Introduction to XPath

Web Engineering, SS 2007



Tomáš Pitner

                                            XPath – Why?

n     Part of the core XML standards family

  q    XML Path Language (XPath) Version 1.0

  q    W3C Recommendation 16 November 1999



n     Mean to navigate in the document tree

  

                                     Example of an XML Document

<?xml version='1.0' encoding='UTF-8'?>

<staff organization="Bundesregierung">

  <person id="agu">

    <name>Alfred Gusenbauer</name>

    <party url="http://www.spoe.at">SPÖ</party>

  </person>

  <person id="wmo">

    <name>Wilhelm Molterer</name>

    <party url="http://www.oevp.at">ÖVP</party>

  </person>

</staff>



                                          XPath – The Goal

n    Navigation in XML documents

  q    Similar to file addressing in a FS

  q    Selects nodes (not just elements) in the tree

  q    Further node processing often takes place:

    n      XSLT, XQuery – both heavily depend on XPath



n    Works on Logical Structure (XML tree)

n    Example: an XPath  expression against the last document

  q    /staff/person[@id=‘wmo’]





                                          XPath Processing

n    XPath Processor/Engine

  q     Takes XPath expression

  q     Takes context node (position in the doc)

  q     Accesses the source XML tree

  q     Produces node-set/sequence as result

                                  XPath – Anatomy of an Expression

n    XPath Expression

  q    Consists of steps separated by slash /

  q    example – two steps

    n     /staff/person

    n      semantics: Select all person elements under staff (which is the root element).

  

  q    resembles traversing directories in a FS

  q    applicable to both elements and other nodes!

  q    may be absolute (starting with /) or relative

    n      i.e. evaluated without or against context nodes

                                         XPath – Predicates

n    XPath Expression with Predicates

  q    each step may contain a predicate

    n     It filters the nodes selected at the step

    

    n     /staff/person[@id=‘wmo’]

    n     Select just the person under staff with id attribute = ‘wmo’

    

    n     Predicates may be compound

    n     //person[@id=‘wmo’ or @id=‘agu’]

                                         XPath – Functions

n    To select non-element nodes

  q    comment()

  q    text()

  q    processing-instruction() …

n    To compute a value

  q    count()

  q    … much more

n    Example – select just the text of the name of the ‘agu’ person:

n     /staff/person[@id=‘agu’]/name/text()

                                  XPath Processing – Context, Axes

n    Context nodes and Axes

  q    Context nodes

    n     “central point” for evaluating relative expressions

  q    Axes

    n     “directions” leading from the context node out

    n     node-sets relative to the context node(s)

    

n    Steps in the expression mean traversing along axes

  

                                     XPath Expression Examples

n      All against the doc on the first slide… absolute expressions



n     //name  … selects all name elements

n     //person[party] … selects all person elements                                           having
(any) party child element

n     /staff/person[party[@url=‘xxx’]]

  q     selects all person elements having party with url xxx

  

n     /person … does not select anything from this doc

  

                                     XPath Expression Examples

n      All against the doc on the first slide… relative expressions

n     Context node: first person



n     name  … selects this person’s name element

n     ../staff … selects the staff element

n     ../* … selects also the staff element

n     / … also the staff



n     .//* … selects all descendant elements

  

n     staff/name … does not select anything from this doc

  

                                   XPath Axes Example (ancestor)

<?xml version='1.0' encoding='UTF-8'?>

<staff organization="Bundesregierung">

  <person id="agu">

    <name>Alfred Gusenbauer</name>

    <party url="http://www.spoe.at">SPÖ</party>

  </person>

  <person id="wmo">

    <name>Wilhelm Molterer</name>

    <party url="http://www.oevp.at">ÖVP</party>

  </person>

</staff>



                                  XPath Axes Example (descendant)

<?xml version='1.0' encoding='UTF-8'?>

<staff organization="Bundesregierung">

  <person id="agu">

    <name>Alfred Gusenbauer</name>

    <party url="http://www.spoe.at">SPÖ</party>

  </person>

  <person id="wmo">

    <name>Wilhelm Molterer</name>

    <party url="http://www.oevp.at">ÖVP</party>

  </person>

</staff>



                                   XPath Axis Example (attribute)

<?xml version='1.0' encoding='UTF-8'?>

<staff organization="Bundesregierung">

  <person id="agu">

    <name>Alfred Gusenbauer</name>

    <party url="http://www.spoe.at">SPÖ</party>

  </person>

  <person id="wmo">

    <name>Wilhelm Molterer</name>

    <party url="http://www.oevp.at">ÖVP</party>

  </person>

</staff>



                               XPath Axis Example (following-sibling)

