Mybatis源码解析之配置文件解析补充

上次大体讲了如何解析配置文件，比较粗略，跳跃比较多，这篇过下整体流程。

上次我们已经知道解析XML文件的是XMLConfigBuilder类，而我们需要向XMLConfigBuilder构造方法传递三个值：XMLConfigBuilder parser = new XMLConfigBuilder(reader, environment, properties);
跟进该构造器，

public XMLConfigBuilder(Reader reader, String environment, Properties props) {
    this(new XPathParser(reader, true, props, new XMLMapperEntityResolver()), environment, props);
  }

我们可以看到又调用另外构造方法：

private XMLConfigBuilder(XPathParser parser, String environment, Properties props) {
    super(new Configuration());
    ErrorContext.instance().resource("SQL Mapper Configuration");
    this.configuration.setVariables(props);
    this.parsed = false;
    this.environment = environment;
    this.parser = parser;
}

该构造方法有调用父类的构造方法，主要是赋个Configuration 对象，再是注册基本类型别名：

public BaseBuilder(Configuration configuration) {
    this.configuration = configuration;
    this.typeAliasRegistry = this.configuration.getTypeAliasRegistry();
    this.typeHandlerRegistry = this.configuration.getTypeHandlerRegistry();
}

但XPathParser 对象怎么来的，需要查看上个构造器的 new XPathParser(reader, true, props, new XMLMapperEntityResolver())

public XPathParser(Reader reader, boolean validation, Properties variables, EntityResolver entityResolver) {
    commonConstructor(validation, variables, entityResolver);
    this.document = createDocument(new InputSource(reader));
}

再看commonConstructor方法，给参数赋值：

private void commonConstructor(boolean validation, Properties variables, EntityResolver entityResolver) {
    this.validation = validation;
    this.entityResolver = entityResolver;
    this.variables = variables;
    XPathFactory factory = XPathFactory.newInstance();
    this.xpath = factory.newXPath();
}

createDocument解析的是传过去的Reader对象或流文件，最终解析成Document对象，使用的是jdk自带的解析器DocumentBuilder

  private Document createDocument(InputSource inputSource) {
    // important: this must only be called AFTER common constructor
    try {
      DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();
      factory.setValidating(validation);

      factory.setNamespaceAware(false);
      factory.setIgnoringComments(true);
      factory.setIgnoringElementContentWhitespace(false);
      factory.setCoalescing(false);
      factory.setExpandEntityReferences(true);

      DocumentBuilder builder = factory.newDocumentBuilder();
      builder.setEntityResolver(entityResolver);
      builder.setErrorHandler(new ErrorHandler() {
        @Override
        public void error(SAXParseException exception) throws SAXException {
          throw exception;
        }

        @Override
        public void fatalError(SAXParseException exception) throws SAXException {
          throw exception;
        }

        @Override
        public void warning(SAXParseException exception) throws SAXException {
        }
      });
      return builder.parse(inputSource);
    } catch (Exception e) {
      throw new BuilderException("Error creating document instance.  Cause: " + e, e);
    }
}

创建XMLConfigBuilder对象完成后，进行解析，调用parse()方法，开始解析configuration的子节点

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

进入parseConfiguration(parser.evalNode("/configuration"))方法

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);
    }
}

这个就是解析具体节点的方法，后面我们一个一个分析。