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mybatis学习笔记(15)-查询缓存之二级缓存

标签： mybatis

mybatis学习笔记15-查询缓存之二级缓存
- 二级缓存原理
- 开启二级缓存
- 调用pojo类实现序列化接口
- 测试方法
- useCache配置
- 刷新缓存就是清空缓存
- 应用场景和局限性

本文主要讲mybatis的二级缓存，二级缓存是mapper级别的缓存，多个SqlSession去操作同一个Mapper的sql语句，多个SqlSession可以共用二级缓存，二级缓存是跨SqlSession的。

二级缓存原理

首先开启mybatis的二级缓存.

sqlSession1去查询用户id为1的用户信息，查询到用户信息会将查询数据存储到二级缓存中。

如果SqlSession3去执行相同mapper下sql，执行commit提交，清空该mapper下的二级缓存区域的数据。

sqlSession2去查询用户id为1的用户信息，去缓存中找是否存在数据，如果存在直接从缓存中取出数据。

二级缓存与一级缓存区别，二级缓存的范围更大，多个sqlSession可以共享一个UserMapper的二级缓存区域。

UserMapper有一个二级缓存区域（按namespace分），其它mapper也有自己的二级缓存区域（按namespace分）。每一个namespace的mapper都有一个二缓存区域，两个mapper的namespace如果相同，这两个mapper执行sql查询到数据将存在相同的二级缓存区域中。

开启二级缓存

mybaits的二级缓存是mapper范围级别，除了在SqlMapConfig.xml设置二级缓存的总开关，还要在具体的mapper.xml中开启二级缓存。

在核心配置文件SqlMapConfig.xml中加入

设置项	描述	允许值	默认值
cacheEnabled	对在此配置文件下的所有cache 进行全局性开/关设置。	true/false	true


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

在UserMapper.xml中开启二缓存，UserMapper.xml下的sql执行完成会存储到它的缓存区域（HashMap）。

<mapper namespace="com.iot.mybatis.mapper.UserMapper">

<cache />

...

mapper>

调用pojo类实现序列化接口

public class User implements Serializable{
    ....
}

为了将缓存数据取出执行反序列化操作，因为二级缓存数据存储介质多种多样，不一定在内存。

测试方法

// 二级缓存测试
@Test
public void testCache2() throws Exception {
    SqlSession sqlSession1 = sqlSessionFactory.openSession();
    SqlSession sqlSession2 = sqlSessionFactory.openSession();
    SqlSession sqlSession3 = sqlSessionFactory.openSession();
    // 创建代理对象
    UserMapper userMapper1 = sqlSession1.getMapper(UserMapper.class);
    // 第一次发起请求，查询id为1的用户
    User user1 = userMapper1.findUserById(1);
    System.out.println(user1);

    //这里执行关闭操作，将sqlsession中的数据写到二级缓存区域
    sqlSession1.close();


//      //使用sqlSession3执行commit()操作
//      UserMapper userMapper3 = sqlSession3.getMapper(UserMapper.class);
//      User user  = userMapper3.findUserById(1);
//      user.setUsername("张明明");
//      userMapper3.updateUser(user);
//      //执行提交，清空UserMapper下边的二级缓存
//      sqlSession3.commit();
//      sqlSession3.close();



    UserMapper userMapper2 = sqlSession2.getMapper(UserMapper.class);
    // 第二次发起请求，查询id为1的用户
    User user2 = userMapper2.findUserById(1);
    System.out.println(user2);

