Mybatis一级缓存及其存在的问题

青旅半醒 2023-02-12 14:25 68阅读 0赞

# **概要：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。** #

*  **作用域**  
    取值有两个：**SqlSession**和**Statement**
    
    > 文档：https://mybatis.org/mybatis-3/configuration.html\#settings
    
    Mybatis plus 基于springboot的配置为：
    
        mybatis-plus:
          configuration:
            local-cache-scope: statement
    
    或者
    
        mybatis-plus:
          configuration:
            local-cache-scope: session
 *  **SqlSession**  
    每个**SqlSession**中持有了**Executor**，每个**Executor**中有一个**LocalCache**  
    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**HashMap**的缓存实现。  
    ![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl8zODQyMjI1OA_size_16_color_FFFFFF_t_70]  
    类图如下：  
    ![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl8zODQyMjI1OA_size_16_color_FFFFFF_t_70 1]

## **测试** ##

*  **SqlSessionConfig**
    
        package com.example.demo.config;
        
        import com.baomidou.mybatisplus.extension.plugins.PaginationInterceptor;
        import com.baomidou.mybatisplus.extension.spring.MybatisSqlSessionFactoryBean;
        
        import org.apache.ibatis.plugin.Interceptor;
        import org.mybatis.spring.mapper.MapperScannerConfigurer;
        import org.slf4j.Logger;
        import org.slf4j.LoggerFactory;
        import org.springframework.beans.factory.annotation.Qualifier;
        import org.springframework.boot.context.properties.ConfigurationProperties;
        import org.springframework.boot.jdbc.DataSourceBuilder;
        import org.springframework.context.annotation.Bean;
        import org.springframework.context.annotation.Configuration;
        import org.springframework.core.io.support.PathMatchingResourcePatternResolver;
        
        import javax.sql.DataSource;
        
        /** * @author liuteng */
        @Configuration
        public class SqlSessionConfig { 
        
            private Logger logger = LoggerFactory.getLogger(SqlSessionConfig.class);
        
            @Bean("sqlSessionFactoryBean")
            public MybatisSqlSessionFactoryBean createSqlSessionFactory(@Qualifier("dataSource") DataSource dataSource,
                                                                        @Qualifier("paginationInterceptor") PaginationInterceptor paginationInterceptor) { 
        
                // MybatisSqlSessionFactory
                MybatisSqlSessionFactoryBean sqlSessionFactoryBean = null;
                try { 
                    // 实例SessionFactory
                    sqlSessionFactoryBean = new MybatisSqlSessionFactoryBean();
                    // 配置数据源
                    sqlSessionFactoryBean.setDataSource(dataSource);
                    // 设置 MyBatis-Plus 分页插件
                    Interceptor [] plugins = { paginationInterceptor};
                    sqlSessionFactoryBean.setPlugins(plugins);
                    // 加载MyBatis配置文件
                    PathMatchingResourcePatternResolver resourcePatternResolver = new PathMatchingResourcePatternResolver();
                    sqlSessionFactoryBean.setMapperLocations(resourcePatternResolver.getResources("classpath*:mapper/*.xml"));
                } catch (Exception e) { 
                    logger.error("创建SqlSession连接工厂错误：{}", e.getMessage());
                }
                return sqlSessionFactoryBean;
            }
        
            @Bean("dataSource")
            @ConfigurationProperties(prefix = "spring.datasource")
            public DataSource dataSource(){ 
                return DataSourceBuilder.create().build();
            }
        
            @Bean
            public MapperScannerConfigurer myGetMapperScannerConfigurer() { 
                MapperScannerConfigurer myMapperScannerConfigurer = new MapperScannerConfigurer();
                myMapperScannerConfigurer.setBasePackage("com.example.demo.mapper");
                myMapperScannerConfigurer.setSqlSessionFactoryBeanName("mySqlSessionFactoryBean");
                return myMapperScannerConfigurer;
            }
        
        }
 *  **UserServiceImpl**
    
        package com.example.demo.service.impl;
        
        import com.alibaba.fastjson.JSON;
        import com.baomidou.mybatisplus.extension.spring.MybatisSqlSessionFactoryBean;
        import com.example.demo.mapper.User1Mapper;
        import com.example.demo.mapper.UserMapper;
        import com.example.demo.entity.User;
        import com.example.demo.service.UserService;
        import lombok.extern.slf4j.Slf4j;
        import org.apache.ibatis.session.SqlSession;
        import org.springframework.beans.factory.annotation.Autowired;
        import org.springframework.stereotype.Service;
        
