MyBatis的缓存分析

一：MyBatis缓存简介

MyBatis支持声明式数据缓存（declarative data caching）。当一条SQL语句被标记为“可缓存”后，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，而不是再次命中数据库。MyBatis提供了默认下基于Java HashMap的缓存实现，以及用于与OSCache、Ehcache、Hazelcast和Memcached连接的默认连接器。MyBatis还提供API供其他缓存实现使用。

　　MyBatis执行SQL语句之后，这条语句就是被缓存，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，这就是所说的一级缓存。一级缓存的作用域scope是SqlSession。

一级缓存

测试

同个session进行两次相同查询：

 @Test
 public void test() {
     SqlSession sqlSession = sqlSessionFactory.openSession();
     try {
         User user = (User)sqlSession.selectOne("org.format.mybatis.cache.UserMapper.getById", 1);
         log.debug(user);
         User user2 = (User)sqlSession.selectOne("org.format.mybatis.cache.UserMapper.getById", 1);
         log.debug(user2);
     } finally {
         sqlSession.close();
     }
 }

test

MyBatis只进行1次数据库查询：

 ==>  Preparing: select * from USERS WHERE ID = ?
 ==> Parameters: 1(Integer)
 <==      Total: 1
 User{id=1, name='format', age=23, birthday=Sun Oct 12 23:20:13 CST 2017}
 User{id=1, name='format', age=23, birthday=Sun Oct 12 23:20:13 CST 2017}

结果

同个session进行两次不同的查询：

 @Test
 public void test() {
     SqlSession sqlSession = sqlSessionFactory.openSession();
     try {
         User user = (User)sqlSession.selectOne("org.format.mybatis.cache.UserMapper.getById", 1);
         log.debug(user);
         User user2 = (User)sqlSession.selectOne("org.format.mybatis.cache.UserMapper.getById", 2);
         log.debug(user2);
     } finally {
         sqlSession.close();
     }
 }

test

MyBatis进行两次数据库查询：

 ==>  Preparing: select * from USERS WHERE ID = ?
 ==> Parameters: 1(Integer)
 <==      Total: 1
 User{id=1, name='format', age=23, birthday=Sun Oct 12 23:20:13 CST 2017}
 ==>  Preparing: select * from USERS WHERE ID = ?
 ==> Parameters: 2(Integer)
 <==      Total: 1
 User{id=2, name='FFF', age=50, birthday=Sat Dec 06 17:12:01 CST 2017}

结果

不同session，进行相同查询：

 @Test
 public void test() {
     SqlSession sqlSession = sqlSessionFactory.openSession();
     SqlSession sqlSession2 = sqlSessionFactory.openSession();
     try {
         User user = (User)sqlSession.selectOne("org.format.mybatis.cache.UserMapper.getById", 1);
         log.debug(user);
         User user2 = (User)sqlSession2.selectOne("org.format.mybatis.cache.UserMapper.getById", 1);
         log.debug(user2);
     } finally {
         sqlSession.close();
         sqlSession2.close();
     }
 }

test

MyBatis进行了两次数据库查询：

 ==>  Preparing: select * from USERS WHERE ID = ?
 ==> Parameters: 1(Integer)
 <==      Total: 1
 User{id=1, name='format', age=23, birthday=Sun Oct 12 23:20:13 CST 2017}
 ==>  Preparing: select * from USERS WHERE ID = ?
 ==> Parameters: 1(Integer)
 <==      Total: 1
 User{id=1, name='format', age=23, birthday=Sun Oct 12 23:20:13 CST 2017}

结果

同个session,查询之后更新数据，再次查询相同的语句：

 @Test
 public void test() {
     SqlSession sqlSession = sqlSessionFactory.openSession();
     try {
         User user = (User)sqlSession.selectOne("org.format.mybatis.cache.UserMapper.getById", 1);
         log.debug(user);
         user.setAge(100);
         sqlSession.update("org.format.mybatis.cache.UserMapper.update", user);
         User user2 = (User)sqlSession.selectOne("org.format.mybatis.cache.UserMapper.getById", 1);
         log.debug(user2);
         sqlSession.commit();
     } finally {
         sqlSession.close();
     }
 }

test

更新操作之后缓存会被清除：

 ==>  Preparing: select * from USERS WHERE ID = ?
 ==> Parameters: 1(Integer)
 <==      Total: 1
 User{id=1, name='format', age=23, birthday=Sun Oct 12 23:20:13 CST 2017}
 ==>  Preparing: update USERS SET NAME = ? , AGE = ? , BIRTHDAY = ? where ID = ?
 ==> Parameters: format(String), 23(Integer), 2017-10-12 23:20:13.0(Timestamp), 1(Integer)
 <==    Updates: 1
 ==>  Preparing: select * from USERS WHERE ID = ?
 ==> Parameters: 1(Integer)
 <==      Total: 1
 User{id=1, name='format', age=23, birthday=Sun Oct 12 23:20:13 CST 2017}

结果

很明显，结果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