3.2、MyBatis 初始化过程

第一步：MyBatis的初始化的过程其实就是解析配置文件和初始化Configuration的过程，MyBatis的初始化过程可用以下几行代码来表述：

String resource = "mybatis.xml";

// 加载mybatis的配置文件（它也加载关联的映射文件）
InputStream inputStream = null;
try {
    inputStream = Resources.getResourceAsStream(resource);
} catch (IOException e) {
    e.printStackTrace();
}

// 构建sqlSession的工厂
sessionFactory = new SqlSessionFactoryBuilder().build(inputStream);

首先会创建SqlSessionFactory建造者对象，然后由它进行创建SqlSessionFactory。这里用到的是建造者模式，建造者模式最简单的理解就是不手动new对象，而是由其他类来进行对象的创建。

// SqlSessionFactoryBuilder类
public SqlSessionFactory build(InputStream inputStream, String environment, Properties properties) {
    try {
        XMLConfigBuilder parser = new XMLConfigBuilder(inputStream, environment, properties);
        return build(parser.parse()); // 开始进行解析了 :)
    } 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.
        }
    }
}

第二步：XMLConfigBuilder对象会进行XML配置文件的解析，实际为configuration节点的解析操作。

// XMLConfigBuilder类
public Configuration parse() {
    if (parsed) {
        throw new BuilderException("Each XMLConfigBuilder can only be used once.");
    }
    parsed = true;
    parseConfiguration(parser.evalNode("/configuration"));
    return configuration;
}

private void parseConfiguration(XNode root) {
    try {
        //issue #117 read properties first
        propertiesElement(root.evalNode("properties"));
        Properties settings = settingsAsProperties(root.evalNode("settings"));
        loadCustomVfs(settings);
        typeAliasesElement(root.evalNode("typeAliases"));
        pluginElement(root.evalNode("plugins"));
        objectFactoryElement(root.evalNode("objectFactory"));
        objectWrapperFactoryElement(root.evalNode("objectWrapperFactory"));
        reflectorFactoryElement(root.evalNode("reflectorFactory"));
        settingsElement(settings);
        // read it after objectFactory and objectWrapperFactory issue #631

        /* 处理environments节点数据 */
        environmentsElement(root.evalNode("environments"));
        databaseIdProviderElement(root.evalNode("databaseIdProvider"));
        typeHandlerElement(root.evalNode("typeHandlers"));
        mapperElement(root.evalNode("mappers"));
    } catch (Exception e) {
        throw new BuilderException("Error parsing SQL Mapper Configuration. Cause: " + e, e);
    }
}

在configuration节点下会依次解析properties/settings/…/mappers等节点配置。在解析environments节点时，会根据transactionManager的配置来创建事务管理器，根据dataSource的配置来创建DataSource对象，这里面包含了数据库登录的相关信息。在解析mappers节点时，会读取该节点下所有的mapper文件，然后进行解析，并将解析后的结果存到configuration对象中。

// XMLConfigBuilder类
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)) {

                /* 创建事务管理器 */
                TransactionFactory txFactory = transactionManagerElement(child.evalNode("transactionManager"));
                DataSourceFactory dsFactory = dataSourceElement(child.evalNode("dataSource"));
                DataSource dataSource = dsFactory.getDataSource();

                /* 建造者模式 设计模式 */
                Environment.Builder environmentBuilder = new Environment.Builder(id)
                        .transactionFactory(txFactory)
                        .dataSource(dataSource);
                configuration.setEnvironment(environmentBuilder.build());
            }
        }
    }
}

// 解析单独的mapper文件
private void mapperElement(XNode parent) throws Exception {
    if (parent != null) {
      for (XNode child : parent.getChildren()) {
        if ("package".equals(child.getName())) {
          String mapperPackage = child.getStringAttribute("name");
          configuration.addMappers(mapperPackage);
        } else {
          String resource = child.getStringAttribute("resource");
          String url = child.getStringAttribute("url");
          String mapperClass = child.getStringAttribute("class");
          if (resource != null && url == null && mapperClass == null) {
            ErrorContext.instance().resource(resource);
            InputStream inputStream = Resources.getResourceAsStream(resource);
            XMLMapperBuilder mapperParser = new XMLMapperBuilder(inputStream, configuration, resource, configuration.getSqlFragments());
            mapperParser.parse(); // 开始解析mapper文件了 :)
          } else if (resource == null && url != null && mapperClass == null) {
            ErrorContext.instance().resource(url);
            InputStream inputStream = Resources.getUrlAsStream(url);
            XMLMapperBuilder mapperParser = new XMLMapperBuilder(inputStream, configuration, url, configuration.getSqlFragments());
            mapperParser.parse();
          } else if (resource == null && url == null && mapperClass != null) {
            Class<?> mapperInterface = Resources.classForName(mapperClass);
            configuration.addMapper(mapperInterface);
          } else {
            throw new BuilderException("A mapper element may only specify a url, resource or class, but not more than one.");
          }
        }
      }
    }
  }

解析完MyBatis配置文件后，configuration就初始化完成了，然后根据configuration对象来创建SqlSession，到这里时，MyBatis的初始化的征程已经走完了。