mybatis缓存

正如大多数持久层框架一样，MyBatis 同样提供了一级缓存和二级缓存的支持。
1. 一级缓存: 基于PerpetualCache 的 HashMap本地缓存，其存储作用域为 Session，当 Session flush 或 close 之后，该Session中的所有 Cache 就将清空。
2. 二级缓存与一级缓存其机制相同，默认也是采用 PerpetualCache，HashMap存储，不同在于其存储作用域为 Mapper(Namespace)，并且可自定义存储源，如 Ehcache。
3. 对于缓存数据更新机制，当某一个作用域(一级缓存Session/二级缓存Namespaces)的进行了 C/U/D 操作后，默认该作用域下所有 select 中的缓存将被clear。 

一级缓存

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MyBatis的一级缓存指的是在一个Session域内,session未关闭的时候执行的查询会根据SQL为key被缓存(跟mysql缓存一样,修改任何参数的值都会导致缓存失效)

1. 默认开启
2. 必须同一个session，如果session对象已经close()过了就不能用了
3. 查询条件必须一致
4. 执行过session.cleanCache();会清理缓存
5. 执行过增删改操作(这些操作都会清理缓存)

案例1.单独使用MyBatis而不继承Spring,使用原生的MyBatis的SqlSessionFactory来构造sqlSession查询,是可以使用以及缓存的,示例代码如下

public class Test {
    public static void main(String[] args) throws IOException {
        String config = "mybatis-config.xml";
        InputStream is = Resources.getResourceAsStream(config);
        SqlSessionFactory factory = new SqlSessionFactoryBuilder().build(is);
        SqlSession session = factory.openSession();
        System.out.println(session.selectOne("selectUserByID", 1));
        // 同一个session的相同sql查询,将会使用一级缓存 
        System.out.println(session.selectOne("selectUserByID", 1));
        // 参数改变,需要重新查询
        System.out.println(session.selectOne("selectUserByID", 2));
        // 清空缓存后需要重新查询
        session.clearCache();
        System.out.println(session.selectOne("selectUserByID", 1));
        // session close以后,仍然使用同一个db connection
        session.close();
        session = factory.openSession();
        System.out.println(session.selectOne("selectUserByID", 1));
    }
}

输入日志

DEBUG - Openning JDBC Connection
DEBUG - Created connection 10044878.
DEBUG - ooo Using Connection [com.mysql.jdbc.JDBC4Connection@9945ce]
DEBUG - ==> Preparing: SELECT * FROM user WHERE id = ?
DEBUG - ==> Parameters: 1(Integer)
1|test1|19|beijing
1|test1|19|beijing
DEBUG - ooo Using Connection [com.mysql.jdbc.JDBC4Connection@9945ce]
DEBUG - ==> Preparing: SELECT * FROM user WHERE id = ?
DEBUG - ==> Parameters: 2(Integer)
2|test2|18|guangzhou
DEBUG - ooo Using Connection [com.mysql.jdbc.JDBC4Connection@9945ce]
DEBUG - ==> Preparing: SELECT * FROM user WHERE id = ?
DEBUG - ==> Parameters: 1(Integer)
1|test1|19|beijing
DEBUG - Resetting autocommit to true on JDBC Connection [com.mysql.jdbc.JDBC4Connection@9945ce]
DEBUG - Closing JDBC Connection [com.mysql.jdbc.JDBC4Connection@9945ce]
DEBUG - Returned connection 10044878 to pool.
DEBUG - Openning JDBC Connection
DEBUG - Checked out connection 10044878 from pool.
DEBUG - Setting autocommit to false on JDBC Connection [com.mysql.jdbc.JDBC4Connection@9945ce]
DEBUG - ooo Using Connection [com.mysql.jdbc.JDBC4Connection@9945ce]
DEBUG - ==> Preparing: SELECT * FROM user WHERE id = ?
DEBUG - ==> Parameters: 1(Integer)

