MyBatis 是如何初始化的

// mybatis初始化String resource = "mybatis-config.xml";  InputStream inputStream = Resources.getResourceAsStream(resource);  SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
// 创建SqlSessionSqlSession sqlSession = sqlSessionFactory.openSession();  
// 执行SQL语句List list = sqlSession.selectList("com.foo.bean.BlogMapper.queryAllBlogInfo");

public SqlSessionFactory build(InputStream inputStream)  {      return build(inputStream, null, null);  }  
public SqlSessionFactory build(InputStream inputStream, String environment, Properties properties)  {      try  {          //2. 创建XMLConfigBuilder对象用来解析XML配置文件，生成Configuration对象          XMLConfigBuilder parser = new XMLConfigBuilder(inputStream, environment, properties);          //3. 将XML配置文件内的信息解析成Java对象Configuration对象          Configuration config = parser.parse();          //4. 根据Configuration对象创建出SqlSessionFactory对象          return build(config);      } catch (Exception e) {          throw ExceptionFactory.wrapException("Error building SqlSession.", e);      } finally {          ErrorContext.instance().reset();          try {              inputStream.close();          } catch (IOException e) {              // Intentionally ignore. Prefer previous error.          }      }}
// 从此处可以看出，MyBatis内部通过Configuration对象来创建SqlSessionFactory,用户也可以自己通过API构造好Configuration对象，调用此方法创SqlSessionFactory  public SqlSessionFactory build(Configuration config) {      return new DefaultSqlSessionFactory(config);  }  

public Configuration parse() {      if (parsed) {          throw new BuilderException("Each XMLConfigBuilder can only be used once.");      }      parsed = true;      //源码中没有这一句，只有 parseConfiguration(parser.evalNode("/configuration"));      //为了让读者看得更明晰，源码拆分为以下两句      XNode configurationNode = parser.evalNode("/configuration");      parseConfiguration(configurationNode);      return configuration;  }  /**  * 解析 "/configuration"节点下的子节点信息，然后将解析的结果设置到Configuration对象中  */  private void parseConfiguration(XNode root) {      try {          //1.首先处理properties 节点             propertiesElement(root.evalNode("properties")); //issue #117 read properties first          //2.处理typeAliases          typeAliasesElement(root.evalNode("typeAliases"));          //3.处理插件          pluginElement(root.evalNode("plugins"));          //4.处理objectFactory          objectFactoryElement(root.evalNode("objectFactory"));          //5.objectWrapperFactory          objectWrapperFactoryElement(root.evalNode("objectWrapperFactory"));          //6.settings          settingsElement(root.evalNode("settings"));          //7.处理environments          environmentsElement(root.evalNode("environments")); // read it after objectFactory and objectWrapperFactory issue #631          //8.database          databaseIdProviderElement(root.evalNode("databaseIdProvider"));          //9.typeHandlers          typeHandlerElement(root.evalNode("typeHandlers"));          //10.mappers          mapperElement(root.evalNode("mappers"));      } catch (Exception e) {          throw new BuilderException("Error parsing SQL Mapper Configuration. Cause: " + e, e);      }  } 

/**  * 解析environments节点，并将结果设置到Configuration对象中  * 注意：创建envronment时，如果SqlSessionFactoryBuilder指定了特定的环境（即数据源）；  *      则返回指定环境（数据源）的Environment对象，否则返回默认的Environment对象；  *      这种方式实现了MyBatis可以连接多数据源  */  private void environmentsElement(XNode context) throws Exception {      if (context != null)      {          if (environment == null)          {              environment = context.getStringAttribute("default");          }          for (XNode child : context.getChildren())          {              String id = child.getStringAttribute("id");              if (isSpecifiedEnvironment(id))              {                  //1.创建事务工厂 TransactionFactory                  TransactionFactory txFactory = transactionManagerElement(child.evalNode("transactionManager"));                  DataSourceFactory dsFactory = dataSourceElement(child.evalNode("dataSource"));                  //2.创建数据源DataSource                  DataSource dataSource = dsFactory.getDataSource();                  //3. 构造Environment对象                  Environment.Builder environmentBuilder = new Environment.Builder(id)                  .transactionFactory(txFactory)                  .dataSource(dataSource);                  //4. 将创建的Envronment对象设置到configuration 对象中                  configuration.setEnvironment(environmentBuilder.build());              }          }      }  }
private boolean isSpecifiedEnvironment(String id)  {      if (environment == null)      {          throw new BuilderException("No environment specified.");      }      else if (id == null)      {          throw new BuilderException("Environment requires an id attribute.");      }      else if (environment.equals(id))      {          return true;      }      return false;  } 

String resource = "mybatis-config.xml";  InputStream inputStream = Resources.getResourceAsStream(resource);  // 手动创建XMLConfigBuilder，并解析创建Configuration对象  XMLConfigBuilder parser = new XMLConfigBuilder(inputStream, null,null); // 看这里 Configuration configuration = parser.parse();  // 使用Configuration对象创建SqlSessionFactory  SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(configuration);  // 使用MyBatis  SqlSession sqlSession = sqlSessionFactory.openSession();  List list = sqlSession.selectList("com.foo.bean.BlogMapper.queryAllBlogInfo");