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Namespace in XML

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  1. Namespace in XML Cheng-Chia Chen

  2. Abstract • XML namespaces provide a simple method for qualifyingelement and attribute names used in XML documents by associating them with namespaces identified by URI references.

  3. Table of Contents 0. Quick reviews 1. Motivation and Summary 2. Declaring Namespaces 3. Qualified Names 4. Using Qualified Names 5. Applying Namespaces to Elements and Attributes 5.1 Namespace Scoping 5.2 Namespace Defaulting 5.3 Uniqueness of Attributes 6. Conformance of Documents

  4. 0. quick reviews: 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"/></big> <info> <head><title>Description of gadget</title> </head> <body> <h1>Gadget</h1> A gadget contains a big gizmo </body> </info></widget> • Meaning of head and body depends on context! • complicates things for processors and might even cause ambiguities. • The solution: multiple namespaces. • name = <namespace name, local name>

  5. Qualifying names • lexical rules for namespace and local names: • require every namespace to be a URIs (Universal Resource Identifiers) called its [namespace URI]. • => name = [namespace URI]:[local part] http://www.w3.org/TR/xhtml1 : head namespace URI local part • same with attribute names. • Notes: • URI = URL  URN (extended to IRI (W3c draft) at 1.1). • URI here used only for identification - doesn't have to point at anything. • Not all URIs are legal Attribute/element names. (XML names do not allow/restrict the use of special characters: • (.:_- ok) (/,#,%,… no) • use namespace prefix as a proxy for namespace URI.

  6. Namespace declarations • indirection: namespaces are declared by special namespace attributes (xmlns:* or xmlns) and associated prefixes. • Example: <... 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 • the element containing the declaration and all descendants • can be overridden by nested declaration • 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.

  7. The default namespace • for backward compatibility and simplicity. • declaration: • xmlns=“aURI" • Unprefixed element names are assigned the default namespace aURI. • default value: "" (means: disable default namespace) • or xmlns= "" • (means: disable default namespace)

  8. Example • <ex xmlns=“http://test.com/” att1=“…” xmlns:s=“http://test.com/” > <ex att1=“abc” > … </ex> <ex xmlns=“” s:att1=“abc”>…</ex> </ex> • Default namespace affects element names only; it does not apply to attributes.in Ex: • 1. the 1st and 2nd <ex> belong to the same namespace (http;;//test.com/) but the 3rd <ex> belongs to no namespace. • 2. Global attribute belongs to namespace: http;//test.com. • 3. Both <att1> attributes are local in the sense that they belong to the local namespace of ex and are different from s:att1. • 4. note the asymmetry of default namspace on elements and attributes

  9. An example: WidgetML with namespaces <widget xmlns="http://www.widget.org" xmlns:xhtml="http://www.w3.org/TR/xhtml1" type="gadget"> <head size="medium"/> <body><subwidget ref="gizmo"/></big> <info><xhtml:head> <xhtml:title>Description of gadget</xhtml:title> </xhtml:head> <xhtml:body> <xhtml:h1>Gadget</xhtml:h1> A gadget contains a big gizmo </xhtml:body> </info></widget> • The main part of WidgetML uses the default namespace which has the URIhttp://www.widget.org; • XHTML uses the namespace prefix xhtml which is assigned the URIhttp://www.w3.org/TR/xhtml1.

  10. Consequences for other XML technologies • - namespace awareness. • XML languages and applications should consider Namespaces as an inherent part of XML. • DTD should take namespace into considerations • parsers • need an extra processing layer on top to resolve namespace declarations and uses • schemas • schemas should be used to declare all possible element/attribute names of a namespace. • namespace URIs could be tied together with schemas • … • ...

  11. 1. Summary [Definition:] An XML namespace • Is a collection of names, • used in XML documents as element types and attribute names. • has a namespace name identified by a URI reference [RFC2396], [Definition:] URI references which identify namespaces are considered identical when they are exactly the same character-for-character. • E.g. • 1. http://a.b.c/~wine/d , 2. http://a.B.c/%7Ewine/d, • 3. http://a.b.c/%7ewine/d , 4. d (relative URI deprecated) • All 4 URIs are treated as equal in URI spec, but are seen as different namespace URIs in xml namespace. • Note: Relative URI should not be used.