    sqlSession2.close();
}

1.无更新，输出

DEBUG [main] - Cache Hit Ratio [com.iot.mybatis.mapper.UserMapper]: 0.0
DEBUG [main] - Opening JDBC Connection
DEBUG [main] - Created connection 103887628.
DEBUG [main] - Setting autocommit to false on JDBC Connection [com.mysql.jdbc.JDBC4Connection@631330c]
DEBUG [main] - ==>  Preparing: SELECT * FROM user WHERE id=? 
DEBUG [main] - ==> Parameters: 1(Integer)
DEBUG [main] - <==      Total: 1
User [id=1, username=测试用户22, sex=2, birthday=null, address=null]
DEBUG [main] - Resetting autocommit to true on JDBC Connection [com.mysql.jdbc.JDBC4Connection@631330c]
DEBUG [main] - Closing JDBC Connection [com.mysql.jdbc.JDBC4Connection@631330c]
DEBUG [main] - Returned connection 103887628 to pool.
DEBUG [main] - Cache Hit Ratio [com.iot.mybatis.mapper.UserMapper]: 0.5
User [id=1, username=测试用户22, sex=2, birthday=null, address=null]

2.有更新，输出

DEBUG [main] - Cache Hit Ratio [com.iot.mybatis.mapper.UserMapper]: 0.0
DEBUG [main] - Opening JDBC Connection
DEBUG [main] - Created connection 103887628.
DEBUG [main] - Setting autocommit to false on JDBC Connection [com.mysql.jdbc.JDBC4Connection@631330c]
DEBUG [main] - ==>  Preparing: SELECT * FROM user WHERE id=? 
DEBUG [main] - ==> Parameters: 1(Integer)
DEBUG [main] - <==      Total: 1
User [id=1, username=测试用户22, sex=2, birthday=null, address=null]
DEBUG [main] - Resetting autocommit to true on JDBC Connection [com.mysql.jdbc.JDBC4Connection@631330c]
DEBUG [main] - Closing JDBC Connection [com.mysql.jdbc.JDBC4Connection@631330c]
DEBUG [main] - Returned connection 103887628 to pool.
DEBUG [main] - Cache Hit Ratio [com.iot.mybatis.mapper.UserMapper]: 0.5
DEBUG [main] - Opening JDBC Connection
DEBUG [main] - Checked out connection 103887628 from pool.
DEBUG [main] - Setting autocommit to false on JDBC Connection [com.mysql.jdbc.JDBC4Connection@631330c]
DEBUG [main] - ==>  Preparing: update user set username=?,birthday=?,sex=?,address=? where id=? 
DEBUG [main] - ==> Parameters: 张明明(String), null, 2(String), null, 1(Integer)
DEBUG [main] - <==    Updates: 1
DEBUG [main] - Committing JDBC Connection [com.mysql.jdbc.JDBC4Connection@631330c]
DEBUG [main] - Resetting autocommit to true on JDBC Connection [com.mysql.jdbc.JDBC4Connection@631330c]
DEBUG [main] - Closing JDBC Connection [com.mysql.jdbc.JDBC4Connection@631330c]
DEBUG [main] - Returned connection 103887628 to pool.
DEBUG [main] - Cache Hit Ratio [com.iot.mybatis.mapper.UserMapper]: 0.3333333333333333
DEBUG [main] - Opening JDBC Connection
DEBUG [main] - Checked out connection 103887628 from pool.
DEBUG [main] - Setting autocommit to false on JDBC Connection [com.mysql.jdbc.JDBC4Connection@631330c]
DEBUG [main] - ==>  Preparing: SELECT * FROM user WHERE id=? 
DEBUG [main] - ==> Parameters: 1(Integer)
DEBUG [main] - <==      Total: 1
User [id=1, username=张明明, sex=2, birthday=null, address=null]
DEBUG [main] - Resetting autocommit to true on JDBC Connection [com.mysql.jdbc.JDBC4Connection@631330c]
DEBUG [main] - Closing JDBC Connection [com.mysql.jdbc.JDBC4Connection@631330c]
DEBUG [main] - Returned connection 103887628 to pool.

useCache配置

在statement中设置useCache=false可以禁用当前select语句的二级缓存，即每次查询都会发出sql去查询，默认情况是true，即该sql使用二级缓存。