Mybatis3源码分析（一）：从sqlSession说起

  



分析MyBatis3的源码首先得从sqlSessionFactory开始，先来看一段spring配置文件中Mybaits的sqlSessionFactory的配置。

<bean id="dataSource" class="org.apache.commons.dbcp.BasicDataSource"  destroy-method="close">  
   <property name="driverClassName" value="${jdbc.driver}" />  
   <property name="url" value="${jdbc.url}" />  
   <property name="username" value="${jdbc.username}" />  
   <property name="password" value="${jdbc.password}" />  
</bean>  
<bean id="sqlSessionFactory" class="org.mybatis.spring.SqlSessionFactoryBean">  
   <property name="dataSource" ref="dataSource" />
   <property name="configLocation" value="classpath:configuration.xml"></property> 
   <property name="mapperLocations" value="classpath:com/tiantian/mybatis/mapper/*.xml"/>  
   <property name="typeAliasesPackage" value="com.tiantian.mybatis.model" />  
</bean>
<bean id="blogMapper" class="org.mybatis.spring.mapper.MapperFactoryBean">  
       <property name="mapperInterface"  
           value="com.tiantian.mybatis.mapper.BlogMapper" />  
      <property name="sqlSessionFactory" ref="sqlSessionFactory" />  
</bean>   

    spring会调用sqlSessionFactory的无参构造函数创建实例，并将dataSource和映射文件路径注入进去初始化sqlSessionFactory。这里的SqlSessionFactoryBean的实现依赖于mybatis社区自行开发的支持包，我们来看看它是怎么读取xml配置文件创建sessionFactory的:

protected SqlSessionFactory buildSqlSessionFactory() throws IOException {
    //代表配置的对象
    Configuration configuration;
    //xml配置创建者
    XMLConfigBuilder xmlConfigBuilder = null;
    //spring中注入的配置文件路径，如果不存在使用默认配置
    if (this.configLocation != null) {
      xmlConfigBuilder = new XMLConfigBuilder(this.configLocation.getInputStream(), null, this.configurationProperties);
      configuration = xmlConfigBuilder.getConfiguration();
    } else {
      if (logger.isDebugEnabled()) {
        logger.debug("Property 'configLocation' not specified, using default MyBatis Configuration");
      }
      configuration = new Configuration();
      configuration.setVariables(this.configurationProperties);
    }
    //如果配置了objectFactory使用此配置类
    if (this.objectFactory != null) {
      configuration.setObjectFactory(this.objectFactory);
    }
    //同上
    if (this.objectWrapperFactory != null) {
      configuration.setObjectWrapperFactory(this.objectWrapperFactory);
    }
    //如果配置了typeAliases（实体类别名标签）的基包则不用在configuration文件中配置
    if (hasLength(this.typeAliasesPackage)) {
      String[] typeAliasPackageArray = tokenizeToStringArray(this.typeAliasesPackage,
          ConfigurableApplicationContext.CONFIG_LOCATION_DELIMITERS);
      for (String packageToScan : typeAliasPackageArray) {
        configuration.getTypeAliasRegistry().registerAliases(packageToScan,
                typeAliasesSuperType == null ? Object.class : typeAliasesSuperType);
        if (logger.isDebugEnabled()) {
          logger.debug("Scanned package: '" + packageToScan + "' for aliases");
        }
      }
    }
    //如果是直接配置的实体类别名则在配置对象中注册
    if (!isEmpty(this.typeAliases)) {
      for (Class<?> typeAlias : this.typeAliases) {
        configuration.getTypeAliasRegistry().registerAlias(typeAlias);
        if (logger.isDebugEnabled()) {
          logger.debug("Registered type alias: '" + typeAlias + "'");
        }
      }
    }
    //是否配置mybaits的一些插件
    if (!isEmpty(this.plugins)) {
      for (Interceptor plugin : this.plugins) {
        configuration.addInterceptor(plugin);
        if (logger.isDebugEnabled()) {
          logger.debug("Registered plugin: '" + plugin + "'");
        }
      }
    }
    //是否配置typeHandler的基包
    if (hasLength(this.typeHandlersPackage)) {
      String[] typeHandlersPackageArray = tokenizeToStringArray(this.typeHandlersPackage,
          ConfigurableApplicationContext.CONFIG_LOCATION_DELIMITERS);
      for (String packageToScan : typeHandlersPackageArray) {
        configuration.getTypeHandlerRegistry().register(packageToScan);
        if (logger.isDebugEnabled()) {
          logger.debug("Scanned package: '" + packageToScan + "' for type handlers");
        }
      }
    }
    //是否直接配置了typeHandler
    if (!isEmpty(this.typeHandlers)) {
      for (TypeHandler<?> typeHandler : this.typeHandlers) {
        configuration.getTypeHandlerRegistry().register(typeHandler);
        if (logger.isDebugEnabled()) {
          logger.debug("Registered type handler: '" + typeHandler + "'");
        }
      }
    }
    //如果xmlConfigBuilder存在则开始分析创建Configuration对象
    if (xmlConfigBuilder != null) {
      try {
        xmlConfigBuilder.parse();

        if (logger.isDebugEnabled()) {
          logger.debug("Parsed configuration file: '" + this.configLocation + "'");
        }
      } catch (Exception ex) {
        throw new NestedIOException("Failed to parse config resource: " + this.configLocation, ex);
      } finally {
        ErrorContext.instance().reset();
      }
    }
    //事务工厂是否配置
    if (this.transactionFactory == null) {
      this.transactionFactory = new SpringManagedTransactionFactory();
    }
    //设置Configuration的环境对象
    Environment environment = new Environment(this.environment, this.transactionFactory, this.dataSource);
    configuration.setEnvironment(environment);
 
    if (this.databaseIdProvider != null) {
      try {
        configuration.setDatabaseId(this.databaseIdProvider.getDatabaseId(this.dataSource));
      } catch (SQLException e) {
        throw new NestedIOException("Failed getting a databaseId", e);
      }
    }
    //是否配置mapperLocations，如果存在解析创建这些mapper，mapper是mybaits管理各种sql的容器
    if (!isEmpty(this.mapperLocations)) {
      for (Resource mapperLocation : this.mapperLocations) {
        if (mapperLocation == null) {
          continue;
        }

        try {
          XMLMapperBuilder xmlMapperBuilder = new XMLMapperBuilder(mapperLocation.getInputStream(),
              configuration, mapperLocation.toString(), configuration.getSqlFragments());
          xmlMapperBuilder.parse();
        } catch (Exception e) {
          throw new NestedIOException("Failed to parse mapping resource: '" + mapperLocation + "'", e);
        } finally {
          ErrorContext.instance().reset();
        }

        if (logger.isDebugEnabled()) {
          logger.debug("Parsed mapper file: '" + mapperLocation + "'");
        }
      }
    } else {
      if (logger.isDebugEnabled()) {
        logger.debug("Property 'mapperLocations' was not specified or no matching resources found");
      }
    }
    //创建sqlSessionFactoryBean
    return this.sqlSessionFactoryBuilder.build(configuration);
  }

    从mybatis创建sqlSessionFactoryBean的过程可以看出，mybaits默认使用spring的事务管理功能或者由第三方实现，它自身并没有提供事务管理能力。其次，它拥有跟其它框架差不多的解析xml过程，都是将xml解析成一个Configuration对象随时取用。到这里我们就得到了一个sqlSessionFactoryBean对象（实际是mybatis本身的DefaultSqlSessionFactory对象），我们可以在DAO中注入这个对象并利用openSession的方法获取SqlSession对象从而进行各种数据库操作。