XPathParser类是mybatis对 javax.xml.xpath.XPath的包装类。
接下来我们来看下XPathParser类的结构

1、属性

  //  存放读取到的整个XML文档
  private final Document document;
  //  是否开启验证
  private boolean validation;
  // 自定义的DTD约束文件实体解析器，与validation配合使用
  private EntityResolver entityResolver;
  // MyBatis配置文件中的properties节点的信息
  private Properties variables;
  //  XPath工具
  private XPath xpath;

2、构造函数

 public XPathParser(String xml) {
   commonConstructor(false, null, null);
   this.document = createDocument(new InputSource(new StringReader(xml)));
 }

构造方法包含两部分：

• 初始化属性

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();
}

• 构建XML文档对应的Document

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);
    }
}

3、方法

基本类型的解析方法，最终调用的都是方法 evalString(Object root, String expression)

  private Properties variables;
  private XPath xpath;
  
  public String evalString(Object root, String expression) {
    // 解析结果
    String result = (String) evaluate(expression, root, XPathConstants.STRING);
    // 对${}变量从Properties配置文件中进行查找相应的属性值替换,若variables为空或未查找到${}直接返回result
    result = PropertyParser.parse(result, variables);
    return result;
  }

  private Object evaluate(String expression, Object root, QName returnType) {
    try {
      // 调用XPath进行解析
      return xpath.evaluate(expression, root, returnType);
    } catch (Exception e) {
      throw new BuilderException("Error evaluating XPath.  Cause: " + e, e);
    }
  }

4、总结

可使用XPathParser类来解析自己定义的XML文件，同时也可以仿照mybatis定义自己的Properties文件类替换XML文件中的变量值

        String path = "E:\\GitResource\\springboot-bucket\\springboot-mybatis\\src\\main\\resources\\mapper\\20240729.xml";
        DocumentBuilderFactory documentBuilderFactory = DocumentBuilderFactory.newInstance();
        DocumentBuilder documentBuilder = documentBuilderFactory.newDocumentBuilder();
        Document document = documentBuilder.parse(path);

        XPathParser xPathParser = new XPathParser(document,false,null,null);
        String evalString = xPathParser.evalString("/mapper/sql[1]");
        System.out.println(evalString);