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XML Fundamentals

XML Fundamentals. Cheng-Chia Chen September 2011. Contents. Well-formed XML concrete textual representation of XML XML Data Model Conceptual tree model Namespaces How does XML avoid name conflicts?. Well-formed XML Document. An XML document is a sequence of characters:

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XML Fundamentals

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  1. XML Fundamentals Cheng-Chia Chen September 2011

  2. Contents • Well-formed XML • concrete textual representation of XML • XMLData Model • Conceptual tree model • Namespaces • How does XML avoid name conflicts?

  3. Well-formed XML Document An XML document is a sequence of characters: • Each character is an atomic unit of text as specified by ISO/IEC 10646 [unicode]. • usually given a .xml extension file name • MIME media type: application/xml or text/xml • Ex: <?xml version=“1.0” encoding=“UTF-8”> <student> 張得功 </student>

  4. Characters used in XML • A character is an atomic unit of text as specified by ISO/IEC 10646 [ISO/IEC 10646]. • Legal characters are tab, carriage return, line feed, and the legal graphic characters of Unicode and ISO/IEC 10646. • Character Range [2] Char ::= #x9 | #xA | #xD | [#x20-#xD7FF] | [#xE000-#xFFFD] | [#x10000-#x10FFFF] /* any Unicode character, excluding the surrogate blocks, FFFE, and FFFF. */ • character encoding may vary from entity to entity. • All XML processors must accept the UTF-8 and UTF-16 encodings.

  5. ASCII code ASCII – Needs 7-bits of storage Codes 0 – 127 used

  6. Whitespace • White Space: [3] S ::= (#x20 | #x9 | #xD | #xA)+ • S (white space) consists of one or more space (#x20) characters, tabs, carriage returns or line feeds. • Whitespace can used to separate otherwise indistinguishable parts of an XML Document. • <student age=“15”>…</student> • <studentage=“15”>…</student>

  7. Version of the XML specification 1.0 or 1.1 no: parsing affected by external DTD subset yes: not affected . character encoding of the document, expressed in Latin characters, e.g., UTF-8, UTF-16, iso-8859-1, XML Declaration <?xml version=“1.0”encoding=“Big5”standalone=“no” ?> • Besides using file extension name, an xml document may use an XML declaration to identify itself as an XML document. • If used, it should occur first (no proceding whitespace allowed) in the document.

  8. Elements, tags and character data • The example : <?xml version=“1.0” encoding=“UTF-8” ?> <student> 張得功 </student> is composed of a single element named student • Start-tag: <student> • End-tag: </student> • Everything between start-tag and end-tag is called content • Content encompasses real information • Whitespace is part of the content, though many applications will choose to ignore it • <student> and </student> are markups • 張得功 and its surrounding whitespace are character data

  9. Structure of an element • Each XML document contains one or more elements, the boundaries of which are either delimited by start-tags and end-tags, or, for empty elements, by an empty-element tag. • Each element has a type, identified by name, and may have a set of attribute specifications. • The name used in start-tag and end-tag must be identical. • Note: xml is case sensitive, so <student> != <Student> • Each attribute specification has a name and a value. • Element [39] element ::= EmptyElemTag | STag content ETag

  10. Element (cont’d) • Content of Elements : those between the start-tag and end-tag is called the element's content: [43] content ::= CharData? ((element | Reference | CDSect | PI | Comment) CharData?)* [14] CharData ::= [^<&]* - ([^<&]* ']]>' [^<&]*) • i.e., Any string containing none of <, & and ]]>. • If an element has empty content, it is represented either by a start-tag immediately followed by an end-tag or by an empty-element tag. • Tags for Empty Elements • [44] EmptyElemTag ::= '<' Name (SAttribute)* S? '/>‘

  11. Examples of empty elements <IMG align="left” src="http://www.w3.org/Icons/WWW/w3c_home" /> <IMG align="left” src="http://www.w3.org/Icons/WWW/w3c_home" ></IMG> 1. <br></br> 2. <br/> • <br> </br> • Note: 1 = 2 != 3.