        
        @Slf4j
        @Service
        public class UserServiceImpl implements UserService { 
        
        
            @Resource
        	DefaultSqlSessionFactory sqlSessionFactoryBean;
            
            @Override
            public int updateById(User user) { 
                try { 
                    SqlSession sqlSession1 = sqlSessionFactoryBean.openSession();
                	SqlSession sqlSession2 = sqlSessionFactoryBean.openSession();
                    UserMapper userMapper1 = sqlSession1.getMapper(UserMapper.class);
                    UserMapper userMapper2 = sqlSession2.getMapper(UserMapper.class);
        
                    User firstUser = userMapper1.selectById(user.getId());
                    log.debug("=========SqlSession1查询======== User:{}", JSON.toJSONString(firstUser));
                    userMapper2.updateById(user);
                    log.debug("=========SqlSession2修改======== User:{}", JSON.toJSONString(user));
                    User secondUser = userMapper2.selectById(user.getId());
                    log.debug("=========SqlSession2查询======== User:{}", JSON.toJSONString(secondUser));
                    User thirdUser = userMapper1.selectById(user.getId());
                    log.debug("=========SqlSession1查询======== User:{}", JSON.toJSONString(thirdUser));
                } catch (Exception e) { 
                    log.error(e.getMessage());
                }
                return 1;
            }
        }
    
    数据库预先录入数据，然后测试修改用户名
 *  **执行结果**
    
        2020-05-26 14:52:21.869 DEBUG 3992 --- [nio-8080-exec-7] o.s.jdbc.datasource.DataSourceUtils      : Fetching JDBC Connection from DataSource
        o.m.s.t.SpringManagedTransaction         : JDBC Connection [HikariProxyConnection@1725242542 wrapping com.mysql.cj.jdbc.ConnectionImpl@7bd24a07] will not be managed by Spring
        c.e.demo.mapper.UserMapper.selectById    : ==>  Preparing: SELECT id,name,sex,phone,create_time,enable,version FROM user WHERE id=? AND enable=1 
        c.e.demo.mapper.UserMapper.selectById    : ==> Parameters: 12121212(Long)
        c.e.demo.mapper.UserMapper.selectById    : <==      Total: 1
        c.e.demo.service.impl.UserServiceImpl    : =========SqlSession1查询======== User:{ "createTime":1590475920000,"enable":true,"id":12121212,"name":"wangwu","phone":"18111111111","sex":"MAN","version":1}
        o.s.jdbc.datasource.DataSourceUtils      : Fetching JDBC Connection from DataSource
        o.m.s.t.SpringManagedTransaction         : JDBC Connection [HikariProxyConnection@691008575 wrapping com.mysql.cj.jdbc.ConnectionImpl@4a631491] will not be managed by Spring
        c.e.demo.mapper.UserMapper.updateById    : ==>  Preparing: UPDATE user SET name=?, phone=?, create_time=? WHERE id=? AND enable=1 
        c.e.demo.mapper.UserMapper.updateById    : ==> Parameters: lisi(String), 18111111111(String), 2020-05-26 14:52:21.869(Timestamp), 12121212(Long)
        c.e.demo.mapper.UserMapper.updateById    : <==    Updates: 1
        c.e.demo.service.impl.UserServiceImpl    : =========SqlSession2修改======== User:{ "createTime":1590475941869,"enable":false,"id":12121212,"name":"lisi","phone":"18111111111"}
        c.e.demo.mapper.UserMapper.selectById    : ==>  Preparing: SELECT id,name,sex,phone,create_time,enable,version FROM user WHERE id=? AND enable=1 
        c.e.demo.mapper.UserMapper.selectById    : ==> Parameters: 12121212(Long)
        c.e.demo.mapper.UserMapper.selectById    : <==      Total: 1
        c.e.demo.service.impl.UserServiceImpl    : =========SqlSession2查询======== User:{ "createTime":1590475942000,"enable":true,"id":12121212,"name":"lisi","phone":"18111111111","sex":"MAN","version":1}
        c.e.demo.service.impl.UserServiceImpl    : =========SqlSession1查询======== User:{ "createTime":1590475920000,"enable":true,"id":12121212,"name":"wangwu","phone":"18111111111","sex":"MAN","version":1}
    
