;
// ============================== Shift ==============================//
// Left shift num of worker id
private long workerIdShift;
// Left shift num of datacenter id
private long datacenterIdShift;
// Left shift num of timestamp
private long timestampShift;
// ============================== Other ==============================//
// Epoch
private long epoch;
// The timestamp that last get snowflake id
private long lastTimestamp;
// ============================== End ==============================//
public SnowflakeUtils(long dataCenterId, long workerId) {
// Default epoch: 2022-07-22 00:00:00
this(1658419200000L, -1L, dataCenterId, workerId, 5L, 5L, 5L);
}
public SnowflakeUtils(long epoch, long lastTimestamp, long datacenterId, long workerId,
long datacenterIdBits, long workerIdBits, long sequenceBits) {
this.epoch = epoch;
this.lastTimestamp = lastTimestamp;
this.datacenterId = datacenterId;
this.workerId = workerId;
this.sequence = 0L;
this.datacenterIdBits = datacenterIdBits;
this.workerIdBits = workerIdBits;
this.sequenceBits = sequenceBits;
this.largestDatacenterId = ~(-1L << datacenterIdBits);
this.largestWorkerId = ~(-1L << workerIdBits);
this.largestSequence = ~(-1L << sequenceBits);
if (datacenterId > largestDatacenterId || datacenterId < 0) {
throw new IllegalArgumentException(
String.format("The datacenter id param can't be greater than %s or less than 0",
largestDatacenterId));
}
if (workerId > largestWorkerId || workerId < 0) {
throw new IllegalArgumentException(
String.format("The worker id param can't be greater than %s or less than 0",
largestWorkerId));
}
this.workerIdShift = sequenceBits;
this.datacenterIdShift = workerIdShift + workerIdBits;
this.timestampShift = datacenterIdShift + datacenterIdBits;
}
/**
* Get snowflake id
* @return
*/
public synchronized long nextId() {
long timestamp = System.currentTimeMillis();
// 若时钟回退
if (timestamp < lastTimestamp) {
throw new RuntimeException(
"System clock moved backward, cannot to generate snowflake id");
}
// 若当前毫秒内多次生成雪花id
if (timestamp == lastTimestamp) {
sequence = (sequence + 1) & largestSequence;
// 序列溢出
if (sequence == 0) {
timestamp = waitUntilNextMilli(timestamp);
}
}
// 若当前毫秒内首次生成雪花id
else {
sequence = 0L;
}
// 更新获取雪花id的时间戳
lastTimestamp = timestamp;
// 生成雪花id (通过位或运算符进行拼接)
return ((timestamp - epoch) << timestampShift) // 时间戳段
| (datacenterId << datacenterIdShift) // 机器码段
| (workerId << workerIdShift) // 机器码段
| sequence; // 自增序列段
}
/**
* Wait until next millisecond
* @param lastTimestamp
* @return
*/
private long waitUntilNextMilli(long lastTimestamp) {
long currentTimeMillis;
do {
currentTimeMillis = System.currentTimeMillis();
}
while (currentTimeMillis <= lastTimestamp);
return currentTimeMillis;
}
/**
* Get util instance
* @param dataCenterId
* @param workerId
* @return
*/
public static SnowflakeUtils getInstance(long dataCenterId, long workerId) {
return new SnowflakeUtils(dataCenterId, workerId);
}
}
