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XML.com: What Is XSLT

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1. 1 1 7 Stylesheet association 2 Transformation data flows 1 2 1 Transformation from XML to XML 1 2 2 Transformation from XML to XSL formatting semantics 1 2 3 Transformation from XML to non XML 1 2 4 Three tiered architectures 2 Getting started with XSLT and XPath 2 1 Stylesheet examples 2 1 1 Some simple examples 2 1 2 Some more complex examples 2 2 Syntax basics stylesheets templates instructions 2 2 1 Explicitly declared stylesheets 2 2 2 Implicitly declared stylesheets 2 2 3 Stylesheet requirements 2 2 4 Instructions and literal result elements 2 2 5 Templates and template rules 2 2 6 Approaches to stylesheet design 1 The Context of XSL Transformations and the XML Path Language This first chapter examines the context of two W3C Recommendations Extensible Stylesheet Language Transformations XSLT and XML Path Language XPath within the growing family of Recommendations related to the Extensible Markup Language XML Later we will look at detailed examples but first let s focus on XSLT and XPath in the context of a few of the Recommendations in the XML family and examine how these two Recommendations work together to address separate and distinct functionality required when working with structured information technologies This chapter does not attempt to address all of the numerous XML related Recommendations currently rele
2. 1 2 1 Transformation from XML to XML As we will see when looking at the data model the normative behavior of XSLT is to transform an XML source into an abstract hierarchical result We can request that result to be serialized into an XML file thus we achieve XML results from XML sources source document transformation f result script documents source document transformation script Figure 1 3 Transformation from XML to XML An XSLT stylesheet can be applied to more than one XML document each stylesheet producing a possibly usually different result Nothing in XSLT inherently ties the stylesheet to a single instance though the stylesheet writer can employ techniques to abort processing based on processing undesirable input An XML document can have more than one XSLT stylesheet applied each stylesheet producing a possible usually different result Even when stylesheet association indicates an author s preference for a stylesheet to use for processing tools should provide the facility to override the preference with the reader s preference for a stylesheet Nothing in XML prevents more than a single stylesheet to be applied Note 12 In all cases in this chapter the depictions show the normative result of the XSLT processor s as the dotted triangle attached to the process rectangle This serves to remind the reader that the serialization of the result into an XML file is a separate task one that is
3. An example of using such a legacy vocabulary for the XSLT processor would be 01 lt xsl template match customer gt 02 lt buyer gt lt xsl value of select db gt lt buyer gt 03 lt xsl template gt Example 1 11 Example XSLT template rule for a legacy vocabulary The transformation would then produce the following result acceptable to the legacy application 01 lt buyer gt cust123 lt buyer gt Example I 12 Example legacy vocabulary for customer information The designers of XSLT have focused on the results of delivering parsed XML information to a rendering agent or to some other application employing an XML processor as the means to access information in an XML instance The information being delivered represents the parsed result of working with the entire XML instance and if supplied the XML document model The actual markup within the source XML instance is not considered material to the application All that counts is the result of having processed the XML instance to find the underlying content the actual markup represents By focusing on this parsed result for downstream applications there is little or no regard in an XSLT stylesheet for the actual XML syntax constructs found within the source input documents or for the actual XML syntax constructs utilized in the resulting output document This prevents a stylesheet from being aware of such constructs or controlling how such constructs are used
4. The xs1 value of instruction uses an XPath expression in the select attribute to calculate a string value from our source information XPath views the source hierarchy using parent child relationships The XSLT processor s initial focus is the root of the document which is considered the parent of the document element Our XPath expression value greet ing selects the child named greeting from the current focus thus returning the value of the document element named greeting from the instance Using an MS DOS command line invocation to execute the standalone processor we see the following result 01 X samp gt xt hello xml hellohtm xsl hellohtm htm 02 X samp gt type hellohtm htm 03 lt html gt 04 lt head gt 05 lt title gt Greeting lt title gt 06 lt head gt 07 lt body gt 08 lt p gt Words of greeting lt br gt 09 lt b gt lt i gt lt u gt Hello world lt u gt lt i gt lt b gt 10 lt p gt 11 lt body gt 12 lt html gt 13 14 X samp gt Example 2 3 Explicit invocation of Example 2 2 Note how the end result contains a mixture of the stylesheet markup and the source instance content without any use of the XSLT vocabulary The processor has recognized the use of HTML by the name of the document element and has engaged SGML lexical conventions The SGML lexical conventions are evidenced on line 8 where the lt br gt empty element has been serialized without the XML lexical convention for
5. recognized URI 02 lt xsl output method prefix serialization method name gt Example 1 17 Using namespaces to specify an extension serialization method Comment the namespace declaration attribute on line 1 must be somewhere in the element or the ancestry of the instruction on line 2 Using the same semantics described for the outboard CVML Interpreter program depicted in Figure 1 7 this translation facility can be incorporated into the XSLT processor itself as an inboard extension The code itself may be directly portable based on the nature of how the outboard program is written Such an extended processor would directly emit the custom format without reifying the intermediate structure though this would be convenient for diagnostic purposes transformation script Custom Vocabuky Semantics XSLT Process source document Customized XSLT Engine result file Figure 1 8 Built in Transformation from XML to Arbitrary Non XML The XT XSLT processor implements an extension serialization method named NXML for non XML 01 xmins prefix http www jclark com xt 02 lt xsl output method prefix nxml gt Example 1 18 Using the XT namespace to specify the NXML extension serialization method Comment the namespace declaration attribute on line 1 must be somewhere in the element or the ancestry of the instruction on line 2 Second this extensibility opens up the opportunity
6. whether the information captures an invoice related transaction between computers or the content of a user manual rendered on paper XML Path Language XPath XPath is a string syntax for building addresses to the information found in an XML document We use this language to specify the locations of document structures or data found in an XML document when processing that Related Reading information using XSLT XPath allows us from any location to address any other location or content Extensible Stylesheet Language Family XSLT XSL Two vocabularies specified in separate W3C Recommendations provide for the two distinct styling processes of transforming and rendering XML instances We can transform information using one vocabulary into an alternate form by using the Extensible Stylesheet Language Transformations XSLT The Extensible Stylesheet Language XSL is a rendering vocabulary describing the semantics of formatting information for different media Namespaces We use XML namespaces to distinguish information when mixing multiple vocabularies in a single instance Without namespaces our processes would find the information ambiguous when identical names have been chosen by the designers of the vocabularies we use Stylesheet Association We declare our choice of an associated stylesheet for an XML instance by embedding the construct described in eS m XPath andy XPointer O REILLY the Stylesheet Associa
7. 4 Transformation from XML to XSL Formatting Semantics Without employing extension techniques or supplemental documentation the stylesheets used in this scenario contain only the transformation vocabulary and the resulting formatting vocabulary There are no other element types from other vocabularies in the result including from the source vocabulary For example rendering processors would not inherently know what to do with an element of type custnbr representing a customer number it is the stylesheet writer s responsibility to transform the information into information recognized by the rendering agent There is no obligation for the rendering processor to serialize the result tree created during transformation The feature of serializing the result tree to XML syntax is however quite useful as a diagnostic tool revealing to us what we really asked to be rendered instead of what we thought we were asking to be rendered when we saw incorrect results There may also be performance considerations of taking the reified result tree in XML syntax and rendering it in other media without incurring the overhead of performing the transformation repeatedly 1 2 3 Transformation from XML to non XML An XSLT processor may choose to recognize the stylesheet writer s desire to serialize a non XML representation of the result tree transformation transformation script script source result source result document document document document F
8. both logically markup and physically entities such as files or other resources they are not the only means by which relationships are specified For example a quantum of information can arbitrarily point or refer to other information elsewhere through use of unique identifiers Two basic objectives of representing information hierarchically are satisfied by the XML Recommendation It provides e an unambiguous mechanism for constraining structure in a stream of information XML defines the concept of well formedness Well formedness dictates the syntax used for markup languages within the content of an instance of information This is the syntax of using angle brackets lt and gt and the ampersand 4 to demarcate and identify constituent components of information within a file a resource or a bound data stream Users of the Hypertext Markup Language HTML will recognize the use of these characters for marking the vocabulary described by the designers of the World Wide Web in their web documents e a language for specifying how a system can constrain the allowed logical hierarchy of information structures XML defines the concept of validity with a syntax for a meta markup language used to specify vocabularies A Document Type Definition DTD describes the structural schema mandating the user defined constraints on well formed information The designers of HTML have formalized their vocabulary through such a DTD thus declaring the allow
9. changes to each product for lines 25 through 28 A local scope variable is bound with the focus position in the tree Each column is then calculated using a built in function as the sum of all lt prodsale gt elements that reference the column being totaled The XPath designers having provided the sum function in the language keep the stylesheet writer from having to implement complex counting and summing code rather the writer merely declares the need for the summed value to be added to the result on demand by using the appropriate XPath expression The file prod exp xs1 is an explicit XSLT stylesheet with a number of result templates for handling source information 01 lt xml version 1 0 gt lt prod exp xsl gt 02 lt XSLT 1 0 http www CraneSoftwrights com training gt 03 lt xsl stylesheet xmlns xsl http www w3 org 1999 XSL Transform 04 version 1 0 gt 05 06 lt xsl template match gt lt root rule gt 07 lt html gt lt head gt lt title gt Record of Sales lt title gt lt head gt 08 lt body gt lt h2 gt Record of Sales lt h2 gt 09 lt xsl apply templates select sales record gt LO lt body gt lt html gt lt xsl template gt AD 2 lt xsl template match record gt lt processing for each record gt 3 lt ul gt lt xsl apply templates gt lt ul gt lt xsl template gt 4 L5 lt xsl template match prodsale gt lt processing for each sale gt 6 lt li gt lt xsl v
10. expression sales record cust The current focus moves to the lt cust gt element for the processing on lines 15 through 21 A local scope variable is bound on line 15 with the tree location of the current focus note how this instruction uses the same XPath expression as on line 11 but with a different result A table row is started on line 16 with the leftmost column calculated from the num attribute of the lt cust gt element being processed The stylesheet then builds in lines 17 through 20 a column for each of the same columns created for the table header on line 10 The focus moves to each product in turn for the processing of lines 18 through 20 Each column s value is then calculated with the expression Scustomer prodsale idref current id which could be expressed as follows from the customer location bound to the variable Scustomer from all of the lt prodsale gt children of that customer find that child whose idref attribute is the value of the id attribute of the focus element When there is no such child the column value is empty and processing continues As many columns are produced for a body row as for the header row and our output becomes perfectly aligned Finally lines 23 through 28 build the bottom row of the table with the totals calculated for each product After the boilerplate leftmost column line 24 uses the same product expression as on lines 10 and 17 to generate the same number of table columns The focus
11. instance Therefore we do not have to model the presence of this construct when we design our document model Any instance can have any number of stylesheet associations added into the document during or after creation or even removed without impacting on the XML content itself An application respecting this construct will process the document content with the stylesheet before delivering the content to the application logic Two cases of this are the use of a stylesheet for rendering to a browser canvas and the use of a transformation script at the front end of an e commerce application The following two examples illustrate stylesheet associations that respectively reference an XSL resource and a Cascading Stylesheet CSS resource 01 lt xml stylesheet href fancy xsl type text xsl gt Example I 13 Associating an XSL stylesheet 01 lt xml stylesheet href normal css type text css gt Example 1 14 Associating a CSS stylesheet The following example naming the association for later reference and indicating that it is not the primary stylesheet resource is less typical but is allowed for in the specification 01 lt xml stylesheet alternate yes title small 02 href small xsl type text xs1 gt Example I 15 Alternative stylesheet association A URL that does not include a reference to another resource but rather is defined exclusively by a local named reference specifies a style
