Spring Boot项目下JPA自定义雪花算法ID生成器详解
作者: 白石(https://github.com/wjw465150)本文分享下Spring boot项目下使用JPA操作数据库时关于雪花ID生成器的相关实现代码。
SnowFlake 算法（雪花算法），是 Twitter 开源的分布式 id 生成算法。其核心思想就是：使用一个 64 bit 的 long 型的数字作为全局唯一 id。在分布式系统中的应用十分广泛，且ID 引入了时间戳，基本上保持自增。
文章图片
在JPA中一个数据表必须要有主键，主键类型一般是推荐使用Long类型，那么在分布式微服务下需要保证ID的唯一性，此时往往需要自定义主键生成策略。
首先实现一个实体类的基类，在基类中定义ID的生成策略，子类继承其实现，这样就不用每个实体类都去写一遍了。
实体类的基类
AbstractBaseEntity
package org.wjw.jpa.snowflake;
import java.io.Serializable;
import javax.persistence.Column;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.MappedSuperclass;
import org.hibernate.annotations.GenericGenerator;
@MappedSuperclass
public abstract class AbstractBaseEntity implements Serializable {
@Id
@GenericGenerator(name = "snowFlakeIdGenerator", strategy = "org.wjw.jpa.snowflake.SnowFlakeIdGenerator")
@GeneratedValue(generator = "snowFlakeIdGenerator")
@Column(name = "id", length=18)
private String id;
/**
* 获取主键id
*
* @return id 因为如果是Long类型,前端js能处理的长度低于Java，防止精度丢失;
java的Long类型是18位， 而 js的Long类型(虽然没有明确定义的Long类型)是16位， 所以会造成丢失精度，
*/
public String getId() {
return id;
}/**
* 设置主键id
*
* @param id 主键id
*/
public void setId(String id) {
this.id = id;
}
}
使用雪花算法要注意的是，保证机器的时钟是一直增加的，也就是说不可以将时钟往前调，不然就不能保证ID的自增，并且有可能发生ID冲突（产生了重复的ID）。因此，上面的代码中，在检查到时钟异常时会抛出异常。strategy表示生成策略实现类的全路径名。
雪花算法类
SnowFlake:
package org.wjw.jpa.snowflake;
import javax.annotation.PostConstruct;
import org.hibernate.HibernateException;
import org.hibernate.internal.CoreLogging;
import org.hibernate.internal.CoreMessageLogger;
import org.springframework.beans.BeansException;
import org.springframework.beans.factory.annotation.Value;
import org.springframework.context.ApplicationContext;
import org.springframework.context.ApplicationContextAware;
import org.springframework.stereotype.Component;
/**
* 雪花算法
*/
@Component
public class SnowFlake implements ApplicationContextAware {
private static final CoreMessageLogger LOG = CoreLogging.messageLogger(SnowFlake.class);
private static ApplicationContext _applicationContext;
@Override
public void setApplicationContext(ApplicationContext applicationContext) throws BeansException {
SnowFlake._applicationContext = applicationContext;
}public static SnowFlake getBean() {
return _applicationContext.getBean(SnowFlake.class);
}/**
* 起始的时间戳
*/
private final long twepoch = 1557825652094L;
/**
* 每一部分占用的位数
*/
private final long workerIdBits = 5L;
private final long datacenterIdBits = 5L;
private final long sequenceBits = 12L;
/**
* 每一部分的最大值
*/
private final long maxWorkerId = -1L ^ (-1L << workerIdBits);
private final long maxDatacenterId = -1L ^ (-1L << datacenterIdBits);
private final long maxSequence = -1L ^ (-1L << sequenceBits);
/**
* 每一部分向左的位移
*/
private final long workerIdShift = sequenceBits;
private final long datacenterIdShift = sequenceBits + workerIdBits;
private final long timestampShift = sequenceBits + workerIdBits + datacenterIdBits;
@Value("${snowflake.datacenter-id:1}")
private long datacenterId;
// 数据中心ID@Value("${snowflake.worker-id:0}")
private long workerId;
// 机器IDprivate long sequence = 0L;
// 序列号
private long lastTimestamp = -1L;
// 上一次时间戳@PostConstruct
public void init() {
String msg;
if (workerId > maxWorkerId || workerId < 0) {
msg = String.format("worker Id can't be greater than %d or less than 0", maxWorkerId);
LOG.unsuccessful(msg);
}
if (datacenterId > maxDatacenterId || datacenterId < 0) {
