Getting the value of an attribute in Java from a XML using XPath

Question

I'm currently using XPath to get some information from a podcast feed using Java and XPath. I'm trying to read the attribute of a node:

<?xml version="1.0" encoding="UTF-8"?>
<rss xmlns:dc="http://purl.org/dc/elements/1.1/" xmlns:sy="http://purl.org/rss/1.0/modules/syndication/" xmlns:admin="http://webns.net/mvcb/" xmlns:atom="http://www.w3.org/2005/Atom/" xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#" xmlns:content="http://purl.org/rss/1.0/modules/content/" xmlns:itunes="http://www.itunes.com/dtds/podcast-1.0.dtd" version="2.0">
  <channel>
    [....]
    <itunes:image href="http://icebox.5by5.tv/images/broadcasts/14/cover.jpg" />
[...]

I want to get the value of the href attribute in <itunes:image>. Currently, I'm using the following code:

private static String IMAGE_XPATH = "//channel/itunes:image/@href";
String imageUrl = xpath.compile(IMAGE_XPATH).evaluate(doc, XPathConstants.STRING).toString();

The result of imageUrl is null. What happens in the code? Do I have an error in the XPath code, or in the Java code?

Thanks! :)

jspboix · Accepted Answer · 2012-06-27 10:11:55Z

4

Disable namespace awarness:

DocumentBuilderFactory xmlFact = DocumentBuilderFactory.newInstance();
xmlFact.setNamespaceAware(false);

Your xpath expression should look like this now:

"//channel/image/@href"

If you need to use it as namespace aware, just implement your own NameSpaceContext, should look like this:

NamespaceContext ctx = new ItunesNamespaceContext();

XPathFactory xpathFact = XPathFactory.newInstance();
XPath xpath = xpathFact.newXPath();
xpath.setNamespaceContext(ctx);
String IMAGE_XPATH = "//channel/itunes:image/@href";
String imageUrl = path.compile(IMAGE_XPATH).evaluate(doc,XPathConstants.STRING).toString();

EDIT: Here is a test code that proves my point:

String a ="<?xml version=\"1.0\" encoding=\"UTF-8\"?><rss xmlns:dc=\"http://purl.org/dc/elements/1.1/\" xmlns:sy=\"http://purl.org/rss/1.0/modules/syndication/\" xmlns:admin=\"http://webns.net/mvcb/\" xmlns:atom=\"http://www.w3.org/2005/Atom/\" xmlns:rdf=\"http://www.w3.org/1999/02/22-rdf-syntax-ns#\" xmlns:content=\"http://purl.org/rss/1.0/modules/content/\" xmlns:itunes=\"http://www.itunes.com/dtds/podcast-1.0.dtd\" version=\"2.0\"><channel><itunes:image href=\"http://icebox.5by5.tv/images/broadcasts/14/cover.jpg\" /></channel></rss>";
DocumentBuilderFactory xmlFact = DocumentBuilderFactory.newInstance();
xmlFact.setNamespaceAware(false);
DocumentBuilder builder = xmlFact.newDocumentBuilder();
XPathFactory xpathFactory = XPathFactory.newInstance();
String expr = "//channel/image/@href";
XPath xpath = xpathFactory.newXPath();
Document doc = builder.parse(new InputSource(new StringReader(a)));
String imageUrl = (String) xpath.compile(expr).evaluate(doc ,XPathConstants.STRING);
System.out.println(imageUrl);

The output is:

http://icebox.5by5.tv/images/broadcasts/14/cover.jpg

edited Jun 27, 2012 at 10:11

answered Jun 26, 2012 at 22:07

jspboix

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2 Comments

Francesc Over a year ago

Hi. I don't think the namespaces are the problem, because, I'm already using other XPath expressions to get itunes: tags.

jspboix Over a year ago

I've just added a piece of code proving you that this answer is correct, <itunes: does not define a tag but the type that a tag belongs to, read this tutorial if you don't understand what namespaces are: w3schools.com/xml/xml_namespaces.asp

DoctorMick · Accepted Answer · 2012-06-26 15:54:05Z

0

The XPath should include the root element, so rss/channel/itunes:image/@href.

Alternatively, you could start the xpath with a // so that all levels are searched for the xpath (//channel/itunes:image/@href) but if the root will always be the same it is more efficient to use the first option.

answered Jun 26, 2012 at 15:54

DoctorMick

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2 Comments

Francesc Over a year ago

Hi. I tried changing the XPath to use the your code but it still returns null

Francesc Over a year ago

Could it be that the XPath returns the whole attribute, I mean, not just the value of it, but, as an object that contains that part of the DOM?

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