  12. Start tag with attribute ( in document) and end tag • <tagattributeName=“attrbute-value“ … > • </tag> name of the attribute value or values of the attribute Each element may contain zero or more attributes name(or type) of the element single or double quotes, ‘ or “ must match start tag and end tag name must match

  13. Attributes • Attach additional information to elements • An attribute is a name-value pair attached to an element’s start-tag • One element can have more than one attribute • Name and value are separated by = and optional whitespace • Attribute value is enclosed in double or single quotation marks • <tel type=“office”>02-29381111</tel> • Attribute order is not significant • <student age=“20” gender=“male”> • 趙得勝 • </student>

  14. Start Tag • Start-tag [40] STag ::= '<' Name (S Attribute)* S? '>' [ WFC: Unique Att Spec ] [41] Attribute ::= Name Eq AttValue Example: <termdef id=“dt-dog” term=“dog”> • End-tag [42] ETag ::= '</' NameS? '>’ Example: </termdef> </termdef > vs </termdef> </termdef>

  15. XML Names • Rules for naming elements, attributes… • Names and Tokens [4] NameChar ::= Letter | Digit | '.' | '-' | '_' | ':' | CombiningChar | Extender [5] Name ::= (Letter | '_' | ':') (NameChar)* [6] Names ::= Name ( #x20Name)* [7] Nmtoken ::= (NameChar)+ [8] Nmtokens ::= Nmtoken (#x20Nmtoken)* • Names beginning with (x|M)(m|M)(l|L) are reserved. • Name is used for naming elements, attributes, entities etc. • Nmtoken (Nmtokens) is used for values of special attributes(ID,IDREFS,NMTOKEN,NMTOKENS).

  16. AttValues (attribute value literal) • are those that can occur as an attribute value. [10] AttValue ::= '"' ([^<&"] | Reference)* '"' | "'" ([^<&'] | Reference)* "'" • Enclosed by double or single quotes. • Can contain • entity/char references (see later slide)or • any char data but excluding<and& and (’or”).

  17. Comments • Comments may appear • 1. anywhere in a document outside other markup; • 2. within the document type declaration at places allowed by the grammar. • They are not part of the document's character data. • The string "--" (double-hyphen) must not occur within comments. • Comments [15] Comment ::= '<!--' ( (Char - '-') | ('-' (Char - '-')) )* '-->' • Example: • <!--declarations for <head> & <body>--> • <error <!-- comments cannot appear here! --> a=“aa”> ..

  18. Processing Instructions (PIs) • Processing instructions (PIs) allow documents to contain instructions for applications. • Processing Instructions: [16] PI ::= '<?' PITarget (S (Char* - (Char* '?>' Char*)))? '?>' [17] PITarget ::= Name - (('X' | 'x') ('M' | 'm') ('L' | 'l')) • The PI begins with a target (PITarget) used to identify the application. • The remaing part is called PIData, which should not contain substring “?>”. • The target names "XML", "xml", and so on are reserved for standardization. • Ex: <?xml-stylesheet type=“text/css” href=“style.css” ?> • xml-stylesheet is reserved for XSLT stylesheet.

  19. Processing Instruction and comment <?PItarget ***other staff*** ?> <!-- 這是說明或註解--> may contain any characters except the substring “?>” may contain any characters except the string “--”

  20. XML Document [1]document ::= prologelementMisc* • elemet is called the root or document element of the document [22] prolog ::= XMLDecl? Misc* (doctypedeclMisc*)? [23] XMLDecl ::= '<?xml' VersionInfoEncodingDecl? SDDecl? S? '?>' [27] Misc ::= Comment | PI | S

  21. Character references • What if the character data inside an element contains < or & ? • <expr> x+1 < z </expr> • Instead of using ‘<‘, we can use its character code (60) reference: • &#60; --- decimal #60 • &#x3c; --- hexadecimal #X3c or #x3C • Rule: if C is a char with code point dddd (decimal) or yyyy (hexideciaml), then we can represent C using • & #dddd; or &#Xyyyy; • Cf: in C or Java, we use • \t or \011 to represent HT (#09). • \\ or \x5c to represent back slash \ (#x5c)