12. markup into an arbitrary delivery oriented markup This delivery markup in turn is translated using XSLT on the user agent for consumption by the operator This approach can reduce bandwidth utilization and increase distributed processing without sacrificing privacy Note 14 There is no consensus in our XML community that semantic firewalls are a good thing Peers of mine preach that the World Wide Web must always be a semantic web with rich markup processed in a distributed fashion among user agents being de rigueur Personally I do not subscribe to this point of view We have the flexibility to weigh the technical and business perspectives of our customers needs for our information our own infrastructure and processing capabilities and our own commercial and privacy concerns We can choose to dumb down our information for consumption and the installed base of user agents supporting presentation oriented semantic less HTML can be the perfect delivery vehicle to protect these concerns of ours This is a prose version of an excerpt from the book Practical Transformation Using XSLT and XPath Eighth Edition ISBN 1 894049 05 5 at the time of this writing published by Crane Softwrights Ltd written by G Ken Holman this excerpt was edited by Stan Swaren and reviewed by Dave Pawson 2 Getting started with XSLT and XPath Examining working stylesheets can help us understand how we use XSLT and XPath to perform transformations Th
13. of 27 select sum prodsale idref pid gt lt i gt 28 lt td gt lt xsl for each gt lt tr gt 29 lt table gt 30 lt body gt lt html gt Example 2 7 Tabular presentation of the sample product sales source information Recall that a stylesheet is oriented according to the desired result producing the result in result parse order The entire document is an HTML file whose document element begins on line 3 and ends on line 30 The XSLT namespace and version declarations are included in the document element The naming of the document element as html triggers the default use of HTML result tree serialization conventions Lines 5 and 6 are fixed boilerplate information for the mandatory lt title gt element Lines 7 through 29 build the result table from the content A single header row lt th gt is generated in lines 9 through 12 with the columns of that row generated by traversing all of the lt product gt elements of the source The focus moves on line 11 to each lt product gt source element in turn and the markup associated with the traversal builds each lt td gt result element The content of each column is specified as which for an element evaluates to the string value of that element Having completed the table header the table body rows are then built one at a time traversing each lt cust gt child of a lt record gt child of the lt sales gt child of the root of the document according to the XPath
14. of the document 2 2 2 Implicitly declared stylesheets The simplest kind of XSLT stylesheet is an XML file implicitly representing the entire outcome of transformation The result vocabulary is arbitrary and the stylesheet tree forms the template used by the XSLT processor to build the result tree If no XSLT or extension instructions are found therein the stylesheet tree becomes the result tree If instructions are present the processor replaces the instructions with the outcomes of their execution lt html xmlns xsl http www w3 org 1999 XSL Transform xsliversion 1 0 gt Version of XSLT required lt head gt lt title gt Greeting lt title gt lt head gt by the Stylesheet lt body gt lt p gt Words of greeting lt br gt Result Tree lt b gt lt i gt lt u gt lt xslivalue of select greeting gt lt u gt lt i gt lt b gt Template lt p gt lt hoays lt html gt Literal Result Elements XML Declaration XSLT Namespace Declaration lt xml version 1 0 gt Figure 2 5 Components of an Implicit Stylesheet The XML declaration is consumed by the XML processor embedded within the XSLT processor thus the XSLT processor never sees it The remainder of the file is considered the result tree template for an implicit rule for the root of the document describing the shape of the entire outcome of the transformation The document element is named htm1 and contains the namespace and version declarations of
15. processor to do its job does not minimize the importance of source validation when developing a stylesheet Often when working incrementally to develop a stylesheet by simultaneously working on the test source file and stylesheet algorithm time can be lost by inadvertently introducing well formed but invalid source content Because there is no validation in the XSLT processor all well formed source will be processed without errors producing a result based on the data found The first reaction of the stylesheet writer is often that a problem has been introduced in the stylesheet logic when in fact the stylesheet works fine for the intended source data The real problem is that the source data being used isn t as intended Note 6 Personally I run a separate post process source file validation after running the source file through a given stylesheet While I am examining the results of stylesheet processing the post process determines whether or not the well formed file validates against the model to which I m designing the stylesheet When anomalies are seen I can check the validation for the possible source of a problem before diagnosing the stylesheet itself 1 1 5 6 Multiple source files possible The first source file fed to the XSLT processor defines the first abstract tree of nodes the stylesheet uses The stylesheet may access arbitrary other source files or even itself as a source file to supplement the information found in the primar
16. purc pdf 7 File Edit Document Tools Plugins View Windov wi From Customer Reference cust123 Figure 1 2 XSL rendering for example The above again illustrates the two distinctive styling steps transforming the instance of the XML vocabulary into a new instance according to a vocabulary of rendering semantics and formatting the instance of the rendering vocabulary in the user agent The rendering semantics of much of the XSL vocabulary are device independent so we can use one set of constructs regardless of the rendering medium It is the rendering agent s responsibility to interpret these constructs accordingly In this way the XSL semantics can be interpreted for print display aural or other presentations There are indeed some specialized semantics we can use to influence rendering on particular media though these are just icing on the cake 1 1 5 Extensible Stylesheet Language Transformations XSLT We all have needs to transform our structured information when it is not appropriately ordered for a purpose other than how it is created The XSLT 1 0 Recommendation describes a transformation instruction vocabulary of constructs that can be expressed in an XML model of elements and attributes http www w3 org TR xslt 1 1 5 1 Transformation by example We can characterize XSLT from other techniques for transmuting our information by regarding it simply as Transformation by Example differentiating many other
17. techniques as Transformation by Program Logic This perspective focuses on the distinction that our obligation is not to tell an XSLT processor how to effect the changes we need rather we tell an XSLT processor what we want as an end result and it is the processor s responsibility to do the dirty work The XSLT Recommendation gives us a vocabulary for specifying templates that function as examples of the result Based on how we instruct the XSLT processor to access the source of the data being transformed the processor will incrementally build the result by adding the filled in templates We write our stylesheets or transformation specifications primarily with declarative constructs though we can employ procedural techniques if and when needed We assert the desired behavior of the XSLT processor based on conditions found in our source We supply examples of how each component of our result is formulated and indicate the conditions of the source that trigger which component is next added to our result Alternatively we can selectively add components to the result on demand Consider once again the customer information in our example purchase order at Example 1 1 An example of the HTML vocabulary supplied to the XSLT processor to produce the markup in Example 1 7 would be 01 lt xsl template match customer gt 02 lt p gt lt xsl itext gt From lt xsl text gt 03 lt i gt lt xsl text gt Customer Reference lt xsl text gt 04 l
18. the responsibility of the XSLT processor and not the stylesheet writer In all diagrams the left pointing triangle represents a hierarchically marked up document such as an XML or HTML document This convention stems from considering the apex of the hierarchy at the left with the sub elements nesting within each other towards the lowest leaves of the hierarchy at the right of the triangle Processes are depicted in rectangles while arbitrary data files of some binary or text form are depicted in parallelograms Other symbols representing screen display print and auditory output are drawn with hopefully obvious shapes 1 2 2 Transformation from XML to XSL formatting semantics When the result tree is specified to utilize the XSL formatting vocabulary the normative behavior of an XSL processor incorporating an XSLT processor is to interpret the result tree This interpretation reifies the semantics expressed in the constructs of the result tree to some medium be it pixels on a screen dots on paper sound through a synthesis device or another medium that makes sense for presentation transformation script Aural Process result tree XSLT Process Print Process i 1 i 1 1 i 1 1 1 1 i i x Display source Process document XSL Formatting and optional Flow Object Semantics serialized _ Interpretation result tree in Each Domain Figure 1
19. the XSLT language Any element type within the result tree template that is qualified by the prefix assigned to the XSLT namespace URI is recognized as an XSLT instruction No extension instruction namespaces are declared thus all other element types in the instance are literal result elements Indeed the document element is a literal result element as it too is not an instruction 2 2 3 Stylesheet requirements Every XSLT stylesheet must identify the namespace prefix used therein for XSLT instructions The default namespace cannot be used for this purpose The namespace URI associated with the prefix must be the value http www w3 org 1999 XSL Transform It is a common practice to use the prefix xs1 to identify the XSLT vocabulary though this is only convention and any valid prefix can be used XSLT processor extensions are outside the scope of the XSLT vocabulary so other URI values must be used to identify extensions The stylesheet must also declare the version of XSLT required by the instructions used therein The attribute is named version and must accompany the namespace declaration in the wrapper element instruction as version version number In an implicit stylesheet where the XSLT namespace is declared in an element that is not an XSLT instruction the namespace qualified attribute declaration must be used as prefix version version number The version number is a numeric floating point value representing the latest version of X
20. the value start Those identified are the question elements whose answer attribute is equal to the string equal to the lower case letter y The value returned is the set of nodes representing the elements meeting the conditions expressed by the address The address is used in a select attribute thus the XSLT processor is selecting all of the addressed elements for some kind of processing 1 1 2 3 XPath is not a query language It is important to remember that addressing information is only one aspect of querying information Other aspects include query operators that massage intermediate results into a final result While a few operators and functions are available in XSLT to use values identified in documents these are oriented to string processing not to complex operations required by some applications Note 3 When query Recommendations are developed I would hope that the addressing portion is based on XPath as a core just as with XSLT This is a prose version of an excerpt from the book Practical Transformation Using XSLT and XPath Eighth Edition ISBN 1 894049 05 5 at the time of this writing published by Crane Softwrights Ltd written by G Ken Holman this excerpt was edited by Stan Swaren and reviewed by Dave Pawson 1 The Context of XSL Transformations and the XML Path Language cont d 1 1 3 Styling structured information 1 1 3 1 Styling is transforming and formatting information Styling is the rendering of infor
21. to the result hierarchy as nodes not as syntax Our objective as XSLT transformation writers is to create a result node tree that may or may not be serialized externally as markup syntax The XSLT processor is not obliged to externalize the result tree if the processor is integral to some process interpreting the result tree for other purposes For example an XSL rendering agent may embed an XSLT processor for interpreting the inputs to produce the intermediate hierarchy of XSL rendering vocabulary to be reified in a given medium In such cases serializing the intermediate tree in syntax is not material to the process of rendering though having the option to serialize the hierarchy is a useful diagnostic tool The stylesheet writer has little or no control over the constructs chosen by the XSLT processor for serializing the result tree There are some behaviors the stylesheet can request of the processor though the processor is not obliged to respect the requests The stylesheet can request a particular output method be used for the serialization and if supported the processor guarantees the final result complies with the lexical requirements of that method Note 7 It is possible to coerce the XSLT processor to violate the lexical rules through certain stylesheet controls that I personally avoid using at all costs For every XML and HTML instance construct not including the document model syntax constructs there are proper XSLT methodologies
22. to use an XSLT processor as a front end to any application that can be modified to access the result tree The intermediate result tree of CVML is not serialized externally rather it is fed directly to the application and the application interprets the internal representation of the content that would have been serialized to a custom format Time is saved by not serializing the result tree and having the application parse the reified file back into a memory representation performance is enhanced by the application directly accessing the result of transformation When generalized a vendor s non XML based application can use this approach to accommodate arbitrary customers XML vocabularies merely by writing W3C conforming XSLT stylesheets as the interpretation specification Some XSLT processors can build a DOM representation of result tree or deliver the result tree as Simple API for XML SAX events thus giving an application developer standardized interfaces to the transformed information expressed using the application s custom semantics vocabulary The developer s programming is then complete and the vendor accommodates each customer vocabulary with an appropriate stylesheet for translation to the application semantics 1 2 4 Three tiered architectures A three tiered architecture can meet technical and business objectives by delivering structured information to web browsers by using XSLT on the host or on the user agent or even on both
