XPath Evaluator in Java

As we know XML document is used to store and transport data. So, to access data from XML, we need something which could access each node and respective attributes data. Then the solution is XPath. XPath can be used to traverse through XML documents, to select nodes/elements, and attribute data. It is a W3C Recommendation and a flexible way of accessing different parts of an XML document. To write XPath is similar to writing path expression in your computer system to traverse to a specific location like (C:/School/Homework/assignment.docx). 

Consider the Following XML Document

XML

<?xml version="1.0" encoding="UTF-8"?>
<students>
    <student branch="CSE">
        <name>Divyank Singh Sikarwar</name>
        <age>18</age>
        <city>Agra</city>
    </student>
    <student branch="CSE">
        <name>Aniket Chauhan</name>
        <age>20</age>
        <city>Shahjahanpur</city>
    </student>
    <student branch="CSE">
        <name>Simran Agarwal</name>
        <age>23</age>
        <city>Buland Shar</city>
    </student>
    <student branch="CSE">
        <name>Abhay Chauhan</name>
        <age>17</age>
        <city>Shahjahanpur</city>
    </student>
    <student branch="IT">
        <name>Himanshu Bhatia</name>
        <age>25</age>
        <city>Indore</city>
    </student>
    <student branch="IT">
        <name>Anuj Modi</name>
        <age>22</age>
        <city>Ahemdabad</city>
    </student>
    <student branch="ECE">
        <name>Manoj Yadav</name>
        <age>23</age>
        <city>Kota</city>
    </student>
</students>

XPath symbols that are used to access different parts of an XML document:

Symbol	Description	Example	Result
name	Selects all tags from XML having name ‘name’	/students/student/name	Displays all names
/	This represents the root of the document	/students/student/city	Display each student’s city
//	Selects node irrespective of where it is.	//age	Selects and display all ages
@	To access attribute value of XML tags	/students/student/@branch	Display each student’s branch
[ ]	It is used to select specific nodes	/students/student[2]/name	Displays Aniket Chauhan

Let’s Practice XPath

Consider above mentioned XML document:

Select 2nd student

/students/student[2]/name

Select all the students with branch IT

/students/student[@branch = /”IT/”]/name

Select all the students whose age is less than equal to 20

/students/student[age <= 20]/name

First 4 students

/students/student[position() <= 4]/name

Java Code to Evaluate XPath Expression

Java

import java.io.File;
import javax.xml.parsers.DocumentBuilder;
import javax.xml.parsers.DocumentBuilderFactory;
import javax.xml.xpath.XPath;
import javax.xml.xpath.XPathConstants;
import javax.xml.xpath.XPathExpression;
import javax.xml.xpath.XPathExpressionException;
import javax.xml.xpath.XPathFactory;
import org.w3c.dom.Document;
import org.w3c.dom.NodeList;

public class XPathDemo {

    public static void main(String[] args) throws Exception
    {
        File xmlFile = new File("student.xml");

        // Get DOM
        DocumentBuilderFactory dbf = DocumentBuilderFactory.newInstance();
        DocumentBuilder db = dbf.newDocumentBuilder();
        Document xml = db.parse(xmlFile);
        xml.getDocumentElement().normalize();

        // Get XPath
        XPathFactory xpf = XPathFactory.newInstance();
        XPath xpath = xpf.newXPath();

        // Find 2nd Student's name
        String name = (String)xpath.evaluate(
            "/students/student[2]/name", xml,
            XPathConstants.STRING);

        System.out.println("2nd Student Name: " + name);

        // find specific students name whose branch is IT
        NodeList nodes = (NodeList)xpath.evaluate(
            "/students/student[@branch = \"IT\"]/name", xml,
            XPathConstants.NODESET);

        System.out.println("\nStudents with branch IT:");
        printNodes(nodes);

        // find specific students
        // name whose age is less
        // than equal to 20
        nodes = (NodeList)xpath.evaluate(
            "/students/student[age <= 20]/name", xml,
            XPathConstants.NODESET);

        System.out.println(
            "\nStudents of age less than equal to 20:");
        printNodes(nodes);

        // First 4 students from XML document
        nodes = (NodeList)xpath.evaluate(
            "/students/student[position() < 5]/name", xml,
            XPathConstants.NODESET);

        System.out.println("\nFirst Four Students: ");
        printNodes(nodes);
    }

    // prints nodes
    public static void printNodes(NodeList nodes)
    {
        for (int i = 0; i < nodes.getLength(); i++) {
            System.out.println(
                (i + 1) + ". "
                + nodes.item(i).getTextContent());
        }
    }
}

Output:

Explanation of classes and methods used in the above code:

• The javax.xml.Parsers.DocumentBuilder class defines the API to obtain DOM instances from an XML document.
• parse() method parse the content of the given file as an XML document and return a new DOM object.
• normalize() method normalize the content of the given file as an XML document.
• The javax.xml.xpath.XPathFactory class instance can be used to create XPath objects which contain evaluate() method to evaluate our written xpath and return string/Node/NodeSet, anyone, according to the passed parameter (see evaluate() method inside code).
• position() is an XPath function that returns the position of the currently specified tag. (In the above code specified tag is ‘student’). Similarly, XPath provides a list of useful functions, you can explore it.