看以看出来,当参数不变的时候只进行了一次查询,参数变更以后,则需要重新进行查询,而清空缓存以后,参数相同的查询过的SQL也需要重新查询。

案例2.跟Spring集成的时候(使用mybatis-spring)

@Repository
public class UserDao extends SqlSessionDaoSupport {
    public User selectUserById(int id) {
        SqlSession session = getSqlSession();
        session.selectOne("dao.userdao.selectUserByID", id);
        // 由于session的实现是SqlSessionTemplate的动态代理实现
        // 它已经在代理类内执行了session.close(),所以无需手动关闭session
        return session.selectOne("dao.userdao.selectUserByID", id);
    }
}

DEBUG - Creating a new SqlSession
DEBUG - SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@1e389b8] was not registered for synchronization because synchronization is not active
DEBUG - Fetching JDBC Connection from DataSource
DEBUG - JDBC Connection [jdbc:mysql://127.0.0.1:3306/mybatistest?characterEncoding=utf8, UserName=root@localhost, MySQL-AB JDBC Driver] will not be managed by Spring
DEBUG - ooo Using Connection [jdbc:mysql://127.0.0.1:3306/mybatistest?characterEncoding=utf8, UserName=root@localhost, MySQL-AB JDBC Driver]
DEBUG - ==> Preparing: SELECT * FROM user WHERE id = ?
DEBUG - ==> Parameters: 1(Integer)
DEBUG - Closing non transactional SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@1e389b8]
DEBUG - Returning JDBC Connection to DataSource
DEBUG - Creating a new SqlSession
DEBUG - SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@169da74] was not registered for synchronization because synchronization is not active
DEBUG - Fetching JDBC Connection from DataSource
DEBUG - JDBC Connection [jdbc:mysql://127.0.0.1:3306/mybatistest?characterEncoding=utf8, UserName=root@localhost, MySQL-AB JDBC Driver] will not be managed by Spring
DEBUG - ooo Using Connection [jdbc:mysql://127.0.0.1:3306/mybatistest?characterEncoding=utf8, UserName=root@localhost, MySQL-AB JDBC Driver]
DEBUG - ==> Preparing: SELECT * FROM user WHERE id = ?
DEBUG - ==> Parameters: 1(Integer)
DEBUG - Closing non transactional SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@169da74]
DEBUG - Returning JDBC Connection to DataSource

这里执行了2次sql查询,看似我们使用了同一个sqlSession,但是实际上因为我们的dao继承了SqlSessionDaoSupport,而SqlSessionDaoSupport内部sqlSession的实现是使用用动态代理实现的,这个动态代理sqlSessionProxy使用一个模板方法封装了select()等操作,每一次select()查询都会自动先执行openSession(),执行完close()以后调用close()方法,相当于生成了一个新的session实例,所以我们无需手动的去关闭这个session()(关于这一点见下面mybatis的官方文档),当然也无法使用mybatis的一级缓存,也就是说mybatis的一级缓存在spring中是没有作用的.

二级缓存

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二级缓存就是global caching,它超出session范围之外,可以被所有sqlSession共享,它的实现机制和mysql的缓存一样,开启它只需要在mybatis的配置文件开启settings里的。

1. 默认不开启

2. 全局开关配置

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE configuration PUBLIC "-//mybatis.org//DTD  Config 3.0//EN" "http://mybatis.org/dtd/mybatis-3-config.dtd">
<configuration>
    <settings>
        <!--  默认是true，如果它配成false，其余各个Mapper XML文件配成支持cache也没用 -->
        <setting name="cacheEnabled" value="true"/>
    </settings>
</configuration>