23. A text contained therein This example instruction is executed in the context of the root of the source document being the focus The child of the root of the document is the document element The expression requests the value of the child named greeting of the root of the document hence the value of the document element named greeting For any source document where greet ing is not the document element the value returned is the empty string For any source document where it is the document element as is our example the value returned is the concatenation of all PCDATA text in the entire instance A literal result element is any element in a stylesheet that is not a top level element and is not either an XSLT instruction or an extension instruction A literal result element can use the default namespace or any namespace not declared in the stylesheet to be an instruction namespace When the XSLT processor reads the stylesheet and creates the abstract nodes in the stylesheet tree those nodes that are literal result elements represent the nodes that are added to the result tree Though the definition of those nodes is dictated by the XML syntax in the stylesheet entity the syntax used does not necessarily represent the syntax that is serialized from the result tree nodes created from the stylesheet nodes Literal result elements marked up in the stylesheet entity may have attributes that are targeted for the XML processor used by the XSLT pro
24. Any transformation requirement that includes original markup syntax preservation would not be suited for XSLT transformations Note 5 Is not being able to support original markup syntax preservation really a problem That depends how you regard the original markup syntax used in an XML instance XML allows you to use various markup techniques to meet identical information representation requirements If you treat this as merely syntactic sugar for human involvement in the markup process then it will not be important how information is specifically marked up once it is out of the hands of the human involved If however you are working with transformations where such issues are more than just a sugar coating and it is necessary to utilize particular constructs based on particular requirements of how the result looks in syntactic form then XSLT will not provide the kind of control you will need 1 1 5 3 Document model and vocabulary independent While checking source documents for validity can be very useful for diagnostic purposes all of the hierarchical relationships of content are based on what is found inside of the instance not what is found in the document model The behavior of the stylesheet is specified against the presence of markup in an instance as the implicit model not against the allowed markup prescribed by any explicit model Because of this an XSLT stylesheet is independent of any Document Type Definition DTD or other ex
25. Considering technical issues first the server can distribute the processing load to XML XSLT aware user agents by delivering a combination of the stylesheet and the source information to be transformed on the recipient s platform Alternatively the server can perform the transformations centrally to accommodate those user agents supporting only HTML or HTML CSS vocabularies transformation script XML Browser ll a source Process document HTML Browser XSLT Process transformation script Figure 1 9 Server side Transformation Architecture There may be good business reasons to selectively deliver richly marked up XML to the user agent or to arbitrarily transform XML to HTML on the server regardless of the user agent capabilities Even if it is technically possible to send semantically rich information in XML protecting your intellectual property by hiding the richness behind the security of a semantic firewall must be considered Perhaps there are revenue opportunities by only delivering a richly marked up rendition of your information to your customers Perhaps you could even scale the richness to differing levels of utility for customers who value your information with different granularity or specificity while preserving the most detailed normative version of the data away of view Lastly there are no restrictions to using two XSLT processes one on the server to translate our organization s rich
26. ISBN 1 894049 05 5 at the time of this writing published by Crane Softwrights Ltd written by G Ken Holman this excerpt was edited by Stan Swaren and reviewed by Dave Pawson XML com Copyright 1998 2006 O Reilly Media Inc
27. Published on XML com http www xml com pub a 2000 08 holman index htm See this if you re having trouble printing code examples What Is XSLT By G Ken Holman August 16 2000 Introduction Now that we are successfully using XML to mark up our information according to our own vocabularies we are taking control and responsibility for our information instead of abdicating such control to product vendors These vendors would rather lock our information into their proprietary schemes to keep us beholden to their solutions and technology But the flexibility inherent in the power given to each of us to develop our own vocabularies and for industry associations e commerce consortia and the W3C to develop their own vocabularies presents the need to be able to transform information marked up in XML from one vocabulary to another Two W3C Recommendations XSLT the Extensible Stylesheet Language Transformations and XPath the XML Path Language meet that need They provide a powerful implementation of a tree oriented transformation language for transmuting instances of XML using one vocabulary into either simple text the legacy HTML vocabulary or XML instances using any other vocabulary imaginable We use the XSLT language which itself uses XPath to specify how an implementation of an XSLT processor is to create our desired output from our given marked up input XSLT enables and empowers interoperability This XML com introduction strives to o
28. SLT defining the instructions used in the stylesheet It need not declare the most capable version supported by the XSLT processor 2 2 4 Instructions and literal result elements XSLT instructions are only detected in the stylesheet tree and are not detected in the source tree Instructions are specified using the namespace prefix associated with the XSLT namespace URI The XSLT Recommendation describes the behavior of the XSLT processor for each of the instructions defined based on the instruction s element type name Top level instructions are considered and or executed by the XSLT processor before processing begins on the source information For better performance reasons a processor may choose to not consider a top level instruction until there is need within the stylesheet to use it All other instructions are found somewhere in a result tree template and are not executed until that point at which the processor is asked to add the instruction to the result tree Instructions themselves are never added to the result tree Some XSLT instructions are control constructs used by the processor to manage our stylesheets The wrapper and top level elements declare our globally scoped constructs Procedural and process control constructs give us the ability to selectively add only portions of templates to the result rather than always adding an entire template Logically oriented constructs give us facilities to share the use of values and declarations wit
29. alue of select num gt lt use parent s attr gt 7 lt xsl text gt lt xsl text gt 18 lt xsl value of select id idref gt lt go indirect gt 19 lt xsl text gt lt xsl text gt 20 lt xsl value of select gt lt li gt lt xsl template gt 21 22 lt xsl stylesheet gt Example 2 8 List oriented presentation of the sample product sales source information A The document element on line 3 includes the requisite declarations of the language namespace and the version being used in the stylesheet The children of the document element are the template rules describing the source tree event handlers for the transformation Each event handler associates a template with an event trigger described by an XPath expression Lines 6 through 10 describe the template rule for processing the root of the document as indicated by the trigger in the mat ch attribute on line 6 The result document element and boilerplate is added to the result tree on lines 7 and 8 Line 9 instructs the XSLT processor in lt xs1 apply templates gt to visit all lt record gt element children of the lt sales gt document element as specified in the select attribute For each location visited the processor pushes that location through the stylesheet thus triggering the template of result markup it can match for each location Lines 12 and 13 describe the result markup when matching a lt record gt element The focus moves to the lt r
30. apply to the result the templates associated with the nodes that are visited The lt xsl apply templates select XPath node expression gt instruction visits the source tree nodes described by the node expression in the select attribute The writer can choose any relative absolute or arbitrary location or locations to be visited Each node visited is pushed through the stylesheet to be caught by template rules Template rules specify the template to be processed and added to the result tree The template added is dictated by the template rule matched for the node being pushed not by a template supplied by the instruction when a node is being pulled This distinguishes the behavior as being a data driven approach to creating the result in that the source determines the ultimate order of the result An implicitly declared stylesheet can only push information through built in template rules which is of limited value As well the built in rules can be mimicked entirely by using pull constructs thus they need never be used There is no room in the stylesheet to declare template rules in an implicitly declared stylesheet since there is no wrapper stylesheet instruction An explicitly declared stylesheet can either push or pull information because there is room in the stylesheet to define the top level elements including any number of template rules required for the transformation Putting it all together We are not obliged to use only one approa
31. ased or in development Specifically we will be looking at only the following as they relate to XSLT and XPath Extensible Markup Language XML For years applications and vendors have imposed their constraints on the way we can represent our information Our data has been created maintained stored and archived according to the rules enforced by others The advent of the Extensible Markup Language XML moves the control of our information out of the hands of others and into our own by providing two basic facilities XML describes rules for structuring our information using embedded markup of our own choice We can take control of our information representation by creating and using a vocabulary we design of elements and attributes that makes sense for the way we do our business and use our data In addition XML describes a language for formally declaring the vocabularies we use This allows our tools to constrain the creation of an instance of our information and allows our users to validate a properly created instance of information against our set of constraints Note 1 An XML document is just an instance of well formed XML The two terms document and instance could be used interchangeably but this reference material uses the term instance to help readers remember that XML isn t just for documents or documentation With XML we describe a related set of information in a tree like hierarchical fashion and gain the benefits of having done so
32. bes a vocabulary recognized by an XSLT processor to transform information from an organization in the source file into a different organization suitable for continued downstream processing The Extensible Stylesheet Language XSL Working Draft describes a vocabulary recognized by a rendering agent to reify abstract expressions of format into a particular medium of presentation Both XSLT and XSL are endorsed by members of WSSSL an association of researchers and developers passionate about the application of markup technologies in today s information technology infrastructure 1 1 4 Extensible Stylesheet Language XSL When we need to present our structured information in a given medium or different media we all have common needs for how the result appears and way the result flows through that appearance The XSL Working Draft describes the current work developing a vocabulary of formatting and flow semantics that can be expressed using an XML model of elements and attributes http www w3 org TR WD xsl 1 1 4 1 Formatting and flow semantics vocabulary This hierarchical vocabulary captures formatting semantics for rendering textual and graphic information in different media A rendering agent is responsible for interpreting an instance of the vocabulary for a given medium to reify a final result This is no different in concept and architecture than using HTML and Cascading Stylesheets CSS as a hierarchical vocabulary for rendering a se
33. cessor targeted for the XSLT processor or targeted for use in the result tree Some attributes are consumed and acted upon as the stylesheet file is processed to build the stylesheet tree while the others remain in the stylesheet tree for later use Those literal result attributes remaining in the stylesheet tree that are qualified with an instruction namespace are acted on when they are asked to be added to the result tree 2 2 5 Templates and template rules Many XSLT instructions are container elements The collection of literal result elements and other instructions being contained therein comprises the XSLT template for that instruction A template can contain only literal result elements only instruction elements or a mixture of both The behavior of the stylesheet can ask that a template be added to the result tree at which point the nodes for literal result elements are added and the nodes for instructions are executed Consider again the simple example in our stylesheets used earlier in this chapter where the following template is used 01 lt b gt lt i gt lt u gt lt xsl value of select greeting gt lt u gt lt i gt lt b gt Example 2 10 Simple template in Example 2 4 This template contains a mixture of literal result elements and an instruction element When the XSLT processor adds this template to the result tree the nodes for the lt b gt lt i gt and lt u gt elements are simply added to the tree while t
34. ch when we write our stylesheets It is very appropriate to push where the order is dictated by the source information and to pull when responding to a push where the order is known by the stylesheet The most common use of this combination in a template is localized pull access to values that are relative to the focus being matched by nodes being pushed Note that push oriented stylesheets more easily accommodate changes to the data and are more easily exploited by others who wish to reuse the stylesheets we write The more granularity we have in our template rules the more flexibly our stylesheets can respond to changes in the order of data The more we pull data from our source tree the more dependent we are on how we have coded the access to the information The more we push data through our stylesheet the less that changes in our data impact our stylesheet code Look again at the examples discussed earlier in this article and analyze the use of the above pull and push constructs to meet the objectives of the transformations These introductions and samples in this article have set the context and only scratch the surface of the power of XSLT to effect the transformations we need when working with our structured information XML com has continuing coverage and tutorials about XPath and XSLT in its regular column Transforming XML This is a prose version of an excerpt from the book Practical Transformation Using XSLT and XPath Eighth Edition
