（一）缓存
缓存的作用：从内存中查询数据，而不从数据库查询，减轻服务器压力
一级缓存：基于SqlSession级别
二级缓存：基于Mapper级别
（二）一级缓存
（三）二级缓存
- Mybatis中默认关闭二级缓存，首先在mybatis-config.xml中开启二级缓存
<settings>
<!--开启二级缓存-->
<setting name="cacheEnabled" value="true"/>
</settings>
- 然后在XXXMapper.xml中配置缓存
<mapper namespace="com.mio4.mapper.UserMapper">
<cache eviction="LRU" flushInterval="60000" size="512" readOnly="true"/>
</mapper>
- 实现二级缓存的POJO类必须要实现io.Serializable接口
- 测试二级缓存
@Test
public void testCache2(){
SqlSession sqlSession = mySqlSessionFactory.getSqlSession();
UserMapper userMapper = sqlSession.getMapper(UserMapper.class);
User user1 = userMapper.selectUserById(1);
System.out.println(user1);
sqlSession.close(); //关闭一级缓存
sqlSession = mySqlSessionFactory.getSqlSession();
userMapper = sqlSession.getMapper(UserMapper.class);
User user2 = userMapper.selectUserById(1);
System.out.println(user2);
//Cache Hit Ratio [com.mio4.mapper.UserMapper]: 0.5
sqlSession.close();
}