零教程

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>

%uA0

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