@Bean
@ConditionalOnMissingBean
public IdentifierGenerator identifierGenerator(InetUtils inetUtils) {
    return new DefaultIdentifierGenerator(inetUtils.findFirstNonLoopbackAddress());
}

public DefaultIdentifierGenerator(InetAddress inetAddress) {
    this.sequence = new Sequence(inetAddress);
}

public Sequence(InetAddress inetAddress) {
    this.inetAddress = inetAddress;
    this.datacenterId = getDatacenterId(maxDatacenterId);
    this.workerId = getMaxWorkerId(datacenterId, maxWorkerId);
    // 打印初始化語句
    initLog();
}

/**
* 數(shù)據(jù)標(biāo)識id部分
*/
protected long getDatacenterId(long maxDatacenterId) {
long id = 0L;
try {
    if (null == this.inetAddress) {
        this.inetAddress = InetAddress.getLocalHost();
    }
    NetworkInterface network = NetworkInterface.getByInetAddress(this.inetAddress);
    if (null == network) {
        id = 1L;
    } else {
        // 獲取mac地址
        byte[] mac = network.getHardwareAddress();
        // 混淆
        if (null != mac) {
            id = ((0x000000FF & (long) mac[mac.length - 2]) | (0x0000FF00 & (((long) mac[mac.length - 1]) << 8))) >> 6;
            id = id % (maxDatacenterId + 1);
        }
    }
} catch (Exception e) {
    logger.warn(" getDatacenterId: " + e.getMessage());
}
return id;
}

/**
 * 獲取 maxWorkerId
 */
protected long getMaxWorkerId(long datacenterId, long maxWorkerId) {
    StringBuilder mpid = new StringBuilder();
    mpid.append(datacenterId);
    String name = ManagementFactory.getRuntimeMXBean().getName();
    if (StringUtils.isNotBlank(name)) {
        /*
         * GET jvmPid
         */
        mpid.append(name.split(StringPool.AT)[0]);
    }
    /*
     * MAC + PID 的 hashcode 獲取16個(gè)低位
     */
    return (mpid.toString().hashCode() & 0xffff) % (maxWorkerId + 1);
}

/**
 * 獲取唯一ID
 *
 * @return id
 */
public static long getId(Object entity) {
    return IDENTIFIER_GENERATOR.nextId(entity).longValue();
}

@Override
public Long nextId(Object entity) {
    return sequence.nextId();
}

/**
 * 獲取下一個(gè) ID
 *
 * @return 下一個(gè) ID
 */
public synchronized long nextId() {
    long timestamp = timeGen();
    // 閏秒
    // 這里會判斷是否發(fā)生時(shí)鐘偏移，若偏移在5ms以內(nèi)會重新嘗試重新獲取時(shí)間，看是否能夠重新獲取正確的時(shí)間。
    // 因?yàn)榕紶枙虚c秒的存在
    if (timestamp < lastTimestamp) {
        long offset = lastTimestamp - timestamp;
        if (offset <= 5) {
            try {
                wait(offset << 1);
                timestamp = timeGen();
                if (timestamp < lastTimestamp) {
                    throw new RuntimeException(String.format("Clock moved backwards.  Refusing to generate id for %d milliseconds", offset));
                }
            } catch (Exception e) {
                throw new RuntimeException(e);
            }
        } else {
            throw new RuntimeException(String.format("Clock moved backwards.  Refusing to generate id for %d milliseconds", offset));
        }
    }
    if (lastTimestamp == timestamp) {
        // 相同毫秒內(nèi)，序列號自增
        sequence = (sequence + 1) & sequenceMask;
        if (sequence == 0) {
            // 同一毫秒的序列數(shù)已經(jīng)達(dá)到最大
            // 序列數(shù)(毫秒內(nèi)自增位)為12位，最大每毫秒分配4096個(gè)
            // 序列數(shù)最大的時(shí)候會等待到下一毫秒才會分配時(shí)間戳
            timestamp = tilNextMillis(lastTimestamp);
        }
    } else {
        // 不同毫秒內(nèi)，序列號置為 1 - 2 隨機(jī)數(shù)
        // 這里序列號置為1-2的隨機(jī)數(shù)是為了方便后續(xù)分庫分表的時(shí)候hash比較均勻
        sequence = ThreadLocalRandom.current().nextLong(1, 3);
    }
    lastTimestamp = timestamp;
    // twepoch 是 時(shí)間起始標(biāo)記點(diǎn)，作為基準(zhǔn)，一般取系統(tǒng)的最近時(shí)間（一旦確定不能變動）
    // 因?yàn)榍懊嬉呀?jīng)說過41位的時(shí)間戳可以分配69.7年，如果從1970.1.1開始數(shù)，那么時(shí)間戳可能在未來某一天大于41位
    // 時(shí)間戳部分 | 數(shù)據(jù)中心部分 | 機(jī)器標(biāo)識部分 | 序列號部分
    return ((timestamp - twepoch) << timestampLeftShift)
        | (datacenterId << datacenterIdShift)
        | (workerId << workerIdShift)
        | sequence;
}

protected long timeGen() {
    return SystemClock.now();
}

/**
 * 高并發(fā)場景下System.currentTimeMillis()的性能問題的優(yōu)化
 *
 * <p>System.currentTimeMillis()的調(diào)用比new一個(gè)普通對象要耗時(shí)的多（具體耗時(shí)高出多少我還沒測試過，有人說是100倍左右）</p>
 * <p>System.currentTimeMillis()之所以慢是因?yàn)槿ジ到y(tǒng)打了一次交道</p>
 * <p>后臺定時(shí)更新時(shí)鐘，JVM退出時(shí)，線程自動回收</p>
 * <p>10億：43410,206,210.72815533980582%</p>
 * <p>1億：4699,29,162.0344827586207%</p>
 * <p>1000萬：480,12,40.0%</p>
 * <p>100萬：50,10,5.0%</p>
 *
 * @author hubin
 * @since 2016-08-01
 */
public class SystemClock {
	// 定期更新時(shí)間戳的時(shí)間單位
    private final long period;
    // 記錄當(dāng)前時(shí)間戳的原子類，因?yàn)榭赡艽嬖诓l(fā)線程使用
    private final AtomicLong now;
    private SystemClock(long period) {
        this.period = period;
        this.now = new AtomicLong(System.currentTimeMillis());
        scheduleClockUpdating();
    }
    private static SystemClock instance() {
        return InstanceHolder.INSTANCE;
    }
    public static long now() {
        return instance().currentTimeMillis();
    }
    public static String nowDate() {
        return new Timestamp(instance().currentTimeMillis()).toString();
    }
    // 這里是有一個(gè)定期更新方法
    // 里面有一個(gè)定時(shí)線程池，它會以固定的時(shí)間間隔(period)在類里面更新當(dāng)前的時(shí)間戳
    private void scheduleClockUpdating() {
        ScheduledExecutorService scheduler = Executors.newSingleThreadScheduledExecutor(runnable -> {
            Thread thread = new Thread(runnable, "System Clock");
            thread.setDaemon(true);
            return thread;
        });
        scheduler.scheduleAtFixedRate(() -> now.set(System.currentTimeMillis()), period, period, TimeUnit.MILLISECONDS);
    }
    // 獲取事件
    private long currentTimeMillis() {
        return now.get();
    }
    // 默認(rèn)事件間隔為1ms
    private static class InstanceHolder {
        public static final SystemClock INSTANCE = new SystemClock(1);
    }
}