[XML] XPath expression

Introduction to XPath

XPath is the XML Path Language (XML Path Language), which is an XML document is used to determine the position of a part of the language.

XPath-based XML tree structure, providing the ability to find a node in the data tree. XPath mind initially proposed that it as a general-purpose, between XPointer and XSL grammar model between. But XPath soon be employed to developers as a small query language .

The basic XPath expression

expression	description
nodename	Select all the child nodes of this node.
/	Choose from the root node.
//	Select the document matches the selected node from the current node, regardless of their location.
.	Select the current node.
..	Select the parent of the current node.
@	Select Properties.

The basic XPath expression case

Path expression	result
bookstore	Selects all child nodes of the bookstore element.
/bookstore	Select the root element bookstore. Note: If the path starts with a forward slash (/), then this path is always representative of the absolute path to an element!
bookstore/book	Select the sub-elements belonging to the bookstore of the book all the elements.
//book	Select all book sub-elements, regardless of their position in the document.
bookstore//book	Select all book elements that belong to the descendants of the bookstore element, and no matter what position they are located below the bookstore.
// @ lang	Select all of the property named lang.

XPath predicate expression

Path expression	result
/bookstore/book[1]	A first element selected book child elements belonging bookstore
/bookstore/book[late()]	The last book element selected sub-elements belonging to bookstore
/bookstore/book[late()-1]	Select the sub-elements belonging to the reciprocal of the second bookstore book element
/bookstore/book[position()<3]	The first two elements selected book belonging to the bookstore element subelements
//title[@lang]	Select all of lang has a property named title element
// title [@ lang = 'What']	Select all the title elements, and these elements have a lang attribute value eng
/bookstore/book[price>35.00]	Select all the book elements bookstore element, and wherein the price element value must be greater than 35.00
/bookstore/book[price>35.00]/title	Select book element bookstore element of all the title elements, and wherein the price element value must be greater than 35.00

Dom4j underlying rely Jaxen achieve XPath query

Jaxen Introduction

• Jaxen is written in Java open source XPath library. Here adapt to a variety of different object model, including DOM, XOM, dom4j and JDOM.
• Dom4j underlying rely Jaxen achieve XPath query
• Jaxen Download: jaxen.codehaus.org , because it is outside the network, it is generally from Ali cloud repository download

XPath Queries

import java.util.List;

import org.dom4j.Document;
import org.dom4j.DocumentException;
import org.dom4j.Element;
import org.dom4j.Node;
import org.dom4j.io.SAXReader;

public class XPathTestor {
    public void xpath(String xpathExp){
        String file = "E:/workspace/eclipse/HelloWorld/src/test2/hr.xml";
        SAXReader reader = new SAXReader();
        try {
            Document document = reader.read(file);
            
            List<Node> nodes = document.selectNodes(xpathExp);//解析xpath表达式
            for(Node node : nodes){
                Element emp = (Element)node;
                System.out.println(emp.attributeValue("no"));
                System.out.println(emp.elementText("name"));
                System.out.println(emp.elementText("age"));
                System.out.println(emp.elementText("salary"));
                System.out.println("==============================");
            }
            
            
        } catch (DocumentException e) {
            e.printStackTrace();
        }
    }
    
    public static void main(String[] args) {
        XPathTestor testor = new XPathTestor();
//      testor.xpath("/hr/employee");//
//      testor.xpath("//employee");
//      testor.xpath("//employee[salary<4000]");
//      testor.xpath("//employee[name='李铁柱']");
//      testor.xpath("//employee[@no=3304]");
//      testor.xpath("//employee[1]");
//      testor.xpath("//employee[last()]");
        //testor.xpath("//employee[position()<3]");
        testor.xpath("//employee[3] | //employee[8]");
        
    }
}

to sum up

When writing XML applications, query priority when it comes to using XPath expressions.

references:

https://baike.baidu.com/item/XPath/5574064