mybatis 启动流程源码分析（二）之 Configuration-Properties解析

一. 配置文件

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

二. 源码解析

1. XMLConfigBuilder 就是解析Xml的主类。

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节点
  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节点
    private void propertiesElement(XNode context) throws Exception {
    if (context != null) {
      // 读取properties节点，返回一个Properties对象
      Properties defaults = context.getChildrenAsProperties();
      // 读取resource属性值
      String resource = context.getStringAttribute("resource");
      // 读取url属性值
      String url = context.getStringAttribute("url");
      // 这儿就是为什么不能在文件中同时配置resource和url,同时配置时抛异常
      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.");
      }
      // 优先获取resource,之所以先判断Resource,我觉得是因为本地文件读取的更快。
      if (resource != null) {
        defaults.putAll(Resources.getResourceAsProperties(resource));
      } else if (url != null) {
        // 从远程url中读取文件流
        defaults.putAll(Resources.getUrlAsProperties(url));
      }
      Properties vars = configuration.getVariables();
      if (vars != null) {
        defaults.putAll(vars);
      }
      // 将读取到的属性值放到parser（XPathParser）中
      parser.setVariables(defaults);
      // 同时也放入configuration中。
      configuration.setVariables(defaults);
    }
  }