spring|Spring Boot项目下JPA自定义雪花算法ID生成器详解 java|spring|hibernate|boot

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。

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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; } }

strategy 表示生成策略实现类的全路径名。

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。
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); } }