Snowflake id生成器

最下面代码是mybatisplus根据 李景枫 的开源稍改写的id生成器，核心算法就： Snowflake算法

Snowflake算法核心

把时间戳，工作机器id，序列号组合在一起。

Snowflake算法核心

除了最高位bit标记为不可用以外，其余三组bit占位均可浮动，看具体的业务需求而定。默认情况下41bit的时间戳可以支持该算法使用到2082年，10bit的工作机器id可以支持1023台机器，序列号支持1毫秒产生4095个自增序列id。下文会具体分析。

Snowflake – 时间戳

这里时间戳的细度是毫秒级，具体代码如下，建议使用64位linux系统机器，因为有vdso，gettimeofday()在用户态就可以完成操作，减少了进入内核态的损耗。

Snowflake – 工作机器id

严格意义上来说这个bit段的使用可以是进程级，机器级的话你可以使用MAC地址来唯一标示工作机器，工作进程级可以使用IP+Path来区分工作进程。如果工作机器比较少，可以使用配置文件来设置这个id是一个不错的选择，如果机器过多配置文件的维护是一个灾难性的事情。

Snowflake – 序列号

序列号就是一系列的自增id（多线程建议使用atomic），为了处理在同一毫秒内需要给多条消息分配id，若同一毫秒把序列号用完了，则“等待至下一毫秒”。

datacenterId是获取的机器网卡然后计算

id = ((0x000000FF & (long) mac[mac.length - 1]) | (0x0000FF00 & (((long) mac[mac.length - 2]) << 8))) >> 6;
id = id % (maxDatacenterId + 1);

getMaxWorkerId

StringBuilder mpid = new StringBuilder();
mpid.append(datacenterId);
String name = ManagementFactory.getRuntimeMXBean().getName();
if (StringUtils.isNotEmpty(name)) {
/*
* GET jvmPid
*/
mpid.append(name.split("@")[0]);
}
/*
* MAC + PID 的 hashcode 获取16个低位
*/
return (mpid.toString().hashCode() & 0xffff) % (maxWorkerId + 1);

