ID生成器之雪花算法

简介

雪花算法是分布式ID生成算法，原理可百度，比较详细。

实现一,分布式使用

public class IdWorker {

    //下面两个每个5位，加起来就是10位的工作机器id
    private long workerId;    //工作id
    private long datacenterId;   //数据id
    //12位的序列号
    private long sequence;

    public IdWorker(long workerId, long datacenterId) {
        if (workerId > maxWorkerId || workerId < 0) {
            throw new IllegalArgumentException(String.format("workerId取值范围为[ 0 , %d ]", maxWorkerId));
        }
        if (datacenterId > maxDatacenterId || datacenterId < 0) {
            throw new IllegalArgumentException(String.format("datacenterId取值范围为[ 0 , %d ]", maxDatacenterId));
        }

        this.workerId = workerId;
        this.datacenterId = datacenterId;
        this.sequence = 0L;
    }

    //初始时间戳
    private long twepoch = 1288834974657L;

    private long workerIdBits = 5L;
    private long maxWorkerId = -1L ^ (-1L << workerIdBits);//31

    private long datacenterIdBits = 5L;
    private long maxDatacenterId = -1L ^ (-1L << datacenterIdBits);//31

    private long sequenceBits = 12L;
    private long sequenceMask = -1L ^ (-1L << sequenceBits);//4095

    //工作id需要左移的位数，12位
    private long workerIdShift = sequenceBits;
    //数据id需要左移位数 12+5=17位
    private long datacenterIdShift = sequenceBits + workerIdBits;
    //时间戳需要左移位数 12+5+5=22位
    private long timestampLeftShift = sequenceBits + workerIdBits + datacenterIdBits;

    //上次时间戳，初始值为负数
    private long lastTimestamp = -1L;

    public long getWorkerId() {
        return workerId;
    }

    public long getDatacenterId() {
        return datacenterId;
    }

    public long getTimestamp() {
        return System.currentTimeMillis();
    }


    //下一个ID生成算法
    public synchronized long nextId() {
        long timestamp = System.currentTimeMillis();

        //获取当前时间戳如果小于上次时间戳，则表示时间戳获取出现异常
        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));
        }

        //获取当前时间戳如果等于上次时间戳（同一毫秒内），则在序列号加一；否则序列号赋值为0，从0开始。
        if (lastTimestamp == timestamp) {
            sequence = (sequence + 1) & sequenceMask;
            if (sequence == 0) {
                timestamp = tilNextMillis(lastTimestamp);
            }
        } else {
            sequence = 0;
        }

        //将上次时间戳值刷新
        lastTimestamp = timestamp;

        /**
         * 返回结果：
         * (timestamp - twepoch) << timestampLeftShift) 表示将时间戳减去初始时间戳，再左移相应位数
         * (datacenterId << datacenterIdShift) 表示将数据id左移相应位数
         * (workerId << workerIdShift) 表示将工作id左移相应位数
         * | 是按位或运算符，例如：x | y，只有当x，y都为0的时候结果才为0，其它情况结果都为1。
         * 因为个部分只有相应位上的值有意义，其它位上都是0，所以将各部分的值进行 | 运算就能得到最终拼接好的id
         */
        return ((timestamp - twepoch) << timestampLeftShift) |
                (datacenterId << datacenterIdShift) |
                (workerId << workerIdShift) |
                sequence;
    }

    //获取时间戳，并与上次时间戳比较
    private long tilNextMillis(long lastTimestamp) {
        long timestamp = System.currentTimeMillis();
        while (timestamp <= lastTimestamp) {
            timestamp = System.currentTimeMillis();
        }
        return timestamp;
    }

