一次读懂mybatis中的缓存机制

缓存功能针对于查询（没听说果UPDATE，INSERT语句要缓存什么，都是直接执行的）

默认情况下，mybatis会启用一级缓存。

如果使用同一个session对象调用了相同的SELECT语句，则直接会从缓存中返回结果，而不是再查询一次数据库。

注意:session调用commit或close方法后,这个session中的一级缓存就会被清空

例如: 根据日志输出可以看出,下面代码只会发出一条sql查询语句

SqlSession sqlSession = MyBatisSqlSessionFactory.openSession();
OneToManyMapper mapper = sqlSession.getMapper(OneToManyMapper.class);
Tutor findTutorById1 = mapper.findTutorById(1);
System.out.println(findTutorById1);
Tutor findTutorById2 = mapper.findTutorById(1);
System.out.println(findTutorById2);

日志文件输出：

2019-10-16 19:15:39,870 [DEBUG] com.briup.mappers.OneToManyMapper.findTutorById - ==> Preparing: select * from tutors where tutor_id=?
2019-10-16 19:15:39,999 [DEBUG] com.briup.mappers.OneToManyMapper.findTutorById - ==> Parameters: 1(Integer)
2019-10-16 19:15:40,218 [DEBUG] com.briup.mappers.OneToManyMapper.findAddressById - ====> Preparing: select * from addresses where addr_id = ?
2019-10-16 19:15:40,218 [DEBUG] com.briup.mappers.OneToManyMapper.findAddressById - ====> Parameters: 1(Integer)
2019-10-16 19:15:40,220 [DEBUG] com.briup.mappers.OneToManyMapper.findAddressById - <==== Total: 1
2019-10-16 19:15:40,221 [DEBUG] com.briup.mappers.OneToManyMapper.findCoursesByTutor - ====> Preparing: select * from courses where tutor_id=?
2019-10-16 19:15:40,221 [DEBUG] com.briup.mappers.OneToManyMapper.findCoursesByTutor - ====> Parameters: 1(Integer)
2019-10-16 19:15:40,226 [DEBUG] com.briup.mappers.OneToManyMapper.findCoursesByTutor - <==== Total: 2
2019-10-16 19:15:40,226 [DEBUG] com.briup.mappers.OneToManyMapper.findTutorById - <== Total: 1

Tutor [tutorId=1, name=null, email=zs@briup.com, phone=PhoneNumber [countryCode=123, stateCode=456, number=7890], address=Address [addrId=1, street=redSt, city=kunshan, state=W, zip=12345, country=china], courses=[Course [courseId=1, name=JavaSE, description=JavaSE, startDate=Thu Jan 10 00:00:00 CST 2019, endDate=Sun Feb 10 00:00:00 CST 2019], Course [courseId=3, name=MyBatis, description=MyBatis, startDate=Thu Jan 10 00:00:00 CST 2019, endDate=Wed Feb 20 00:00:00 CST 2019]]]

Tutor [tutorId=1, name=null, email=zs@briup.com, phone=PhoneNumber [countryCode=123, stateCode=456, number=7890], address=Address [addrId=1, street=redSt, city=kunshan, state=W, zip=12345, country=china], courses=[Course [courseId=1, name=JavaSE, description=JavaSE, startDate=Thu Jan 10 00:00:00 CST 2019, endDate=Sun Feb 10 00:00:00 CST 2019], Course [courseId=3, name=MyBatis, description=MyBatis, startDate=Thu Jan 10 00:00:00 CST 2019, endDate=Wed Feb 20 00:00:00 CST 2019]]]

从以上日志可以分析出来,这个select语句只发了一次,第二次的查询用的是第一次查询的缓存

二级缓存: 在不同的session对象之间可以共享缓存的数据
1.mybatis-config.xml文件中保证<setting name="cacheEnabled" value="true"/>设置中是缓存功能是开启的,默认就是开启的true
2.在需要二级缓存的xml映射文件中,手动开启缓存功能,在根元素中加入一个标签即可:<cache/>
3.一个session查询完数据之后,需要调用commit或者close方法后,这个数据才会进入到二级缓存中,然后其他session就可以共享到这个缓存数据了

注意:默认情况下,被二级缓存保存的对象需要实现序列化接口。

例如:
mybatis-config.xml:

<settings>
<setting name="cacheEnabled" value="true"/>
</settings>

xml映射文件:

<mapper namespace="com.briup.mappers.SpecialMapper">
<cache/>
<select> ..</select>
<select> ..</select>
<select> ..</select>
</mapper>

测试代码:

@Test
public void test_cache2(){
SqlSession session1 = null;
SqlSession session2 = null;
try {
session1 = MyBatisSqlSessionFactory.openSession();
session2 = MyBatisSqlSessionFactory.openSession();
 
SpecialMapper mapper1 = session1.getMapper(SpecialMapper.class);
SpecialMapper mapper2 = session2.getMapper(SpecialMapper.class);
 