  12. 1. Summary (cont’d) • Names from XML namespaces appear as • qualified names • (= [ namespace prefix ’:’] local part ) • URI references can contain characters not allowed in names, so cannot be used directly as namespace prefixes. Therefore, the namespace prefix serves as a proxy for a URI reference. • Special attributes (xmlns & xmlns:xxx) used to declare the association of the namespace prefix with a URI reference.

  13. 2. Declaring Namespaces A namespace is declared using two types of reserved attributes. • 1. xmlns = “default_URI” • 2. xmlns:aPrefix = “aURI” • Notes: • default_URI and aURI are URI references used to identify a namespace [name]. • Type 1 is used to bind default_URI to all unprefixed elements. • Type 2 is used to bind aURI to all elements/attributes whose common prfix part is aPrfix. • Namespace declarations may occur directly (in document instance) or by default (in DTD).

  14. Example <?xml versin=“1.0” ?> <!DOCTYPE x [ <!ATTLIST x xmlns CDATA FIXED “http://test.com”> ]> <x xmlns:edi='http://ec.org/schema'> <edi:order >… </edi:order> … </x> Notes: Two namespace declarations in <x …> 1. the one occurring indirectly at DTD is a default namespace, while 2. the one inside <x …> declares all elements inside <x>…</x> with the prefix : edi be assocaited with the namepspace : http://ec.org/schema.

  15. Attribute Names for Namespace Declaration [1] NSAttName ::= PrefixedAttName | DefaultAttName [2] PrefixedAttName ::= 'xmlns:' NCName [ NSC: Leading "XML" ] [3] DefaultAttName ::= 'xmlns' [4] NCName ::= (Letter | '_') (NCNameChar)* /* An XML Name, minus the ":" */ [5] NCNameChar ::= Letter | Digit | '.' | '-' | '_' | CombiningChar | Extender • Note: NCName is any XML Name containing no char ‘:’. <x xmlns:edi='http://ec.org/schema'> <!-- the "edi” prefix is bound to http://ec.org/schema for the "x" element and contents --> </x>

  16. 2. Declaring Namespaces (cont’d) • Notes on the properties of a namespace name: • Must be a URI reference, • should be unique and persistent. • Need not directly usable for retrieval of a schema ) • syntax designed with these goals: Uniform Resource Names [RFC2141]. • Ordinary URLs can be managed in such a way as to achieve these same goals.

  17. 3. Qualified Names • Qualified Name: [6] QName ::= (Prefix ':')? LocalPart [7] Prefix ::= NCName [8] LocalPart ::= NCName • The Prefix provides the namespace prefix part of the qualified name, and must be associated with a namespace URI reference in a namespace declaration. • Note that the prefix functions only as a placeholderfor a namespace name. • Applications should use the namespace name, not the prefix,in constructing names whose scope extends beyond the containing document.

  18. 4. Using Qualified Names [for element type] Element types are given as qualified names, as follows: • Element Types : [9] STag ::= '<' QName (S Attribute)* S? '>' [10] ETag ::= '</' QName S? '>' [11] EmptyElemTag ::= '<' QName (S Attribute)* S? '/>' [NSC: Prefix Declared ] • Example: <x xmlns:edi='http://ecommerce.org/schema'> <!-- the 'price' element's namespace is http://ecommerce.org/schema --> <edi:price units='Euro'>32.18</edi:price> </x>

  19. 4. Using Qualified Names [for attribute names] • Attributes are either namespace declarations or their names are given as qualified names: • Attribute: [12] Attribute ::= NSAttName Eq AttValue | QName Eq AttValue [ NSC: Prefix Declared ] Example of a qualified name serving as an attribute name: <x xmlns:edi='http://ecommerce.org/schema'> <!-- the 'taxClass' attribute's namespace is http://ecommerce.org/schema --> <lineItem edi:taxClass="exempt">Baby food</lineItem> </x>

