一个全局ID生成器

public class IdGenUtil {
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; // 节点ID长度
private long datacenterIdBits = 5L; // 数据中心ID长度
private long maxWorkerId = -1L ^ (-1L << workerIdBits); // 最大支持机器节点数0~31，一共32个
private long maxDatacenterId = -1L ^ (-1L << datacenterIdBits); // 最大支持数据中心节点数0~31，一共32个
private long sequenceBits = 12L; // 序列号12位
private long workerIdShift = sequenceBits; // 机器节点左移12位
private long datacenterIdShift = sequenceBits + workerIdBits; // 数据中心节点左移17位
private long timestampLeftShift = sequenceBits + workerIdBits + datacenterIdBits; // 时间毫秒数左移22位
private long sequenceMask = -1L ^ (-1L << sequenceBits); // 4095
private long lastTimestamp = -1L;
private static class IdGenHolder {
private static final IdGenUtil instance = new IdGenUtil();
}
public static IdGenUtil get() {
return IdGenHolder.instance;
}
public IdGenUtil() {
this(0L, 0L);
}
public IdGenUtil(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(); // 获取当前毫秒数
// 如果服务器时间有问题(时钟后退) 报错。
if (timestamp < lastTimestamp) {
throw new RuntimeException(String.format(
"Clock moved backwards.  Refusing to generate id for %d milliseconds", lastTimestamp - timestamp));
}
// 如果上次生成时间和当前时间相同,在同一毫秒内
if (lastTimestamp == timestamp) {
// sequence自增，因为sequence只有12bit，所以和sequenceMask相与一下，去掉高位
sequence = (sequence + 1) & sequenceMask;
// 判断是否溢出,也就是每毫秒内超过4095，当为4096时，与sequenceMask相与，sequence就等于0
if (sequence == 0) {
timestamp = tilNextMillis(lastTimestamp); // 自旋等待到下一毫秒
}
} else {
sequence = 0L; // 如果和上次生成时间不同,重置sequence，就是下一毫秒开始，sequence计数重新从0开始累加
}
lastTimestamp = timestamp;
// 最后按照规则拼出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();
}
}