35. common but misdirected practice to model information based on how you plan to use it downstream It does not matter if your target is a presentation oriented structure for example or a structure that is appropriate for another markup based system Modeling practice should focus on both the business reasons and inherent relationships existing in the semantics behind the information being described as such the vocabularies are then content oriented For example emphasized text is often confused with a particular format in which it is rendered Where we could model information using a lt b gt element type for eventual rendering in a bold face we would be better off modeling the information using an lt emph gt element type In this way we capture the reason for marking up information that it is emphasized from surrounding information and we do not lock the downstream targets into only using a bold face for rendering Many times the midstream or downstream processes need only rearrange re label or synthesize the information for a target purpose and never apply any semantics of style for rendering purposes Transformation tasks stand alone in such cases meeting the processing needs without introducing rendering issues One caveat regarding modeling content oriented information is that there are applications where the content orientation is indeed presentation oriented Consider book publishing where the abstract content is based on presentational se
36. ct greeting gt lt u gt lt i gt lt b gt 03 lt xsl template gt Example 2 11 Simple template rule in Example 2 4 This template rule defines the template to be added to the result tree when the root of the document is visited This written rule overrides the built in rule implicitly available in the XSLT processor The template is the same template we were discussing earlier a set of result tree nodes and an instruction The XSLT processor begins processing by visiting the root of the document This gives control to the stylesheet writer Either the supplied template tule or built in template rule for the root of the document is processed based on what the writer has declared in the stylesheet The writer is in complete control at this early stage and all XSLT processor behavior is dictated what the writer asks to be calculated and where the writer asks the XSLT processor to visit 2 2 6 Approaches to stylesheet design The last discussion in this two chapter introduction regards how to approach using templates and instructions when writing a stylesheet Two distinct approaches can be characterized Choosing which approach to use when depends on your own preferences the nature of the source information and the nature of the desired result Note I refer to these two approaches as either stylesheet driven or data driven though the former might be misconstrued Of course all results are stylesheet driven because the styl
37. e XSLT processor interpreting the stylesheet A conforming processor may or may not support such extensions and is only obliged to accommodate error and fallback processing in such a way that a stylesheet writer can reconcile the behavior if needed An XSLT processor can implement extension instructions functions serialization conventions and sorting schemes that provide functionality beyond what is defined in XSLT 1 0 all accessed through standardized facilities A stylesheet writer must not rely on any extension facilities if the XSLT processor being used for the stylesheet is not known or is outside of the stylesheet writer s control If an end user base utilizes different brands of XSLT processors and the stylesheet needs to be portable across all processors only the standardized facilities can be used Standardized presence testing and fallback facilities can be used by the stylesheet writer to accommodate the ability of a processor to act on extension facilities used in the stylesheet 1 1 5 9 Abstract structure result In the same way our stylesheets are insulated from the syntax of our source files our stylesheets are insulated from the syntax of our result We do not focus on the syntax of the file to be produced by the XSLT processor rather we create a result tree of abstract nodes which is similar to the tree of abstract nodes of our input information Our examples of transformation converted to nodes from our stylesheet are added
38. e product description information and have unique identifiers according to the ID 1DREF rules Lines 16 through 23 summarize customer purchases product sales each entry referring to the product having been sold by use of the idref attribute Not all customers have been sold all products Consider the following two renderings of the same data using two orientations each produced with different stylesheets vix Netscape Product Sales Summary Miel E3 Netscape Record of Sales iof x File Edit View Go Bookmarks Options Directory File Edit View Go Bookmarks Options Window Help Directory Window Help Location WEARTEN Product Sales Summary Record of Sales e C1001 Packing Tape 200 Bores Tepe e C1002 Packing Tape 50 C1001 100 200 e C1003 Packing Boxes 75 C1002 50 e C1003 Packing Tape 15 C1003 T5 15 Totals 175 265 Ti Document Done BP Z sats Document E BM Z Figure 2 3 Different HTML results from the same XML source Note how the same information is projected into a table orientation on the left canvas and a list orientation on the right canvas The one authored order is delivered in two different presentation orders Both results include titles from boilerplate text not found in the source The table information on the left includes calculations of the sums of quantities in the columns generated by the stylesheet and not present explicitly in the source T
39. e two very dissimilar renderings 01 lt xml version 1 0 gt lt prod xml gt 02 lt DOCTYPE sales 03 lt ELEMENT sales products record gt lt sales information gt 04 lt ELEMENT products product gt lt product record gt 05 lt ELEMENT product PCDATA gt lt product information gt 06 lt ATTLIST product id ID REQUIRED gt 07 lt ELEMENT record cust gt lt sales record gt 08 lt ELEMENT cust prodsale gt lt customer sales record gt 09 lt ATTLIST cust num CDATA REQUIRED gt lt customer number gt 0O lt ELEMENT prodsale PCDATA gt lt product sale record gt 1 lt ATTLIST prodsale idref IDREF REQUIRED gt 2 J gt 3 lt sales gt 14 lt products gt lt product id p1 gt Packing Boxes lt product gt 5 lt product id p2 gt Packing Tape lt product gt lt products gt 6 lt record gt lt cust num C1001 gt 7 lt prodsale idref p1 gt 100 lt prodsale gt 8 lt prodsale idref p2 gt 200 lt prodsale gt lt cust gt t9 lt cust num C1002 gt 20 lt prodsale idref p2 gt 50 lt prodsale gt lt cust gt 21 lt cust num C1003 gt 22 lt prodsale idref p1 gt 75 lt prodsale gt 23 lt prodsale idref p2 gt 15 lt prodsale gt lt cust gt lt record gt 24 lt sales gt Example 2 6 Sample product sales source information Lines 2 through 11 describe the document model for the sales information Lines 14 and 15 summariz
40. ecord gt element being visited The template rule on line 13 adds the markup for the HTML unordered list lt u1 gt element to the result tree The content of the list is created by instructing the processor to visit all children of the focus location implicitly by not specifying any select attribute and apply the templates of result markup it triggers for each child The only children of lt record gt are lt cust gt elements The stylesheet does not provide any template rule for the lt cust gt element so built in template rules automatically process the children of each location being visited in turn Implicitly then our source information is being traversed in the depth first order visiting the locations in parse order and pushing each location through any template rules that are then found in the stylesheet The children of the lt cust gt elements are lt prodsale gt elements The stylesheet does provide a template rule in lines 15 through 20 to handle a lt prodsale gt element when it is pushed so the XSLT processor adds the markup triggered by that rule to the result The focus changes when the template rule handles it thus lines 16 18 and 20 each pull information relative to the lt prodsale gt element respectively the parent s num attribute the lt cust gt element s attribute the string value of the target element being pointed to by the lt prodsale gt element s idref attribute indirectly obtaining the lt product gt e
41. ed or expected relationships between components of a hypertext document There is an implicit document model for an instance of well formed XML defined by the mere presence of nested elements found in the information There is no need to declare this model because the syntax rules governing well formedness guarantee the information to be seen properly as a hierarchy As with all hierarchies there are family tree like relationships of parent child and sibling constructs relative to each construct found Consider the following well formed XML instance purc xml 01 lt xml version 1 0 gt 02 lt purchase id p001 gt 03 lt customer db cust123 gt 04 lt product db prod345 gt 05 lt amount gt 23 45 lt amount gt 06 lt product gt 07 lt purchase gt Example 1 1 A well formed XML purchase order instance Observe the content nesting whitespace has been added only for illustrative purposes The instance follows the lexical rules for XML markup and the hierarchical model is implicit by the nesting of elements Pay particular attention to the markup on line 3 for the empty element named customer with the attribute named ap It will be used later in examples throughout this chapter The customer element is a child of the document element which is named purchase Although the presence of an explicit formal document model is useful to an XML processor or to a system working with XML instances that model has no impac
42. esheet dictates what to do so the use of the term involves some nuance By stylesheet driven I mean that the order of the result is a result of the stylesheet tree having explicitly instructed the adding of information to the result tree By data driven I mean that the order of the result is a result of the source tree ordering having dictated the adding of information to the result tree 2 2 6 1 Pulling the input data When the stylesheet writer knows the location of and order of data found in the source tree and the writer wants to add to the result a value from or collection of that data then information can be pulled from the source tree on demand Two instructions are provided for this purpose one for obtaining or calculating a single string value to add to the result and one for adding rich markup to the result based on obtaining as many values as may exist in the tree The writer uses the lt xsl value of select XPath expression gt instruction in a stylesheet s element content to calculate a single value to be added to the result tree The instruction is always empty and therefore does not contain a template This value calculated can be the result of function execution the value of a variable or the value of a node selected from the source tree When used in the template of various XSLT instructions the outcome becomes part of the value of a result element attribute comment or processing instruction Note there is also a shorthand
43. g the constructs available While there will always be a trade off between extending the processor to implement something internally and writing an elaborate stylesheet to implement something portably there is sufficient expressive power to implement some algorithmic business rules and semantic processing in the XSLT syntax In short straightforward and common requirements can be satisfied in a straightforward fashion while unconventional requirements can be satisfied to an extent as well with some programming styled effort Note 4 Theory aside the necessarily verbose XSLT syntax dictated by its declarative nature and use of XML syntax makes the coding of some complex algorithms a bit awkward I have implemented some very complex traversals and content generation with successful results but with code that could be difficult to maintain my own valiant if not always satisfactory documentation practices notwithstanding The designers of XSLT recognized the need to maintain large transformation specifications and the desire to tap prior accomplishments when writing stylesheets so they have included a number of constructs supporting the management maintenance and exploitation of existing stylesheets Organizations can build libraries of stylesheet components for sharing among their colleagues Stylesheet writers can tweak the results of a transformation by writing shell specifications that include or import other stylesheets known to solve proble
44. h the XSLT namespace URI This wrapper element is the document element in a standalone stylesheet but may in other cases be embedded inside an XML document XML Declaration XSLT Namespace Declaration lt xml version 1 0 7 gt lt xsli transform xmlns xsl http wuw w3 org XSL Transform Document Element Top level version 1 0 gt Elements z wersion of XSLT x required by the lt xsl output method xml indent yes gt Stylesheet Result Tree Template lt b gt lt i gt lt u gt lt xslivalue of select greeting gt lt u gt lt i gt lt b gt lt xsl template gt Literal Result Elements Iera lt xsl transform gt a l template match gt Template Rule Figure 2 4 Components of an Explicit Stylesheet The XML declaration is consumed by the XML processor embedded within the XSLT processor thus the XSLT processor never sees it The wrapper element must include the XSLT namespace and version declarations for the element to be recognized as an instruction The children of the wrapper element are the top level elements comprised of global constructs serialization information and certain maintenance instructions Template rules supply the stylesheet behavior for matching source tree conditions The content of a template rule is a result tree template containing both literal result elements and XSLT instructions The example above has only a single template rule that being for the root
45. he implicit stylesheet prod imp xs1 is an XHTML file utilizing the XSLT vocabulary for instructions to fill in the one result template by pulling data from the source 01 lt xml version 1 0 gt lt prod imp xsl gt 02 lt XSLT 1 0 http www CraneSoftwrights com training gt 03 lt html xmilns xsl http www w3 org 1999 XSL Transform 04 xsl version 1 0 gt 05 lt head gt lt title gt Product Sales Summary lt title gt lt head gt 06 lt body gt lt h2 gt Product Sales Summary lt h2 gt 07 lt table summary Product Sales Summary border 1 gt 08 lt list products gt 09 lt th align center gt 10 lt xsl for each select product gt T lt td gt lt b gt lt xsl value of select gt lt b gt lt td gt 2 lt xsl for each gt lt th gt 3 lt list customers gt 4 lt xsl for each select sales record cust gt 5 lt xsl variable name customer select gt 6 lt tr align right gt lt td gt lt xsl value of select num gt lt td gt 7 lt xsl for each select product gt lt each product gt 8 lt td gt lt xsl value of select customer prodsale 19 idref current id gt 20 lt td gt lt xsl for each gt Zi lt tr gt lt xsl for each gt 22 lt summarize gt 23 lt tr align right gt lt td gt lt b gt Totals lt b gt lt td gt 24 lt xsl for each select product gt 25 lt xsl variable name pid select id gt 26 lt td gt lt i gt lt xsl value
