JAVA读取XML文件（水文，Mybatis源码）

　　这篇文章主要是为了写Mybatis源码解析配置文件xml所需的背景知识，表达式部分的文档 https://www.w3.org/TR/1999/REC-xpath-19991116/ JAVA  JSR 206: Java API for XML Processing (JAXP) 1.3 https://docs.oracle.com/cd/E17802_01/webservices/webservices/docs/2.0/jaxp/ReleaseNotes.html#migrationFrom13
　　先看下这个是xml文件，然后看这个文档如何解析<?xml version=＂1.0＂ encoding=＂UTF-8＂?>           insert into users (id, name) values (#{id}, #{name})           insert into users (id, name) values (#{id}, #{name})            select * from users    
　　读文件的demo/**  * Copyright 2009-2022 the original author or authors.  * 
  * Licensed under the Apache License, Version 2.0 (the ＂License＂);  * you may not use this file except in compliance with the License.  * You may obtain a copy of the License at  * 
  * http://www.apache.org/licenses/LICENSE-2.0  * 
  * Unless required by applicable law or agreed to in writing, software  * distributed under the License is distributed on an ＂AS IS＂ BASIS,  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.  * See the License for the specific language governing permissions and  * limitations under the License.  */ package org.apache.ibatis.begincode;  import org.apache.ibatis.parsing.XPathParser; import org.junit.jupiter.api.Test; import org.w3c.dom.Document; import org.w3c.dom.Node; import org.w3c.dom.NodeList; import org.xml.sax.EntityResolver; import org.xml.sax.SAXException;  import javax.xml.parsers.DocumentBuilder; import javax.xml.parsers.DocumentBuilderFactory; import javax.xml.parsers.ParserConfigurationException; import javax.xml.xpath.XPath; import javax.xml.xpath.XPathConstants; import javax.xml.xpath.XPathExpressionException; import javax.xml.xpath.XPathFactory; import java.io.File; import java.io.FileInputStream; import java.io.IOException;  public class XMLTest { //  https://www.w3.org/TR/1999/REC-xpath-19991116/   @Test   public void resolveXml() throws ParserConfigurationException, IOException, SAXException, XPathExpressionException {     String fileName = ＂Mapper.xml＂;     String xmlFilePath = this.getClass().getResource(＂＂).getPath() + fileName;      DocumentBuilderFactory documentBuilderFactory = DocumentBuilderFactory.newInstance();     documentBuilderFactory.setValidating(false);     DocumentBuilder db = documentBuilderFactory.newDocumentBuilder();     Document doc = db.parse(new FileInputStream(new File(xmlFilePath)));      XPathFactory factory = XPathFactory.newInstance();     XPath xpath = factory.newXPath();       String expression;     Node node;     NodeList nodeList;       // 1. 根目录     expression = ＂/*＂;     node = (Node) xpath.evaluate(expression, doc, XPathConstants.NODE);     System.out.println(＂1. ＂ + node.getNodeName());      // 2.根据标签名获取根目录     expression = ＂/mapper＂;     node = (Node) xpath.evaluate(expression, doc, XPathConstants.NODE);     System.out.println(＂2. ＂ + node.getNodeName());      // 3. 根据标签层级获取节点      expression = ＂/mapper/select＂;     node = (Node) xpath.evaluate(expression, doc, XPathConstants.NODE);     System.out.println(＂3. ＂ + node.getNodeName());      // 4. 获取标签下所有节点     expression = ＂/mapper/*＂;     nodeList = (NodeList) xpath.evaluate(expression, doc, XPathConstants.NODESET);     System.out.print(＂4. ＂);     for (int i = 0; i < nodeList.getLength(); i++) {       System.out.print(nodeList.item(i).getNodeName() + ＂ ＂);     }     System.out.println();      // 5. 获取所有指定节点     expression = ＂//insert＂;     nodeList = (NodeList) xpath.evaluate(expression, doc, XPathConstants.NODESET);     System.out.print(＂5. ＂);     for (int i = 0; i < nodeList.getLength(); i++) {       System.out.print(nodeList.item(i).getNodeName() + ＂ ＂);     }     System.out.println();      // 6. 获取所有非指定名字节点     expression = ＂//*[name() != ＂insert＂]＂;     nodeList = (NodeList) xpath.evaluate(expression, doc, XPathConstants.NODESET);     System.out.print(＂6. ＂);     for (int i = 0; i < nodeList.getLength(); i++) {       System.out.print(nodeList.item(i).getNodeName() + ＂ ＂);     }     System.out.println();      // 7. 获取至少有一个子节点的节点     expression = ＂//*[*]＂;     nodeList = (NodeList) xpath.evaluate(expression, doc, XPathConstants.NODESET);     System.out.print(＂7. ＂);     for (int i = 0; i < nodeList.getLength(); i++) {       System.out.print(nodeList.item(i).getNodeName() + ＂ ＂);     }     System.out.println();      // 8.获取指定层级的节点     expression = ＂/*/*＂;     nodeList = (NodeList) xpath.evaluate(expression, doc, XPathConstants.NODESET);     System.out.print(＂8. ＂);     for (int i = 0; i < nodeList.getLength(); i++) {       System.out.print(nodeList.item(i).getNodeName() + ＂ ＂);       System.out.print(nodeList.item(i).getFirstChild().getNodeValue() + ＂ ＂);     }     System.out.println();    } }
　　执行结果
　　代码执行结果
　　其他使用方法 ，直接用 Document处理@Test public void documentTest() throws ParserConfigurationException, IOException, SAXException {   String fileName = ＂Mapper.xml＂;   String xmlFilePath = this.getClass().getResource(＂＂).getPath() + fileName;    DocumentBuilderFactory docbuilderFactory = DocumentBuilderFactory.newInstance();   DocumentBuilder dombuilder = docbuilderFactory.newDocumentBuilder();   InputStream is = new FileInputStream(xmlFilePath);   Document doc = dombuilder.parse(is);   Element root = doc.getDocumentElement();   System.out.println(root.getChildNodes().item(1).getFirstChild().getNodeValue()); }