# Java XPath解析器 - 解析XML文档 - Java XML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(" ");
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：
```
dinkar
kad
dinkar
85
Vaneet
Gupta
vinni
95
jasvir
singh
jazz
90
```
**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
```