  22. Entity reference • Numeric code is hard to remember. • Can use a name to denote a char or a string • Such name is called an entity. • Entity reference – • If xxx is an entity => &xxx; is its entity reference • While parsing an XML document, xml processor would replace every encountered entity reference with its actual character. • XML predefines 5 entity references – you can define your own. • &lt; – the less-than sign (<) • &amp; – the ampersand (&) • &gt; – the greater-than sign(>) -- not needed in general • &quot; – the straight, double quotation marks (") • &apos; – the straight single quote (')

  23. CDATA Section • What if my element content has a lot of special characters ? • Ex: <expr> x < y && z < 1 </expr> • Solution 1: • <expr> x &lt; y &amps;&amps; z &lt; 1 </expr> • Hard to read/comprehend • Solution 2: • <expr><![CDATA[x < y && z < 1 ]]></expr>

  24. CDATA Sections • CDATA sections • may occur as part of the content of an element; • used to escape blocks of text containing many special characters. • begin with the string "<![CDATA[" and end with the string "]]>": • CDATA Sections [18] CDSect ::= CDStartCData CDEnd [19] CDStart ::= '<![CDATA[' [20] CData ::= (Char* - (Char* ']]>' Char*)) [21] CDEnd ::= ']]>' • What cannot occur inside a CDATA section? • Ans: ']]>' • Every character inside CDATA section is recognized as a literal character, so ‘<‘ and ‘&’ may and must occur in their literal form. • Example: <![CDATA[<greeting>Hello, world!</greeting>]]>

  25. Character Data and Markup • XML Document consists of intermingled character data and markup. • Markup takes the form of • start-tags, end-tags, empty-element tags, • entity references, character references, • comments, CDATA section delimiters, • document type declarations, • processing instructions, • XML declarations, text declarations and • white space outside root element • All text that is not markup constitutes the character data of the document. I.e., it may occur • in the content of an element or • In the content of an CDATA Section.

  26. Character Data and Markup (cont’d) • In the content of elements, character data is any string of characters, which does not contain the start-delimiter (< and & ) of any markup. • In a CDATA section, character data is any string of characters not including the CDATA-section-close delimiter, "]]>". • To allow attribute values to contain both single and double quotes, the apostrophe or single-quote character (') may be represented as "&apos;", and the double-quote character (") as "&quot;". • Character Data : [14] CharData ::= [^<&]* - ([^<&]* ']]>' [^<&]*) • i.e., Any string containing none of <, & and ]]>.

  27. Possible contents of an element • Element [39] element ::= EmptyElemTag | STag content ETag • Content of Elements [43] content ::= CharData? ((element | Reference | CDSect | PI | Comment) CharData?)* • In addition to char data and child elements, an element may contain as children also • references, • PIs, • comments or • CDATA sections.

  28. General rules for well-formed XML Documents 1: balanced start and end tags • The set of tags is unlimited but all start tags must have matching end tags Example of legal XML • <student><name> DeTsi Wang</name><email> wang@cs.nccu.edu.tw</email><age> 20 </age></student> 2: There must be exactly one root element

  29. Rules for well-formed XML Documents • Rule 3: Proper element nesting • All tags must be nested correctly. Like HTML, XML can intermix tags and text, but tags may not overlap each other. • Legal XML <student><name> DeTsi Wang</name><email> wang@cs.nccu.edu.tw</email><age> 20 </age> </student> • Illegal XML <b><i>This text is bold and italic</b> and italic</i>

  30. Rules for well-formed XML Documents • Rule 4: Attribute values must be single or double quoted • Legal <tag attribute=“value”> <tag attribute=‘value’> • Illegal <font size=6> <font size=“60’> • Rule 5: An element may not have two attributes with the same name • <font size=“6” size = “10”/> • Rule 6: Comments and processing instructions may not appear inside tags • <font <!– error comment --> size = “6” /> • Rule 7: No unescaped < or & signs may occur in the character data of an element or attributes • <font zise=“<20”> 20&3 </font>

  31. An example XML document : Recipes in XML • Define our own “Recipe Markup Language” • Choose markup tags that correspond to concepts in this application domain • recipe, ingredient, amount, ... • No canonical choices • granularity of markup? • simply <date>14 Jun 95</date> or • <date><y>95</y><m>6</m><d>14</d></date> • structuring? • elements or attributes? • ...

