SDPL 20112: XML Basics1 2 Basics of XML and XML documents Survivor's Guide to XML, or XML for Computer Scientists / Dummies 2.1 XML and XML documents 2.2 Basics of XML DTDs 2.3 XML Namespaces

SDPL 20112: XML Basics2 2.1 XML and XML documents n XML Extensible Markup Language, W3C Recommendation, February 1998 –not an official standard, but a stable industry standard –2 nd Ed 2000, 3 rd Ed 2004, 4 th Ed 2006, 5 th Ed Nov 2008 »editorial revisions, not new versions of XML 1.0 n a simplified subset of SGML, Standard Generalized Markup Language, ISO 8879:1987 –what is said later about valid XML documents applies to SGML documents, too

SDPL 20112: XML Basics3 What is XML? n Extensible Markup Language is not a markup language! –does not fix a tag set nor its semantics (unlike markup languages, e.g. HTML) n XML documents have no inherent (processing or presentation) semantics –even though many think that XML is semantic or self- describing; See next

SDPL 20112: XML Basics4 Semantics of XML Markup n Meaning of this XML fragment? –The computer cannot know it either! –Implementing the semantics is the topic of this course

SDPL 20112: XML Basics5 What is XML (2)? n XML is a (A) way to use markup to represent information (B) metalanguage »supports definition of specific markup languages through XML DTDs or Schemas »E.g. XHTML a reformulation of HTML using XML n (C) Often “XML”  XML + XML technology –that is, processing models and languages we’re studying (and many others...)

SDPL 20112: XML Basics6 How does it look? Pekka Kilpeläinen Pekka Kilpeläinen </client> XML Handbook XSLT Programmer’s Ref XML Handbook XSLT Programmer’s Ref </invoice>

SDPL 20112: XML Basics7 Essential Features of XML n Overview of XML essentials –many details skipped »some to be discussed in exercises or with other topics when the need arises –Learn to consult original sources (specifications, documentation etc) for details! »The XML specification is easy to browse

SDPL 20112: XML Basics8 XML Document Characters n XML documents are made of ISO (32-bit) characters; in practice of 16-bit Unicode chars (cf. Java) n Three aspects or characters (see next): representation by bytes/octets numeric code points visual presentation

SDPL 20112: XML Basics9 External Aspects of Characters n Documents are stored/transmitted as a sequence of bytes (or octets). An encoding determines how characters are represented by bytes. –UTF-8 (  7-bit ASCII) is the XML default encoding – encoding="iso " ~> 256 Western European chars as single bytes n A font (collection of character images called glyphs) determines the visual presentation of characters

SDPL 20112: XML Basics10 XML Encoding of Structure 1 n XML is, essentially, just a textual encoding scheme of labelled, ordered and attributed trees, in which –internal nodes are elements labelled by type names –leaves are text nodes labelled by string values, or empty element nodes –the left-to-right order of children of a node matters –element nodes may carry attributes (= name-string-value pairs)

SDPL 20112: XML Basics11 XML Encoding of Structure 2 n XML encoding of a tree –corresponds to a pre-order walk –start of an element node with type name A denoted by a start tag, and its end denoted by end tag –start of an element node with type name A denoted by a start tag, and its end denoted by end tag –possible attributes written within the start tag: –possible attributes written within the start tag: »names must be unique: attr k  attr h when k  h – text nodes written as their string value

SDPL 20112: XML Basics12 XML Encoding of Structure: Example <S> S E <W> W Hello Hello </W> W world! </W> <W> world! </S> A=1

SDPL 20112: XML Basics13 XML: Logical Document Structure n Elements –indicated by matching (case-sensitive!) tags … –indicated by matching (case-sensitive!) tags … –can contain text and/or subelements –can be empty: or (e.g. in XHTML) –unique root element  document a single tree

