Namespaces in XML 1.1
Namespaces in XML 1.1
W3C Recommendation 4 February 2004
This version:
Latest version:
Previous version:
Editors:
Tim Bray, Textuality
Dave Hollander, Contivo, Inc.
Andrew Layman, Microsoft
Richard Tobin, University of Edinburgh and Markup Technology Ltd
- Version 1.1
Please refer to the
errata
for this document, which may
include some normative corrections.
See also
translations
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rules apply.
Abstract
XML namespaces provide a simple method for qualifying
element and attribute
names used in Extensible Markup Language documents by associating them
with namespaces identified by
IRI
references.
Status of this Document
This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the
W3C technical reports index
at http://www.w3.org/TR/.
This document is a
Recommendation
of the W3C.
It has been reviewed by W3C Members and other interested parties, and has
been endorsed by the Director as a W3C Recommendation. It is a stable document and may be used as reference material or cited as a normative reference from another document. W3C's role in making the
Recommendation is to draw attention to the specification and to promote its widespread deployment.
This enhances the functionality and interoperability of the Web.
This document
is a product of the
W3C XML Activity
The English version of this specification is the only normative version. However,
for translations of this document, see
Documentation of intellectual property possibly relevant to this recommendation
may be found at the Working Group's public
IPR disclosure page
Known implementations are documented in the
Namespaces 1.1 implementation report
A test suite is also available via the
XML Test Suite
page.
Please report errors in this document to
xml-names-editor@w3.org
public
archives
are available. The errata list
for this document is available
at
Table of Contents
Motivation and Summary
1.1
A Note on Notation and Usage
XML Namespaces
2.1
Basic Concepts
2.2
Use of IRIs as Namespace Names
2.3
Comparing IRI References
Declaring Namespaces
Qualified Names
Using Qualified Names
Applying Namespaces to Elements and Attributes
6.1
Namespace Scoping
6.2
Namespace Defaulting
6.3
Uniqueness of Attributes
Conformance of Documents
Conformance of Processors
Internationalized Resource Identifiers (IRIs)
Appendices
Normative References
Other references
(Non-Normative)
The Internal Structure of XML Namespaces
(Non-Normative)
Changes since version 1.0
(Non-Normative)
Acknowledgements
(Non-Normative)
1 Motivation and Summary
We envision applications of Extensible Markup Language (XML) where
a single XML document may
contain elements and attributes
(here referred to as a "markup vocabulary")
that are defined for and used by multiple software modules.
One motivation for this is modularity: if such a markup vocabulary exists
which is well-understood and for which there is useful software
available, it is better to re-use this markup rather than re-invent it.
Such documents, containing multiple markup vocabularies,
pose problems of recognition and collision. Software modules need to
be able to recognize the elements and attributes which they are designed
to process, even in the face
of "collisions" occurring when markup intended for some other software
package uses the same element
name
or attribute name.
These considerations require that document constructs
should have names constructed so as to avoid clashes
between names from different markup vocabularies.
This specification describes a mechanism,
XML namespaces
, which accomplishes this
by assigning
expanded names
to elements and attributes.
1.1 A Note on Notation and Usage
Where
EMPHASIZED
, the key words
MUST
MUST NOT
REQUIRED
SHOULD
SHOULD NOT
MAY
in this document are to be interpreted as described in
[Keywords]
Note that many of the
nonterminals in the productions in
this specification are defined not here but in
the XML specification
[XML]
When nonterminals defined here have the same names as nonterminals
defined in the XML specification, the productions here
in all cases match a subset of the strings matched by the
corresponding ones there.
In this document's productions,
the
NSC
is a "Namespace Constraint",
one of the rules that documents conforming to this specification
MUST
follow.
2 XML Namespaces
2.1 Basic Concepts
Definition
An
XML namespace
is identified by
an
IRI reference
element and attribute names
may be placed in an XML namespace using the mechanisms described
in this specification.
Definition
An
expanded name
is a pair consisting of a
namespace name
and a
local name
Definition
For a name
in a namespace identified
by an IRI
, the
namespace name
is
. For a name
that is not in a namespace, the
namespace name
has no value.
Definition
In either case the
local name
is
It is this combination of the universally managed IRI namespace
with the vocabulary's local names that is effective in avoiding
name clashes.