46. he node for the xs1 value of instruction triggers the processor to add the outcome of instruction execution to the tree A template rule is a declaration to the XSLT processor of a template to be added to the result tree when certain conditions are met by source locations visited by the processor Template rules are either top level elements explicitly written in the stylesheet or built in templates assumed by the processor and implicitly available in all stylesheets The criteria for adding a written template rule s template to the result tree are specified in a number of attributes one of which must be the mat ch attribute This attribute is an XPath pattern expression which is a subset of XPath expressions in general The pattern expression describes preconditions of source tree nodes The stylesheet writer is responsible for writing the preconditions and other attribute values in such a way as to unambiguously provide a single written or built in template for each of the anticipated source tree conditions In an implicitly declared stylesheet the entire file is considered the template for the template rule for the root of the document This template rule overrides the built in rule implicitly available in the XSLT processor Back to the simple example in our explicitly declared stylesheet used earlier in this chapter the following template rule is declared 01 lt xsl template match gt 02 lt b gt lt i gt lt u gt lt xsl value of sele
47. hin our own stylesheet files Physically oriented constructs give us the power to share entire stylesheet fragments Other XSLT instructions are result tree value placeholders We declare how a value is calculated by the processor or obtained from a source tree or both calculated by the processor from a value from a source tree The value calculation is triggered when the XSLT processor is about to add the instruction to the result tree The outcome of the calculation which may be nothing is added to the result tree All other instructions engage customized non standard behaviors and are specified using extension elements in a standardized fashion These elements use namespace prefixes declared by our stylesheets to be instruction prefixes Extension instructions may be either control constructs or result tree value placeholders Consider the simple example in our stylesheets used earlier in this chapter where the following instruction is used 01 lt xsl value of select greeting gt Example 2 9 Simple value calculation instruction in Example 2 4 This instruction uses the select attribute to specify the XPath expression of some value to be calculated and added to the result tree When the expression is a location in the source tree as is this example the value returned is the value of the first location identified using the criteria When that location is an element the value returned is the concatenation of all of the PCDAT
48. icult to use and would only delay the problem to the point that these weakened naming conventions would still eventually result in vocabulary collisions Note 9 Enter the dreaded namespaces a Recommendation undeserving of its sullied reputation This is a powerful yet very simple technique for disambiguating element type names in vocabularies Perhaps the reputation spread from those unfamiliar with the requirements being satisfied Perhaps concerns were spread by those who made assumptions about the values used in namespace declarations As unjustified as it is evoking namespaces unnecessarily and unfortunately strikes fear in many people It is my goal to help the reader understand that not only are namespaces easy to define and easy to use but that they are easy to understand and are not nearly as complex as others have believed The Namespaces in XML Recommendation describes a technique for exploiting the established uniqueness of Uniform Resource Identifier URI values under the purview of the Internet Engineering Task Force IETF We users of the Internet accept the authority of the registrar of Internet domain names to allot unique values to organizations and it is in our best interest to not arrogate or usurp values allotted to others as our own We can therefore assume a published URI value belongs to the owner of the domain used as the basis of the value The value is not a Uniform Resource Locator URL which is a URI that identif
49. ies an actual addressed location on the Internet rather the URI is being used merely as a unique string value To set the stage for how these URI values are used consider an example of two vocabularies that could easily be used together in an XML instance the Scalable Vector Graphics SVG vocabulary and the Mathematical Markup Language MathML In SVG the lt set gt element type is used to scope a value for reference by descendent elements In MathML the lt set gt element type defines a set in the mathematical sense of a collection Remembering that names in XML follow rigid lexical constraints we pick out of thin air a prefix we use to distinguish each element type from their respective vocabulary The prefix we choose is not mandated by any organization or any authority in our instances we get to choose any prefix we wish We should however make the prefix meaningful or we will obfuscate our information so let s choose in this example to distinguish the two element types as lt svg set gt and lt math set gt Note that making the prefix short is a common convention supporting human legibility and using the won colon separating the prefix from the rest of the name is prescribed by the Namespaces in XML recommendation While we are talking about names let s not forget that some Recommendations utilize the XML name lexical construct for other purposes such as naming facilities that may be available to a processor We get to use thi
50. igure 1 5 Transformation from XML to Aware Non XML The XSLT Recommendation documents two non XML tree serialization methods that can be requested by the stylesheet writer When the processor offers serialization it is only obliged to reify the result using XML lexical and syntax rules and may support producing output following either HTML lexical and syntax rules or simple text 1 2 3 1 HTML lexical and syntactic conventions Internet web browsers are specific examples of the generic HTML user agent User agents are typically designed for instances of HTML following the precursor to XML the Standard Generalized Markup Language SGML lexical conventions Certain aspects of the HTML document model also dictate syntactic shortcuts available when working with SGML While some more recently developed user agents will accept XML lexical conventions thus accepting Extensible Hypertext Markup Language XHTML output from an XSLT processor older user agents will not Some of these user agents will not accept XML lexical conventions for empty elements while some require SGML syntax minimization techniques to compress certain attribute specifications Additionally user agents recognize a number of general entity references as built in characters supporting accented letters the non breaking space and other characters from public entity sets defined or used by the designers of HTML An XSLT processor recognizes the use of these characters in the re
51. information and any other piece of helpful information deemed suitable for the public consumption This convention should help clear up many misperceptions about the URI being used to obtain some kind of machine readable resource or schema though it will not dispel the misperception that there needs to be some resource of some kind at the URL represented by a namespace URI So now a processor can unambiguously distinguish an element s type as being from a particular vocabulary by knowing the URI associated with the vocabulary Our choice of prefix is arbitrary and of no relevance The URI we have associated with the prefix used in a namespace qualified XML name often called a QName informs the processor of the identity of the name Our choice of prefix is used and then discarded by the processor while the URI persists and is the basis of namespace aware processing We have achieved uniqueness and identity in our element type names and other XML names in a succinct legible fashion without violating the lexical naming rules of XML 1 1 6 3 Namespaces in XSL and XSLT Namespaces identify different constructs for the processors interpreting XSL formatting specifications and XSLT stylesheets An XSL rendering agent responsible for interpreting an XSL formatting specification will recognize those constructs identified with the http www w3 org 1999 XSL Format namespace Note that the year value used in this URI value is not used as a version indic
52. is article first dissects some example stylesheets before introducing basic terminology and design principles 2 1 Stylesheet examples Let s first look at some example stylesheets using two implementations of XSLT 1 0 and XPath 1 0 the XT processor from James Clark and the third web release of Internet Explorer 5 s MSXML Technology Preview These two processors were chosen merely as examples of respectively standalone and browser based XSLT XPath implementations without prejudice to other conforming implementations The code samples only use syntax conforming to XSLT 1 0 and XPath 1 0 recommendations and will work with any conformant XSLT processor Note The current 4 14 2000 Internet Explorer 5 production release supports only an archaic experimental dialect of XSLT based on an early working draft of the recommendation The examples in this book will not run on the production release of IES The production implementation of the old dialect is described in http msdn microsoft com xml XSLGuide conformance asp 2 1 1 Some simple examples Consider the following XML file he11o xm1 obtained from the XML 1 0 Recommendation and modified to declare an associated stylesheet 01 lt xml version 1 0 gt 02 lt xml stylesheet type text xsl href hello xsl gt 03 lt greeting gt Hello world lt greeting gt Example 2 1 The first sample instance in XML 1 0 modified We
53. lement s value and the value of the lt prodsale gt element itself This is a prose version of an excerpt from the book Practical Transformation Using XSLT and XPath Eighth Edition ISBN 1 894049 05 5 at the time of this writing published by Crane Softwrights Ltd written by G Ken Holman this excerpt was edited by Stan Swaren and reviewed by Dave Pawson 2 Getting started with XSLT and XPath cont d 2 2 Syntax basics Stylesheets Templates Instructions Next we ll look at some basic terminology both helpful in understanding the principles of writing an XSLT stylesheet and recognizing the constructs used therein This section is not meant as tutelage for writing stylesheets but only as background information nomenclature and practice guidelines 4 Note I use two pairs of diametric terms not used as such in the XSLT Recommendation itself explicit implicit stylesheets and push pull design approaches Students of my instructor led courses have found these distinctions helpful even though they are not official terms Though these terms are documented here with apparent official status such status is not meant to be conferred 2 2 1 Explicitly declared stylesheets An explicitly declared XSLT stylesheet is comprised of a distinct wrapper element containing the stylesheet specification This wrapper element must be an XSLT instruction named either stylesheet or transform thus it must be qualified by the prefix associated wit
54. lesheet already represent examples of the nodes that we want added to the result tree but we must ensure these examples are triggered to be added to the result tree in result parse order otherwise we will not get the desired result We can peruse and traverse our source files in any predictable order we need to produce the result but we can only produce the result tree once and then only in result tree parse order It is often difficult to change traditional perspectives of transformation that focus on the source tree yet we must look at XSLT transformations focused on the result tree The predictable orders we traverse the source trees are not restricted to only source tree parse order also called document order Information in the source trees can be ignored or selectively processed The order of the result tree dictates the order in which we must access our source trees Note 8 I personally found this required orientation difficult to internalize having been focused on the creation of my source information long before addressing issues of transforming the sources to different results Understanding this orientation is key to quickly producing results using XSLT It is not however an XSLT processor implementation constraint to serially produce the result tree This is an important distinction in the language design that supports parallelism An XSLT processor supporting parallelism can simultaneously produce portions of the result tree pro
55. line 6 that the db attribute is indeed required if the attribute is absent the XML processor can report syntactic model constraint violation even if the element is otherwise lexically well formed The document model can also provide additional information not evident without a document model such as the information on line 4 that the ia attribute for purchase is of XML type Ip 1 1 1 2 No built in meanings or concepts The area of semantics associated with XML instances is very gray A document model is but one component used to help describe the semantics of the information found in an instance While well formed instances do not have a formal document model often the names of the constructs used within the instances give hints to the associated semantics Without a formalism yet available in our community to express semantics in a rigorous fashion we users of XML do or should capture the semantics of a given vocabulary in prose whether or not the document model is formalized The XML 1 0 Recommendation only describes the behavior required of an XML processor acting on an XML stream and how it must identify constituent data and provide that data to an application using the processor Since there are no formalized semantic description facilities in XML any XML that is used is not tied to any one particular concept or application There are no rendition or transformation rules or constructs defined in XML The only purpose of XML is to unambiguo
56. mantics This is meaningful because there is no abstraction beyond the appearance or presentation of the content Consider the customer information in Example 1 1 A web user agent doesn t know how to render an element named lt customer gt The HTML vocabulary used to render the customer information could be as follows 01 lt p gt From lt i gt Customer Reference lt b gt cust123 lt b gt lt i gt 02 lt p gt Example 1 7 HTML rendering semantics markup for example The rendering result would then be as follows with the rendering user agent interpreting the markup for italics and boldface presentation semantics big Netscape Purchase p001 File Edit View Go Bookmarks Options Directory Wi From Customer Reference cust 23 Figure 1 1 HTML rendering for example The above illustrates these two distinct styling steps transforming the instance of the XML vocabulary into a new instance according to a vocabulary of rendering semantics and formatting the instance of the rendering vocabulary in the user agent 1 1 3 2 Two W3C Recommendations In order to meet these two distinct processes in a detached yet related fashion the W3C Working Group responsible for the Extensible Stylesheet Language XSL split the original drafts of their work into two separate Recommendations one for transforming information and the other for rendering information The XSL Transformations XSLT 1 0 Recommendation descri
