XML namespaces, in a way, are similar to Java packages because they allow creating an additional context for grouping a set of elements. We already noted some differences in namespace handling for the XmlParser and XmlSlurper classes in the Reading XML using XmlParser and Reading XML using XmlSlurper recipes.
In this recipe, we dig a bit deeper into the details of XML namespace support in Groovy.
Let's use the same shakespeare.xml file we used for the Reading XML using XmlParser and Reading XML using XmlSlurper recipes.
XmlParser requires you to specify an element name exactly as it appears in the parsed XML, including the name of the prefix used in the actual XML content. This makes the code fragile because the namespace prefixes have to match.
- In order to make code that is based on
XmlParsermore reliable in respect to namespaces, we can resort to thegroovy.xml.Namespaceclass as shown in the following code:import groovy.xml.Namespace def xmlSource = new File('shakespeare.xml') def bibliography = new XmlParser().parse(xmlSource) def bib = new Namespace('http://bibliography.org', 'bib') def lit = new Namespace('http://literature.org', 'lit') println bibliography[bib.author].text() println bibliography[lit.play].findAll { it[lit.year].text().toInteger() > 1592 }.size() XmlSlurperhas a similar API for declaring the prefixes and namespaces required to navigate the nodes, shown in the following code:def xmlSource = new File('shakespeare.xml') def bibliography = new XmlSlurper().parse(xmlSource) bibliography.declareNamespace( bib: 'http://bibliography.org', lit: 'http://literature.org') println bibliography.'bib:author' println bibliography.'lit:play'.findAll { it.'lit:year'.toInteger() > 1592 }.size()- The output of both scripts is indistinguishable:
William Shakespare 3
Both of the previous code snippets extract the author's name and the number of plays written after 1592 from our reference bibliography data XML document.
In the case of XmlParser, we declare two instances of the groovy.xml.Namespace class. When we fetch a property (for example, bibliography[bib.author]) from the Namespace object, this is really what happens:
- The
bib.authorexpression returns a value ofjavax.xml.namespace.QNametype. - The array reference
bibliography[bib.author]is translated by Groovy into a call to thegetAtmethod of thegroovy.util.Nodeclass. This method accepts aQNameas an argument and returns a node if theQNameis found, as shown next:QName ns = bib.author Node n = bibliography.getAt(ns)
In the case of XmlSlurper, the groovy.util.slurpersupport.GPathResult class instance (returned by the parse method) has an additional method to declare namespaces called, not surprisingly, declareNamespace.
Note
Please note that unlike XmlParser, the XmlSlurper implementation (or more specifically GPathResult) does not force you to depend on namespaces or prefixes at all. You can refer to elements and attributes using their local names, and only resort to using namespace prefixes if there are same local names under different namespaces.
If you try to use a fully qualified name (for example. bib:author) before declaring the namespace within an XmlSlurper instance, you'll get no result back. Also, namespace prefixes defined by declareNamespace do not have to match prefixes appearing in the actual XML file.
The declareNamespace method takes a map of prefixes and namespaces. When those are defined, you can use them to reference elements using their fully qualified names.
If you plan to switch between XmlParser and XmlSlurper implementations and you need to parse XML that uses namespaces, then the safest approach is to use the *: prefix for element or attribute queries. For example:
println bibliography.'*:author'.text()
- Reading XML using XmlParser
- Reading XML using XmlSlurper
- Searching in XML with GPath
- http://groovy.codehaus.org/api/groovy/util/XmlParser.html
- http://groovy.codehaus.org/api/groovy/util/XmlSlurper.html
- http://groovy.codehaus.org/api/groovy/util/Node.html
- http://groovy.codehaus.org/api/groovy/xml/Namespace.html