IRI references can contain characters not allowed in names, and are often
inconveniently long, so expanded names are not used directly to name
elements and attributes in XML documents. Instead
qualified names
are used.
Definition
qualified name
is a name subject to namespace interpretation.
In documents conforming to this specification,
element and attribute names appear as qualified names.
Syntactically, they are either
prefixed names
or
unprefixed names
An attribute-based declaration syntax is provided to bind prefixes to
namespace names and to bind a default namespace that applies to
unprefixed element names;
these declarations are scoped by the elements on which they appear so that
different bindings may apply in different parts of a document.
Processors conforming to this specification
MUST
recognize and act on these declarations and prefixes.
2.2 Use of IRIs as Namespace Names
The empty string, though it is a legal IRI
reference, cannot be used as a namespace name.
The use of relative IRI references,
including same-document references, in namespace declarations is
deprecated.
Note:
This deprecation of relative URI references was decided on by a
W3C XML Plenary Ballot
[Relative URI deprecation]
. It also declares that
"later specifications such as DOM, XPath, etc. will define no
interpretation for them".
2.3 Comparing IRI References
IRI references identifying namespaces are compared when determining
whether a name belongs to a given namespace, and whether two names
belong to the same namespace.
Definition
The two IRIs are treated as strings, and they are
identical
if and only if the strings are identical, that is, if they
are the same sequence of characters.
The comparison is case-sensitive, and no %-escaping is done or undone.
A consequence of this is that IRI references which are not identical
in this sense may resolve to the same resource. Examples include
IRI references which differ only in case or %-escaping, or which are
in external entities which have different base URIs (but note that
relative IRIs are deprecated as namespace names).
In a namespace declaration, the IRI reference is the
normalized value
of the attribute, so replacement of XML character and entity references
has already been done before any comparison.
Examples:
The IRI references below are all different for the purposes of identifying
namespaces, since they differ in case:
The IRI references below are also all different for the purposes of identifying
namespaces:
As are these:
If the entity
eacute
has been defined to be
the start tags below all contain namespace declarations binding
the prefix
to the same IRI reference,
Because of the risk of confusion between IRIs that would be equivalent
if dereferenced, the use of %-escaped characters in namespace names is
strongly discouraged.
3 Declaring Namespaces
Definition
: A namespace
(or more precisely, a namespace binding)
is
declared
using
a family of reserved attributes.
Such an attribute's name must either
be
xmlns
or
begin
xmlns:
These attributes, like any other XML attributes, may be provided
directly or by
default
Attribute Names for Namespace Declaration
[1]
NSAttName
::=
PrefixedAttName
DefaultAttName
[2]
PrefixedAttName
::=
'xmlns:'
NCName
[NSC: Reserved Prefixes and Namespace Names]
[3]
DefaultAttName
::=
'xmlns'
[4]
NCName
::=
NCNameStartChar
NCNameChar
/* An XML
Name
, minus the ":" */
[5]
NCNameChar
::=
NameChar
- ':'
[5a]
NCNameStartChar
::=
NameStartChar
- ':'
The attribute's
normalized value
MUST
be either an IRI reference — the
namespace name
identifying the namespace —
or an empty string.
The namespace name, to serve its
intended purpose,
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).
Uniform Resource Names
[RFC2141]
is an example of a syntax that
is designed with these goals in mind.
However, it should be noted that ordinary URLs can be managed in such a way as
to achieve these same goals.
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 the scope of the element to which the declaration
is attached.]
Definition
: If the
attribute name matches
DefaultAttName
then the
namespace name
in the
attribute value is
that of the
default namespace
in the scope of the element to which the declaration
is attached.]
Default namespaces and overriding of declarations are discussed in
6 Applying Namespaces to Elements and Attributes
An example namespace declaration, which associates the
namespace prefix
edi
with the namespace name
Namespace constraint: Reserved Prefixes and Namespace Names
The prefix
xml
is by definition bound to the namespace name
. It
MAY
but need not, be
declared, and
MUST NOT
be undeclared or bound to any other namespace name. Other prefixes
MUST NOT
be bound to this namespace name.
The prefix
xmlns
is used only to declare namespace bindings and is by
definition bound to the namespace name
. It
MUST NOT
be declared or undeclared. Other prefixes
MUST NOT
be bound to this namespace name.
