ID生成 雪花算法

/**
 * ID生成 雪花算法
 */
public class SnowFlake
{
    public static SnowFlake getInstance()
    {
        return SingletonSnowFlake.instance;
    }


    private static class SingletonSnowFlake
    {
        private static final SnowFlake instance = new SnowFlake(1,1);
    }

    /**
     * 起始的时间戳
     */
    private final static long START_STMP = 1480166465631L;

    /**
     * 每一部分占用的位数
     */
    private final static long SEQUENCE_BIT = 12; //序列号占用的位数

    private final static long MACHINE_BIT = 5;   //机器标识占用的位数

    private final static long DATACENTER_BIT = 5;//数据中心占用的位数

    /**
     * 每一部分的最大值
     */
    private final static long MAX_DATACENTER_NUM = -1L ^ (-1L << DATACENTER_BIT);

    private final static long MAX_MACHINE_NUM = -1L ^ (-1L << MACHINE_BIT);

    private final static long MAX_SEQUENCE = -1L ^ (-1L << SEQUENCE_BIT);

    /**
     * 每一部分向左的位移
     */
    private final static long MACHINE_LEFT = SEQUENCE_BIT;

    private final static long DATACENTER_LEFT = SEQUENCE_BIT + MACHINE_BIT;

    private final static long TIMESTMP_LEFT = DATACENTER_LEFT + DATACENTER_BIT;

    private long datacenterId;  //数据中心

    private long machineId;     //机器标识

    private long sequence = 0L; //序列号

    private long lastStmp = -1L;//上一次时间戳

    public SnowFlake(long datacenterId, long machineId)
    {
        if (datacenterId > MAX_DATACENTER_NUM || datacenterId < 0)
        {
            throw new IllegalArgumentException("datacenterId can't be greater than MAX_DATACENTER_NUM or less than 0");
        }

        if (machineId > MAX_MACHINE_NUM || machineId < 0)
        {
            throw new IllegalArgumentException("machineId can't be greater than MAX_MACHINE_NUM or less than 0");
        }

        this.datacenterId = datacenterId;

        this.machineId = machineId;
    }

    /**
     * 产生下一个ID
     *
     * @return
     */
    public synchronized long nextId()
    {

        long currStmp = getNewstmp();

        if (currStmp < lastStmp)
        {
            throw new RuntimeException("Clock moved backwards.  Refusing to generate id");
        }

        if (currStmp == lastStmp)
        {
            //相同毫秒内，序列号自增
            sequence = (sequence + 1) & MAX_SEQUENCE;

            //同一毫秒的序列数已经达到最大
            if (sequence == 0L)
            {
                currStmp = getNextMill();
            }
        }
        else
        {
            //不同毫秒内，序列号置为0
            sequence = 0L;
        }

        lastStmp = currStmp;

        return (currStmp - START_STMP) << TIMESTMP_LEFT //时间戳部分
               | datacenterId << DATACENTER_LEFT       //数据中心部分
               | machineId << MACHINE_LEFT             //机器标识部分
               | sequence;                             //序列号部分
    }

    private long getNextMill()
    {
        long mill = getNewstmp();

        while (mill <= lastStmp)
        {
            mill = getNewstmp();
        }
        return mill;
    }

    private long getNewstmp()
    {
        return System.currentTimeMillis();
    }

    public static void main(String[] args)
    {
        SnowFlake snowFlake = new SnowFlake(1, 1);

        long start = System.currentTimeMillis();

        for (int i = 0; i < 1000000; i++)
        {
            System.out.println(snowFlake.nextId());
        }

        System.out.println(System.currentTimeMillis() - start);

    }
}