config.xml理会为org.w3c.dom.Document

本文首先来简朴看一下MyBatis中将config.xml理会为org.w3c.dom.Document的流程，代码为上文的这部门：

static {
    try {
        reader = Resources.getResourceAsReader("mybatis/config.xml");
        ssf = new SqlSessionFactoryBuilder().build(reader);
    } 
    catch (IOException e) {
        e.printStackTrace();
    }
}

第3行的代码实现为：

public static Reader getResourceAsReader(String resource) throws IOException {
    Reader reader;
    if (charset == null) {
      reader = new InputStreamReader(getResourceAsStream(resource));
    } else {
      reader = new InputStreamReader(getResourceAsStream(resource), charset);
    }
    return reader;
}

相当于就是将输入的路径转换为一个字符输入流并返回。

接着继承看静态块第4行的代码，new SqlSessionFactoryBuilder().build(reader)，把代码定位到SqlSessionFactoryBuilder类的builder要领，这里利用了多态，直接跟到build要领：

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) {
        // Intentionally ignore. Prefer previous error.
      }
    }
}

理会config.xml的代码在第3行XMLConfigBuilder类的结构要领中，看一下XMLConfigBuilder类的结构要领做了什么：

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

这里的要害是第二行代码的第一个参数XPathParser，看一下实例化XPathParser类的代码：

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

第2行的代码commonConstructor要领没什么悦目标，将validation、variables、entityResolver配置到XPathParser类的参数中罢了，顺便再实例化一个javax.xml.xpath.XPath出来，XPath用于在XML文档中通过元素和属性举办导航，并对元素和属性举办遍历。

接着看第3行的createDocument要领：

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);
    }
}

看一下第5行~第11行的代码配置DocumentBuilderFactory中参数的寄义：

第13行的代码由配置的参数从DocumentBuilderFactory中获取一个DocumentBuilder实例DocumentBuilderImpl，并由第14行的代码配置一个实体理会器，由第15行~第29行的代码配置一个错误处理惩罚器。

最后看一下第30行的代码parse要领：

public Document parse(InputSource is) throws SAXException, IOException {
        if (is == null) {
            throw new IllegalArgumentException(
                DOMMessageFormatter.formatMessage(DOMMessageFormatter.DOM_DOMAIN,
                "jaxp-null-input-source", null));
        }
        if (fSchemaValidator != null) {
            if (fSchemaValidationManager != null) {
                fSchemaValidationManager.reset();
                fUnparsedEntityHandler.reset();
            }
            resetSchemaValidator();
        }
        domParser.parse(is);
        Document doc = domParser.getDocument();
        domParser.dropDocumentReferences();
        return doc;
}