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本文主要研究一下mybatis-plus的DefaultIdentifierGenerator
MybatisSqlSessionFactoryBuilder
com/baomidou/mybatisplus/core/MybatisSqlSessionFactoryBuilder.java
代码语言：javascript复制 @Override
public SqlSessionFactory build(Configuration configuration) {
GlobalConfig globalConfig = GlobalConfigUtils.getGlobalConfig(configuration);
final IdentifierGenerator identifierGenerator;
if (null == globalConfig.getIdentifierGenerator()) {
identifierGenerator = new DefaultIdentifierGenerator();
globalConfig.setIdentifierGenerator(identifierGenerator);
} else {
identifierGenerator = globalConfig.getIdentifierGenerator();
}
IdWorker.setIdentifierGenerator(identifierGenerator);
if (globalConfig.isEnableSqlRunner()) {
new SqlRunnerInjector().inject(configuration);
}
SqlSessionFactory sqlSessionFactory = super.build(configuration);
// 缓存 sqlSessionFactory
globalConfig.setSqlSessionFactory(sqlSessionFactory);
return sqlSessionFactory;
}MybatisSqlSessionFactoryBuilder的build方法，在globalConfig.getIdentifierGenerator()为null的时候创建并使用DefaultIdentifierGenerator
IdentifierGenerator
com/baomidou/mybatisplus/core/incrementer/IdentifierGenerator.java
代码语言：javascript复制public interface IdentifierGenerator {
/**
* 判断是否分配 ID
*
* @param idValue 主键值
* @return true 分配 false 无需分配
*/
default boolean assignId(Object idValue) {
return StringUtils.checkValNull(idValue);
}
/**
* 生成Id
*
* @param entity 实体
* @return id
*/
Number nextId(Object entity);
/**
* 生成uuid
*
* @param entity 实体
* @return uuid
*/
default String nextUUID(Object entity) {
return IdWorker.get32UUID();
}
}IdentifierGenerator接口定义了nextId方法，同时提供了assignId、nextUUID默认方法
DefaultIdentifierGenerator
com/baomidou/mybatisplus/core/incrementer/DefaultIdentifierGenerator.java
代码语言：javascript复制public class DefaultIdentifierGenerator implements IdentifierGenerator {
private final Sequence sequence;
public DefaultIdentifierGenerator() {
this.sequence = new Sequence(null);
}
public DefaultIdentifierGenerator(InetAddress inetAddress) {
this.sequence = new Sequence(inetAddress);
}
public DefaultIdentifierGenerator(long workerId, long dataCenterId) {
this.sequence = new Sequence(workerId, dataCenterId);
}
public DefaultIdentifierGenerator(Sequence sequence) {
this.sequence = sequence;
}
@Override
public Long nextId(Object entity) {
return sequence.nextId();
}
}DefaultIdentifierGenerator实现了IdentifierGenerator接口，其nextId方法委托给了Sequence
Sequence
com/baomidou/mybatisplus/core/toolkit/Sequence.java
代码语言：javascript复制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 static final long twepoch = 1288834974657L;
/**
* 机器标识位数
*/
private final long workerIdBits = 5L;
private final long datacenterIdBits = 5L;
private final long maxWorkerId = -1L ^ (-1L << workerIdBits);
private final long maxDatacenterId = -1L ^ (-1L << datacenterIdBits);
/**
* 毫秒内自增位
*/
private final long sequenceBits = 12L;
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 final long workerId;
/**
* 数据标识 ID 部分
*/
private final long datacenterId;
/**
* 并发控制
*/
private long sequence = 0L;
/**
* 上次生产 ID 时间戳
*/
private long lastTimestamp = -1L;
/**
* IP 地址
*/
private InetAddress inetAddress;
public Sequence(InetAddress inetAddress) {
this.inetAddress = inetAddress;
this.datacenterId = getDatacenterId(maxDatacenterId);
this.workerId = getMaxWorkerId(datacenterId, maxWorkerId);
}
/**
* 有参构造器
*
* @param workerId 工作机器 ID
* @param datacenterId 序列号
*/
public Sequence(long workerId, long datacenterId) {
Assert.isFalse(workerId > maxWorkerId || workerId < 0,
String.format("worker Id can't be greater than %d or less than 0", maxWorkerId));
Assert.isFalse(datacenterId > maxDatacenterId || datacenterId < 0,
String.format("datacenter Id can't be greater than %d or less than 0", maxDatacenterId));
this.workerId = workerId;
this.datacenterId = datacenterId;
}
//......
