使用XPath的步骤

以下是使用XPath解析器在解析文档时使用的步骤。

• 导入XML相关的软件包。

• 创建DocumentBuilder

• 从文件或数据流创建一个文档

• 创建XPath对象和XPath的路径表达式

• 编译XPath表达式使用XPath.compile() ，并由XPath.evaluate()评估计算获得一个节点列表

• 遍历节点列表。

• 检查属性

• 检查子元素

导入XML相关的软件包

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

创建DocumentBuilder

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

从文件或数据流创建一个文档

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，并计算它

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

遍历节点列表

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

检查属性

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

检查子元素

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

演示示例：

这里是我们需要分析输入文本文件：

<?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>

 

演示示例：

XPathParserDemo.java

package com.yiibai.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();
      }
   }
}

这将产生以下结果:

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

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