  32. Example (1/2) <collection> <description>Recipes suggested by Jane Dow</description> <recipe id="r117"> <title>Rhubarb Cobbler</title> <date>Wed, 14 Jun 95</date> <ingredient name="diced rhubarb" amount="2.5" unit="cup"/> <ingredient name="sugar" amount="2" unit="tablespoon"/> <ingredient name="fairly ripe banana" amount="2"/> <ingredient name="cinnamon" amount="0.25" unit="teaspoon"/> <ingredient name="nutmeg" amount="1" unit="dash"/> <preparation> <step> Combine all and use as cobbler, pie, or crisp. </step> </preparation>

  33. Example (2/2) <comment> Rhubarb Cobbler made with bananas as the main sweetener. It was delicious. </comment> <nutrition calories="170" fat="28%" carbohydrates="58%" protein="14%"/> <related ref="42">Garden Quiche is also yummy</related> </recipe> </collection>

  34. Building on the XML Notation • Defining the syntax of our recipe language • DTD, XML Schema, ... • Showing recipe documents in browsers • XPath, XSLT • Recipe collections as databases • XQuery • Building a Web-based recipe editor • HTTP, Servlets, JSP, ... • ...

  35. XML data models • An XML document may contain lots of information which not all applications would need/like to use. • eg: <abc>abc <![CDATA[ def ]]> end</abc> • <abc>abc def end</abc> • need to be differentiated? • XML data models are abstracted views of XML documents so that unintended information of an XML document is ignored in the model. • There are more than one XML data model. • DOM (document object model) • XPath 1.0 ; • XPath 2.0; • XML information set • … • All uses tree structure to model an XML document. • though we could also model XML documents as graphs.

  36. XML Trees • Conceptually, an XML document is a tree structure • node, edge • root, leaf • child, parent • sibling (ordered), ancestor,descendant

  37. An Analogy: File Systems

  38. Tree View of the XML Recipes

  39. Nodes in XML Trees • Root nodes: every XML tree has one root node that represents the entire tree • Element nodes: define hierarchical logical groupings of contents, each have a name • Text nodes: carry the actual contents, leaf nodes • Attribute nodes: unordered, each associated with an element node, has a name and a value • Namepace nodes: effective namespace associated with an element. • Comment nodes: ignorable meta-information • Processing instructions: instructions to specific processors, each have a target and a value

  40. Types of node in an XML tree • The tree contains nodes. • Types of nodes and their possible children: • root nodes : element ( = 1), comment, PI • element nodes: element, text, PI, comment, [attribute, namespace] • text nodes: leaves • attribute nodes : leaves • namespace nodes: leaves • processing instruction nodes : leaves • comment nodes : leaves

  41. XML Applications Rough classification: • Data-oriented languages • inventory, customer and employee records in a company • regular flat wide tree ; traditionally stored in db • Document-oriented languages • XHTML, DOCBook, WML, XML formats of word, openOffice • loosely structured, tags ignorable, mixed content • Protocols and programming languages • XML Schema, XSLT, WDSL • ebXML, XMI, BML • Hybrids • patient record : billing info; notes from doctor • article collection: isbn, name; abstract

  42. Example: XHTML • XMLification of HTML • end tag must not be omitted • element/attribute names all in lower case • attribute values must be present and quoted. • decomposed into modules reuseable by other applications <?xml version="1.0" encoding="UTF-8"?> <html xmlns="http://www.w3.org/1999/xhtml"> <head><title>Hello world!</title></head> <body> <h1>This is a heading</h1> This is some text. </body> </html>

