8. Mybatis缓存

1.一级缓存

在mybatis中，一级缓存默认是开启的，并且无法关闭。一级缓存存在于SqlSession的生命周期中，在同一个SqlSession中查询时，mybatis会把执行的方法和参数生成缓存的键值，将键值和查询结果存入一个Map对象中。如果同一个SqlSession中执行的方法和参数完全一致，那么通过算法会生成相同的键值，当Map缓存对象中已存在该键值时，则会返回缓存中的对象。

测试：

@Test
public void testQueryUserById() {
    System.out.println(this.userMapper.queryUserById("1"));
    System.out.println(this.userMapper.queryUserById("1"));
}

执行日志：

2018-07-01 17:08:50,156 [main] [org.apache.ibatis.transaction.jdbc.JdbcTransaction]-[DEBUG] Opening JDBC Connection
2018-07-01 17:08:50,421 [main] [org.apache.ibatis.datasource.pooled.PooledDataSource]-[DEBUG] Created connection 242355057.
2018-07-01 17:08:50,423 [main] [com.zpc.mybatis.dao.UserMapper.queryUserById]-[DEBUG] ==> Preparing: select * from tb_user where id = ? 
2018-07-01 17:08:50,476 [main] [com.zpc.mybatis.dao.UserMapper.queryUserById]-[DEBUG] ==> Parameters: 1(String)
2018-07-01 17:08:50,509 [main] [com.zpc.mybatis.dao.UserMapper.queryUserById]-[DEBUG] <== Total: 1
User{id='1', userName='bigGod222', password='123456', name='鹏程', age=20, sex=1, birthday='2018-07-01', created='2018-07-01 13:35:40.0', updated='2018-07-01 13:35:40.0'}
User{id='1', userName='bigGod222', password='123456', name='鹏程', age=20, sex=1, birthday='2018-07-01', created='2018-07-01 13:35:40.0', updated='2018-07-01 13:35:40.0'}

使用：sqlSession.clearCache();可以强制清除缓存

测试：

@Test
public void testQueryUserById() {
    System.out.println(this.userMapper.queryUserById("1"));
    sqlSession.clearCache();
    System.out.println(this.userMapper.queryUserById("1"));
}

日志：

2018-07-01 17:10:51,065 [main] [org.apache.ibatis.transaction.jdbc.JdbcTransaction]-[DEBUG] Opening JDBC Connection
2018-07-01 17:10:51,359 [main] [org.apache.ibatis.datasource.pooled.PooledDataSource]-[DEBUG] Created connection 242355057.
2018-07-01 17:10:51,360 [main] [com.zpc.mybatis.dao.UserMapper.queryUserById]-[DEBUG] ==> Preparing: select * from tb_user where id = ? 
2018-07-01 17:10:51,408 [main] [com.zpc.mybatis.dao.UserMapper.queryUserById]-[DEBUG] ==> Parameters: 1(String)
2018-07-01 17:10:51,437 [main] [com.zpc.mybatis.dao.UserMapper.queryUserById]-[DEBUG] <== Total: 1
User{id='1', userName='bigGod222', password='123456', name='鹏程', age=20, sex=1, birthday='2018-07-01', created='2018-07-01 13:35:40.0', updated='2018-07-01 13:35:40.0'}
2018-07-01 17:10:51,438 [main] [com.zpc.mybatis.dao.UserMapper.queryUserById]-[DEBUG] ==> Preparing: select * from tb_user where id = ? 
2018-07-01 17:10:51,438 [main] [com.zpc.mybatis.dao.UserMapper.queryUserById]-[DEBUG] ==> Parameters: 1(String)
2018-07-01 17:10:51,440 [main] [com.zpc.mybatis.dao.UserMapper.queryUserById]-[DEBUG] <== Total: 1
User{id='1', userName='bigGod222', password='123456', name='鹏程', age=20, sex=1, birthday='2018-07-01', created='2018-07-01 13:35:40.0', updated='2018-07-01 13:35:40.0'}

执行update、insert、delete的时候，会清空缓存
测试：

@Test
public void testQueryUserById() {
    System.out.println(this.userMapper.queryUserById("1"));
    //sqlSession.clearCache();

    User user=new User();
    user.setName("美女");
    user.setId("1");
    userMapper.updateUser(user);

