MyBatis 3 解析 mybatis-config.xml 配置

public SqlSessionFactory build(Reader reader, String environment, Properties properties) {
    try {
      XMLConfigBuilder parser = new XMLConfigBuilder(reader, environment, properties);
      return build(parser.parse());
    } catch (Exception e) {
      throw ExceptionFactory.wrapException("Error building SqlSession.", e);
    } finally {
      ErrorContext.instance().reset();
      try {
        reader.close();
      } catch (IOException e) {
      }
    }
  }

public abstract class BaseBuilder {
  protected final Configuration configuration;  //全局唯一配置对象
  protected final TypeAliasRegistry typeAliasRegistry;  //解析mybatis-config.xml中<typeAliases>标签
  protected final TypeHandlerRegistry typeHandlerRegistry;  //解析mybatis-config.xml中<typeHandlers>标签
  
}

public class Configuration {
  
	protected final TypeAliasRegistry typeAliasRegistry = new TypeAliasRegistry();
  protected final TypeHandlerRegistry typeHandlerRegistry = new TypeHandlerRegistry();
}

public class XMLConfigBuilder extends BaseBuilder {
  //是否已经解析过mybatis-config.xml文件
  private boolean parsed;
  //用来解析mybatis-config.xml文件的对象
  private final XPathParser parser;
  //标记<environments>标记的名称，默认读取default属性
  private String environment;
  //用来创建和缓存Reflector对象
  private final ReflectorFactory localReflectorFactory = new DefaultReflectorFactory();
  
}

  //解析配置
  public Configuration parse() {
    //只解析加载一次
    if (parsed) {
      throw new BuilderException("Each XMLConfigBuilder can only be used once.");
    }
    parsed = true;
    //根节点是configuration
    parseConfiguration(parser.evalNode("/configuration"));
    return configuration;
  }

//解析配置中各个节点
  private void parseConfiguration(XNode root) {
    try {
      //1.解析<properties>节点
      propertiesElement(root.evalNode("properties"));
      //2.解析<typeAliases>节点
      typeAliasesElement(root.evalNode("typeAliases"));
      //3.解析<plugins>节点
      pluginElement(root.evalNode("plugins"));
      //4.解析<objectFactory>节点
      objectFactoryElement(root.evalNode("objectFactory"));
      //5.解析<objectWrapperFactory>节点
      objectWrapperFactoryElement(root.evalNode("objectWrapperFactory"));
      //6.解析<settings>节点
      settingsElement(root.evalNode("settings"));
      //7.解析<environments>节点
      environmentsElement(root.evalNode("environments"));
      //8.解析<databaseIdProvider>节点
      databaseIdProviderElement(root.evalNode("databaseIdProvider"));
      //9.解析<typeHandlers>节点
      typeHandlerElement(root.evalNode("typeHandlers"));
      //10.解析<mappers>节点
      mapperElement(root.evalNode("mappers"));
    } catch (Exception e) {
      throw new BuilderException("Error parsing SQL Mapper Configuration. Cause: " + e, e);
    }
  }

private void propertiesElement(XNode context) throws Exception {
  //<properties resource="org/mybatis/example/config.properties">
  //    <property name="username" value="aaaaa"/>
  //    <property name="password" value="bbbbb"/>
  //</properties>
    if (context != null) {
      //如果在这些地方,属性多于一个的话,MyBatis 按照如下的顺序加载它们:
      //1.读取property属性。
      //2.读取url或resource对应的属性，第19行指明二者不能同时配置。
      //3.作为方法参数传递的属性最后被读取
      //如果属性名称相同，后加载的值会覆盖前者
      //1.XNode.getChildrenAsProperties函数方便得到孩子所有Properties
      Properties defaults = context.getChildrenAsProperties();
      //2.然后查找resource或者url,加入前面的Properties
      String resource = context.getStringAttribute("resource");
      String url = context.getStringAttribute("url");
      if (resource != null && url != null) {
        throw new BuilderException("The properties element cannot specify both a URL and a resource based property file reference.  Please specify one or the other.");
      }
      if (resource != null) {
        defaults.putAll(Resources.getResourceAsProperties(resource));
      } else if (url != null) {
        defaults.putAll(Resources.getUrlAsProperties(url));
      }
      //3.Variables也全部加入Properties
      Properties vars = configuration.getVariables();
      if (vars != null) {
        defaults.putAll(vars);
      }
      parser.setVariables(defaults);
      configuration.setVariables(defaults);
    }
  }