  20. 4. Using Qualified Names [ in DTD] • Element names and attribute types are also given as qualified names in the DTD: • Qualified Names in Declarations [13] doctypedecl ::= '<!DOCTYPE' S QName (S ExternalID)? S? ('[' (markupdecl | PEReference | S)* ']' S?)? '>' [14] elementdecl ::= '<!ELEMENT' S QName S contentspec S? '>' [15] cp ::= (QName | choice | seq) ('?' | '*' | '+')? [16] Mixed ::= '(' S? '#PCDATA' (S? '|' S? QName)* S? ')*' | '(' S? '#PCDATA' S? ')' [17] AttlistDecl ::= '<!ATTLIST' S QName AttDef* S? '>' [18] AttDef ::= S (QName | NSAttName) S AttType S DefaultDec

  21. 5. Applying Namespaces to Elements and Attributes 5.1 Namespace Scoping: • The namespace declaration is considered to apply to the element where it is specified and to all elements within the content of that element, unless overridden by another namespace declarationwith the same NSAttName part. • Example: <?xml version="1.0"?> <!-- all elements here are explicitly in the HTML namespace --> <html:html xmlns:html='http://www.w3.org/TR/REC-html40'> <html:head><html:title>Frobnostication</html:title></html:head> <html:body><html:p>Moved to <html:a href='http://frob.com'>here.</html:a></html:p></html:body> </html:html>

  22. 5.1 Namespace Scoping: • Multiple different namespace prefixes can be declared in the same start tag. • Example: <?xml version="1.0"?> <!-- both namespace prefixes are available throughout --> <bk:book xmlns:bk='urn:loc.gov:books' xmlns:isbn='urn:ISBN:0-395-36341-6'> <bk:title>Cheaper by the Dozen</bk:title> <isbn:number>1568491379</isbn:number> </bk:book>

  23. 5.2 Namespace Defaulting • A default namespace is applied to • the element where it is declared (if that element has no namespace prefix), and to • all elements with no prefix within the content of that element. • If the URI reference in a default namespace declaration is empty, then unprefixed elements are not considered to be in any namespace. • Note : Default namespaces do not apply directly to attributes !!!

  24. 5.2 Namespace Defaulting: examples <?xml version="1.0"?> <!-- elements are in the HTML namespace, by default --> <htmlxmlns='http://www.w3.org/TR/REC-html40'> <head><title>Frobnostication</title></head> <body><p>Moved to <a href='http://frob.com'>here</a>.</p></body> </html> <?xml version="1.0"?> <!-- unprefixed element types are from "books" --> <bookxmlns='urn:loc.gov:books' xmlns:isbn='urn:ISBN:0-395-36341-6'> <title>Cheaper by the Dozen</title> <isbn:number>1568491379</isbn:number> </book>

  25. 5.2 Namespace Defaulting: a larger example <?xml version="1.0"?> <!-- initially, the default namespace is "book" --> <book xmlns='urn:loc.gov:books' xmlns:isbn='urn:ISBN:0-395-36341-6'> <title>Cheaper by the Dozen</title> <isbn:number>1568491379</isbn:number> <notes> <!-- make HTML the default namespace for some commentary --> <p xmlns='urn:w3-org-ns:HTML'> This is a <i>funny</i> book! </p> </notes> </book>

  26. 5.2 Namespace Defaulting: • The default namespace can be set to the empty string to disable default namespace. <?xml version='1.0'?> <Beers> <!-- the default namespace is now that of HTML --> <table xmlns='http://www.w3.org/TR/REC-html40'> <th><td>Name</td><td>Origin</td><td>Description</td></th> <tr> <!-- no default namespace inside table cells --> <td> <brandName xmlns="” >Huntsman</brandName> </td> <td> <origin xmlns="” >Bath, UK</origin> </td> <td> <details xmlns="” ><class>Bitter</class><hop>Fuggles</hop> <pro>Wonderful hop, light alcohol, good summer beer</pro> <con>Fragile; excessive variance pub to pub</con> </details> </td> </tr> </table> </Beers>

