Question

I want to parse a Feedburner feed with HtmlUnit.
The feed is this one: http://feeds.feedburner.com/alcoanewsreleases

From this feed I want to read all item nodes, so normally a //item XPath should do the trick. Unfortunately that does not work in this case.

groovy code snippet:

def page = webClient.getPage("http://feeds.feedburner.com/alcoanewsreleases")
def elements = page.getByXPath("//item")

Sample of the XML feed:

<?xml version="1.0" encoding="UTF-8"?>
<?xml-stylesheet type="text/xsl" media="screen" href="/~d/styles/rss1full.xsl"?>
<?xml-stylesheet type="text/css" media="screen" href="http://feeds.feedburner.com/~d/styles/itemcontent.css"?>

<rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#" xmlns:dc="http://purl.org/dc/elements/1.1/" xmlns="http://purl.org/rss/1.0/" xmlns:feedburner="http://rssnamespace.org/feedburner/ext/1.0">

[...SNIP...]

<item rdf:about="http://www.alcoa.com/global/en/news/news_detail.asp?newsYear=2011&amp;pageID=20110518006002en">
    <title>Chris L. Ayers Named President, Alcoa Global Primary Products</title>
    <dc:date>2011-05-18</dc:date
    <link>http://feedproxy.google.com/~r/alcoanewsreleases/~3/PawvdhpJrkc/news_detail.asp</link>
    <description>NEW YORK--(BUSINESS WIRE)--Alcoa (NYSE:AA) announced today that Chris L. Ayers has been named President of Alcoa’s Global Primary Products (GPP) business, effective May 18, 2011. Ayers, previously Chief Operating Officer of GPP, succeeds John Thuestad, who will be handling special projects for the Company. Ayers joined Alcoa in February 2010 as Chief Operating Officer of Alcoa Cast, Forged and Extruded Products, a new position. He was elected a Vice President of Alcoa in April 2010 and Executive</description>
    <feedburner:origLink xmlns:feedburner="http://rssnamespace.org/feedburner/ext/1.0">http://www.alcoa.com/global/en/news/news_detail.asp?newsYear=2010&amp;pageID=20100104006194en</feedburner:origLink>
</item>

[...SNIP...]

</rdf:RDF>

I suspect this to be an issue with the namespaces because this document has 4 namespaces. The namespaces are

• (this is the default) xmlns=”http://purl.org/rss/1.0/”
• xmlns:rdf=”http://www.w3.org/1999/02/22-rdf-syntax-ns#”
• xmlns:dc=”http://purl.org/dc/elements/1.1/”
• xmlns:feedburner=”http://rssnamespace.org/feedburner/ext/1.0″

I have tried to use Nokogiri with this (another XML Parser that I use for ruby scripts).
With Nokogiri I could just us the XPath //xmlns:item which works and returns all nodes from the feed.

I have tried the same XPath with HtmlUnit but it does not work.

So I think I can phrase my question as:
How can I select a node from the default namespace with HtmlUnit?

Any ideas?

Answer 1

From this feed I want to read all item
nodes, so normally a //item XPath
should do the trick. Unfortunately
that does not work in this case.

In XPath, that means “select all elements whose local name is item that are in no namespace“. In RSS, the item elements must be in a namespace. So the above should never work with a conforming XML parser and XPath engine.

What’s confusing is that in XML, <item> means “an element named item that is in the default namespace, i.e. whatever default namespace is in scope at this place in the document;” whereas in XPath, “item” means an element in no namespace. (Or, you could say, it means an element in the default namespace, but unless you have a way to tell XPath what the default namespace is, the default namespace is no namespace. Usually (always?) in XPath 1.0 there is no way to declare the default namespace for XPath expressions.)

The other confusing thing to beginners is that the namespace prefix mappings in the source XML document are not considered significant by the XPath processor. When the XML document is parsed, a data structure is built that remembers the name and namespace of every element (and other nodes). The namespace prefixes used, including the empty prefix of the default namespace, are considered mere syntactic convenience. More on this below…

With Nokogiri I could just us the
XPath //xmlns:item which works and
returns all nodes from the feed.

Whatever that is, it’s not XPath. Maybe it’s a Nokogiri extension to it (a very convenient one, but its syntax is really counter-intuitive).

So I think I can phrase my question
as: How can I select a node from the
default namespace with HtmlUnit?

Let’s phrase it as: How can I select the RSS item elements with HtmlUnit? I phrase it that way because the RSS spec (actually in general any conforming XML vocabulary spec) does not require that its elements will be in the default namespace. That happens to be true in the sample you received, but the service provider could change that tomorrow and still be perfectly conformant to RSS. Tomorrow, the service provider could use the “rss” namespace prefix for that namespace; or any other arbitrary prefix. What RSS does specify is what namespace its elements will be in: the namespace whose URI is http://purl.org/rss/1.0/.

It’s kind of like asking, “How do I write a function (in Javascript, C, Java, etc.) that can tell me the value of the variable a?” Usually a function has no idea what variable name was used for what in the caller. All it knows are the values of its arguments. If you call sqrt(4), you’ll get the same answer as with a = 4; sqrt(a) or rumpelstiltzkin = 4; sqrt(rumpelstiltzkin). Clearly, the name of the variable argument has no direct effect on the result of the function call. It just needs to be the name of a variable that holds the right value. If a compiler complained because you wrote b = 4; return sqrt(b) instead of using a, you’d think that compiler was nuts. It’s not supposed to care about variable names as long as you use valid identifiers.

In the same way, when processing RSS, we’re not supposed to care about what namespace prefix is used, as long as it’s a prefix that identifies the right namespace. It could be no prefix (which identifies the default namespace).

In XPath 2.0, you can wildcard the namespace. This is very handy if you know you’re not going to need namespaces for disambiguation. In that case you can select //*:item. However, I don’t think HTMLUnit supports XPath 2.0. Also in XPath 2.0 environments like XSLT 2.0, you can specify a default namespace for XPath expressions, but that won’t help you in HTMLUnit.

So you have a couple of choices:

• Use an XPath expression that ignores namespaces, such as //*[local-name() = 'item'].

or

• The robust way: Register a namespace prefix for http://purl.org/rss/1.0/ and use it in your XPath expression: //rss:item. The question then becomes, how do you register a namespace prefix in HTMLUnit and pass it to the XPath processor? I took a quick look in the docs and didn’t find any facility for doing that.

Caveat: I should add that the above is in regard to conforming XPath processors. I have no idea what XPath processor HTMLUnit uses. There are some XPath processors out there that ignore the specs and make the world more confusing for everybody.

I saw here that someone used the following syntax for elements in the default namespace in HTMLUnit:

//:item

But I wouldn’t recommend that, for three reasons:

1. It’s not valid XPath, so you can’t expect it to work with other programs.

2. It will only work on RSS feeds that declare the RSS namespace to be the default namespace. RSS feeds that use a namespace prefix will cause the above to fail.

3. It will hold you back from learning how XML namespaces really work, and it will help preserve the status quo of tools that don’t adequately support namespaces.

HTMLUnit is primarily designed for HTML, so incomplete handling of XML is understandable. But claiming to support XPath and then not providing ways to declare namespace prefixes is a bug. HTMLUnit uses an XPath package that seems to be part of Xalan-J. That package has ways to provide namespace mappings to XPath, but I don’t know if HTMLUnit exposes that functionality.