 3. 各个Mapper XML文件，默认是不采用cache。在配置文件加一行就可以支持cache

<mapper namespace="com.test">
    <!-- 
        eviction="FIFO"  回收策略为先进先出，（LRU，最近最少使用的算法）
        flushInterval="300000"   自动刷新时间5分钟
        size="16"  最多缓存16个引用对象
        readOnly="true"  只读
     -->
    <cache eviction="FIFO" flushInterval="300000" size="16" readOnly="true" />
    <select id=""></select>
</mapper>

 4. Mapper XML文件配置支持cache后，该文件中所有的Mapper statement就支持了。此时要个别sql不需要缓存的话，需要进行配置

<!-- useCache="false" 配置后则没有缓存-->
<select id="check" parameterType="string" resultType="int" useCache="false">
</select>

案例

@RequestMapping("/getUser")
    public String getUser(Model model) {
        User user = userDao.selectUserById(1);
        model.addAttribute(user);
        return "index";
    }  
}

当我们访问两次 /getUser 这个url,查看日志输出

DEBUG - Creating a new SqlSession
DEBUG - SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@659812] was not registered for synchronization because synchronization is not active
DEBUG - Cache Hit Ratio [dao.userdao]: 0.0
DEBUG - Fetching JDBC Connection from DataSource
DEBUG - JDBC Connection [jdbc:mysql://127.0.0.1:3306/mybatistest?characterEncoding=utf8, UserName=root@localhost, MySQL-AB JDBC Driver] will not be managed by Spring
DEBUG - ooo Using Connection [jdbc:mysql://127.0.0.1:3306/mybatistest?characterEncoding=utf8, UserName=root@localhost, MySQL-AB JDBC Driver]
DEBUG - ==> Preparing: SELECT * FROM user WHERE id = ?
DEBUG - ==> Parameters: 1(Integer)
DEBUG - Closing non transactional SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@659812]
DEBUG - Returning JDBC Connection to DataSource
DEBUG - Invoking afterPropertiesSet() on bean with name 'index'
DEBUG - Rendering view [org.springframework.web.servlet.view.JstlView: name 'index'; URL [/index.jsp]] in DispatcherServlet with name 'dispatcher'
DEBUG - Added model object 'org.springframework.validation.BindingResult.user' of type [org.springframework.validation.BeanPropertyBindingResult] to request in view with name 'index'
DEBUG - Added model object 'user' of type [bean.User] to request in view with name 'index'
DEBUG - Forwarding to resource [/index.jsp] in InternalResourceView 'index'
DEBUG - Successfully completed request
DEBUG - Returning cached instance of singleton bean 'sqlSessionFactory'
DEBUG - DispatcherServlet with name 'dispatcher' processing GET request for [/user/getUser]
DEBUG - Looking up handler method for path /user/getUser
DEBUG - Returning handler method [public java.lang.String controller.UserController.getUser(org.springframework.ui.Model)]
DEBUG - Returning cached instance of singleton bean 'userController'
DEBUG - Last-Modified value for [/user/getUser] is: -1
DEBUG - Creating a new SqlSession
DEBUG - SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@539a92] was not registered for synchronization because synchronization is not active
DEBUG - Cache Hit Ratio [dao.userdao]: 0.5
DEBUG - Closing non transactional SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@539a92]
DEBUG - Rendering view [org.springframework.web.servlet.view.JstlView: name 'index'; URL [/index.jsp]] in DispatcherServlet with name 'dispatcher'
DEBUG - Added model object 'org.springframework.validation.BindingResult.user' of type [org.springframework.validation.BeanPropertyBindingResult] to request in view with name 'index'
DEBUG - Added model object 'user' of type [bean.User] to request in view with name 'index'
DEBUG - Forwarding to resource [/index.jsp] in InternalResourceView 'index'
DEBUG - Successfully completed request

可以看出第二次访问同一个url的时候相同的查询 hit cache了,这就是global cache的作用

总结

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一级，二级缓存意义都不是很大

面对一定规模的数据量，内置的cache方式就派不上用场了

数据缓存在内存中，是增加的应用的压力

一般缓存采用Memcached、redis等