Java code using Xerces's getElementsByTagName does not return child node

Question

I'm using Xerces parser in trying to parse the following XML snippet. I can find the "location-info" element by when I use the findElementsByTagName method to find Circle, I am getting an empty NodeList back. Can someone please spot check and see what I am doing wrong?

<urn:locationResponse xmlns:urn="urn:ietf:params:xml:ns:geopriv:held">
<presence entity="pres:www.telecomsys.com" xmlns="urn:ietf:params:xml:ns:pidf"
          xmlns:gp="urn:ietf:params:xml:ns:pidf:geopriv10" xmlns:dm="urn:ietf:params:xml:ns:pidf:data-model"
          xmlns:gs="http://www.opengis.net/pidflo/1.0" xmlns:ca="urn:ietf:params:xml:ns:pidf:geopriv10:civicAddr"
          xmlns:gml="http://www.opengis.net/gml">
    <tuple id="FIRST_LOCATION">
        <status>
            <gp:geopriv>
                <location-info xmlns="urn:ietf:params:xml:ns:pidf:geopriv10">
                    <gs:Circle srsName="urn:ogc:def:crs:EPSG::4326">
                        <gml:pos>00.000000 -00.00000</gml:pos>
                        <gs:radius uom="urn:ogc:def:uom:EPSG::9001">200</gs:radius>
                    </gs:Circle>
                    <gp:confidence>95</gp:confidence>
                </location-info>
                <gp:usage-rules>
                    <gp:retransmission-allowed>yes</gp:retransmission-allowed>
                </gp:usage-rules>
                <gp:method>Derived</gp:method>
            </gp:geopriv>
        </status>
        <timestamp>2001-01-00T00:00Z</timestamp>
    </tuple>
</presence>

Following is my code which attempts to get the "Circle" tag out of this XML

    private static final Log logger = LogFactory.getLog(PIDFLOParser.class);
    private static final String LOCATION_INFO = "location-info";
    private static final String CIRCLE = "Circle";

    // Use of the Document BuilderFactory to create a DocumentBuilder class.
    DocumentBuilderFactory fact = DocumentBuilderFactory.newInstance();
    DocumentBuilder builder = fact.newDocumentBuilder();

    Document doc = builder.parse(new InputSource(new StringReader(xmlDoc)));
    doc.getDocumentElement().normalize();

    Node node = doc.getDocumentElement();

    String root = node.getNodeName();
    System.out.println("Root Node: " + root);

    NodeList listResponse = doc.getElementsByTagName(LOCATION_INFO);
    if (listResponse.getLength() == 0) {
        System.out.println(String.format("%s doesn't exist in the Document.", LOCATION_INFO));
    }

    Node firstNode = listResponse.item(0);
    listResponse = ((Element) firstNode).getElementsByTagName(CIRCLE);
    if (listResponse.getLength() == 0) {
        System.out.println(String.format("%s doesn't exist in the Document.", CIRCLE));
    }

    listResponse = ((Element) firstNode).getElementsByTagNameNS("gs", CIRCLE);
    if (listResponse.getLength() == 0) {
        System.out.println(String.format("%s doesn't exist in the Document when searching with namespace.", CIRCLE));
    }

Output of this code is:

    Root Node: urn:locationResponse
Circle doesn't exist in the Document.
Circle doesn't exist in the Document when searching with namespace.

What am I doing wrong? Thanks in advance for help!

Updated after guido's comment about full URI of namespace

...
    private static final String NS_GS = "http://www.opengis.net/pidflo/1.0";
...
    listResponse = ((Element) firstNode).getElementsByTagNameNS(NS_GS, CIRCLE);
    if (listResponse.getLength() == 0) {
        System.out.println(String.format("%s doesn't exist in the Document when searching with namespace.", CIRCLE));
    }

Output is still the same:

Root Node: urn:locationResponse
Circle doesn't exist in the Document.
Circle doesn't exist in the Document when searching with namespace.

Answer 1

When you call getElementsByTagNameNS you should specify the URI for the namespace, not the prefix used in the xml, so:

getElementsByTagNameNS("gs", CIRCLE);

should be:

getElementsByTagNameNS("http://www.opengis.net/pidflo/1.0", CIRCLE);

because gs:Circle element is defined under the namespace URI:

xmlns:gs="http://www.opengis.net/pidflo/1.0"

To make namespaces to work, you need to set the factory for it:

 DocumentBuilderFactory fact = DocumentBuilderFactory.newInstance();
 fact.setNamespaceAware(true);

Or alternatively, you can just use simply (without namespaces) the full qualified name:

 getElementsByTagName("gs:Circle");

Note: also note that your xml is invalid in your question because it is missing the closing root element </urn:locationResponse>