Namespace in XML

1
Namespace in XML

• Cheng-Chia Chen

2
Abstract

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

3
Table of Contents

• 0. Quick views
• 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 views Motivation

• name clashes.
• Consider an XML language WidgetML which uses
  XHTML as a sublanguage for help
• messages
• ltwidget type"gadget"gt
• lthead size"medium"/gt
• ltbodygtltsubwidget ref"gizmo"/gtlt/biggt
• ltinfogt ltheadgtlttitlegtDescription of
  gadgetlt/titlegt
• lt/headgt ltbodygt lth1gtGadgetlt/h1gt
• A gadget contains a big gizmo
• lt/bodygt
• lt/infogtlt/widgetgt
• Meaning of head and body depends on context!
• complicates things for processors and might even
  cause ambiguities.
• The solution multiple namespaces.
• name ltnamespace name, local namegt

5
Qualifying names

• lexical rules for namespace and local names
• require every namespace to be a URIs (Universal
  Resource Identifiers) called its namespace URI.
• gt name namespace URIlocal part
• http//www.w3.org/TR/xhtml1head
• 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
• lt... xmlnsfoo"http//www.w3.org/TR/xhtml1"gt
• ...
• ltfooheadgt...lt/fooheadgt
• ...
• lt/...gt
• xmlnsprefix"URI" declares a namespace with a
  prefix and a URI.
• 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 xmlnsaURI"
• Unprefixed element names are assigned the default
  namespace aURI.
• default value "" (means disable default
  namespace)
• or xmlns ""
• Ex
• ltex xmlnshttp//test.com/ att1
  xmlnsshttp//test.com/gt
• ltex att1abc gt lt/ex1gt
• ltex xmlns satt1abcgtlt/ex1gt
• lt/examplegt
• Default namespace affects element names only it
  does not affect unprefixed attribute names.
• in Ex 1. the 1st and 2nd ltexgt belong to the same
  namespace ! namespace URI of 3rd ltexgt
• 2. all ltattgtattributes belong to the same
  namespace.

8
An example WidgetML with namespaces

• ltwidget xmlns"http//www.widget.org"
• xmlnsxhtml"http//www.w3.org/TR/xhtml1"
• type"gadget"gt
• lthead size"medium"/gt
• ltbodygtltsubwidget ref"gizmo"/gtlt/biggt
• ltinfogtltxhtmlheadgt
• ltxhtmltitlegtDescription of
  gadgetlt/xhtmltitlegt
• lt/xhtmlheadgt
• ltxhtmlbodygt ltxhtmlh1gtGadgetlt/xhtmlh1gt
• A gadget contains a big gizmo
• lt/xhtmlbodygt lt/infogtlt/widgetgt
• The main part of WidgetML uses the default
  namespace which has the URI http//www.widget.org
  
• XHTML uses the namespace prefix xhtml which is
  assigned the URI http//www.w3.org/TR/xhtml1.

9
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
• ...

10
1. Summary

• Definition An XML namespace is a collection of
  names, identified by a URI reference RFC2396,
  which are used in XML documents as element types
  and attribute names.
• Definition URI references which identify
  namespaces are considered identical when they are
  exactly the same character-for-character.
• Examples include URI references which differ
  only in case, or which are in external entities
  which have different effective base URIs.
• E.g. http//a.b.c/wine/d http//a.B.c/7Ewine/d
  
• http//a.b.c/7ewine/d d (relative URI
  deprecated)

11
1. Summary (contd)

• Names from XML namespaces appear as
• qualified names,
• which contain a single colon, separating the name
  into a namespace prefix and a 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.
• attribute-based syntax used to declare the
  association of the namespace prefix with a URI
  reference.

12
2. Declaring Namespaces

• Definition A namespace is declared using a
  family of reserved attributes.
• Such an attribute's name must either be xmlns or
  have xmlns as a prefix.
• These attributes, like any other XML attributes,
  may be provided directly or by default.

13
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 NCNames are legal XML Name containing no
  char .
• ltx xmlnsedi'http//ecommerce.org/schema'gt
• lt!-- the "edi prefix is bound to
  http//ecommerce.org/schema for the "x" element
  and contents --gt
• lt/xgt

14
2. Declaring Namespaces (contd)

• Definition The attribute's value, a URI
  reference, is the namespace name identifying the
  namespace.
• The namespace name should have the
  characteristics of uniqueness and persistence.
• It is not a goal that it be directly usable for
  retrieval of a schema (if any exists).
• syntax designed with these goals Uniform
  Resource Names RFC2141.
• Ordinary URLs can be managed in such a way as to
  achieve these same goals.