SDPL 20112: XML Basics14 Logical document structure (2) n Attributes –name-value pairs attached to elements –‘‘metadata’’, usually not treated as content –in start-tag after the element type name … … n Also: – –

SDPL 20112: XML Basics15 CDATA Sections n “CDATA Sections” to include XML markup characters as textual content if (Count 0) ]]> if (Count 0) ]]>

SDPL 20112: XML Basics16 Two levels of correctness (1) n Well-formed documents – roughly: follows the syntax of XML, markup correct (elements properly nested, tag names match, attributes of an element have unique names,...) –violation is a fatal error n Valid documents –(in addition to being well-formed) obey an associated grammar (DTD/Schema)

SDPL 20112: XML Basics17 XML docs and valid XML docs XML docs and valid XML docs XML documents = well-formed XML documents DTD-valid documents Schema-valid documents

SDPL 20112: XML Basics18 An XML Processor (Parser) n Reads XML documents and reports their contents to an application –relieves the application from details of markup –XML Recommendation specifies, partly, the behaviour of XML processors: –recognition of characters as markup or data; what information to pass to applications; how to check the correctness of documents; –validation based on comparing document against its grammar

SDPL 20112: XML Basics Basics of XML DTDs n A Document Type Declaration provides a grammar (document type definition, DTD) for a class of documents [Defined in XML Rec] n Syntax (in the prolog of a document instance): [ ]> n DTD = union of the external and internal subset (could be empty); internal has higher precedence –can override entity and attribute declarations (see next)

SDPL 20112: XML Basics20 Markup Declarations n DTD consists of markup declarations –element type declarations »similar to productions of ECFGs –attribute-list declarations »for declared element types –entity declarations »short-hand notations and physical storage units –notation declarations »information about external (binary) objects

SDPL 20112: XML Basics21 How do Declarations Look Like? <!ATTLIST item price NMTOKEN #REQUIRED unit (FIM | EUR) ”EUR” > string of name charactersname characters; name characters (Letter | [0-9] |. | - | _ | : ) +

SDPL 20112: XML Basics22 Element type declarations n The general form is where E is a content model (regular expr.); (≈ ECFG production elemType  E ) n Content model operators: E | F : alternationE, F: concatenation E? : optionalE* : zero or more E+ : one or more(E) : grouping n No priorities: either (A,B)|C or A,(B|C), but no A,B|C

SDPL 20112: XML Basics23 Attribute-List Declarations n Can declare attributes for elements: –Name, data type and possible default value: –Name, data type and possible default value: n Semantics mainly up to the application –parser checks that ID attributes are unique and that targets of IDREF and IDREFS attributes exist

SDPL 20112: XML Basics24 Mixed, Empty and Arbitrary Content n Mixed content: n Mixed content: –may contain text ( #PCDATA ) and elements n Empty content: n Empty content: n Arbitrary content: (= )

SDPL 20112: XML Basics25 Entities (1) n Named storage units or XML fragments (~ macros in some languages) –character entities: »< < and < all expand to ‘ < ‘ (treated as data, not as start-of-markup) »other predefined entities: & > &apos; &quote; for & > ' " –general entities are shorthand notations: School of Computing">

SDPL 20112: XML Basics26 Entities (2) n physical storage units comprising a document –parsed entities –parsed entities –document entity is the starting point of processing –entities and elements must nest properly: <!DOCTYPE doc [ <!ENTITY chap1 ( … as above …) > ] <doc>&chap1;</doc> …</sec> …</sec>

SDPL 20112: XML Basics27 Unparsed Entities n For connecting external binary objects to XML documents; (XML processor handles only text) n Usage: n Usage: –parser provides notation and address to the application –an SGML legacy technique(?)