All other prefixes beginning with the three-letter sequence x, m, l,
in any case combination, are reserved. This means that:
users
SHOULD NOT
use them except as defined by later specifications
processors
MUST NOT
treat them as fatal errors.
Though they are not themselves reserved, it is inadvisable to use
prefixed names whose LocalPart begins with the letters x, m, l, in any
case combination, as
these names would be reserved if used without a prefix.
4 Qualified Names
In XML
documents conforming to this specification, some
names (constructs corresponding to the nonterminal
Name
MUST
be
given as
qualified names
defined as follows:
Qualified Name
[6]
QName
::=
PrefixedName
UnprefixedName
[6a]
PrefixedName
::=
Prefix
':'
LocalPart
[6b]
UnprefixedName
::=
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
IRI
reference in a
namespace declaration
Definition
The
LocalPart
provides the
local part
of the qualified name.]
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.
5 Using Qualified Names
In XML documents conforming to this specification,
element
names
are given as
qualified names
, as
follows:
Element Names
[9]
STag
::=
'<'
QName
Attribute
)*
? '>'
[NSC: Prefix Declared]
[10]
ETag
::=
'QName
? '>'
[NSC: Prefix Declared]
[11]
EmptyElemTag
::=
'<'
QName
Attribute
)*
? '/>'
[NSC: Prefix Declared]
An example of a qualified name serving as an element name:
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]
An example of a qualified name serving as an attribute name:
Namespace constraint: Prefix Declared
The namespace prefix, unless it is
xml
or
xmlns
MUST
have been
declared in a
namespace declaration
attribute in either the start-tag of the element where the prefix
is used or in an ancestor element (i.e. an element in whose
content
the
prefixed markup occurs).
Furthermore, the attribute value in the innermost such declaration
MUST NOT
be
an empty string
This constraint may lead to operational difficulties in the case where
the namespace declaration attribute is provided, not directly in the XML
document entity
, but
via a default attribute declared in an external entity.
Such declarations may not be read by software which is based on a
non-validating XML processor.
Many XML applications, presumably including namespace-sensitive ones, fail to
require validating processors.
If correct operation with such applications is required
namespace declarations
MUST
be
provided either directly or via default attributes declared in the
internal subset of the DTD
Element names and attribute
names
are also given as qualified names when
they appear in declarations in the
DTD
Qualified Names in Declarations
[13]
doctypedecl
::=
'QName
ExternalID
)?
? ('['
markupdecl
PEReference
)*
']'
?)? '>'
[14]
elementdecl
::=
'QName
contentspec
? '>'
[15]
cp
::=
QName
choice
seq
('?' | '*' | '+')?
[16]
Mixed
::=
'('
'#PCDATA'
'|'
QName
)*
')*'
| '('
? '#PCDATA'
? ')'
[17]
AttlistDecl
::=
'QName
AttDef
? '>'
[18]
AttDef
::=
QName
NSAttName
AttType
DefaultDecl
Note that DTD-based validation is not namespace-aware in the following
sense: a DTD constrains the elements and attributes that may appear in
a document by their uninterpreted names, not by (namespace name, local
name) pairs. To validate a document that uses namespaces against a
DTD, the same prefixes must be used in the DTD as in the instance.
A DTD may however indirectly constrain the namespaces used in a valid
document by providing
#FIXED
values for attributes that
declare namespaces.
6 Applying Namespaces to Elements and Attributes
6.1 Namespace Scoping
The scope of a namespace declaration declaring a prefix extends from
the beginning of the start-tag in which it appears to the end of the
corresponding end-tag, excluding the scope of any inner declarations
with the same NSAttName part.
In the case of an empty tag, the scope is the tag itself.
Such a namespace declaration applies to all element and attribute
names within its scope whose prefix matches that specified in the
declaration.
The
expanded name
corresponding to a prefixed element or attribute name has the
IRI to which the
prefix
is bound as its
namespace name
and the
local part
as its
local name
1.1
"?>
Multiple namespace prefixes can be declared as attributes of a single element,
as shown in this example:
1.1
"?>
The attribute value in a namespace declaration for a prefix
MAY
be empty.