57. mation into a form suitable for consumption by a target audience Because the audience can change for a given set of information we often need to apply different styling for that information in order to obtain dissimilar renderings in order to meet the needs of each audience Perhaps some information needs to be rearranged to make more sense for the reader Perhaps some information needs to be highlighted differently to bring focus to key content It is important when we think about styling information to remember that two distinct processes are involved not just one First we must transform the information from the organization used when it was created into the organization needed for consumption Second when rendering we must express whatever the target medium the aspects of the appearance of the reorganized information Consider the flow of information as a streaming process where information is created upstream and processed or consumed downstream Upstream in the early stages we should be expressing the information abstractly thus preventing any early binding of concrete or final form concepts Midstream or even downstream we can exploit the information as long as it remains flexible and abstract Late binding of the information to a final form can be based on the target use of the final product by delaying this binding until late in the process we preserve the original information for exploitation for other purposes along the way It is a
58. ms they are addressing Stylesheet fragments can be written for particular vocabulary fragments these fragments can subsequently be used in concert as part of an organization s strategy for common information description in numerous markup models 1 1 5 2 Not intended for general purpose XML transformations It is important to remember that XSLT was designed primarily for transforming XML vocabularies to the XSL formatting vocabulary This doesn t preclude us from using XSLT for other transformation requirements but it does influence the design of the language and it does constrain some of the functionality from being truly general purpose For this reason the designers do not claim XSLT is a general purpose transformation language However it is still powerful enough for most downstream processing transformation needs and XSLT stylesheets are often called XSLT transformation scripts because they can be used in many areas not at all related to stylesheet rendering Consider an electronic commerce environment where transformation is not used for presentation purposes In this case the XSLT processor may transform a source instance which is based on a particular vocabulary and deliver the results to a legacy application that expects a different vocabulary as input In other words we can use XSLT in a non rendering situation when it doesn t matter what syntax is utilized to represent the content when only the parsed result of the syntax is material
59. nd not a constraint 1 1 5 4 XML source and stylesheet The input files to an XSLT processor are one or more stylesheet files and one or more source files The initial inputs are a single stylesheet file and a single source file Other stylesheet files are assimilated before the first source file is processed The XML processor will then access other source files according to the first file s XML content The XSLT processor may then access other source files at any time under stylesheet control All of the inputs must be well formed but not necessarily valid XML documents This precludes using an HTML file following non XML lexical conventions but does not rule out processing an Extensible Hypertext Markup Language XHTML file as an input Many users of existing HTML files that are not XML compliant will need to manipulate or transform them all that is needed to use XSLT for this is a preprocess to convert existing Standard Generalized Markup Language SGML markup conventions into XML markup conventions XHTML can be created from HTML using a handy free tool on the W3C site http www w3 org People Raggett tidy This tool corrects whatever improperly coded HTML it can and flags any that it cannot correct When the output is configured to follow XML lexical conventions the resulting file can be used as an input to the XSLT processor 1 1 5 5 Validation unnecessary but convenient That an XSLT processor need not incorporate a validating XML
60. notation called an attribute value template that allows the equivalent to lt xs1 value of gt to be used in a stylesheet s attribute content To iterate over locations in the source tree the lt xsl for each select XPath node set expression gt instruction defines a template to be processed for each instance possibly repeated of the selected locations This template can contain literal result elements or any instruction to be executed When processing the given template the focus of the processor s view of the source tree shifts to the location being visited thus providing for relative addressing while moving through the information These instructions give the writer control over the order of information in the result The data is being pulled from the source on demand and added to the result tree in the stylesheet determined order When collections of nodes are iterated the nodes are visited in document order This implements a stylesheet driven approach to creating the result An implicitly declared stylesheet is obliged to use only these pull instructions and must dictate the order of the result with the above instructions in the lone template 2 2 6 2 Pushing the input data The stylesheet writer may not know the order of the data found in the source tree or may want to have the source tree dictate the ordering of content of the result tree In these situations the writer instructs the XSLT processor to visit source tree nodes and to
61. nsider our example as it might be internally represented by an application e lt http www w3 org 2000 svg 20000629 set gt e lt http www w3 org 1998 Math MathML set gt The specification of a URI instead of a URL means that the namespace aware processor will never look at the URI as a URL to accomplish its work There never need be any resource available at the URI used in a namespace declaration The URI is just a string and its value is used only as a string and the fact that there may or may not be any resource at the URL identified by the URI is immaterial to namespace processing The URI does not identify the location of a schema or a DTD or any file whatsoever when used by a namespace aware processor Note 10 Perhaps some of the confusion regarding namespaces is rooted in the overloading of the namespace URI by some Recommendations These Recommendations require that the URI represent a URL where a particular resource is located fetched and utilized to some purpose This behavior is outside the scope of namespaces and is mandated solely by the Recommendations that require it Practice has however indicated an end user friendly convention regarding the URI used in namespace declarations The W3C has placed a documentation file at every URL represented by a namespace URI Requesting the resource at the URL returns an HTML document discussing the namespace being referenced perhaps a few pointer documents to specifications or user help
62. ntify and iterate over structures found in the source information The information being transformed can be traversed in any order needed and as many times as required to produce the desired result We can visit source information numerous times if the result of transformation requires that information to be present numerous times We users of XSLT don t have the burden of implementing numerous practical algorithms required to present information The designers of XSLT have specified that such algorithms be implemented within the processor itself and have enabled us to engage these algorithms declaratively High level functions such as sorting and counting are available to us on demand when we need them Low level functions such as memory management node manipulation and garbage collection are all integral to the XSLT processor This declarative nature of the stylesheet markup makes XSLT so very much more accessible to non programmers than the imperative nature of procedurally oriented transformation languages Writing a stylesheet is as simple as using markup to declare the behavior of the XSLT processor much like HTML is used to declare the behavior of the web browser to paint information on the screen The designers have also accommodated the programmer as well as the non programmer in that there are procedural constructs specified XSLT is in theory Turing complete thus any arbitrarily complex algorithm could theoretically be implemented usin
63. of the resulting format We must not be thinking of our source XML vocabularies rather our focus is entirely on the semantics of what exactly makes up our target custom format Let s refer to this custom format s XML vocabulary we divine from our analysis as the Custom Vocabulary Markup Language CVML Using our programming resources we can then write a single transformation application responsible for interpreting XML instances of CVML to produce a file following the custom format This transformation application could be written using the Document Object Model DOM as a basis for tree oriented access to the information Alternatively a SAX based application can interpret the instances to produce the outputs if the nature of CVML lends itself to that orientation The key is that regardless of how instances of CVML are created the interpretation of CVML markup to produce an output file never changes Our one CVML Instance Interpreter application can produce any custom format output file expressible in the CVML semantics Getting back to our own or others XML vocabularies we have now reduced the problem to XML instance transformation Our objective is simplified to produce XML instances of CVML from instances of our many input XML vocabularies This is a classical XSLT situation and we need only write XSLT stylesheets combining the XSLT instructions with CVML as the result vocabulary Our investment in XSLT for our colleagues is leveraged by the CVML In
64. oints in the tree having unique identifiers We convey XPath addresses in a simple and compact non XML syntax This allows us to use an XPath expression as the value of an attribute in an XML vocabulary as in the following examples 01 select answer Example 1 3 A simple XPath expression in a select attribute The above attribute value expresses all children named answer of the current focus element 01 match question answer Example 1 4 An XPath expression in a mat ch attribute The above attribute value expresses a test of an element being in the union of the element types named question and answer The XPath syntax looks a lot like addressing subdirectories in a file system or as part of a Universal Resource Identifier URI Multiple steps in a location path are separated by either one or two oblique characters Filters can be specified to further refine the nature of the components of our information being addressed 01 select question 3 answer 1 Example 1 5 A multiple step XPath expression in a select attribute The above example selects only the first answer child of the third question child of the focus element 01 select id start question answer y Example 1 6 A more complex XPath expression in a select attribute The above example uses an XPath address identifying some descendants of the element in the instance that has the unique identifier with
65. ons with our XML data Not using XSLT to produce custom output Consider first the scenario without using XSLT where we must write individual XML aware applications to accommodate our information vocabularies For each of our vocabularies we need separate programs to convert to the common custom format required by the application This incurs programming resources to accommodate any and every change to our vocabularies in order to meet the new inputs to satisfy the same custom output XML Vocabulary 1 Conversion XML Vocabulary n Conversion source program 1 resulting source program n resulting document file document file vocabulary 1 vocabulary n Figure 1 6 Accommodating multiple inputs with different XML vocabularies Using XSLT to produce custom output If however we focus on the custom output instead of focusing on our vocabulary inputs we can leverage a single investment in programming across all of our vocabularies Moreover by being independent of the vocabulary used in the source we can accommodate any of our or others vocabularies we may have to deal with in the future The approach involves us creating our own custom markup language based on a critical analysis of the target custom format to distill the semantics of how information is represented in the resulting file These semantics can be expressed using an XML vocabulary whose elements and attributes engage the features and functions
66. plicit schema that may have been used to constrain the instance at other stages This is very handy when working with well formed XML that doesn t have an explicit document model If an explicit document model is supplied certain information such as attribute types and defaulted values enhance the processor s knowledge of the information found in the input documents Without this information the processor can still perform stylesheet processing as long as the absence of the information does not influence the desired results Without a reliance on the document model for the instance we can design a single stylesheet that can process instances of different models When the models are very similar much of the stylesheet operates the same way each time and the rest of the stylesheet only processes that which it finds in the sources It may be obvious but should be stated for completeness that a given source file can be processed with multiple stylesheets for different purposes This means though that it is possible to successfully process a source file with a stylesheet designed for an entirely different vocabulary The results will probably be totally inappropriate but there is nothing inherent to an instance that ties it to a single stylesheet or a set of stylesheets Stylesheet designers might well consider how their stylesheets could validate input perhaps issuing error messages when unexpected content arrives However this is a matter of practice a
67. rosoft Internet Explorer Of x File Edt View Favorites Tool Help Address 2 x samp hella xm x ooo E Hello worid Sma a ae hello xml Notepad 00 04 11 07 36 el E3 File Edit Search Help lt xml version 1 6 gt lt xml stylesheet type text xsl href hello xsl gt lt greeting gt Hello world lt greeting gt E Done Figure 2 2 An XML aware browser viewing the source of a document The canvas content matches what the non XML browser rendered in Figure 2 1 Note that View Source displays the raw XML source and not the transformed XHTML result of applying the stylesheet Note I found it very awkward when first using browser based stylesheets to diagnose problems in my stylesheets Without access to the intermediate results of transformation it is often impossible to ascertain the nature of the faulty HTML generation One of the free resources found on the Crane Softwrights Ltd web site is a script for standalone command line invocation of the MSXML XSLT processor This script is useful for diagnosing problems by revealing the result of transformation This script has also been used extensively by some to create static HTML snapshots of their XML for delivery to non XML aware browsers This is a prose version of an excerpt from the book Practical Transformation Using XSLT and XPath Eighth Edition ISBN 1 894049 05 5 at the time of this writing published by Crane Softwrights Ltd