15
2. Declaring Namespaces (contd)

• Definition If the attribute name matches
  PrefixedAttName, then the NCName gives the
  namespace prefix, used to associate element and
  attribute names with the namespace name in the
  attribute value.
• In such declarations, the namespace name may not
  be empty.
• Definition If the attribute name matches
  DefaultAttName, then the namespace name in the
  attribute value is that of the default namespace.
  
• In such a default declaration, the attribute
  value may be empty. ? Disable default namespace.

16
3. Qualified Names

• Definition In XML documents, some names may be
  given as qualified names, defined as follows
• 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
  placeholder for a namespace name. Applications
  should use the namespace name, not the prefix,in
  constructing names whose scope extends beyond the
  containing document.

17
4. Using Qualified Names for element type

• Element types are given as qualified names, as
  follows
• Element Types
• 9 STag 'lt' QName (S Attribute) S? 'gt'
• 10 ETag 'lt/' QName S? 'gt'
• 11 EmptyElemTag 'lt' QName (S Attribute)
  S? '/gt'
• NSC Prefix Declared
• Example
• ltx xmlnsedi'http//ecommerce.org/schema'gt
• lt!-- the 'price' element's namespace is
  http//ecommerce.org/schema --gt
• ltediprice units'Euro'gt32.18lt/edipricegt
• lt/xgt

18
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
• ltx xmlnsedi'http//ecommerce.org/schema'gt
• lt!-- the 'taxClass' attribute's namespace is
  http//ecommerce.org/schema --gt
• ltlineItem editaxClass"exempt"gtBaby
  foodlt/lineItemgt
• lt/xgt

19
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 'lt!DOCTYPE' S QName (S
  ExternalID)?
• S? ('' (markupdecl PEReference
  S) '' S?)? 'gt'
• 14 elementdecl 'lt!ELEMENT' S QName S
  contentspec
• S? 'gt'
• 15 cp (QName choice seq) ('?'
  '' '')?
• 16 Mixed '(' S? 'PCDATA' (S? '' S?
  QName) S? ')'
• '(' S? 'PCDATA' S?
  ')'
• 17 AttlistDecl 'lt!ATTLIST' S QName AttDef
  S? 'gt'
• 18 AttDef S (QName NSAttName) S AttType S
• DefaultDec

20
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
  declaration with the same NSAttName part.
• Example
• lt?xml version"1.0"?gt
• lt!-- all elements here are explicitly in the HTML
  namespace --gt
• lthtmlhtml xmlnshtml'http//www.w3.org/TR/REC-ht
  ml40'gt
• lthtmlheadgtlthtmltitlegtFrobnosticationlt/htmltitle
  gtlt/htmlheadgt
• lthtmlbodygtlthtmlpgtMoved to lthtmla
  href'http//frob.com'gthere.lt/htmlagtlt/htmlpgtlt/ht
  mlbodygt
• lt/htmlhtmlgt

21
5.1 Namespace Scoping

• Multiple namespace prefixes can be declared as
  attributes of a single element.
• Example
• lt?xml version"1.0"?gt
• lt!-- both namespace prefixes are available
  throughout --gt
• ltbkbook xmlnsbk'urnloc.govbooks'
• xmlnsisbn'urnISBN0-395-36341-6'gt
• ltbktitlegtCheaper by the Dozenlt/bktitlegt
• ltisbnnumbergt1568491379lt/isbnnumbergt
• lt/bkbookgt

22
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 !!!

