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e1074. Finding Elements by Absolute Location in a DOM Document Using XPath

XPath is an expression language for selecting nodes in an XML file. For example, the XPath expression /a/b selects all b elements under the root element a. This example demonstrates some common XPath expressions for selecting elements based on the document root. For examples of XPath expressions relative to a particular node, see eX X. For more information about XPath, see the specification at http://www.w3c.org/TR/xpath. The XPath package used in this example selects nodes in a DOM document. Hence, the XML file is first parsed into a DOM document and the XPath expression is then applied to the DOM document. The result of an XPath expression is shown next to the expression; the numbers are ids of elements in the sample file shown at the end of this example.

Note: This example uses the XPath packages (org.apache.xpath.*) available in J2SE 1.4. However, these packages are not yet part of the Java 2 API and so this example may not work in future versions of J2SE.

    // Get the root element (without specifying its name)
    String xpath = "/*";                       // 1
    
    // Get the root element (using its name)
    xpath = "/root";                           // 1
    
    // Get all elements directly under the root
    xpath = "/root/*";                         // 2 8 12
    
    // Get all e elements directly under the root
    xpath = "/root/e";                         // 12
    
    // Get all e elements in the document
    xpath = "//e";                             // 4 6 10 11 12
    
    // Get all non-e elements in the document
    xpath = "//*[name() != 'e']";              // 1 2 3 5 7 8 9
    
    // Get all e elements directly under an elem1 element
    xpath = "//elem1/e";                       // 10 11
    
    // Get all e elements anywhere under an elem1 element
    xpath = "//elem1//e";                      // 4 6 10 11
    
    // Get all elements with at least one child element
    xpath = "//*[*]";                          // 1 2 3 5 8
    
    // Get all elements without a child element
    xpath = "//*[not(*)]";                     // 4 6 7 9 10 11 12
    
    // Get all elements with at least one child e element
    xpath = "//*[e]";                          // 1 3 5 8
    
    // Get all elements with more than one child e elements
    xpath = "//*[count(e)>1]";                 // 8
    
    // Get all non-e elements without an e child element
    xpath = "//*[not(e) and name() != 'e']";   // 2 7 9
    
    // Get all level-4 e elements (the root being at level 1)
    xpath = "/*/*/*/e";                        // 4
    
    // Get all elements with more than one child e elements
    xpath = "//*[count(e)>1]";                 // 8

XPath 1.0 does not support regular expressions to match element names. However, it is possible to perform some very simple matches on element names.

    // Get all elements whose name starts with el
    xpath = "//*[starts-with(name(), 'el')]";  // 2 3 5 7 8 9
    
    // Get all elements whose name contains with lem1
    xpath = "//*[contains(name(), 'lem1')]";   // 2 8

Sets of elements can also be combined using the union operator |

    // Get all e elements directly under either the root or an elem2 element
    xpath = "/*/e | //elem2/e";                // 4 12

Elements are returned in document order, that is, if the location of A's start tag appears before the location of B's start tag in the XML document, element A is returned before element B. Moreover, the returned elements are unique.

    // Read an XML document; this method is implemented in
    // e510 The Quintessential Program to Create a DOM Document from an XML File
    Document doc = parseXmlFile("infilename.xml", false);
    
    try {
        // Get the matching elements
        NodeList nodelist = org.apache.xpath.XPathAPI.selectNodeList(doc, xpath);
    
        // Process the elements in the nodelist
        for (int i=0; i<nodelist.getLength(); i++) {
            // Get element
            Element elem = (Element)nodelist.item(i);
        }
    } catch (javax.xml.transform.TransformerException e) {
    }

Here is the sample XML file used in the example:

    <?xml version="1.0" encoding="UTF-8"?>
    <root id="1">
        <elem1 id="2">
            <elem2 id="3">
                <e id="4"/>
                <elem3 id="5">
                    <e id="6"/>
                </elem3>
                <elem3 id="7"/>
            </elem2>
        </elem1>
        <elem1 id="8">
            <elem2 id="9"/>
            <e id="10"/>
            <e id="11"/>
        </elem1>
        <e id="12"/>
    </root>