具体算法代码

/**
* Copy
* right (c) 2011-2020, hubin (jobob@qq.com).
* <p>
* Licensed under the Apache License, Version 2.0 (the "License"); you may not
* use this file except in compliance with the License. You may obtain a copy of
* the License at
* <p>
* http://www.apache.org/licenses/LICENSE-2.0
* <p>
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
* WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
* License for the specific language governing permissions and limitations under
* the License.
*/
package com.baomidou.mybatisplus.toolkit;
import java.lang.management.ManagementFactory;
import java.net.InetAddress;
import java.net.NetworkInterface;
import java.util.concurrent.ThreadLocalRandom;
import org.apache.ibatis.logging.Log;
import org.apache.ibatis.logging.LogFactory;
/**
* <p>
* 分布式高效有序ID生产黑科技(sequence) <br>
* 优化开源项目：http://git.oschina.net/yu120/sequence
* </p>
*
* @author hubin
* @date 2016-08-18
*/
public class Sequence {
private static final Log logger = LogFactory.getLog(Sequence.class);
/* 时间起始标记点，作为基准，一般取系统的最近时间（一旦确定不能变动） */
private final long twepoch = 1288834974657L;
private final long workerIdBits = 5L;/* 机器标识位数 */
private final long datacenterIdBits = 5L;
private final long maxWorkerId = -1L ^ (-1L << workerIdBits);
private final long maxDatacenterId = -1L ^ (-1L << datacenterIdBits);
private final long sequenceBits = 12L;/* 毫秒内自增位 */
private final long workerIdShift = sequenceBits;
private final long datacenterIdShift = sequenceBits + workerIdBits;
/* 时间戳左移动位 */
private final long timestampLeftShift = sequenceBits + workerIdBits + datacenterIdBits;
private final long sequenceMask = -1L ^ (-1L << sequenceBits);
private long workerId;
/* 数据标识id部分 */
private long datacenterId;
private long sequence = 0L;/* 0，并发控制 */
private long lastTimestamp = -1L;/* 上次生产id时间戳 */
public Sequence() {
this.datacenterId = getDatacenterId(maxDatacenterId);
this.workerId = getMaxWorkerId(datacenterId, maxWorkerId);
}
/**
* @param workerId
工作机器ID
* @param datacenterId 序列号
*/
public Sequence(long workerId, long datacenterId) {
if (workerId > maxWorkerId || workerId < 0) {
throw new IllegalArgumentException(String.format("worker Id can't be greater than %d or less than 0", maxWorkerId));
}
if (datacenterId > maxDatacenterId || datacenterId < 0) {
throw new IllegalArgumentException(
String.format("datacenter Id can't be greater than %d or less than 0", maxDatacenterId));
}
this.workerId = workerId;
this.datacenterId = datacenterId;
}
/**
* <p>
* 获取 maxWorkerId
* </p>
*/
protected static long getMaxWorkerId(long datacenterId, long maxWorkerId) {
StringBuilder mpid = new StringBuilder();
mpid.append(datacenterId);
String name = ManagementFactory.getRuntimeMXBean().getName();
if (StringUtils.isNotEmpty(name)) {
/*
* GET jvmPid
*/
mpid.append(name.split("@")[0]);
}
/*
* MAC + PID 的 hashcode 获取16个低位
*/
return (mpid.toString().hashCode() & 0xffff) % (maxWorkerId + 1);
}
/**
* <p>
* 数据标识id部分
* </p>
*/
protected static long getDatacenterId(long maxDatacenterId) {
long id = 0L;
try {
InetAddress ip = InetAddress.getLocalHost();
NetworkInterface network = NetworkInterface.getByInetAddress(ip);
if (network == null) {
id = 1L;
} else {
byte[] mac = network.getHardwareAddress();
if (null != mac) {
id = ((0x000000FF & (long) mac[mac.length - 1]) | (0x0000FF00 & (((long) mac[mac.length - 2]) << 8))) >> 6;
id = id % (maxDatacenterId + 1);
}
}
} catch (Exception e) {
logger.warn(" getDatacenterId: " + e.getMessage());
}
return id;
}
/**
* 获取下一个ID
*
* @return
*/
public synchronized long nextId() {
long timestamp = timeGen();
if (timestamp < lastTimestamp) {//闰秒
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) {
// 相同毫秒内，序列号自增
sequence = (sequence + 1) & sequenceMask;
if (sequence == 0) {
// 同一毫秒的序列数已经达到最大
timestamp = tilNextMillis(lastTimestamp);
}
} else {
// 不同毫秒内，序列号置为 1 - 3 随机数
sequence = ThreadLocalRandom.current().nextLong(1, 3);
}
lastTimestamp = timestamp;
return ((timestamp - twepoch) << timestampLeftShift)
// 时间戳部分
| (datacenterId << datacenterIdShift)
// 数据中心部分
| (workerId << workerIdShift)
// 机器标识部分
| sequence;
// 序列号部分
}
protected long tilNextMillis(long lastTimestamp) {
long timestamp = timeGen();
while (timestamp <= lastTimestamp) {
timestamp = timeGen();
}
return timestamp;
}
protected long timeGen() {
return SystemClock.now();
}
}

/**
* Copyright (c) 2011-2020, hubin (jobob@qq.com).
* <p>
* Licensed under the Apache License, Version 2.0 (the "License"); you may not
* use this file except in compliance with the License. You may obtain a copy of
* the License at
* <p>
* http://www.apache.org/licenses/LICENSE-2.0
* <p>
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
* WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
* License for the specific language governing permissions and limitations under
* the License.
*/
package com.baomidou.mybatisplus.toolkit;
import java.sql.Timestamp;
import java.util.concurrent.Executors;
import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.ThreadFactory;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicLong;
/**
* <p>
* 高并发场景下System.currentTimeMillis()的性能问题的优化
* </p>
* <p>
* System.currentTimeMillis()的调用比new一个普通对象要耗时的多（具体耗时高出多少我还没测试过，有人说是100倍左右）<br>
* System.currentTimeMillis()之所以慢是因为去跟系统打了一次交道<br>
* 后台定时更新时钟，JVM退出时，线程自动回收<br>
* 10亿：43410,206,210.72815533980582%<br>
* 1亿：4699,29,162.0344827586207%<br>
* 1000万：480,12,40.0%<br>
* 100万：50,10,5.0%<br>
* </p>
*
* @author lry
*/
public class SystemClock {
private final long period;
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();
}
private void scheduleClockUpdating() {
ScheduledExecutorService scheduler = Executors.newSingleThreadScheduledExecutor(new ThreadFactory() {
@Override
public Thread newThread(Runnable runnable) {
Thread thread = new Thread(runnable, "System Clock");
thread.setDaemon(true);
return thread;
}
});
scheduler.scheduleAtFixedRate(new Runnable() {
@Override
public void run() {
now.set(System.currentTimeMillis());
}
}, period, period, TimeUnit.MILLISECONDS);
}
private long currentTimeMillis() {
return now.get();
}
private static class InstanceHolder {
public static final SystemClock INSTANCE = new SystemClock(1);
}
}

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