  27. 5.3 Uniqueness of Attributes • In XML documents conforming to this specification, no tag may contain two attributes which: • 1.have identical names, or // for well-formedness • 2.have qualified names with the same local part and with prefixes which have been bound to namespace names that are identical. // for namespace conformity • Example, each of the following bad start-tags is illegal: <!-- http://www.w3.org is bound to n1 and n2 --> <x xmlns:n1="http://www.w3.org" xmlns:n2="http://www.w3.org" > <bad a="1" a="2" /> <bad n1:a="1" n2:a="2" /> </x>

  28. 5.3 Uniqueness of Attributes (cont’d) • Each of the following is legal, the second is so because the default namespace does not apply to attribute names!: <!-- http://www.w3.org is bound to n1 and is the default --> <x xmlns:n1="http://www.w3.org" xmlns="http://www.w3.org" > <good a="1" b="2" /> <good a="1" n1:a="2" /> </x>

  29. 6. Conformance of Documents • In XML documents which conform to this specification, • element types and attribute names must match the production for QName and must satisfy the "Namespace Constraints". • all other tokens [in the document which are required to match the XML production for Name] match this specification's production for NCName. • The effect of conformance is that in such a document: • All element types and attribute names contain either zero or one colon. • No entity names, PI targets, or notation names contain any colons.

  30. A.2 XML Namespace Partitions (removed in 1.1) • Each XML namespace contains disjoint partitions:: 1. The All Element Types Partition • combination of the namespace name and the local part uniquely identifies the element type. 2. The Global Attribute Partition • The combination of the namespace name and the attribute name uniquely identifies the global attribute. 3. The Per-Element-Type (local attribute) Partitions • The combination of the attribute name with the element's type and namespace name uniquely identifies each unqualified attribute.

  31. Examples of namespace partitions <!-- 1 --> <sectionxmlns='urn:com:books-r-us'> <!-- 2 --> <title>Book-Signing Event</title> <!-- 3 --> <signing> <!-- 4 --> <author title="Mr" name="Vikram Seth" /> <!-- 5 --> <booktitle="A Suitable Boy" price="$22.95" /> </signing> </section> Note: Titles in 4 and 5 belong to different partitions <!-- 1 --> <RESERVATIONxmlns:HTML="http://www.w3.org/TR/REC-html40"> <!-- 2 --> <NAMEHTML:CLASS="largeSansSerif">Layman, A</NAME> <!-- 3 --> <SEATCLASS="Y" HTML:CLASS="largeMonotype">33B</SEAT> <!-- 4 --> <HTML:AHREF='/cgi-bin/ResStatus'>Check Status</HTML:A> <!-- 5 --> <DEPARTURE>1997-05-24T07:55:00+1</DEPARTURE> </RESERVATION> Notes: 1. two namespace: default and HTML.

  32. Partition of a namespace Global elements: html: body: a: table: … Global attributes: class: stylesheet: … Local attributes for table: … Local attributes for html: … Local attributes for body: … Local attributes for a href name

  33. Links to more information • http://www.w3.org/TR/REC-xml-names • The W3C XML Namespace Recommendation v1.0 • http://www.w3.org/TR/xml-names11/ • Namespaces in XML 1.1W3C Candidate Recommendation 18 December 2002 • IRI draft • Internationalized Resource Identifiers (IRIs). Available at http://www.w3.org/International/iri-edit/draft-duerst-iri-02.txt • RFC2396 • IETF (Internet Engineering Task Force) RFC 2396: Uniform Resource Identifiers (URI): Generic Syntax, eds. T. Berners-Lee, R. Fielding, L. Masinter. August 1998. Available at ftp://ftp.ietf.org/rfc/rfc2396.txt