68. s namespace prefix we ve chosen on these names to guarantee uniqueness just as we have done on the names used to indicate element types 1 1 6 2 URI value association But having the prefix is not enough because we haven t yet guaranteed global identity or singularity by a short string of name characters to do so we must associate the prefix with a globally unique URI before we use that prefix Note that we are unable to use a URI directly as a prefix because the lexical constraints on a URI are looser than those of an XML name the invalid XML name characters in a URI would cause an XML processor to balk We assert the association between a namespace prefix and a namespace URI by using a namespace declaration attribute as in the following examples e xmins svg http www w3 org 2000 svg 20000629 xmins math http www w3 org 1998 Math MathML As noted earlier the prefix we choose is arbitrary and can be any lexically valid XML name The prefix is discarded by the namespace aware processor and is immaterial to the application using the names it is only a syntactic shortcut to get at the associated URI The associated URI supplants the prefix in the internal representation of the name value and the application can distinguish the names by the new composite name that would have been illegal in XML syntax There is no convention for documenting a namespace qualified name using its associated URI but one way to perceive the uniqueness is to co
69. sheet resource that is located inside the XML document being processed as in the following example 01 lt xml stylesheet href stylel type text xs1l gt Example I 16 Associating an internal stylesheet The Recommendation designers expect additional schemes for linking stylesheets and other processing scripts to XML documents to be defined in future specifications Note 11 Embedding stylesheet association information in an XML document and using the XML processing instruction to do so are both considered stopgap measures by the W3C This Recommendation cautions readers that no precedents are set by employing these makeshift techniques and that urgency dictated their choice Indeed there is some question as to the appropriateness of tying processing to data so tightly and we will see what considered approaches become available to us in the future This is a prose version of an excerpt from the book Practical Transformation Using XSLT and XPath Eighth Edition ISBN 1 894049 05 5 at the time of this writing published by Crane Softwrights Ltd written by G Ken Holman this excerpt was edited by Stan Swaren and reviewed by Dave Pawson 1 The Context of XSL Transformations and the XML Path Language cont d 1 2 Transformation data flows Here we look at the interactions between some of the Recommendations we focus on by examining how our information flows through processes engaging or supporting the technologies
70. stance Interpreter so that they can now take their XML and use stylesheets to produce the binary or custom lexical format transformation script Custom Vocabukry Semantics Interpretation CVML Interpreter XSLT Process source result document document result file Figure 1 7 Transformation from XML to an arbitrary format This approach separates the awareness of the lexical and syntactic requirements of the custom output format from the numerous stylesheets we write for all of our possible input XML vocabularies Our colleagues use XSLT just as they would with HTML or XSL as a result vocabulary They leverage the single investment in producing the custom format by using the CVML Interpreter to serialize the results of their transformations to produce the files designed for other applications This in turn leverages the investment in learning and using XSLT in the organization Taking this two steps further First the X in XSLT represents the word extensible and result tree serialization is one of the areas where we can extend an XSLT processor s functionality This allows us to implement non standard vendor specific or application specific output serialization methods and engage these facilities in a standard manner As with all extension mechanisms in XSLT the trigger is the use of an XML namespace recognized by the XSLT processor implementing the extension 01 xmins prefix processor
71. sult tree and serializes them using the assumed built in general entities 1 2 3 2 Text lexical conventions An XSLT processor can be asked to serialize only the PcbatTa content of the entire result tree resulting in a file of simple text without employing any markup techniques All text is represented by the characters individual values even those characters sensitive to XML interpretation I use the text method often for synthesizing MSDOS batch files By walking through my XML source I generate commands to act on Note 13 resources identified therein thus producing an executable batch file tailored to the information 1 2 3 3 Arbitrary binary and custom lexical conventions Many of our legacy systems or existing applications expect information to follow custom lexical conventions according to arbitrary rules Often this format is raw binary not following textual lexical patterns We are usually obliged to write custom programs and transformation applications to convert our XML information to these non standardized formats due to their binary or non structured natures XSLT can play a role even here where the target format is neither structured nor text nor in any format anticipated by the designers of the Recommendation We do have a responsibility to fill in a critical piece of the formula described below but we can leverage this single effort in the equation to allow us and our colleagues to continue to use W3C Recommendati
72. t b gt lt xsl value of select db gt lt b gt lt i gt lt p gt 05 lt xsl template gt Example 1 9 Example XSLT template rule for the HTML vocabulary An example of XSL vocabulary supplied to the XSLT processor to produce the markup in Example 1 8 would be 01 lt xsl template match customer gt 02 lt fo block space before optimum 20pt font size 20pt gt 03 lt xsl text gt From lt xsl text gt 04 lt fo inline sequence font style italic gt OS lt xsl text gt Customer Reference lt xsl text gt 06 lt fo inline sequence font weight bold gt 07 lt xsl value of select db gt 08 lt fo inline sequence gt lt fo inline sequence gt lt fo block gt 09 lt xsl template gt Example 1 10 Example XSLT template rule for the XSL vocabulary Where XSLT is similar to other transmutation approaches is that we deal with our information as trees of abstract nodes We don t deal with the raw syntax of our source data Unlike these other approaches however the primary memory management and information manipulation node traversal and node creation is handled by the XSLT processor not by the stylesheet writer This is a significant difference between XSLT and a transformation programming language or interface like the Document Object Model DOM where the programmer is responsible for handling the low level manipulation of information constructs XSLT includes constructs which we use to ide
73. t of information in a web browser In essence we are transforming our XML documents into their final display form by transforming instances of our XML vocabularies into instances of a particular rendering vocabulary This Working Draft normatively references XSLT as an integral component of XSL A stylesheet could be written with both the transformation vocabulary and the formatting semantics vocabulary together it would style an XML instance by rendering the results of transformation This result need not be serialized in XML syntax rather an XSLT XSL processor can utilize the result of transformation to create a rendered result by interpreting the abstract hierarchy of information without seeing syntax 1 1 4 2 Target of transformation When using a formatting semantics vocabulary as the rendering language the objective for a stylesheet writer is to convert an XML instance of some arbitrary XML vocabulary into an instance of the formatting semantics vocabulary The result of transformation cannot contain any user defined vocabulary construct for example an address customer identifier or purchase order number construct because the rendering agent would not know what to do with constructs labeled with these foreign unknown identifiers Consider two examples HTML for rendering in a web browser and XSL for rendering on screen on paper or audibly In both cases the rendering agents only understand the vocabulary expressing their respective forma
74. t on the implicit structural model and only minor influence on the interpretation of content found in the instance This point holds true whether the model is expressed in a DTD or in some of the other Recommendations for structural and content schemata being developed Consider the following valid XML instance purcdtd xml 01 lt xml version 1 0 gt 02 lt DOCTYPE purchase 03 lt ELEMENT purchase customer product gt 04 lt ATTLIST purchase id ID REQUIRED gt 05 lt ELEMENT customer EMPTY gt 06 lt ATTLIST customer db CDATA REQUIRED gt 07 lt ELEMENT product amount gt 08 lt ATTLIST product db CDATA REQUIRED gt 09 lt ELEMENT amount PCDATA gt 10 J gt 11 lt purchase id p001 gt 12 lt customer db cust123 gt 13 lt product db prod345 gt 14 lt amount gt 23 45 lt amount gt 15 lt product gt 16 lt purchase gt Example 1 2 A valid XML purchase order instance See how the information content is no different from the previous example but in this case an explicit document model using XML 1 0 DTD syntax is included it could have been included by reference to a separate resource A processor can validate that the information content conforms not only to the lexical rules for XML well formedness but also the syntax rules dictated by the supplied document model validity Looking at the same customer element as before now on line 12 the document model indicates on
75. the closing delimiter This corresponds to line 6 of our stylesheet in Example 2 2 where this element is marked up as lt br gt according to XML rules Our inputs are always XML but the XSLT processor may recognize the output as being HTML and serialize the result following SGML rules Consider next the following explicitly declared XSLT file hello xs1 to produce XML output using the HTML vocabulary thus the output is serialized as XHTML 01 lt xml version 1 0 gt lt hello xsl gt 02 lt XSLT 1 0 http www CraneSoftwrights com training gt 03 04 lt xsl transform xmlns xsl http www w3 org 1999 XSL Transform 05 version 1 0 gt 06 07 lt xsl output method xml omit xml declaration yes gt 08 09 lt xsl template match gt 10 lt b gt lt i gt lt u gt lt xsl ivalue of select greeting gt lt u gt lt i gt lt b gt 11 lt xsl template gt 12 13 lt xsl transform gt Example 2 4 An explicitly declared simple stylesheet This file explicitly declares the document element of an XSLT stylesheet with the requisite XSLT namespace and version declarations Line 7 declares the output to follow XML lexical conventions and that the XML declaration is to be omitted from the serialized result Lines 9 through 11 declare the content of the result that is added when the source information position matches the XPath expression in the mat ch attribute on line 9 The value of matches the root of the doc
76. tion Recommendation Recipients and applications can choose to respect or ignore this choice but the declaration indicates that we have tied some process typically rendering to our data which specifies how to consume or work with our information XPath and XPointer Locating Content in XML Documents By John E Simpson 1 1 The XML family of Recommendations Now let s look at the objectives of these selected Recommendations 1 1 1 Extensible Markup Language XML Historically the ways we have expressed created stored and transmitted our electronic information have been constrained and controlled by the vendors we choose and the applications we run Alternatively we now can express our data in a structured fashion oriented around our perspective of the nature of the information itself rather than the nature of an application s choice of how to represent our information With Extensible Markup Language XML we describe our information using embedded markup of elements attributes and other constructs in a tree like structure http www w3 org TR REC xml 1 1 1 1 Structuring information Contrasted to a file format where information identification relies on some proprietary hidden format predetermined ordering or some kind of explicit labeling the tree like hierarchical storage structure infers relationships by the scope of values encompassing the scopes of other values Though trees shape a number of areas of XML
77. to follow though not always as compact as coercing the processor The abstract nature of the node trees representing the input source and stylesheet instances and the hands off nature of serializing the abstract result node tree are the primary reasons that source tree original markup syntax preservation cannot be supported The design of the language does however support the serialization of the result tree in such a way as not to require the XSLT processor to maintain the result tree in the abstract form For example the processor can instantly serialize the start of an element as soon as the element content of the result is defined There is no need to maintain nor is there any ability in the stylesheet to add to the start of an element once the stylesheet begins supplying element content The XSLT 1 0 Recommendation defines three output methods for lexically reifying the abstract result tree as serialized syntax XML conventions HTML conventions and simple text conventions An XSLT processor can be extended to support custom serialization methods for specialized needs 1 1 5 10 Result tree oriented objective This result abstraction impacts how we design our stylesheets We have to always remember that the result of transformation is created in result parse order thus allowing the XSLT processor to immediately serialize the result without maintaining the result for later purposes The examples of transformation that we include in our sty
78. tor rather the W3C convention for assigning namespace URI values incorporates the year the value was assigned to the working group An XSLT processor responsible for interpreting an XSLT stylesheet recognizes instructions and named system properties using the http www w3 org 1999 XSL Transform namespace An XSLT processor will not recognize using an archaic value for working draft specifications of XSLT XSLT use namespace qualified names to identify extensions that implement non standardized facilities A number of kinds of extensions can be defined in XSLT including functions instructions serialization methods sort methods and system properties The XT XSLT processor written by James Clark is an example of a processor implementing extension facilities XT uses the http www jclark com xt namespace to identify the extension constructs it implements Remembering that this is a URI and not a URL you will not find any kind of resource or file when using this value as a URL We also use our own namespaces in an XSLT stylesheet for two other purposes We need to specify the namespaces of the elements and attributes of our result if the process interpreting the result relies on the vocabulary to be identified Furthermore our own non default namespaces distinguish internal XSLT objects we include in our stylesheets Each of these will be detailed later where such constructs are described 1 1 7 Stylesheet association When we wish to associate