23
5.2 Namespace Defaulting examples

• lt?xml version"1.0"?gt
• lt!-- elements are in the HTML namespace, by
  default --gt
• lthtml xmlns'http//www.w3.org/TR/REC-html40'gt
• ltheadgtlttitlegtFrobnosticationlt/titlegtlt/headgt
• ltbodygtltpgtMoved to
• lta href'http//frob.com'gtherelt/agt.lt/pgtlt/body
  gt
• lt/htmlgt
• lt?xml version"1.0"?gt
• lt!-- unprefixed element types are from "books"
  --gt
• ltbook xmlns'urnloc.govbooks'
• xmlnsisbn'urnISBN0-395-36341-6'gt
• lttitlegtCheaper by the Dozenlt/titlegt
• ltisbnnumbergt1568491379lt/isbnnumbergt
• lt/bookgt

24
5.2 Namespace Defaulting a larger example

• lt?xml version"1.0"?gt
• lt!-- initially, the default namespace is "books"
  --gt
• ltbook xmlns'urnloc.govbooks'
• xmlnsisbn'urnISBN0-395-36341-6'gt
• lttitlegtCheaper by the Dozenlt/titlegt
• ltisbnnumbergt1568491379lt/isbnnumbergt
• ltnotesgt
• lt!-- make HTML the default namespace for
  some commentary --gt
• ltp xmlns'urnw3-org-nsHTML'gt
• This is a ltigtfunnylt/igt book!
• lt/pgt
• lt/notesgt
• lt/bookgt

25
5.2 Namespace Defaulting

• The default namespace can be set to the empty
  string to disable default namespace.
• lt?xml version'1.0'?gt ltBeersgt
• lt!-- the default namespace is now that of HTML
  --gt
• lttable xmlns'http//www.w3.org/TR/REC-html40'gt
  
• ltthgtlttdgtNamelt/tdgtlttdgtOriginlt/tdgtlttdgtDescriptio
  nlt/tdgtlt/thgt
• lttrgt lt!-- no default namespace inside table
  cells --gt
• lttdgt ltbrandName xmlns" gtHuntsmanlt/brandNam
  egt lt/tdgt
• lttdgt ltorigin xmlns" gtBath, UKlt/origingt
  lt/tdgt
• lttdgt ltdetails xmlns" gtltclassgtBitterlt/clas
  sgtlthopgtFuggleslt/hopgt
• ltprogtWonderful hop, light alcohol,
  good summer beerlt/progt
• ltcongtFragile excessive variance
  pub to publt/congt
• lt/detailsgt lt/tdgt lt/trgt
• lt/tablegt lt/Beersgt

26
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
• lt!-- http//www.w3.org is bound to n1 and n2 --gt
• ltx xmlnsn1"http//www.w3.org"
• xmlnsn2"http//www.w3.org" gt
• ltbad a"1" a"2" /gt
• ltbad n1a"1" n2a"2" /gt
• lt/xgt

27
5.3 Uniqueness of Attributes (contd)

• Each of the following is legal, the second is so
  because the default namespace does not apply to
  attribute names!
• lt!-- http//www.w3.org is bound to n1 and is the
  default --gt
• ltx xmlnsn1"http//www.w3.org"
• xmlns"http//www.w3.org" gt
• ltgood a"1" b"2" /gt
• ltgood a"1" n1a"2" /gt
• lt/xgt

28
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.

29
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.

30
Examples of namespace partitions

• lt!-- 1 --gt ltsection xmlns'urncombooks-r-us'gt
• lt!-- 2 --gt lttitlegtBook-Signing Eventlt/titlegt
• lt!-- 3 --gt ltsigninggt
• lt!-- 4 --gt ltauthor title"Mr" name"Vikram
  Seth" /gt
• lt!-- 5 --gt ltbook title"A Suitable Boy"
  price"22.95" /gt
• lt/signinggt lt/sectiongt
• Note Titles in 4 and 5 belong to different
  partitions
• lt!-- 1 --gt ltRESERVATION xmlnsHTML"http//www.w3.
  org/TR/REC-html40"gt
• lt!-- 2 --gt ltNAME HTMLCLASS"largeSansSerif"gtLaym
  an, Alt/NAMEgt
• lt!-- 3 --gt ltSEAT CLASS"Y" HTMLCLASS"largeMonot
  ype"gt33Blt/SEATgt
• lt!-- 4 --gt ltHTMLA HREF'/cgi-bin/ResStatus'gtChec
  k Statuslt/HTMLAgt
• lt!-- 5 --gt ltDEPARTUREgt1997-05-24T0755001lt/DEPA
  RTUREgt
• lt/RESERVATIONgt
• Notes 1. two namespace default and HTML.

31
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 1.1 W3C Candidate
  Recommendation 18 December 2002
• IRI draft
• Internationalized Resource Identifiers (IRIs).
  Available at http//www.w3.org/International/iri-e
  dit/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