解析XML文档（Parse XML Document）

使用XPath的步骤

以下是使用XPath Parser解析文档时使用的步骤。

• 导入与XML相关的包。

• 创建一个DocumentBuilder。

• 从文件或流创建文档。

• 创建Xpath对象和XPath路径表达式。

• 使用XPath.compile()编译XPath表达式，并通过XPath.evaluate()评估编译的表达式获取节点列表。

• 迭代节点列表。

• 检查属性。

• 检查子元素。

Import XML-related packages

import org.w3c.dom.*;
import org.xml.sax.*;
import javax.xml.parsers.*;
import javax.xml.xpath.*;
import java.io.*;

创建一个DocumentBuilder (Create a DocumentBuilder)

DocumentBuilderFactory factory =
DocumentBuilderFactory.newInstance();
DocumentBuilder builder = factory.newDocumentBuilder();

从文件或流创建文档

StringBuilder xmlStringBuilder = new StringBuilder();
xmlStringBuilder.append("<?xml version = "1.0"?> <class> </class>");
ByteArrayInputStream input =  new ByteArrayInputStream(
   xmlStringBuilder.toString().getBytes("UTF-8"));
Document doc = builder.parse(input);

构建XPath

XPath xPath =  XPathFactory.newInstance().newXPath();

准备路径表达式并对其进行评估

String expression = "/class/student";	        
NodeList nodeList = (NodeList) xPath.compile(expression).evaluate(
   doc, XPathConstants.NODESET);

迭代NodeList

for (int i = 0; i < nodeList.getLength(); i++) {
   Node nNode = nodeList.item(i);
   ...
}

检查属性 (Examine attributes)

//returns specific attribute
getAttribute("attributeName");
//returns a Map (table) of names/values
getAttributes(); 

检查子元素 (Examine sub-elements)

//returns a list of subelements of specified name
getElementsByTagName("subelementName");
//returns a list of all child nodes
getChildNodes(); 

例子 (Demo Example)

这是我们需要解析的输入文本文件 -

<?xml version = "1.0"?>
<class>
   <student rollno = "393">
      <firstname>dinkar</firstname>
      <lastname>kad</lastname>
      <nickname>dinkar</nickname>
      <marks>85</marks>
   </student>
   <student rollno = "493">
      <firstname>Vaneet</firstname>
      <lastname>Gupta</lastname>
      <nickname>vinni</nickname>
      <marks>95</marks>
   </student>
   <student rollno = "593">
      <firstname>jasvir</firstname>
      <lastname>singh</lastname>
      <nickname>jazz</nickname>
      <marks>90</marks>
   </student>
</class>

XPathParserDemo.java

package cn.xnip.xml;
import java.io.File;
import java.io.IOException;
import javax.xml.parsers.DocumentBuilderFactory;
import javax.xml.parsers.DocumentBuilder;
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 org.w3c.dom.Document;
import org.w3c.dom.NodeList;
import org.w3c.dom.Node;
import org.w3c.dom.Element;
import org.xml.sax.SAXException;
public class XPathParserDemo {
   public static void main(String[] args) {
      try {
         File inputFile = new File("input.txt");
         DocumentBuilderFactory dbFactory = DocumentBuilderFactory.newInstance();
         DocumentBuilder dBuilder;
         dBuilder = dbFactory.newDocumentBuilder();
         Document doc = dBuilder.parse(inputFile);
         doc.getDocumentElement().normalize();
         XPath xPath =  XPathFactory.newInstance().newXPath();
         String expression = "/class/student";	        
         NodeList nodeList = (NodeList) xPath.compile(expression).evaluate(
            doc, XPathConstants.NODESET);
         for (int i = 0; i < nodeList.getLength(); i++) {
            Node nNode = nodeList.item(i);
            System.out.println("\nCurrent Element :" + nNode.getNodeName());
            if (nNode.getNodeType() == Node.ELEMENT_NODE) {
               Element eElement = (Element) nNode;
               System.out.println("Student roll no :" + eElement.getAttribute("rollno"));
               System.out.println("First Name : " 
                  + eElement
                  .getElementsByTagName("firstname")
                  .item(0)
                  .getTextContent());
               System.out.println("Last Name : " 
                  + eElement
                  .getElementsByTagName("lastname")
                  .item(0)
                  .getTextContent());
               System.out.println("Nick Name : " 
                  + eElement
                  .getElementsByTagName("nickname")
                  .item(0)
                  .getTextContent());
               System.out.println("Marks : " 
                  + eElement
                  .getElementsByTagName("marks")
                  .item(0)
                  .getTextContent());
            }
         }
      } catch (ParserConfigurationException e) {
         e.printStackTrace();
      } catch (SAXException e) {
         e.printStackTrace();
      } catch (IOException e) {
         e.printStackTrace();
      } catch (XPathExpressionException e) {
         e.printStackTrace();
      }
   }
}

这会产生以下结果 -

Current Element :student
Student roll no : 393
First Name : dinkar
Last Name : kad
Nick Name : dinkar
Marks : 85
Current Element :student
Student roll no : 493
First Name : Vaneet
Last Name : Gupta
Nick Name : vinni
Marks : 95
Current Element :student
Student roll no : 593
First Name : jasvir
Last Name : singh
Nick Name : jazz
Marks : 90