This has the effect, within the scope of the declaration, of removing
any association of the prefix with a namespace name. Further declarations
MAY
re-declare the prefix again:
6.2 Namespace Defaulting
The scope of a
default namespace
declaration
extends from the beginning of the
start-tag in which it appears to the end of the corresponding end-tag,
excluding the scope of any inner default namespace declarations.
In the case of an empty tag, the scope is the tag itself.
A default namespace declaration applies to all unprefixed element names
within its scope.
Default namespace declarations do not apply directly to attribute names;
the interpretation of unprefixed attributes is
determined by the element on which they appear.
If there is a default namespace declaration in scope, the
expanded name
corresponding to an unprefixed element name has the
IRI of the
default namespace
as its
namespace name
If there is no default namespace declaration in scope, the
namespace name has no value.
The namespace name for an unprefixed attribute name always has no value.
In all cases, the
local name
is
local part
(which is of course the same as the unprefixed name itself).
1.1
"?>
Moved to
here.
1.1
"?>
A larger example of namespace scoping:
1.1
"?>
This is a funny book!
The attribute value in a default namespace declaration
MAY
be empty.
This has the same
effect, within the scope of the declaration, of there being no default
namespace.
1.1
'?>
| Name | Origin | Description |
6.3 Uniqueness of Attributes
In XML documents conforming
to this specification, no tag
may contain two attributes which:
have identical names, or
have qualified names with the same
local part
and with
prefixes
which have been bound
to
namespace names
that
are
identical
This constraint is equivalent to requiring that no element have two
attributes with the same
expanded name
For example, each of the
bad
start-tags is illegal in the
following:
However, each of the following is legal, the second because the default
namespace does not apply to attribute names:
7 Conformance of Documents
This specification applies to XML 1.1 documents. To conform to this
specification, a document
MUST
be well-formed according to the XML 1.1
specification
[XML 1.1]
In XML documents which conform to this specification, element
and attribute names
MUST
match the production for
QName
and
MUST
satisfy the "Namespace Constraints". All other tokens in the
document which are
REQUIRED
for XML 1.1 well-formedness, to match the
XML production for
Name
MUST
match this specification's production for
NCName
Definition
A document is
namespace-well-formed
if it conforms to this specification.
It follows that in a namespace-well-formed document:
All element and attribute names contain either zero or one
colon;
No entity names, processing instruction targets, or notation names contain any colons.
In addition, a namespace-well-formed document may also be namespace-valid.
Definition
A namespace-well-formed document is
namespace-valid
if it is valid according to the XML 1.1 specification, and all tokens
other than element and attribute names which are
REQUIRED
for XML 1.1 validity, to match the XML production for
Name
match this specification's production for
NCName
It follows that in a namespace-valid document:
No attributes with a declared type of
ID
IDREF(S)
ENTITY(IES)
, or
NOTATION
contain any colons.
8 Conformance of Processors
To conform to this specification, a processor
MUST
report
violations of namespace well-formedness, with the exception that it
is not
REQUIRED
to check that namespace names are legal IRIs.
Definition
A validating XML processor that conforms to this specification
is
namespace-validating
if in addition
it reports violations of namespace validity.
9 Internationalized Resource Identifiers (IRIs)
Work is currently in progress to produce an RFC defining
Internationalized Resource Identifiers (IRIs). Since this work is not
yet complete, this section gives a syntactic definition of IRIs
for the purposes of this specification. The XML Core Working Group expects
to issue an erratum
replacing this section with a reference to the RFC when it is
published.
Users defining
namespaces are advised to restrict namespace
names to URIs until the RFC is published and software supporting
IRIs is in common use. Implementors are likewise advised not to reject
namespace names that violate the drafts in terms of the allowed characters.
For a more general definition and discussion of IRIs see
[IRI draft 5]
(work in progress).
URI references are restricted to a subset of the ASCII characters;
IRI references allow most Unicode characters from #xA0 onwards.
Earlier drafts of the IRI RFC (eg
[IRI draft 3]
) also
allowed some of the disallowed ASCII characters, but the current draft
[IRI draft 5]
) does not.
Definition
The
additional characters
allowed in IRIs by
[IRI draft 5]
are:
the Unicode plane 0 characters #xA0 - #xD7FF, #xF900-#xFDCF, #xFDF0-#xFFEF
the Unicode plane 1-14 characters #x10000-#x1FFFD ... #xD0000-#xDFFFD, #xE1000-#xEFFFD
Definition
An
IRI reference
is a string that can be converted to
a URI reference by applying the following steps:
Convert the hostname part, if present, using
the ToASCII operation specified in Section 4.1 of
[RFC3490]
with the flags
UseSTD3ASCIIRules and AllowUnassigned set to TRUE.
