Mybatis一级缓存及其存在的问题

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。
  
  类图如下：
  

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
  

• 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