一个全局ID生成器

版权声明：本文为博主原创文章，未经博主允许不得转载。 https://blog.csdn.net/bloodylzy/article/details/79150945

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();
     }
}