} Sequence的twepoch为1288834974657(
2010-11-04 09:42:54 657)，默认工作机器ID为5bit和数据中心ID均为5bit(与雪花算法一致)，默认maxWorkerId与maxDatacenterId均为2^5-1；毫秒内序列号为12bit；时间戳为41bit
getDatacenterId
代码语言：javascript复制 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 {
byte[] mac = network.getHardwareAddress();
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;
}getDatacenterId会通过NetworkInterface.getByInetAddress获取NetworkInterface，若为null则返回1，否则获取NetworkInterface的getHardwareAddress，之后根据mac地址(
00:16:3e:0e:36:ac)的倒数第2部分的低8位，将倒数第1部分左移8位再取低16位，再将这两部分取或，最后右移6位，再将结果对(maxDatacenterId 1)(即32)取余作为datacenterId
getMaxWorkerId
代码语言：javascript复制 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 获取16个低位
*/
return (mpid.toString().hashCode() & 0xffff) % (maxWorkerId 1);
}getMaxWorkerId实际是在没有指定workerId的时候自动计算一个workerId，它是基于datacenterId及pid的哈希值取低16位，最后对(maxWorkerId 1)(
即32)取余
nextId
代码语言：javascript复制 public synchronized long nextId() {
long timestamp = timeGen();
//闰秒
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 - 2 随机数
sequence = ThreadLocalRandom.current().nextLong(1, 3);
}
lastTimestamp = timestamp;
// 时间戳部分 | 数据中心部分 | 机器标识部分 | 序列号部分
return ((timestamp - twepoch) << timestampLeftShift)
| (datacenterId << datacenterIdShift)
| (workerId << workerIdShift)
| sequence;
}
protected long timeGen() {
return SystemClock.now();
}
protected long tilNextMillis(long lastTimestamp) {
long timestamp = timeGen();
while (timestamp <= lastTimestamp) {
timestamp = timeGen();
}
return timestamp;
} nextId就是雪花算法生成id的部分，它先通过SystemClock.now()获取当前时间戳，判断是同毫秒的则递增sequence，不同毫秒则随机设置为1或者2；最开始会对时间回拨进行判断，若差异大于5则直接抛出异常，在小于等于5的时候会等待offset的两倍，若时间还是小于lastTimestamp则抛出异常；sequence设置完之后更新lastTimestamp，然后按照
时间戳部分 | 数据中心部分 | 机器标识部分 | 序列号部分构造id
SystemClock
com/baomidou/mybatisplus/core/toolkit/SystemClock.java
代码语言：javascript复制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(runnable -> {
Thread thread = new Thread(runnable, "System Clock");
thread.setDaemon(true);
return thread;
});
scheduler.scheduleAtFixedRate(() -> 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);
}
}SystemClock是针对高并发场景下System. currentTimeMillis()的性能问题的优化，其构造器在创建的时候将System.currentTimeMillis()赋值给AtomicLong，其now方法返回的是instance().currentTimeMillis()，即now.get()，它有个定时任务，每隔1毫秒更新一下now为当前System.currentTimeMillis()
小结
mybatis-plus的MybatisSqlSessionFactoryBuilder的build方法，在globalConfig.getIdentifierGenerator()为null的时候创建并使用DefaultIdentifierGenerator，它内部使用的是Sequence来生成id的，Sequence使用的是雪花算法，默认的datacenterId(5bit)是基于mac地址计算而来(取倒数第2部分的低8位，将倒数第1部分左移8位再取低16位，再将这两部分取或，最后右移6位，再将结果对(maxDatacenterId 1)(即32)取余作为datacenterId)，默认的workerId(5bit)是基于datacenterId及pid的哈希值取低16位，最后对(maxWorkerId 1)(即32)取余；nextId就是雪花算法生成id的部分，它先通过SystemClock.now()获取当前时间戳，判断是同毫秒的则递增sequence，不同毫秒则随机设置为1或者2。
doc
- snowflake