    System.out.println(this.userMapper.queryUserById("1"));
}

日志：

2018-07-01 17:18:15,128 [main] [org.apache.ibatis.transaction.jdbc.JdbcTransaction]-[DEBUG] Opening JDBC Connection
2018-07-01 17:18:15,399 [main] [org.apache.ibatis.datasource.pooled.PooledDataSource]-[DEBUG] Created connection 242355057.
2018-07-01 17:18:15,401 [main] [com.zpc.mybatis.dao.UserMapper.queryUserById]-[DEBUG] ==> Preparing: select * from tb_user where id = ? 
2018-07-01 17:18:15,466 [main] [com.zpc.mybatis.dao.UserMapper.queryUserById]-[DEBUG] ==> Parameters: 1(String)
2018-07-01 17:18:15,492 [main] [com.zpc.mybatis.dao.UserMapper.queryUserById]-[DEBUG] <== Total: 1
User{id='1', userName='bigGod222', password='123456', name='鹏程', age=20, sex=1, birthday='2018-07-01', created='2018-07-01 13:35:40.0', updated='2018-07-01 13:35:40.0'}
2018-07-01 17:18:15,527 [main] [com.zpc.mybatis.dao.UserMapper.updateUser]-[DEBUG] ==> Preparing: UPDATE tb_user set name = ?, updated = now() WHERE (id = ?); 
2018-07-01 17:18:15,529 [main] [com.zpc.mybatis.dao.UserMapper.updateUser]-[DEBUG] ==> Parameters: 美女(String), 1(String)
2018-07-01 17:18:15,532 [main] [com.zpc.mybatis.dao.UserMapper.updateUser]-[DEBUG] <== Updates: 1
2018-07-01 17:18:15,532 [main] [com.zpc.mybatis.dao.UserMapper.queryUserById]-[DEBUG] ==> Preparing: select * from tb_user where id = ? 
2018-07-01 17:18:15,533 [main] [com.zpc.mybatis.dao.UserMapper.queryUserById]-[DEBUG] ==> Parameters: 1(String)
2018-07-01 17:18:15,538 [main] [com.zpc.mybatis.dao.UserMapper.queryUserById]-[DEBUG] <== Total: 1
User{id='1', userName='bigGod222', password='123456', name='美女', age=20, sex=1, birthday='2018-07-01', created='2018-07-01 13:35:40.0', updated='2018-07-01 17:18:15.0'}

一级缓存在多线程场景下可能会导致脏读

测试代码：

public class MybatisTest2 {
    public static void main(String[] args) throws Exception {
        // 指定全局配置文件
        String resource = "mybatis-config.xml";
        // 读取配置文件
        InputStream inputStream = Resources.getResourceAsStream(resource);
        // 构建sqlSessionFactory
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
        // 获取sqlSession
        SqlSession sqlSession = sqlSessionFactory.openSession();
        try {
            UserMapper userMapper = sqlSession.getMapper(UserMapper.class);
            User user = userMapper.queryUserById(1);
            System.out.println(user);
            Thread.sleep(1000*60);//此时可以手动修改数据库中数据
            User user2 = userMapper.queryUserById(1);
            System.out.println(user2);
            System.out.println(user==user2);
        } finally {
            sqlSession.close();
        }
    }
}

测试结果：两次返回的对象是同一个，第二次返回的是缓存中数据。

2020-04-21 11:11:18,921 [main] [org.apache.ibatis.datasource.pooled.PooledDataSource]-[DEBUG] Created connection 1738710757.
2020-04-21 11:11:18,921 [main] [org.apache.ibatis.transaction.jdbc.JdbcTransaction]-[DEBUG] Setting autocommit to false on JDBC Connection [com.mysql.jdbc.JDBC4Connection@67a29ee5]
2020-04-21 11:11:18,928 [main] [com.mybatis.UserMapper.queryUserById]-[DEBUG] ==>  Preparing: select * from tb_user where id = ? 
2020-04-21 11:11:19,041 [main] [com.mybatis.UserMapper.queryUserById]-[DEBUG] ==> Parameters: 1(Integer)
2020-04-21 11:11:19,158 [main] [com.mybatis.UserMapper.queryUserById]-[DEBUG] <==      Total: 1
User(id=1, userName=zpc, password=123456, name=鹏程, age=33, sex=1, birthday=Sun Sep 02 00:00:00 CDT 1990, created=2020-04-19 11:14:40.0, updated=2020-04-19 11:14:40.0)
User(id=1, userName=zpc, password=123456, name=鹏程, age=33, sex=1, birthday=Sun Sep 02 00:00:00 CDT 1990, created=2020-04-19 11:14:40.0, updated=2020-04-19 11:14:40.0)
true
2020-04-21 11:12:19,161 [main] [org.apache.ibatis.transaction.jdbc.JdbcTransaction]-[DEBUG] Resetting autocommit to true on JDBC Connection [com.mysql.jdbc.JDBC4Connection@67a29ee5]

