With this example we are going to demonstrate how to find elements by Id using XPath. The XPath language provides a simple, concise syntax for selecting nodes from an XML document. XPath also provides rules for converting a node in an XML document object model (DOM) tree to a boolean, double, or string value. In short, to find elements by Id using XPath you should:
- Obtain a new instance of a DocumentBuilderFactory, that is a factory API that enables applications to obtain a parser that produces DOM object trees from XML documents.
- Set the parser produced so as not to validate documents as they are parsed, using
setValidating(boolean validating)API method of DocumentBuilderFactory, with validating set to false. - Create a new instance of a DocumentBuilder, using
newDocumentBuilder()API method of DocumentBuilderFactory. - Parse the FileInputStream with the content to be parsed, using
parse(InputStream is)API method of DocumentBuilder. This method parses the content of the given InputStream as an XML document and returns a new DOM Document object. - Create an XPathFactory instance to be used to create XPath objects, with
newInstance()API method of XPathFactory. - Create a new XPath object, using the underlying object model determined when the XPathFactory was instantiated.
- Create a String expression and use
evaluate(String expression, Object item, QName returnType)API method of XPath in order to evaluate it in the specified Document object. The method returns a result as the specified type. - In the example, first we evaluate the expression returning the node containing the id, so
XPathConstants.NODEis returned. It returns the Node containing the id. We also evaluate the expression returning the set of nodes containing the id, so thereturnTypeis set toXPathConstants.NODESET. It returns a NodeList containing the collection of Nodes that contain the id. We can also evaluate an expression checking if more than one ids exist in the Document. The returnType is set toXPathConstants.NODESETagain, and a NodeList is also returned, containing the collection of Nodes that contain the ids.
Let’s take a look at the code snippet that follows:
package com.javacodegeeks.snippets.core;
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.XPathFactory;
import org.w3c.dom.Document;
import org.w3c.dom.Node;
import org.w3c.dom.NodeList;
public class FindElementsByIdWithXPath {
public static void main(String[] args) throws Exception {
DocumentBuilderFactory dbf = DocumentBuilderFactory.newInstance();
dbf.setValidating(false);
DocumentBuilder db = dbf.newDocumentBuilder();
Document doc = db.parse(new FileInputStream(new File("in.xml")));
XPathFactory factory = XPathFactory.newInstance();
XPath xpath = factory.newXPath();
String expression;
Node node;
NodeList nodeList;
// 1. elements with id '1'
expression = "id('1')";
node = (Node) xpath.evaluate(expression, doc, XPathConstants.NODE);
System.out.println("1. " + node.getNodeName());
// 2. all elements under element with id '1'
expression = "id('1')/entry";
nodeList = (NodeList) xpath.evaluate(expression, doc, XPathConstants.NODESET);
System.out.print("2. ");
for (int i = 0; i < nodeList.getLength(); i++) {
System.out.print(nodeList.item(i).getNodeName() + " ");
}
System.out.println();
// 3. elements with id 1, 2 or 4
expression = "id('1 2 4')";
nodeList = (NodeList) xpath.evaluate(expression, doc, XPathConstants.NODESET);
System.out.print("3. ");
for (int i = 0; i < nodeList.getLength(); i++) {
System.out.print(nodeList.item(i).getNodeName() + " ");
}
System.out.println();
// 4. element that does not exist
expression = "id('UNKNOWN')";
node = (Node) xpath.evaluate(expression, doc, XPathConstants.NODE);
System.out.println("4. " + node);
}
}
Input:
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE entries [ <!ELEMENT entry (entry*) >
<!ATTLIST entry id ID #REQUIRED>
]>
<entries>
<entry id="1">
<entry id="2">
<entry id="3"/>
</entry>
</entry>
<entry id="4"/>
</entries>
Output:
1. entry
2. entry
3. entry entry entry
4. null
This was an example of how to find elements by Id using XPath in Java.
