MyBatis一级缓存介绍

默认情况下，MyBatis只启用了本地的会话缓存，它仅仅对一个会话中的数据进行缓存。这也就是大家常说的MyBatis一级缓存，一级缓存的作用域是SqlSession。

MyBatis一级缓存的运行过程是这样的：执行SQL语句的过程中，首次执行它时从数据库获取的所有数据会被存储在一段高速缓存中，今后执行这条语句时就会从高速缓存中读取结果，而不是再次查询数据库。MyBatis提供了默认下基于Java HashMap的缓存实现。

重点是要明白：MyBatis执行SQL语句之后，这条语句的执行结果被缓存，以后再执行这条语句的时候，会直接从缓存中拿结果，而不是再次执行SQL。但是一旦执行新增或更新或删除操作，缓存就会被清除。下面将分情况来验证一下。

第1种情况：同个session进行两次相同查询

public class Test
{
    public static void main(String[] args) throws Exception
    {
        InputStream inputStream = Resources.getResourceAsStream("mybatis-config.xml");

        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);

        SqlSession session = sqlSessionFactory.openSession();

        Person p1 = session.selectOne("cn.mybatis.mydemo.mapper.PersonMapper.selectPersonById", 1);

        Person p2 = session.selectOne("cn.mybatis.mydemo.mapper.PersonMapper.selectPersonById", 1);
        
        session.commit();
        
        session.close();
    }
}

结论：MyBatis只进行1次数据库查询。

==> Preparing: select * from t_person where id = ?
==> Parameters: 1(Integer)
<== Total: 1
User{id=1, name='Kit', age=23, birthday=Sun Oct 12 23:20:13 CST 2010}
User{id=1, name='Kit', age=23, birthday=Sun Oct 12 23:20:13 CST 2010}

第2种情况：同个session进行两次不同的查询。

public class Test
{
    public static void main(String[] args) throws Exception
    {
        InputStream inputStream = Resources.getResourceAsStream("mybatis-config.xml");

        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);

        SqlSession session = sqlSessionFactory.openSession();

        Person p1 = session.selectOne("cn.mybatis.mydemo.mapper.PersonMapper.selectPersonById", 1);

        Person p2 = session.selectOne("cn.mybatis.mydemo.mapper.PersonMapper.selectPersonById", 2);
        
        session.commit();
        
        session.close();
    }
}

结论：MyBatis进行两次数据库查询。

==> Preparing: select * from t_person where id = ?
==> Parameters: 1(Integer)
<== Total: 1
User{id=1, name='kit', age=23, birthday=Sun Oct 12 23:20:13 CST 2010}
==> Preparing: select * from t_person where id = ?
==> Parameters: 2(Integer)
<== Total: 1
User{id=2, name='Jim', age=50, birthday=Sat Dec 06 17:12:01 CST 2010}

第3种情况：不同session，进行相同查询。

public class Test
{
    public static void main(String[] args) throws Exception
    {
        InputStream inputStream = Resources.getResourceAsStream("mybatis-config.xml");

        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);

        SqlSession session1 = sqlSessionFactory.openSession();

        Person p1 = session1.selectOne("cn.mybatis.mydemo.mapper.PersonMapper.selectPersonById", 1);

        SqlSession session2 = sqlSessionFactory.openSession();
        Person p2 = session2.selectOne("cn.mybatis.mydemo.mapper.PersonMapper.selectPersonById", 2);

        session1.commit();
        session2.commit();

        session1.close();
        session2.close();
    }
}

结论：MyBatis进行两次数据库查询。

==> Preparing: select * from t_person where id = ?
==> Parameters: 1(Integer)
<== Total: 1
User{id=1, name='kit', age=23, birthday=Sun Oct 12 23:20:13 CST 2010}
==> Preparing: select * from t_person where id = ?
==> Parameters: 2(Integer)
<== Total: 1
User{id=2, name='Jim', age=50, birthday=Sat Dec 06 17:12:01 CST 2010}

第4种情况：同个session，查询之后更新数据，再次查询相同的语句。

public class Test
{
    public static void main(String[] args) throws Exception
    {
        InputStream inputStream = Resources.getResourceAsStream("mybatis-config.xml");

        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);

        SqlSession session = sqlSessionFactory.openSession();

        Person p = null;
        
        p = session.selectOne("cn.mybatis.mydemo.mapper.PersonMapper.selectPersonById", 1);

        p.setAge(30);

        session.update("cn.mybatis.mydemo.mapper.PersonMapper.updatePerson", p);

        p = session.selectOne("cn.mybatis.mydemo.mapper.PersonMapper.selectPersonById", 1);

        session.commit();

        session.close();

    }
}

结论：更新操作之后缓存会被清除：

==> Preparing: select * from t_person where id = ?
==> Parameters: 1(Integer)
<== Total: 1
User{id=1, name='kit', age=23, birthday=Sun Oct 12 23:20:13 CST 2010}
==> Preparing: update t_person set age = ? where ID = ?
==> Parameters: 30(Integer), 1(Integer)
<== Updates: 1
==> Preparing: sselect * from t_person where id = ?
==> Parameters: 1(Integer)
<== Total: 1
User{id=1, name='kit', age=30, birthday=Sun Oct 12 23:20:13 CST 2010}

小结

很明显，以上各种情况验证了一级缓存的概念，在同个SqlSession中，查询语句相同的sql会被缓存，但是一旦执行新增或更新或删除操作，缓存就会被清除。