User user1 = mapper1.findUserById(56);
System.out.println(user1);
session1.commit();
 
User user2 = mapper2.findUserById(56);
System.out.println(user2);
session2.commit();
} catch (Exception e) {
e.printStackTrace();
}finally {
if(session1!=null)session1.close();
if(session2!=null)session2.close();
}
}

日志文件输出:

2019-10-16 19:29:22,443 [DEBUG] com.briup.mappers.OneToManyMapper - Cache Hit Ratio [com.briup.mappers.OneToManyMapper]: 0.0
2019-10-16 19:29:23,449 [DEBUG] com.briup.mappers.OneToManyMapper.findTutorById - ==> Preparing: select * from tutors where tutor_id=?
2019-10-16 19:29:23,648 [DEBUG] com.briup.mappers.OneToManyMapper.findTutorById - ==> Parameters: 1(Integer)
2019-10-16 19:29:23,805 [DEBUG] com.briup.mappers.OneToManyMapper - Cache Hit Ratio [com.briup.mappers.OneToManyMapper]: 0.0
2019-10-16 19:29:23,805 [DEBUG] com.briup.mappers.OneToManyMapper.findAddressById - ====> Preparing: select * from addresses where addr_id = ?
2019-10-16 19:29:23,806 [DEBUG] com.briup.mappers.OneToManyMapper.findAddressById - ====> Parameters: 1(Integer)
2019-10-16 19:29:23,808 [DEBUG] com.briup.mappers.OneToManyMapper.findAddressById - <==== Total: 1
2019-10-16 19:29:23,808 [DEBUG] com.briup.mappers.OneToManyMapper - Cache Hit Ratio [com.briup.mappers.OneToManyMapper]: 0.0
2019-10-16 19:29:23,809 [DEBUG] com.briup.mappers.OneToManyMapper.findCoursesByTutor - ====> Preparing: select * from courses where tutor_id=?
2019-10-16 19:29:23,809 [DEBUG] com.briup.mappers.OneToManyMapper.findCoursesByTutor - ====> Parameters: 1(Integer)
2019-10-16 19:29:23,814 [DEBUG] com.briup.mappers.OneToManyMapper.findCoursesByTutor - <==== Total: 2
2019-10-16 19:29:23,814 [DEBUG] com.briup.mappers.OneToManyMapper.findTutorById - <== Total: 1

Tutor [tutorId=1, name=null, email=zs@briup.com, phone=PhoneNumber [countryCode=123, stateCode=456, number=7890], address=Address [addrId=1, street=redSt, city=kunshan, state=W, zip=12345, country=china], courses=[Course [courseId=1, name=JavaSE, description=JavaSE, startDate=Thu Jan 10 00:00:00 CST 2019, endDate=Sun Feb 10 00:00:00 CST 2019], Course [courseId=3, name=MyBatis, description=MyBatis, startDate=Thu Jan 10 00:00:00 CST 2019, endDate=Wed Feb 20 00:00:00 CST 2019]]]

2019-10-16 19:29:23,825 [DEBUG] com.briup.mappers.OneToManyMapper - Cache Hit Ratio [com.briup.mappers.OneToManyMapper]: 0.25

Tutor [tutorId=1, name=null, email=zs@briup.com, phone=PhoneNumber [countryCode=123, stateCode=456, number=7890], address=Address [addrId=1, street=redSt, city=kunshan, state=W, zip=12345, country=china], courses=[Course [courseId=1, name=JavaSE, description=JavaSE, startDate=Thu Jan 10 00:00:00 CST 2019, endDate=Sun Feb 10 00:00:00 CST 2019], Course [courseId=3, name=MyBatis, description=MyBatis, startDate=Thu Jan 10 00:00:00 CST 2019, endDate=Wed Feb 20 00:00:00 CST 2019]]]

可以看出,sqlSession2这个对象第二次查询的时候,命中了缓存

2019-10-16 19:29:23,825 [DEBUG] com.briup.mappers.OneToManyMapper - Cache Hit Ratio [com.briup.mappers.OneToManyMapper]: 0.25(缓存命中率0.25)

二级缓存补充说明
　　1. 映射语句文件中的所有select语句将会被缓存
　　2. 映射语句文件中的所有insert，update和delete语句会刷新缓存
　　3. 缓存会使用Least Recently Used（LRU，最近最少使用的）算法来收回。
　　4. 缓存会根据指定的时间间隔来刷新。
　　5. 缓存会存储1024个对象

cache标签常用属性：
<cache
eviction="FIFO" <!--回收策略为先进先出-->
flushInterval="60000" <!--自动刷新时间60s-->
size="512" <!--最多缓存512个引用对象-->
readOnly="true"/> <!--true表示对象不能被写出去,即不可以被序列化,false表示可以写出去,即可以被序列化,默认值是false-->