分布式id主键雪花算法

import java.util.HashSet;
import java.util.Set;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

public class SnowFlake {
    // 起始的时间戳
    private final static long START_STMP = 1480166465631L;
    // 每一部分占用的位数，就三个
    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 static long datacenterId; // 数据中心
    private static long machineId; // 机器标识
    private static long sequence = 0L; // 序列号
    private static long lastStmp = -1L;// 上一次时间戳

    private static SnowFlakeStrategy snowFlakeStrategy = SnowFlakeStrategy.getSingleInstance();

    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
    public static synchronized long nextId() {
        long currStmp = getNewstmp();
        if (currStmp < lastStmp) {
            throw new RuntimeException("Clock moved backwards.  Refusing to generate id");
        }

        if (currStmp == lastStmp) {
            //if条件里表示当前调用和上一次调用落在了相同毫秒内，只能通过第三部分，序列号自增来判断为唯一，所以+1.
            sequence = (sequence + 1) & MAX_SEQUENCE;
            //同一毫秒的序列数已经达到最大，只能等待下一个毫秒
            if (sequence == 0L) {
                currStmp = getNextMill();
            }
        } else {
            //不同毫秒内，序列号置为0
            //执行到这个分支的前提是currTimestamp > lastTimestamp，说明本次调用跟上次调用对比，已经不再同一个毫秒内了，这个时候序号可以重新回置0了。
            sequence = 0L;
        }

        lastStmp = currStmp;
        //就是用相对毫秒数、机器ID和自增序号拼接
        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
    }

    /**
     * 产生下一个String ID
     */
    public static String nextStrId(){
        return nextStrId((int)(Math.random()*30),(int)(Math.random()*30));
    }

    /**
     * 根据数据中心ID和机器标识的组合唯一ID获取对应的SnowFlake类的实例，如不存在，则根据组合唯一ID设置对应的SnowFlake类的实例并返回
     * @param datacenterId
     * @param machineId
     * @return
     */
    public static String nextStrId(long datacenterId, long machineId){
        SnowFlake snowFlake = snowFlakeStrategy.takeSnowFlakeInstance(datacenterId,machineId);
        return String.valueOf(snowFlake.nextId());
    }

    private static long getNextMill() {
        long mill = getNewstmp();
        while (mill <= lastStmp) {
            mill = getNewstmp();
        }
        return mill;
    }

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

    public static void main(String[] args) {
        // 构造方法设置机器码：第9个机房的第20台机器
        //SnowFlake  snowFlake = new SnowFlake(9, 20);
        ExecutorService executorService = Executors.newFixedThreadPool(50);
        Set count = new HashSet();
        for(int i =1; i <=5000; i++){
            executorService.submit(new Runnable() {
                @Override
                public void run() {
                    count.add(nextStrId(9,27));
                }
            });
            executorService.submit(new Runnable() {
                @Override
                public void run() {
                    count.add(nextStrId(8,10));

                }
            });
        }
        executorService.shutdown();
        while(true) {
            if (executorService.isTerminated()) {
                System.out.println(count.size());
                break;
            }else{
                System.out.println(count.size());
            }
        }
    }
}