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: 1User{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: 1User{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: 1User{id=2, name='Jim', age=50, birthday=Sat Dec 06 17:12:01 CST 2010}
第3种情况：不同session，进行不同查询。
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: 1User{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: 1User{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: 1User{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: 1User{id=1, name='kit', age=30, birthday=Sun Oct 12 23:20:13 CST 2010}
小结
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。
REF
http://www.mybatis.cn/archives/744.html
http://www.mybatis.cn/archives/746.html
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