<?xml version='1.0' encoding='UTF-8'?>

<staff organization="Bundesregierung">

  <person id="agu">

    <name>Alfred Gusenbauer</name>

    <party url="http://www.spoe.at">SPÖ</party>

  </person>

  <person id="wmo">

    <name>Wilhelm Molterer</name>

    <party url="http://www.oevp.at">ÖVP</party>

  </person>

  <person id="abc">

    <name>ABC</name>

    <party url="http://www.abc.at">ABC</party>

  </person>

</staff>



                               XPath Axis Example (preceding-sibling)

<?xml version='1.0' encoding='UTF-8'?>

<staff organization="Bundesregierung">

  <person id="agu">

    <name>Alfred Gusenbauer</name>

    <party url="http://www.spoe.at">SPÖ</party>

  </person>

  <person id="wmo">

    <name>Wilhelm Molterer</name>

    <party url="http://www.oevp.at">ÖVP</party>

  </person>

  <person id="abc">

    <name>ABC</name>

    <party url="http://www.abc.at">ABC</party>

  </person>

</staff>



                                   XPath Axis Example (preceding)

<?xml version='1.0' encoding='UTF-8'?>

<staff organization="Bundesregierung">

  <person id="agu">

    <name>Alfred Gusenbauer</name>

    <party url="http://www.spoe.at">SPÖ</party>

  </person>

  <person id="wmo">

    <name>Wilhelm Molterer</name>

    <party url="http://www.oevp.at">ÖVP</party>

  </person>

  <person id="abc">

    <name>ABC</name>

    <party url="http://www.abc.at">ABC</party>

  </person>

</staff>



                                   XPath Axis Example (following)

<?xml version='1.0' encoding='UTF-8'?>

<staff organization="Bundesregierung">

  <person id="agu">

    <name>Alfred Gusenbauer</name>

    <party url="http://www.spoe.at">SPÖ</party>

  </person>

  <person id="wmo">

    <name>Wilhelm Molterer</name>

    <party url="http://www.oevp.at">ÖVP</party>

  </person>

  <person id="abc">

    <name>ABC</name>

    <party url="http://www.abc.at">ABC</party>

  </person>

</staff>



                                     XPath Axis Example (self)

<?xml version='1.0' encoding='UTF-8'?>

<staff organization="Bundesregierung">

  <person id="agu">

    <name>Alfred Gusenbauer</name>

    <party url="http://www.spoe.at">SPÖ</party>

  </person>

  <person id="wmo">

    <name>Wilhelm Molterer</name>

    <party url="http://www.oevp.at">ÖVP</party>

  </person>

  <person id="abc">

    <name>ABC</name>

    <party url="http://www.abc.at">ABC</party>

  </person>

</staff>



                                  XPath  Axis Example (namespace)

<?xml version='1.0' encoding='UTF-8'?>

<staff organization="Bundesregierung

       xmlns:breg=“http://www.austria.gv.at/ns/1.0"

       xmlns="http://generic-ns.org/ns/1.0">

  <person id="agu">

    <name>Alfred Gusenbauer</name>

    <breg:party url="http://www.spoe.at">SPÖ</breg:party>

  </person>

  <person id="wmo">

    <name>Wilhelm Molterer</name>

    <breg:party url="http://www.oevp.at">ÖVP</breg:party>

  </person>

</staff>



                                 XPath Processing – Summary of axes

  q     ancestor

  q     ancestor-or-self

  q     attribute

  q     child

  q     descendant

  q     descendant-or-self

  q     following

  q     following-sibling

  q     namespace

  q     parent

  q     preceding

  q     preceding-sibling

  q     self

  

                                        XPath – Long Syntax

n    The expression

  q   /staff/person can be rewritten as

  q   /child::staff/child::person



n    Similar for other axes

  q    there are axes expressible in the long form ONLY

  q    Example: preceding-sibling, ancestor-or-self…



                                         XPath – Resources

n    E. R. Harold – XML in a Nutshell

  q    Chapter on XPath free on the web: http://www.oreilly.com/catalog/xmlnut/chapter/ch09.html



n    XPath (and XSLT) Quick Reference Card

  q     http://www.mulberrytech.com/quickref/XSLT_1quickref-v2.pdf