雪花算法生成分佈式id

目前本單位系統中的id主要生成方式有兩種：自增和UUID，自增對數據遷移不友好，UUID對大量數據情況下查詢不友好，故引入雪花算法，做一次數據遷移，代碼如下

import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;

public class SnowFlake {

    private static final Map<String, SnowFlake> MODULE_INSTANCE = new ConcurrentHashMap<>(100);

    /**
     * 起始的時間戳
     */
    private final static long START_STMP = 1480166465631L;

    /**
     * 每一部分佔用的位數
     */
    private final static long SEQUENCE_BIT = 12; //序列號佔用的位數
    private final static long MACHINE_BIT = 5;   //機器標識佔用的位數
    private final static long DATACENTER_BIT = 5;//數據中心佔用的位數

    /**
     * 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;  //數據中心
    private long machineId;     //機器標識
    private long sequence = 0L; //序列號
    private long lastStmp = -1L;//上一次時間戳

    private 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;
    }

    /**
     * 獲取雪花算法實例
     * @param dataCenterId
     * @param machineId
     * @return
     */
    public static SnowFlake getInstance(long dataCenterId, long machineId) {
        String key = joinKey(dataCenterId,machineId);
        //雙重檢查鎖
        SnowFlake snowFlake = MODULE_INSTANCE.get(key);
        if(null==snowFlake){
            synchronized (MODULE_INSTANCE){
                snowFlake = MODULE_INSTANCE.get(key);
                if(null==snowFlake){
                    snowFlake = new SnowFlake(dataCenterId,machineId);
                    MODULE_INSTANCE.put(joinKey(dataCenterId,machineId),snowFlake);
                }
            }
        }
        return snowFlake;
    }

    /**
     * 產生下一個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) {
            //相同毫秒內，序列號自增
            sequence = (sequence + 1) & MAX_SEQUENCE;
            //同一毫秒的序列數已經達到最大
            if (sequence == 0L) {
                currStmp = getNextMill();
            }
        } else {
            //不同毫秒內，序列號置爲0
            sequence = 0L;
        }

        lastStmp = currStmp;

        return (currStmp - START_STMP) << TIMESTMP_LEFT //時間戳部分
                | datacenterId << DATACENTER_LEFT       //數據中心部分
                | machineId << MACHINE_LEFT             //機器標識部分
                | sequence;                             //序列號部分
    }

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

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

    private static String joinKey(long systemId,long moduleId){
        return systemId + "_" + moduleId;
    }

}