一、基本概念介绍

    XPath 是一门在 XML 文档中查找信息的语言， 可用来在 XML 文档中对元素和属性进行遍历。XPath 是 W3C XSLT 标准的主要元素，并且 XQuery 和 XPointer 同时被构建于 XPath 表达之上。因此，对 XPath 的理解是很多高级 XML 应用的基础。
    XPath非常类似对数据库操作的SQL语言，或者说JQuery，它可以方便开发者抓起文档中需要的东西。（dom4j也支持xpath

   1.节点类型

    XPath中有七种结点类型：元素、属性、文本、命名空间、处理指令、注释以及文档节点（或称为根节点）。文档中存在元素节点，属性节点，根节点

   2.常用路径表达式

 3.限定语

用来查找某个特定的节点或者包含某个指定的值的节点。以方括号括起

二、代码示例

import java.io.File;  import java.io.FileInputStream;    import javax.xml.parsers.DocumentBuilder;  import javax.xml.parsers.DocumentBuilderFactory;  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.Node;  import org.w3c.dom.NodeList;    public class XPathDemo {   private static Document doc;   private static XPath xpath;     public static void main(String[] args) throws Exception {    init();    getRootEle();    getChildEles();    getPartEles();    haveChildsEles();    getLevelEles();    getAttrEles();        //打印根节点下的所有元素节点    System.out.println(doc.getDocumentElement().getChildNodes().getLength());    NodeList nodeList = doc.getDocumentElement().getChildNodes();    for (int i = 0; i < nodeList.getLength(); i++) {     if (nodeList.item(i).getNodeType() == Node.ELEMENT_NODE) {      System.out.print(nodeList.item(i).getNodeName() + " ");     }    }   }     // 初始化Document、XPath对象   public static void init() throws Exception {    // 创建Document对象    DocumentBuilderFactory dbf = DocumentBuilderFactory.newInstance();    dbf.setValidating(false);    DocumentBuilder db = dbf.newDocumentBuilder();    doc = db.parse(new FileInputStream(new File("demo.xml")));      // 创建XPath对象    XPathFactory factory = XPathFactory.newInstance();    xpath = factory.newXPath();   }     // 获取根元素   // 表达式可以更换为/*,/rss   public static void getRootEle() throws XPathExpressionException {    Node node = (Node) xpath.evaluate("/rss", doc, XPathConstants.NODE);    System.out.println(node.getNodeName() + "--------"      + node.getNodeValue());   }     // 获取子元素并打印   public static void getChildEles() throws XPathExpressionException {    NodeList nodeList = (NodeList) xpath.evaluate("/rss/channel/*", doc,      XPathConstants.NODESET);    for (int i = 0; i < nodeList.getLength(); i++) {     System.out.print(nodeList.item(i).getNodeName() + " ");    }    System.out.println();   }     // 获取部分元素   // 只获取元素名称为title的元素   public static void getPartEles() throws XPathExpressionException {    NodeList nodeList = (NodeList) xpath.evaluate("//*[name() = 'title']",      doc, XPathConstants.NODESET);    for (int i = 0; i < nodeList.getLength(); i++) {     System.out.print(nodeList.item(i).getNodeName() + "-->"       + nodeList.item(i).getTextContent());    }    System.out.println();   }     // 获取包含子节点的元素   public static void haveChildsEles() throws XPathExpressionException {    NodeList nodeList = (NodeList) xpath.evaluate("//*[*]", doc,      XPathConstants.NODESET);    for (int i = 0; i < nodeList.getLength(); i++) {     System.out.print(nodeList.item(i).getNodeName() + " ");    }    System.out.println();   }     // 获取指定层级的元素   public static void getLevelEles() throws XPathExpressionException {    NodeList nodeList = (NodeList) xpath.evaluate("/*/*/*/*", doc,      XPathConstants.NODESET);    for (int i = 0; i < nodeList.getLength(); i++) {     System.out.print(nodeList.item(i).getNodeName() + "-->"       + nodeList.item(i).getTextContent() + " ");    }    System.out.println("-----------------------------");   }     // 获取指定属性的元素   // 获取所有大于指定价格的书箱   public static void getAttrEles() throws XPathExpressionException {    NodeList nodeList = (NodeList) xpath.evaluate("//bookstore/book[price>35.00]/title", doc,      XPathConstants.NODESET);    for (int i = 0; i < nodeList.getLength(); i++) {     System.out.print(nodeList.item(i).getNodeName() + "-->"       + nodeList.item(i).getTextContent() + " ");    }    System.out.println();   }  }

使用的XML文档

<?xml version="1.0" encoding="UTF-8"?>  <rss version="2.0">   <channel>    <title>Java Tutorials and Examples 2</title>    <language>en-us</language>    <item>     <title><![CDATA[Java Tutorials 2]]></title>     <link>http://www.javacodegeeks.com/</link>    </item>    <item>     <title><![CDATA[Java Examples 2]]></title>     <link>http://examples.javacodegeeks.com/</link>    </item>   </channel>   <college name="c1">    <class name="class1">     <student name="stu1" sex='male' age="21" />     <student name="stu2" sex='female' age="20" />     <student name="stu3" sex='female' age="20" />    </class>   </college>   <bookstore>    <book>     <title lang="eng">Harry Potter</title>     <price>29.99</price>    </book>    <book>     <title lang="eng">Learning XML</title>     <price>39.95</price>    </book>   </bookstore>  </rss>

三、参考

http://www.w3school.com.cn/xpath/index.asp