此时数据库中真实数据：

 解决方法：

　　根据数据安全性划分，像评论等对用业务影响不大的数据，保持使用一级缓存，提高查询效率。

　　像价格等对业务影响很大的数据，每次查询时清空缓存或者每次查询都新建个sqlSession对象。

清空缓存：

方式一：代码里手动调用下面方法

sqlSession.clearCache();

方式二：mapper.xml配置文件里对应查询sql增加 flushCache="true" 配置

    <select id="queryUserById" flushCache="true" resultType="com.mybatis.User">
        select * from tb_user where id = #{id}
    </select>

2.二级缓存

mybatis 的二级缓存的作用域是一个mapper的namespace ，同一个namespace中查询sql可以从缓存中命中。

开启二级缓存：

<mapper namespace="com.zpc.mybatis.dao.UserMapper">
　　<cache/>
</mapper>

测试：

@Test
public void testCache() {
    System.out.println(this.userMapper.queryUserById("1"));

    sqlSession.close();
    SqlSession sqlSession = sqlSessionFactory.openSession();
    UserMapper mapper = sqlSession.getMapper(UserMapper.class);

    System.out.println(mapper.queryUserById("1"));
}

开启二级缓存，必须序列化：

public class User implements Serializable{
private static final long serialVersionUID = -3330851033429007657L;

日志：

2018-07-01 17:23:39,335 [main] [org.apache.ibatis.transaction.jdbc.JdbcTransaction]-[DEBUG] Opening JDBC Connection
2018-07-01 17:23:39,664 [main] [org.apache.ibatis.datasource.pooled.PooledDataSource]-[DEBUG] Created connection 2092769598.
2018-07-01 17:23:39,665 [main] [com.zpc.mybatis.dao.UserMapper.queryUserById]-[DEBUG] ==> Preparing: select * from tb_user where id = ? 
2018-07-01 17:23:39,712 [main] [com.zpc.mybatis.dao.UserMapper.queryUserById]-[DEBUG] ==> Parameters: 1(String)
2018-07-01 17:23:39,734 [main] [com.zpc.mybatis.dao.UserMapper.queryUserById]-[DEBUG] <== Total: 1
User{id='1', userName='bigGod222', password='123456', name='美女', age=20, sex=1, birthday='2018-07-01', created='2018-07-01 13:35:40.0', updated='2018-07-01 17:18:15.0'}
2018-07-01 17:23:39,743 [main] [org.apache.ibatis.transaction.jdbc.JdbcTransaction]-[DEBUG] Closing JDBC Connection [com.mysql.jdbc.JDBC4Connection@7cbd213e]
2018-07-01 17:23:39,744 [main] [org.apache.ibatis.datasource.pooled.PooledDataSource]-[DEBUG] Returned connection 2092769598 to pool.
2018-07-01 17:23:39,746 [main] [com.zpc.mybatis.dao.UserMapper]-[DEBUG] Cache Hit Ratio [com.zpc.mybatis.dao.UserMapper]: 0.5
User{id='1', userName='bigGod222', password='123456', name='美女', age=20, sex=1, birthday='2018-07-01', created='2018-07-01 13:35:40.0', updated='2018-07-01 17:18:15.0'}

关闭二级缓存：

不开启，或者在全局的mybatis-config.xml 中去关闭二级缓存

<settings>
　　<!--开启驼峰匹配-->
　　<setting name="mapUnderscoreToCamelCase" value="true"/>
　　<!--开启二级缓存,全局总开关，这里关闭，mapper中开启了也没用-->
　　<setting name="cacheEnabled" value="false"/>
</settings>