SDPL 20112: XML Basics28 An Alternative Way n I have rarely used unparsed entities or notations n Easier "HTML-style" linking: n Easier "HTML-style" linking: –the link indicates the format and the address directly –maintenance of numerous entities might be easier with (indirect references through) explicit declarations

SDPL 20112: XML Basics29 Parameter Entities n to parameterize and modularize DTDs: %tableDTD; n to parameterize and modularize DTDs: %tableDTD; (Parameter entities inside a markup declaration allowed only in external DTD subset)

SDPL 20112: XML Basics30 Speculations about XML Parsing n Parsing involves two things: 1. Pulling the entities together, and checking the well- formedness 2. Building a parse tree for the input (a'la DOM), or otherwise informing the application about document content and structure (e.g. a'la SAX) n Task 2 is simple (  simplicity of XML markup; see next) n Checking well-formedness is straightforward; n Implementing validation is a bit more challenging

SDPL 20112: XML Basics31 Building an XML Parse Tree <S> S <W> W Hello Hello </W> <E> E <W> W world! world! </W> </S> </E>

Simplified XML Tree Building Algorithm C ← new Document(); // current node C ← new Document(); // current node Scan document text until at end: case of scanned text: " ":N ← new ElementNode(Type); C.addChildNode(N); C ← N; C ← N; "Txt": C.addChildNode(new TextNode(Txt)); " ":C ← C.parentNode(); return C; SDPL 20112: XML Basics32 Uses DOM like tree operations Uses DOM like tree operations

SDPL 20112: XML Basics33 Sketching XML validation n Treat the document as a tree d n Document is valid w.r.t. a grammar (DTD/Schema) G iff d is a syntax tree over G –Check that the root is labelled by the start symbol of G –For each element node n of the tree, check that its »attributes match with those of the element type »content matches the content model of its type: If n is of type A and its children of type B 1, …, B n, check that the grammar has a production A  E for which B 1 … B n  L(E) (1)

SDPL 20112: XML Basics34 Sketching the validation… (2) n How to check condition (1; matching of children with a content model)? –by an automaton built from content model E –Example: –Example:

SDPL 20112: XML Basics XML Namespaces n Documents often comprise parts processed by different applications (and/or defined in different schemas) –for example, in XSLT scripts: –for example, in XSLT scripts: </xsl:template> –How to manage multiple sets of names? HTMLelements XSLT elements/ instructions

SDPL 20112: XML Basics36 XML Namespaces (2/5) n Solution: XML Namespaces, W3C Rec. Jan’99, for separating possibly overlapping “vocabularies” (sets of element type and attribute names) within a single document n by introducing (arbitrary) local name prefixes, and binding them to (fixed) globally unique URIs –For example, the local prefix “ xsl: ” conventionally used in XSLT scripts

SDPL 20112: XML Basics37 XML Namespaces briefly (3/5) n Namespace identified by a URI (through the associated local prexif) e.g /XSL/Transform for XSLT –conventional but not required to use URLs –the identifying URI has to be unique, but it does not have to be an existing address n Association inherited to sub-elements –see the next example (of an XSLT script)

SDPL 20112: XML Basics38 XML Namespaces (4/5) </xsl:template></xsl:stylesheet>

SDPL 20112: XML Basics39 XML Namespaces briefly (5/5) n Built on top of basic XML –By overloading attribute syntax ( xmlns:foo=... ) –does not affect validation »namespace attributes must be declared for DTD-validity »all element type names must be declared (with their prefixes!) –> Other schema languages (XML Schema, Relax NG) better for validating documents with Namespaces n Processing languages allow to declare NS prefixes; see next

Example: Namespaces in XSLT SDPL : XML Basics 40 aaa aaa bbb bbb <xsl:transform version="1.0" xmlns:xsl=" xmlns:bar=" xmlns:bar=" </xsl:template></xsl:transform> XML:

Example: Namespaces in XQuery SDPL : XML Basics 41 aaa aaa bbb bbb xquery version "1.0"; declare namespace zap = " doc("ns-test.xml")//zap:* ns-test.xml : n For NS support in XML APIs, see next