Escape all
additional characters
as follows:
Each additional character is converted to UTF-8
[RFC3629]
as one or more bytes.
The resulting bytes are escaped with
the URI escaping mechanism (that is, converted to %HH, where HH is
the hexadecimal notation of the byte value).
The original character is replaced by the resulting character sequence.
Note:
The algorithm in
[IRI draft 5]
includes a UCS normalization
step, but this makes no difference to which strings are IRI references.
A Normative References
Keywords
RFC 2119: Key words for use in RFCs to Indicate Requirement Levels
S. Bradner, ed.
IETF (Internet Engineering Task Force),
March 1997.
Available at
RFC2141
RFC 2141: URN Syntax
R. Moats, ed.
IETF (Internet Engineering Task Force),
May 1997.
Available at
RFC2396
RFC 2396: Uniform Resource Identifiers (URI): Generic Syntax
T. Berners-Lee, R. Fielding, and L. Masinter, eds.
IETF (Internet Engineering Task Force),
August 1998.
Available at
RFC2732
RFC 2732: Format for Literal IPv6 Addresses in URL's
R. Hinden, B. Carpenter, and L. Masinter, eds.
IETF (Internet Engineering Task Force),
December 1999.
Available at http://www.rfc-editor.org/rfc/rfc2732.txt.
RFC3490
RFC 3490: Internationalizing Domain Names in Applications (IDNA)
P. Faltstrom, P. Hoffman, and A. Costello, eds.
IETF (Internet Engineering Task Force),
March 2003.
Available at http://www.rfc-editor.org/rfc/rfc3490.txt
RFC3629
RFC 3629: UTF-8, a transformation format of ISO 10646
F. Yergeau, ed.
IETF (Internet Engineering Task Force),
November 2003.
Available at http://www.rfc-editor.org/rfc/rfc3629.txt
XML
Extensible Markup Language
(XML) 1.0 (Third Edition)
, Tim Bray, Jean
Paoli, C. M. Sperberg-McQueen, Eve Maler, and François Yergeau eds.
W3C (World Wide Web Consortium),
4 February 2004.
Available at
XML 1.1
Extensible Markup Language
(XML) 1.1
Tim Bray, Jean Paoli, C. M. Sperberg-McQueen, Eve Maler, and John Cowan eds.
W3C (World Wide Web Consortium),
4 February 2004.
Available at
B Other references (Non-Normative)
IRI draft 3
Internationalized Resource Identifiers (IRIs)
M. Duerst and M. Suignard eds.
March 2, 2003.
Available at
IRI draft 5
Internationalized Resource Identifiers (IRIs)
M. Duerst and M. Suignard eds.
October 26, 2003.
Available at
1.0 Errata
Namespaces in XML Errata
W3C (World Wide Web Consortium).
Available at
Relative URI deprecation
Results of W3C XML Plenary
Ballot on relative URI References
In namespace declarations
3-17 July 2000
Dave Hollander and
C. M. Sperberg-McQueen,
6 September 2000.
Available at
Requirements
Namespaces in XML 1.1 Requirements
Jonathan Marsh, ed.
W3C (World Wide Web Consortium),
March 2002.
Available at
C The Internal Structure of XML Namespaces (Non-Normative)
This appendix has been deleted.
D Changes since version 1.0 (Non-Normative)
This version incorporates the errata to version 1.0 as of 6 December 2002
[1.0 Errata]
. There are two further substantive changes:
A mechanism is provided for undeclaring prefixes;
Namespace names are IRIs, rather than URIs.
There are several editorial changes, including a number
of terminology changes and additions intended to produce greater
consistency. The non-normative appendix "The Internal Structure
of XML Namespaces" has been removed.
E Acknowledgements (Non-Normative)
This work reflects input from a very large number of people,
including especially the participants in the World Wide
Web Consortium XML Working Group and Special Interest Group
and the participants in the W3C Metadata Activity.
The contributions of Charles Frankston of Microsoft
were particularly valuable.