Java解析xml-XPath解析

文章非个人原创，内容copy自：[易百教程]https://www.yiibai.com/java_xml/java_xpath_parser.html 

XPath是万维网联盟(W3C)的官方推荐。它定义了一个语言在XML文件中查找信息。它被用于遍历XML文档的元素和属性。 XPath提供各种类型，可用于从XML文档查询相关的信息表现形式。

XPath使用步骤

1.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.*;

2.创建DocumentBuilder

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

3.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);

4.创建XPath对象

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

5.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

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

6.遍历节点列表。

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

7.检查属性

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

8.检查子元素

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

demo案例

xml.txt

<?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

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("./xml.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("
Current 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();
        }
    }
}

执行的结果：

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