模仿美团的leaf算法实现的，简易版本
基本思想：利用MySQL字段实现自增id，如果每次都去MySQL取id，性能较差，因此每次取id时，获取一个号段，比如一次获取2000个id，缓存在应用里，每次用完后再去MySQL数据库获取新的号段。
知识点：怎么在静态方法中调用非静态属性、怎么将两条sql放在一个事务里面

• 新建表，初始化数据(此处跟leaf算法相同)

CREATE TABLE `leaf_alloc` (
  `biz_tag` varchar(128)  NOT NULL DEFAULT '',
  `max_id` bigint(20) NOT NULL DEFAULT '1',
  `step` int(11) NOT NULL,
  `description` varchar(256)  DEFAULT NULL,
  `update_time` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
  PRIMARY KEY (`biz_tag`)
) ENGINE=InnoDB;

insert into leaf_alloc(biz_tag, max_id, step, description) values('leaf-segment-test', 1686672000000, 2000, 'Test leaf Segment Mode Get Id')

• id生成器实现类

package com.xys.util;

import com.xys.entity.LeafAlloc;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.stereotype.Component;

import javax.annotation.PostConstruct;
import javax.annotation.Resource;
import java.util.concurrent.atomic.AtomicLong;

@Component
public class IDGen {
    private static final Logger LOG = LoggerFactory.getLogger(IDGen.class);
    private static final AtomicLong value = new AtomicLong(0);
    private static volatile long max;
    private static volatile int step;
    private static final String BIZ_TAG = "leaf-segment-test";

    @Resource
    private SqlSessionFactory sqlSessionFactory;
    //此处的idGen对象是为了在静态方法中调用非静态属性sqlSessionFactory
    private static IDGen idGen;

    @PostConstruct
    public void init() throws Exception {
        idGen = this;
        idGen.sqlSessionFactory = this.sqlSessionFactory;
        updateMaxIdAndGetLeafAlloc();
    }

    public static synchronized long nextId() {
        long l = value.incrementAndGet();
        if (l >= max) {
            updateMaxIdAndGetLeafAlloc();
        }
        return l;
    }

    private static void updateMaxIdAndGetLeafAlloc() {
        //更新和查询两条sql放在同一个事务里，目的是当成一次原子操作
        try (SqlSession sqlSession = idGen.sqlSessionFactory.openSession(false)) {
            //此处是mapper的全类名
            sqlSession.update("com.xys.mapper.LeafAllocMapper.updateMaxId", BIZ_TAG);
            LeafAlloc newAlloc = sqlSession.selectOne("com.xys.mapper.LeafAllocMapper.selectByPrimaryKey", BIZ_TAG);
            sqlSession.commit();
            value.set(newAlloc.getMaxId() - newAlloc.getStep());
            max = newAlloc.getMaxId();
            step = newAlloc.getStep();
            LOG.info("更新数据库到id生成器，value:{}, max:{}, step:{}", value, max, step);
        }
    }
}

• mapper的updateMaxId sql

<update id="updateMaxId">
        update leaf_alloc
        set max_id = max_id + step
        where biz_tag = #{bizTag,jdbcType=VARCHAR}
    </update>