Namespace in XML

Namespace in XML

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Namespace in XML

Cheng-Chia Chen

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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.

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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

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0. quick views: Motivation

name clashes. Consider an XML language WidgetML which uses XHTML as a sublang

uage 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>

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lexical rules for namespace and local names: require every namespace to be a URIs (Universal Resource Identi

fiers) called its [namespace URI]. => name = [namespace URI]:[local part]

http://www.w3.org/TR/xhtml1:head namespace URI local partsame 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 anyt

hing. 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.

Qualifying names

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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:prefix="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.

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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= ""

Ex:

<ex xmlns=“http://test.com/” att1=“…” xmlns:s=“http://test.com/”>

<ex att1=“abc” > … </ex1>

<ex xmlns=“” s:att1=“abc”>…</ex1>

</example>

Default namespace affects element names only; it does not affect unprefixed attribute names. in Ex: 1. the 1st and 2nd <ex> belong to the same namespace != namesp

ace URI of 3rd <ex>

2. all <att>attributes belong to the same namespace.

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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 URI

http://www.widget.org; XHTML uses the namespace prefix xhtml which is assigned the URI htt

p://www.w3.org/TR/xhtml1.

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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 decla

rations and usesschemas

schemas should be used to declare all possible element/attribute names of a namespace.

namespace URIs could be tied together with schemas …

...

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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 deprecat

ed)

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1. Summary (cont’d)

Names from XML namespaces appear as qualified names, which contain a single colon, separating the name into a n

amespace prefix and a local part.

URI references can contain characters not allowed in na

mes, 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.

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2. Declaring Namespaces

[Definition:] A namespace is declared using a family of reserved attributes. Such an attribute's name must either be xml

ns or have xmlns: as a prefix.

These attributes, like any other XML attributes, may be provided directly or by default.

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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 | ExtenderNote: NCNames are legal XML Name containing no char ‘:’.<x xmlns:edi='http://ecommerce.org/schema'>  </x>

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2. Declaring Namespaces (cont’d)

[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.

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2. Declaring Namespaces (cont’d)

[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 no

t 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 m

ay be empty. Disable default namespace.

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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 ::= NCNameThe Prefix provides the namespace prefix part of the qua

lified 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.

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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'>



<edi:price units='Euro'>32.18</edi:price>

</x>

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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'>



<lineItem edi:taxClass="exempt">Baby food</lineItem>

</x>

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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

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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:<?xml version="1.0"?>



<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>

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5.1 Namespace Scoping:

Multiple namespace prefixes can be declared as attributes of a single element.

Example:

<?xml version="1.0"?>



<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>

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5.2 Namespace Defaulting

A default namespace is applied to the element where it is declared (if that element ha

s no namespace prefix), and to all elements with no prefix within the content of th

at element. If the URI reference in a default namespace decla

ration is empty, then unprefixed elements are not considered to be in any namespace.

Note :

Default namespaces do not apply directly to attributes !!!

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5.2 Namespace Defaulting: examples

<?xml version="1.0"?> <html xmlns='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"?>  <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> </book>

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5.2 Namespace Defaulting: a larger example

<?xml version="1.0"?>  <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>  <p xmlns='urn:w3-org-ns:HTML'> This is a <i>funny</i> book! </p> </notes> </book>

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5.2 Namespace Defaulting:

The default namespace can be set to the empty string to disable default namespace.

<?xml version='1.0'?> <Beers>



<table xmlns='http://www.w3.org/TR/REC-html40'>

<th><td>Name</td><td>Origin</td><td>Description</td></th>

<tr> 

<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>

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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 pr

efixes which have been bound to namespace names that are identical. // for namespace conformity

Example, each of the following bad start-tags is illegal:



<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>

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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!:



<x xmlns:n1="http://www.w3.org"

xmlns="http://www.w3.org" >

<good a="1" b="2" />

<good a="1" n1:a="2" />

</x>

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6. Conformance of Documents

In XML documents which conform to this specification, element types and attribute names must match the product

ion 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 o

r one colon. No entity names, PI targets, or notation names contain any

colons.

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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.

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Examples of namespace partitions

 <section xmlns='urn:com:books-r-us'>

 <title>Book-Signing Event</title>

 <signing>

 <author title="Mr" name="Vikram Seth" />

 <book title="A Suitable Boy" price="$22.95" />

</signing> </section>

Note: Titles in 4 and 5 belong to different partitions <RESERVATION xmlns:HTML="http://www.w3.org/TR/REC-html40">

 <NAME HTML:CLASS="largeSansSerif">Layman, A</NAME>

 <SEAT CLASS="Y" HTML:CLASS="largeMonotype">33B</SEAT>

 <HTML:A HREF='/cgi-bin/ResStatus'>Check Status</HTML:A>

 <DEPARTURE>1997-05-24T07:55:00+1</DEPARTURE>

</RESERVATION>

Notes: 1. two namespace: default and HTML.

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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 Decembe

r 2002

IRI draft

Internationalized Resource Identifiers (IRIs). Available at http://www.w

3.org/International/iri-edit/draft-duerst-iri-02.txt

RFC2396

IETF (Internet Engineering Task Force) RFC 2396: Uniform Resource Id

entifiers (URI): Generic Syntax, eds. T. Berners-Lee, R. Fielding, L. Mas

inter. August 1998. Available at ftp://ftp.ietf.org/rfc/rfc2396.txt

Namespace in XML

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