msg = String.format("datacenter Id can't be greater than %d or less than 0", maxDatacenterId);
LOG.unsuccessful(msg);
}
}public String nextIdString() {
long id = nextId();
return String.valueOf(id);
}public synchronized long nextId() {
long timestamp = timeGen();
if (timestamp < lastTimestamp) {
throw new HibernateException(String.format("Clock moved backwards.Refusing to generate id for %d milliseconds", lastTimestamp - timestamp));
}
if (timestamp == lastTimestamp) {
sequence = (sequence + 1) & maxSequence;
if (sequence == 0L) {
timestamp = tilNextMillis();
}
} else {
sequence = 0L;
}
lastTimestamp = timestamp;
return (timestamp - twepoch) << timestampShift // 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
}private long tilNextMillis() {
long timestamp = timeGen();
while (timestamp <= lastTimestamp) {
timestamp = timeGen();
}
return timestamp;
}private long timeGen() {
return System.currentTimeMillis();
}}代码中使用***@Value(“ s n o w f l a k e . d a t a c e n t e r ? i d : 1 " ) ? ? ? 和 ? ? ? @ V a l u e ( " {snowflake.datacenter-id:1}")***和***@Value(" snowflake.datacenter?id:1")???和???@Value("{snowflake.worker-id:0}”)***注解从环境配置中读取当前的数据中心id机器id。集成JPA配置SnowFlakeIdGenerator
package org.wjw.jpa.snowflake;
import java.io.Serializable;
import java.util.Properties;
import org.hibernate.HibernateException;
import org.hibernate.MappingException;
import org.hibernate.engine.spi.SharedSessionContractImplementor;
import org.hibernate.id.Configurable;
import org.hibernate.id.IdentifierGenerator;
import org.hibernate.service.ServiceRegistry;
import org.hibernate.type.Type;
/**
* JPA 雪花算法ID生成器
*/
public class SnowFlakeIdGenerator implements IdentifierGenerator, Configurable {@Override
public Serializable generate(SharedSessionContractImplementor session, Object o) throws HibernateException {
return SnowFlake.getBean().nextIdString();
}@Override
public void configure(Type type, Properties params, ServiceRegistry serviceRegistry) throws MappingException {
}
}测试 创建一个
@Entity好的，接下来就是正常实体类继承基类就可以了，如下
User实体类：package org.wjw.jpa.entity;
import javax.persistence.Entity;
import org.wjw.jpa.snowflake.AbstractBaseEntity;
@Entity
public class User extends AbstractBaseEntity {
private String name;
private String email;
public String getName() {
return name;
}public void setName(String name) {
this.name = name;
}public String getEmail() {
return email;
}public void setEmail(String email) {
this.email = email;
}@Override
public String toString() {
return "User [id=" + id + ", name=" + name + ", email=" + email + "]";
}}
创建一个 Repository
仓库类
UserJpaRepository:package org.wjw.jpa.repository;
import org.springframework.data.jpa.repository.JpaRepository;
import org.wjw.jpa.entity.User;
public interface UserJpaRepository extends JpaRepository {
}
进行单元测试
【spring|Spring Boot项目下JPA自定义雪花算法ID生成器详解】单元测试代码:
package org.wjw.jpa;
import org.junit.jupiter.api.Test;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.context.SpringBootTest;
import org.wjw.jpa.entity.User;
import org.wjw.jpa.repository.UserJpaRepository;
@SpringBootTest
public class MyTests {
@Autowired
private UserJpaRepository userRepository;
@Test
public void testAddNewUser() {
User n = new User();
n.setName("Querydsl");
n.setEmail("Querydsl@gamil.com");
userRepository.save(n);
}
}
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