XPath. by Klaus Lüthje Lauri Pitkänen

Transcription

1 XPath by Klaus Lüthje Lauri Pitkänen

2 Agenda Introduction History Syntax Additional example and demo Applications Xpath 2.0 Future

3 Introduction Expression language for Addressing portions of an XML document Computing arithmetic values Based on a tree representation of the document

4 History Originally developed for a common behavior for selecting parts of XML documents in XSLT and XPointer XPath 1.0 W3C Recommendation Nov 1999 Adopted by many programmers as a small query language for XML data XPath 2.0 W3C Recommendation Jan 2007

5 Abbreviated and expanded syntax Reasonable defaults are used in abbreviated syntax Non default options are entered explicitly using abbreviations In expanded syntax all values are defined explicitly using long names Some features can only be specified using expanded syntax as abbreviations do not exist for them /A/B/C Simple abbreviated example /child::a/child::b/child: :C Same as previous in expanded syntax /A//B/*[1] Uses non default axis specifier, wildcard node test and predicate /child::a/descendant orself::node()/child::b/chi ld::*[1] Expanded syntax

6 Axis specifiers Define the direction of navigation within the document tree Child (default) Attribute Descendant (N/A) Descendant or self (//) Parent (..) And 8 others Abbreviated syntax only available for some options /A/B/C C is a child of B is a child of A, which is a root node /A//B/C C is a child of B, which is either a child of A or a deeper descendant /child::a/descendant::b/c hild::c C is a child of B, which must be a descendant of A, but may not be a direct child of A Cannot be abbreviated

7 Node tests and predicates Node tests are specific node names or more general expressions comment(), text(), processing instruction() and node() are available Predicates are arbitrarily complex limiting expressions Predicade order is significant Example will follow /A/B/C A, B and C are node tests /child::a/descendant orself::text() Text() limits the scope to text nodes Star (*) matches all names //a[@href='help.php'] Predicate matches and a element with an href attribute whose value is help.php /A/B/*[1] Node test matches any name, but predicate limits selection to first match

8 Functions and operators Logical, arithmetic and comparison operators Such as (union), and, +, < and!= Also /, // and [ ] as previously defined Functions for node sets, strings, booleans and numbers Some operators can be used in predicates to limit selection scope v[x or y] w[z] Returns a single node set of all v elements with x or y child elements and all w elements with z child elements //item[@price > 2*@discount] Selects items whose price attribute is at least two times the numeric value of the discount attribute

9 Predicate order example get Will only match if the first a element within document The [1] is evaluated first and restricts the selection Also note that the predicate [/html/@lang='en'] refers to the root level! Compare with //a[/html/@lang='en'][@href='help.php'][1]/@tar get

10 Demo Xpath Explorer by Purple Tecnology used for demo Web based evaluators exist, but GUI often not as good Several Eclipse plugins available

11 Applications Addressing elements and computing values in other XML languages Creating bindings between documents for XML languages Addressing elements and computing values from XML documents in general purpose programming languages via suitable APIs Creating queries for databases supporting XML Further refined in XQuery

12 Usage in XML Languages XSLT Template matching and calculation done using XPath Made for transforming XML documents to human displayable formats XQuery Overlaps with XSLT but with different starting point Made as a database query language like SQL W3C Recommendation just came but has lots of implementations

13 Usage in XML Languages XML Schema Used for identity constraint definitions XForms Used to connect the user interface to the model XPointer XLink Schematron

14 Programming Language Support Java: integrated API, multiple implementations Jaxen, Apache Xalan Java,Saxon, VTD XML.NET: integrated implementation Most other languages: 3 rd party libraries C/C++, Perl, PHP, Python, Ruby Web Browsers Firefox through DOM IE ActiveX object Generic Javascript library

15 Usage for Database Queries Many database engines exists that have XML and XSLT/XPath support Some support XQuery XML Engines ontop of a traditional database Oracle, SQL Server Native XML Databases Apache Xindice, Sedna, exist db, XStreamDB

16 XPath 2.0 Part of XQuery 1.0 and XSLT 2.0 Superset of XPath 1.0 and is backwards compatible (with a few exceptions) Richer type system Atomic types, user defined types defined with XML Schema Float, date, time, anyuri, etc. Expanded function and operator set to support the new types For and If then else expressions Sequences Can have the same node multiple times

17 XPath 2.0 Examples At least Saxon supports XPath 2.0 (and XSLT 2.0) For expression that returns a sequence for $a in distinct values(/bib/book/author) return ($a, /bib/book[author = $a]/title) If expression if ($temperature1 < $temperature2) then $temperature1 else $temperature2

18 Future More support for XPath 2.0 XQuery 1.0 and XPath 2.0 Full Text Extends XQuery and XPath with full text search capabilities W3C Working Draft May 2006 More extensions?

19 Web Links XPath 1.0 W3C Recommendation XPath 2.0 W3C Recommendation Wikipedia XPath page (with examples)