Mybatis源码学习(20)-Mybatis初始化时environments的解析过程

一、<environments>元素的结构

在Mybatis-config.xml中，<environments>元素的配置方式如下所示：

<environments default="mysql">
   <environment id="mysql">
        <transactionManager type="JDBC" />
        <!-- 配置数据库连接信息 -->
        <dataSource type="POOLED">
            <property name="driver" value="${driver}" />
            <property name="url" value="${url}" />
            <property name="username" value="${username}" />
            <property name="password" value="${password?:123456}" />
            <property name="driver.encoding" value="UTF8"/> 
        </dataSource>
    </environment>
</environments>

从定义config配置文件文档结构的文件mybatis-3-config.dtd中<environments>元素相关约束可以看出<environments>元素可以允许的结构：

<environments>元素可以出现一次或者零次
<environments>元素必须有属性default
<environments>元素必须有一个或者多个<environment>子元素
<environment>元素必须有属性id
<environment>元素下必须有且只有一个<transactionManager>和<dataSource>子元素
<transactionManager>元素必须有一个type属性
<transactionManager>元素可以有零或多个<property>元素
<dataSource>元素必须有一个type属性
<dataSource>元素可以有零或多个<property>元素

<!ELEMENT configuration (properties?, settings?, typeAliases?, typeHandlers?, objectFactory?, objectWrapperFactory?, reflectorFactory?, plugins?, environments?, databaseIdProvider?, mappers?)>

<!ELEMENT environments (environment+)>
<!ATTLIST environments
default CDATA #REQUIRED
>

<!ELEMENT environment (transactionManager,dataSource)>
<!ATTLIST environment
id CDATA #REQUIRED
>

<!ELEMENT transactionManager (property*)>
<!ATTLIST transactionManager
type CDATA #REQUIRED
>

<!ELEMENT dataSource (property*)>
<!ATTLIST dataSource
type CDATA #REQUIRED
>

关于DTD相关知识请参考《XML DTD》

二、<environments>元素初始化时解析的入口

environmentsElement()方法用来解析<environments>元素，其中包括了解析<transactionManager>和<dataSource>子元素的方法。然后通过Environment的内部构建器类Builder来构建Environment实例，然后把构建的Environment实例，赋值到了configuration配置类的environment属性中，便于后期使用。这个时候就完成了初始化解析的工作了。

/**
   * 解析配置文件中的environments元素
   * @param context
   * @throws Exception
   */
  private void environmentsElement(XNode context) throws Exception {
    if (context != null) {
      if (environment == null) {
        environment = context.getStringAttribute("default");
      }
      for (XNode child : context.getChildren()) {//解析environment元素
        String id = child.getStringAttribute("id");
        if (isSpecifiedEnvironment(id)) {//判断是否是默认配置，即environments元素的default属性值。比如：<environments default="development">
          //解析事务管理器
          TransactionFactory txFactory = transactionManagerElement(child.evalNode("transactionManager"));
          //解析数据源
          DataSourceFactory dsFactory = dataSourceElement(child.evalNode("dataSource"));
          DataSource dataSource = dsFactory.getDataSource();
          //构建environment对象
          Environment.Builder environmentBuilder = new Environment.Builder(id)
              .transactionFactory(txFactory)
              .dataSource(dataSource);
          //赋值到configuration的对应属性中
          configuration.setEnvironment(environmentBuilder.build());
        }
      }
    }
  }

transactionManagerElement()方法用来解析<transactionManager>元素。

/**
   * 解析事务管理器对象
   * @param context
   * @return
   * @throws Exception
   */
  private TransactionFactory transactionManagerElement(XNode context) throws Exception {
    if (context != null) {
      String type = context.getStringAttribute("type");
      Properties props = context.getChildrenAsProperties();
      TransactionFactory factory = (TransactionFactory) resolveClass(type).newInstance();
      factory.setProperties(props);
      return factory;
    }
    throw new BuilderException("Environment declaration requires a TransactionFactory.");
  }

dataSourceElement()方法用来解析<dataSource>元素。

/**
   * 解析数据源元素
   * @param context
   * @return
   * @throws Exception
   */
  private DataSourceFactory dataSourceElement(XNode context) throws Exception {
    if (context != null) {
      String type = context.getStringAttribute("type");
      Properties props = context.getChildrenAsProperties();
      DataSourceFactory factory = (DataSourceFactory) resolveClass(type).newInstance();
      factory.setProperties(props);
      return factory;
    }
    throw new BuilderException("Environment declaration requires a DataSourceFactory.");
  }