• 雪花算法


    public class IdWorker {
    
        private long workerId;
        private long datacenterId;
        private long sequence;
    
        public IdWorker(long workerId, long datacenterId, long sequence) {
            // sanity check for workerId
            // 这儿不就检查了一下,要求就是你传递进来的机房id和机器id不能超过32,不能小于0
            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));
            }
            System.out.printf(
                    "worker starting. timestamp left shift %d, datacenter id bits %d, worker id bits %d, sequence bits %d, workerid %d",
                    timestampLeftShift, datacenterIdBits, workerIdBits, sequenceBits, workerId);
    
            this.workerId = workerId;
            this.datacenterId = datacenterId;
            this.sequence = sequence;
        }
    
        private long twepoch = 1288834974657L;
    
        private long workerIdBits = 5L;
        private long datacenterIdBits = 5L;
    
        // 这个是二进制运算,就是 5 bit最多只能有31个数字,也就是说机器id最多只能是32以内
        private long maxWorkerId = -1L ^ (-1L << workerIdBits);
    
        // 这个是一个意思,就是 5 bit最多只能有31个数字,机房id最多只能是32以内
        private long maxDatacenterId = -1L ^ (-1L << datacenterIdBits);
        private long sequenceBits = 12L;
    
        private long workerIdShift = sequenceBits;
        private long datacenterIdShift = sequenceBits + workerIdBits;
        private long timestampLeftShift = sequenceBits + workerIdBits + datacenterIdBits;
        private long sequenceMask = -1L ^ (-1L << sequenceBits);
    
        private long lastTimestamp = -1L;
    
        public long getWorkerId() {
            return workerId;
        }
    
        public long getDatacenterId() {
            return datacenterId;
        }
    
        public long getTimestamp() {
            return System.currentTimeMillis();
        }
    
        public synchronized long nextId() {
            // 这儿就是获取当前时间戳,单位是毫秒
            long timestamp = timeGen();
    
            if (timestamp < lastTimestamp) {
                System.err.printf("clock is moving backwards.  Rejecting requests until %d.", lastTimestamp);
                throw new RuntimeException(String.format(
                        "Clock moved backwards.  Refusing to generate id for %d milliseconds", lastTimestamp - timestamp));
            }
    
            if (lastTimestamp == timestamp) {
                // 这个意思是说一个毫秒内最多只能有4096个数字
                // 无论你传递多少进来,这个位运算保证始终就是在4096这个范围内,避免你自己传递个sequence超过了4096这个范围
                sequence = (sequence + 1) & sequenceMask;
                if (sequence == 0) {
                    timestamp = tilNextMillis(lastTimestamp);
                }
            } else {
                sequence = 0;
            }
    
            // 这儿记录一下最近一次生成id的时间戳,单位是毫秒
            lastTimestamp = timestamp;
    
            // 这儿就是将时间戳左移,放到 41 bit那儿;
            // 将机房 id左移放到 5 bit那儿;
            // 将机器id左移放到5 bit那儿;将序号放最后12 bit;
            // 最后拼接起来成一个 64 bit的二进制数字,转换成 10 进制就是个 long 型
            return ((timestamp - twepoch) << timestampLeftShift) | (datacenterId << datacenterIdShift)
                    | (workerId << workerIdShift) | sequence;
        }
    
        private long tilNextMillis(long lastTimestamp) {
            long timestamp = timeGen();
            while (timestamp <= lastTimestamp) {
                timestamp = timeGen();
            }
            return timestamp;
        }
    
        private long timeGen() {
            return System.currentTimeMillis();
        }
    
        // ---------------测试---------------
        public static void main(String[] args) {
            IdWorker worker = new IdWorker(1, 1, 1);
            for (int i = 0; i < 30; i++) {
                System.out.println(worker.nextId());
            }
        }
    
    }
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  • 原文地址:https://www.cnblogs.com/wueryuan/p/14929536.html
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