}

实现二，单机使用

import java.sql.Timestamp;
import java.util.concurrent.Executors;
import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicLong;

public class SystemClock {
    private final long period;
    private final AtomicLong now;

    private SystemClock(long period) {
        this.period = period;
        this.now = new AtomicLong(System.currentTimeMillis());
        this.scheduleClockUpdating();
    }

    private static SystemClock instance() {
        return SystemClock.InstanceHolder.INSTANCE;
    }

    public static long now() {
        return instance().currentTimeMillis();
    }

    public static String nowDate() {
        return (new Timestamp(instance().currentTimeMillis())).toString();
    }

    private void scheduleClockUpdating() {
        ScheduledExecutorService scheduler = Executors.newSingleThreadScheduledExecutor((runnable) -> {
            Thread thread = new Thread(runnable, "System Clock");
            thread.setDaemon(true);
            return thread;
        });
        scheduler.scheduleAtFixedRate(() -> {
            this.now.set(System.currentTimeMillis());
        }, this.period, this.period, TimeUnit.MILLISECONDS);
    }

    private long currentTimeMillis() {
        return this.now.get();
    }

    private static class InstanceHolder {
        public static final SystemClock INSTANCE = new SystemClock(1L);

        private InstanceHolder() {
        }
    }
}

public class Sequence {
    private static final Sequence instance = new Sequence(0L, 0L);
    private final long startTime = 1519740777809L;
    private final long workerIdBits = 5L;
    private final long dataCenterIdBits = 5L;
    private final long sequenceBits = 12L;
    private final long maxWorkerId = 31L;
    private final long maxDataCenterId = 31L;
    private final long workerIdShift = 12L;
    private final long dataCenterIdShift = 17L;
    private final long timestampLeftShift = 22L;
    private final long sequenceMask = 4095L;
    private long workerId;
    private long dataCenterId;
    private long sequence = 0L;
    private long lastTimestamp = -1L;
    private boolean isClock = false;

    public Sequence(long workerId, long dataCenterId) {
        if (workerId <= 31L && workerId >= 0L) {
            if (dataCenterId <= 31L && dataCenterId >= 0L) {
                this.workerId = workerId;
                this.dataCenterId = dataCenterId;
            } else {
                throw new IllegalArgumentException(String.format("dataCenter Id can't be greater than %d or less than 0", 31L));
            }
        } else {
            throw new IllegalArgumentException(String.format("worker Id can't be greater than %d or less than 0", 31L));
        }
    }

    public static synchronized Long next() {
        return instance.nextId();
    }

    public void setClock(boolean clock) {
        this.isClock = clock;
    }

    public synchronized Long nextId() {
        long timestamp = this.timeGen();
        if (timestamp < this.lastTimestamp) {
            long offset = this.lastTimestamp - timestamp;
            if (offset > 5L) {
                throw new RuntimeException(String.format("Clock moved backwards.  Refusing to generate id for %d milliseconds", offset));
            }

            try {
                this.wait(offset << 1);
                timestamp = this.timeGen();
                if (timestamp < this.lastTimestamp) {
                    throw new RuntimeException(String.format("Clock moved backwards.  Refusing to generate id for %d milliseconds", offset));
                }
            } catch (Exception var6) {
                throw new RuntimeException(var6);
            }
        }

        if (this.lastTimestamp == timestamp) {
            this.sequence = this.sequence + 1L & 4095L;
            if (this.sequence == 0L) {
                timestamp = this.tilNextMillis(this.lastTimestamp);
            }
        } else {
            this.sequence = 0L;
        }

        this.lastTimestamp = timestamp;
        return timestamp - 1519740777809L << 22 | this.dataCenterId << 17 | this.workerId << 12 | this.sequence;
    }

    private long tilNextMillis(long lastTimestamp) {
        long timestamp;
        for(timestamp = this.timeGen(); timestamp <= lastTimestamp; timestamp = this.timeGen()) {
        }

        return timestamp;
    }

    private long timeGen() {
        return this.isClock ? SystemClock.now() : System.currentTimeMillis();
    }
}

两种方法生成id验证

@Test
public void idGenerator(){
    //此为方法一，web项目中的可以注入此类，则为单例的，分布式部署，不同服务器设置不同的workerId和datacenterId
    IdWorker worker = new IdWorker(0,0);

    int total = 2000000;
    HashSet<Long> ids = new HashSet<>(total);

    long start = System.currentTimeMillis();
    for (int i = 0; i < total; i++) {
        long l = worker.nextId();
    //此为方法二，直接Sequence.next()即可得到id
//      long l = Sequence.next();
        ids.add(l);
    }
    long end = System.currentTimeMillis();

    //若通过断言，则证明产生了total个不同的id
    Assert.assertEquals(ids.size(),total);

    System.out.println(String.format("生成%s个id，耗时%sms",total,end-start));
}