  43. Example: CML CML : XML-based data-oriented language for representation of molecules and chemical reaction. <molecule id="METHANOL"> <atomArray> <stringArray builtin="id">a1 a2 a3 a4 a5 a6</stringArray> <stringArray builtin="elementType">C O H H H H</stringArray> <floatArray builtin="x3" units="pm"> -0.748 0.558 ... </floatArray> <floatArray builtin="y3" units="pm"> -0.015 0.420 ... </floatArray> <floatArray builtin="z3" units="pm"> 0.024 -0.278 ... </floatArray> </atomArray> </molecule>

  44. Example: ebXML ebXML: a worldwide initiative aiming to utilize XML for exchange of electronic business data. It has delivered many XML standards for business processes, core data component, collaboration protocol agreements, messaging, registries and repositories. <MultiPartyCollaboration name="DropShip"> <BusinessPartnerRole name="Customer"> <Performs initiatingRole='//binaryCollaboration[@name="Firm Order"]/ InitiatingRole[@name="buyer"]' /> </BusinessPartnerRole> <BusinessPartnerRole name="Retailer"> <Performs respondingRole='//binaryCollaboration[@name="Firm Order"]/ RespondingRole[@name="seller"]' /> <Performs initiatingRole='//binaryCollaboration[...]/ InitiatingRole[@name="buyer"]' /> </BusinessPartnerRole> <BusinessPartnerRole name="DropShip Vendor"> ... </BusinessPartnerRole> </MultiPartyCollaboration>

  45. Example: ThML A XML-based markup language for theological texts. <h3 class="s05" id="One.2.p0.2">Having a Humble Opinion of Self</h3> <p class="First" id="One.2.p0.3">EVERY man naturally desires knowledge <note place="foot" id="One.2.p0.4"> <p class="Footnote" id="One.2.p0.5"><added id="One.2.p0.6"> <name id="One.2.p0.7">Aristotle</name>, Metaphysics, i. 1. </added></p> </note>; but what good is knowledge without fear of God? Indeed a humble rustic who serves God is better than a proud intellectual who neglects his soul to study the course of the stars. <added id="One.2.p0.8"><note place="foot" id="One.2.p0.9"> <p class="Footnote" id="One.2.p0.10"> Augustine, Confessions V. 4. </p> </note></added> </p>

  46. XML Namespace

  47. Motivation • name clashes. • Consider an XML language WidgetML which uses XHTML as a sublanguage for help • messages: <widget type="gadget"> <head size="medium"/> <body><subwidget ref="gizmo"/></body> <info> <head><title>Description of gadget</title> </head> <body><h1>Gadget</h1> A gadget contains a big gizmo </body> </info></widget> • Meanings of head and body depend on context! • complicates things for processors and might even cause ambiguities. • The solution: different namespaces for different use of the same name.

  48. The Idea • Assign a namespace to each set of elements/attributes (which forms an XML language) http://www.w3.org/1999/xhtml • Each namespace is identified and referenced by a URI • Qualify every element/attribute names with the URI of its namespace:{http://www.w3.org/1999/xhtml}head => name = namespace URI + local part html head body h1 … widget head body info …

  49. Problems for qualifying names • URI as part of a name would use too much space since it is usually a long string. • Not all URIs are legal Attribute/element names. • (XML names do not allow/restrict the use of special characters: • (.:_- ok) (/,#,%,… no) • Solution: use namespace prefix as a proxy for namespace URI. • xmlns:aPfx = “aURI” • Notes: • URI = URL  URN (extended to IRI at 1.1). • URI here used only for identification - doesn't have to point at anything.

  50. Namespace declarations • Namespaces are declared byspecial namespace attributes (xmlns: or xmlns) and associated prefixes. • Example: <foo:e1 xmlns:foo="http://www.w3.org/TR/xhtml1"> ... <foo:head>...</foo:head> ... </...> • xmlns:prefix1="URI1" declares a namespace with a prefix: prefix1 and a URI: URI1. • Scope rule: lexical • A namespace declaration has effect on the element containing the declaration as well as all its descendants unless it is overridden by other declaration in nested declarations. • Both element and attribute names can be qualified with namespaces. • Note: the prefix is just a proxy - applications should use only the URI for identification.

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