一. 配置檔案

參考： https://www.cnblogs.com/wanthune/p/13674243.html

二. 原始碼解析

1. XMLConfigBuilder 就是解析Xml的主類。

public Configuration parse() {
    if (parsed) {
      throw new BuilderException("Each XMLConfigBuilder can only be used once.");
    }
    parsed = true;
    parseConfiguration(parser.evalNode("/configuration"));
    return configuration;
  }
   // 第一步，解析properites節點
  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
      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);
    }
  }

   // 解析properties節點
    private void propertiesElement(XNode context) throws Exception {
    if (context != null) {
      // 讀取properties節點，返回一個Properties物件
      Properties defaults = context.getChildrenAsProperties();
      // 讀取resource屬性值
      String resource = context.getStringAttribute("resource");
      // 讀取url屬性值
      String url = context.getStringAttribute("url");
      // 這兒就是為什麼不能在檔案中同時配置resource和url,同時配置時拋異常
      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.");
      }
      // 優先獲取resource,之所以先判斷Resource,我覺得是因為本地檔案讀取的更快。
      if (resource != null) {
        defaults.putAll(Resources.getResourceAsProperties(resource));
      } else if (url != null) {
        // 從遠端url中讀取檔案流
        defaults.putAll(Resources.getUrlAsProperties(url));
      }
      Properties vars = configuration.getVariables();
      if (vars != null) {
        defaults.putAll(vars);
      }
      // 將讀取到的屬性值放到parser（XPathParser）中
      parser.setVariables(defaults);
      // 同時也放入configuration中。
      configuration.setVariables(defaults);
    }
  }