    **由执行结果可知，同一个Session中，相同查询会使用缓存，不同Session之间互不影响。当有多个SqlSession或者分布式的环境下，数据库写操作会引起脏数据**  
    ![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl8zODQyMjI1OA_size_16_color_FFFFFF_t_70 2]
 *  设置一级缓存级别为**Statement**
    
    修改**SqlSessionConfig**配置，新增如下
    
        MybatisConfiguration configuration = new MybatisConfiguration();
        configuration.setLocalCacheScope(LocalCacheScope.STATEMENT);
        sqlSessionFactoryBean.setConfiguration(configuration);
    
    执行结果：
    
        com.zaxxer.hikari.pool.HikariPool        : HikariPool-1 - Added connection com.mysql.cj.jdbc.ConnectionImpl@a4a722
        com.zaxxer.hikari.HikariDataSource       : HikariPool-1 - Start completed.
        o.m.s.t.SpringManagedTransaction         : JDBC Connection [HikariProxyConnection@1748010355 wrapping com.mysql.cj.jdbc.ConnectionImpl@a4a722] will not be managed by Spring
        c.e.demo.mapper.UserMapper.selectById    : ==>  Preparing: SELECT id,name,sex,phone,create_time,enable,version FROM user WHERE id=? AND enable=1 
        c.e.demo.mapper.UserMapper.selectById    : ==> Parameters: 12121212(Long)
        com.zaxxer.hikari.pool.HikariPool        : HikariPool-1 - Pool stats (total=1, active=1, idle=0, waiting=0)
        c.e.demo.mapper.UserMapper.selectById    : <==      Total: 1
        c.e.demo.service.impl.UserServiceImpl    : =========SqlSession1查询======== User:{ "createTime":1590489590000,"enable":true,"id":12121212,"name":"关羽","phone":"12345678910","sex":"MAN","version":1}
        o.s.jdbc.datasource.DataSourceUtils      : Fetching JDBC Connection from DataSource
        com.zaxxer.hikari.pool.HikariPool        : HikariPool-1 - Added connection com.mysql.cj.jdbc.ConnectionImpl@6eac3382
        o.m.s.t.SpringManagedTransaction         : JDBC Connection [HikariProxyConnection@1875164390 wrapping com.mysql.cj.jdbc.ConnectionImpl@6eac3382] will not be managed by Spring
        c.e.demo.mapper.UserMapper.updateById    : ==>  Preparing: UPDATE user SET name=?, phone=?, create_time=? WHERE id=? AND enable=1 
        c.e.demo.mapper.UserMapper.updateById    : ==> Parameters: 赵云(String), 12345678910(String), 2020-05-26 18:40:20.098(Timestamp), 12121212(Long)
        c.e.demo.mapper.UserMapper.updateById    : <==    Updates: 1
        c.e.demo.service.impl.UserServiceImpl    : =========SqlSession2修改======== User:{ "createTime":1590489620098,"enable":false,"id":12121212,"name":"赵云","phone":"12345678910"}
        c.e.demo.mapper.UserMapper.selectById    : ==>  Preparing: SELECT id,name,sex,phone,create_time,enable,version FROM user WHERE id=? AND enable=1 
        c.e.demo.mapper.UserMapper.selectById    : ==> Parameters: 12121212(Long)
        c.e.demo.mapper.UserMapper.selectById    : <==      Total: 1
        c.e.demo.service.impl.UserServiceImpl    : =========SqlSession2查询======== User:{ "createTime":1590489620000,"enable":true,"id":12121212,"name":"赵云","phone":"12345678910","sex":"MAN","version":1}
        c.e.demo.mapper.UserMapper.selectById    : ==>  Preparing: SELECT id,name,sex,phone,create_time,enable,version FROM user WHERE id=? AND enable=1 
        c.e.demo.mapper.UserMapper.selectById    : ==> Parameters: 12121212(Long)
        c.e.demo.mapper.UserMapper.selectById    : <==      Total: 1
        c.e.demo.service.impl.UserServiceImpl    : =========SqlSession1查询======== User:{ "createTime":1590489620000,"enable":true,"id":12121212,"name":"赵云","phone":"12345678910","sex":"MAN","version":1}
    
    **总结：Mybatis作为ORM框架，为了避免存在的脏读问题，建议修改一级缓存作用域为：Statement**

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