79. tting semantics and wouldn t know what to do with alien element types defined by the user Just as with HTML a stylesheet writer utilizing XSL for rendering must transform each and every user construct into a rendering construct to direct the rendering agent to produce the desired result By learning and understanding the semantics behind the constructs of XSL formatting the stylesheet writer can create an instance of the formatting vocabulary expressing the desired layout of the final result e g area geometry spacing font metrics etc with each piece of information in the result coming from either the source data or the stylesheet itself Consider once more the customer information in Example 1 1 An XSL rendering agent doesn t know how to render a marked up construct named lt customer gt The XSL vocabulary used to render the customer information could be as follows 01 lt fo block space before optimum 20pt font size 20pt gt From 02 lt fo inline sequence font style italic gt Customer Reference 03 lt fo inline sequence font weight bold gt cust123 lt fo inline sequence gt 04 lt fo inline sequence gt 05 lt fo block gt Example 1 8 XSL rendering semantics markup for example The rendering result when using the Portable Document Format PDF would then be as follows with an intermediate PDF generation step interpreting the XSL markup for italics and boldface presentation semantics Adobe Acrobat
80. tured our information using XML how are we going to talk about address what is inside our documents Locating information in an XML document is critical to both transforming it and to associating or relating it to other information When we write stylesheets and use linking languages we can address components of our information for a processor by our use of the XML Path Language also called XPath http www w3 org TR xpath 1 1 2 1 Addressing structured information The W3C working group responsible for stylesheets collaborated with the W3C working group responsible for the next generation of hyperlinking to produce XPath as a common base for addressing requirements shared by their respective Recommendations Both groups extend the core XPath facilities to meet the needs they have in each of their domains the stylesheet group uses XPath as the core of expressions in XSLT the linking group uses XPath as the core of expressions in the XPointer Recommendation In order to address components you have to know the addressing scheme with which the components are arranged The basis of addressing XML documents is an abstract data model of interlinked nodes arranged hierarchically echoing the tree shape of the nested elements in an instance Nodes of different types make up this hierarchy each node representing the parsed result of a syntactic structure found in the bytes of the XML instance This abstraction insulates addressing from the multiple s
81. ument hence this refers to the XSLT processor s initial focus The result we specify on line 10 wraps our source information in the HTML elements without the boilerplate used in the previous example Line 13 ends the formal specification of the stylesheet content Using an MS DOS command line invocation to execute the XT processor we see the following result 01 X samp gt xt hello xml hello xsl hello htm 02 03 X samp gt type hello htm 04 lt b gt lt i gt lt u gt Hello world lt u gt lt i gt lt b gt 05 X samp gt Example 2 5 Explicit invocation of Example 2 4 Using a non XML aware browser to view the resulting HTML in Example 2 5 we see the following on the canvas the child window is opened using the View Source menu item yix Netscape file 7x 7C samp hello htm File Edit View Go Bookmarks Options Directory Window Help Location ile 2 7C samp hello htm X Hello world win Netscape Source of file x 7C samp hello htm E3 lt b gt lt i gt lt u gt Hello world lt u gt lt i gt lt b gt 3 34 Document Done Figure 2 1 An non XML aware browser viewing the source of a document Using an XML aware browser recognizing the W3C stylesheet association processing instruction in Example 2 1 the canvas is painted with the HTML resulting from application of the stylesheet the child window is opened using the View Source menu item J X samp hello_xml Mic
82. usly identify and deliver constituent components of data There are no inherent meanings or semantics of any kind associated with element types defined in a document model There are no defined controls for implying any rendering semantics Even the xml space attribute allowing for the differentiation of whitespace found in a document is not an aspect of rendering but of information description The author or modeler of an instance is indicating with this reserved attribute termed special in XML 1 0 the nature of the information and how the whitespace found in the information is to be either preserved or handled by a processor in a default fashion Some new users of XML who have a background in a markup language such as HTML often assume a magical association of semantics with element types of the same names they have been exposed to in their prior work In a web page they can safely assume that the construct lt p gt will be interpreted as a paragraph or lt em gt as emphasized text However this interpretation is solely the purview of the designers of HTML and user agents attempting to conform to the World Wide Web Consortium W3C published semantics Nothing is imposed by any process when creating a new XML vocabulary that happens to use the same names Applications using XML processors to access XML information must be instructed how to interpret and implement the desired semantics 1 1 2 XML Path Language XPath Assuming that we have struc
83. verview essential aspects of understanding the context in which these languages help us meet our transformation requirements and to introduce substantive concepts and terminology to bolster the information available in the W3C Recommendation documents themselves Since April 1999 Crane Softwrights Ltd has published commercial training material titled Practical Transformation Using XSLT and XPath covering the entire scope of the W3C XSLT and XPath through working drafts and the final 1 0 recommendations This material is delivered by Crane in instructor led sessions and is licensed to other training organizations around the world needing to teach these exciting technologies Crane has rewritten the first two chapters of this material into prose These prose oriented chapters are published on XML com correspondingly as two main sections The material assumes no prior knowledge of XSLT and XPath and guides the reader through background context structure concepts and introductory terminology Table of Contents Related Reading Learning f Learning XSLT By Michael Fitzgerald 1 The context of XSL Transformations and the XML Path Language e1 1 The XML family of Recommendations 1 1 1 Extensible Markup Language XML 1 1 2 XML Path Language XPath 1 1 3 Styling structured information 1 1 4 Extensible Stylesheet Language XSL 1 1 5 Extensible StylesheetLanguage Transformations XSLT 1 1 6 Namespaces
84. vided only that the end result is created as if it were produced serially 1 1 6 Namespaces To successfully use and distinguish element types in our instances as being from given vocabularies the Namespaces in XML Recommendation gives us means to preface our element type names to make them unique The Recommendation and the following widely read discussion document describe the precepts for using this technique http www w3 org TR REC xml names 1 1 6 1 Vocabulary distinction It would be unreasonable to mandate that all document models have mutually unique element type names We design our document models with our own business requirements and our own naming conventions so do other users A W3C working group developing vocabularies has its own conventions and requirements so do other committees An XML based application knowing that an instance is using element types from only a single vocabulary can easily distinguish all elements by the name since each element type is declared in the model by its name But what happens when we need to create an XML instance that contains element types from more than one vocabulary If all the element types are uniquely named then we could guess the vocabulary for a given element by its name But if the same name is used in more than one vocabulary we need a technique to avoid ambiguity Using cryptically compressed or unmanageably elongated element type names to guarantee uniqueness would make XML diff
85. will use this simple file as the source of information for our transformation Note that the stylesheet association processing instruction in line 2 refers to a stylesheet with the name hello xsi of type XSL Recall that an XSLT processor is not obliged to respect the stylesheet association preference so let us first use a standalone XSLT processor with the following stylesheet hellohtm xs1 01 lt xml version 1 0 gt lt hellohtm xsl gt 02 lt XSLT 1 0 http www CraneSoftwrights com training gt 03 lt html xmilns xsl http www w3 org 1999 XSL Transform 04 xsl version 1 0 gt 05 lt head gt lt title gt Greeting lt title gt lt head gt 06 lt body gt lt p gt Words of greeting lt br gt 07 lt b gt lt i gt lt u gt lt xsl ivalue of select greeting gt lt u gt lt i gt lt b gt 08 lt p gt lt body gt 09 lt html gt Example 2 2 An implicitly declared simple stylesheet This file looks like a simple XHTML file an XML file using the HTML vocabulary Indeed it is just that but we are allowed to inject into the instance XSLT instructions using the prefix for the XSLT vocabulary declared on line 3 We can use any XML file as an XSLT stylesheet provided it declares the XSLT vocabulary within and indicates the version of XSLT being used Any prefix can be used for XSLT instructions though convention often sees xs1 as the prefix value Line 7 contains the only XSLT instruction in the instance
86. with our information one or more preferred or suitable stylesheet resources geared to process that information the W3C stylesheet association Recommendation describes the syntax and semantics for a construct we can add to our XML documents http www w3 org TR xml stylesheet 1 1 7 1 Relating documents to their stylesheets XML information in its authored form is often not organized in an appropriate ordering for consumption A stylesheet association processing instruction is used at the start of an XML document to indicate to the recipient which stylesheet resources are to be used when reading the contents of that document The recipient is not obliged to use the resources referenced and can choose to examine the XML using any stylesheet or transformation process they desire by ignoring the preferences stated within Some XML applications ignore the stylesheet association instruction entirely while others choose to steadfastly respect the instruction without giving any control to the recipient A flexible application will let the recipient choose how they wish to view the content of the document The designers of this specification adopted the same semantics of the lt LINK gt construct defined in the HTML 4 0 recommendation lt LINK REL stylesheet gt lt LINK REL alternate stylesheet gt 1 1 7 2 Ancillary markup A processing instruction is ancillary to the XML document model constraining the creation and validation of an
87. written by G Ken Holman this excerpt was edited by Stan Swaren and reviewed by Dave Pawson 2 Getting started with XSLT and XPath cont d 2 1 2 Some more complex examples The following more complex examples are meant merely as illustrations of some of the powerful facilities and techniques available in XSLT These samples expose concepts such as variables functions and process control constructs a stylesheet writer uses to effect the desired result but does not attempt any tutelage in their use Note This subsection can be skipped entirely or for quick exposure to some of the facilities available in XSLT and XPath only briefly reviewed In the associated narratives I ve avoided the precise terminology that hasn t yet been introduced and I overview the stylesheet contents and processor behaviors in only broad terms Subsequent subsections of this chapter review some of the basic terminology and design approaches I hope not to frighten the reader with the complexity of these examples but it is important to realize that there are more complex operations than can be illustrated using our earlier three line source file example The complexity of your transformations will dictate the complexity of the stylesheet facilities being engaged Simple transformations can be performed quite simply using XSLT but not all of us have to meet only simple requirements The following XML source information in prod xm1 is used to produc
88. y file The names of these supplementary resources can be hardwired into the stylesheet passed to the stylesheet as a parameter or the stylesheet can find them in the source files A separate node tree represents every resource accessed as a source file each with its own scope of unique node identifiers and global values When a given resource is identified more than once as a source file the XSLT processor creates only a single representation for that resource In this way a stylesheet is guaranteed to work unambiguously with source information 1 1 5 7 Stylesheet supplements source A given transformation result does not necessarily obtain all of its information from the source files It is often almost always necessary to supplement the source with boilerplate or other hardwired information The stylesheet can add any arbitrary information to the result tree as it builds the result tree from information found in the source trees A stylesheet can be the synthesis of the primary file and any number of supplemental files that are included or imported by the main file This provides powerful mechanisms for sharing and exploiting fragments of stylesheets in different scenarios 1 1 5 8 Extensible language design supplements processing The X in XSLT stands for Extensible for a reason the designers have built in conforming techniques for accessing non conforming facilities requested by a stylesheet writer that may or may not be available in th
89. yntactic forms of given XML constructs allowing us to focus on the information itself and not the syntax used to represent the information Note 2 We see XML documents as a stream or string of bytes that follow the rules of the XML 1 0 Recommendation Stylesheets do not regard instances in this fashion and we have to change the way we think of our XML documents in order to successfully work with our information This leap of understanding ranks high on the list of key aspects of stylesheet writing I needed to internalize before successfully using this technology We are given tools to work in the framework provided by the abstraction a set of data types used to represent values found in the generalization and a set of functions we use to manipulate and examine those values The data types include strings numbers boolean values and sets of nodes of our information The functions allow us to cast these values into other data type representations and to return massaged information according to our needs 1 1 2 2 Addressing identifies a hierarchical position or positions XPath defines common semantics and syntax for addressing XML expressed information and bases these primarily on the hierarchical position of components in the tree This ordering is referred to as document order in XPath while in other contexts this is often termed either parse order or depth first order Alternatively we can access an arbitrary location in the tree based on p

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