Namespaces

Elliotte Rusty Harold

XML & Web Services 2002 London

Tuesday, March 12, 2002

elharo@metalab.unc.edu

http://www.cafeconleche.org/

Raison d'etre

To distinguish between elements and attributes from different vocabularies with different meanings.
To group all related elements and attributes together so that a parser can easily recognize them.

The Need for Namespaces

The XLink specification defines an attribute with the name href. The XHTML specification also uses href attributes on some elements. And the XInclude specification uses href attributes.
An XSLT style sheet that will transform XHTML documents containing both Scalable Vector Graphics (SVG) pictures and MathML equations into XSL-Formatting object documents.
- The a, title, script, style and font elements in XHTML and SVG
- The table element in XHTML and XSL-FO
- The text element in XSLT and SVG
- The set element in MathML and SVG
An XSLT stylesheet that transforms a style sheet in an older version of the XSLT specification to a style sheet in a newer version of the XSLT specification.

Namespaces disambiguate elements

Namespaces disambiguate elements with the same name from each other by attaching different prefixes to names from different XML applications.
Each prefix is associated with a URI.
- Names whose prefixes are associated with the same URI are in the same namespace.
- Names whose prefixes are associated with different URIs are in different namespaces.

URIs

Uniform Resource Identifier
Two kinds:
- URLs: Uniform Resource Locators
- URNs: Uniform Resource Names
Which should be used for namespaces?
Absolute vs. relative

Namespace Syntax

Elements and attributes that are in namespaces have names that contain exactly one colon. They look like this:
```
rdf:description
xlink:type
xsl:template
```
Everything before the colon is called the prefix
Everything after the colon is called the local part.
The complete name including the colon is called the qualified name.

Namespace URIs

Each prefix is associated with a URI (technically a URI reference) called a namespace name.
For example, all elements in XSLT 1.0 style sheets are associated with the http://www.w3.org/1999/XSL/Transform URI.
The customary prefix xsl is a shorthand for the longer URI http://www.w3.org/1999/XSL/Transform.
You can't use the URI in the element name directly.
{http://www.w3.org/1999/XSL/Transform}template

http://www.w3.org/1999/XSL/Transform#template

Binding Prefixes to Namespace URIs

Prefixes are bound to namespace URIs by attaching an xmlns:prefix attribute to the prefixed element or one of its ancestors.

<svg:svg xmlns:svg="http://www.w3.org/2000/svg" 
 width="12cm" height="10cm">
  <svg:ellipse rx="110" ry="130" />
  <svg:rect x="4cm" y="1cm" width="3cm" height="6cm" />
</svg:svg>

Bindings have scope within the element where they're declared.
An SVG processor recognizes all three of these elements as SVG elements because they all have prefixes bound to the particular URI defined by the SVG specification.

Binding Prefixes to Namespace URIs Example

<xhtml:html xmlns:xhtml="http://www.w3.org/1999/xhtml"
            xmlns:mathml="http://www.w3.org/1998/Math/MathML">
  <xhtml:head><xhtml:title>Three Namespaces</xhtml:title></xhtml:head>
  <xhtml:body>
    <xhtml:h1 align="center">An Ellipse and a Rectangle</xhtml:h1>
    <svg:svg xmlns:svg="http://www.w3.org/2000/svg" 
             width="12cm" height="10cm">
      <svg:ellipse rx="110" ry="130" />
      <svg:rect x="4cm" y="1cm" width="3cm" height="6cm" />
    </svg:svg>
    <xhtml:p>The equation for ellipses</xhtml:p>
<mathml:math>
  <mathml:apply>
    <mathml:eq/>
    <mathml:cn> 1 </mathml:cn>
    <mathml:apply>
      <mathml:plus/>
      <mathml:apply>
        <mathml:divide/>
        <mathml:apply>
          <mathml:power/>
          <mathml:ci> x </mathml:ci>
          <mathml:cn> 2 </mathml:cn>
        </mathml:apply>
        <mathml:apply>
          <mathml:power/>
          <mathml:ci> a </mathml:ci>
          <mathml:cn> 2 </mathml:cn>
        </mathml:apply>
      </mathml:apply>
      <mathml:apply>
        <mathml:divide/>
        <mathml:apply>
          <mathml:power/>
          <mathml:ci> y </mathml:ci>
          <mathml:cn> 2 </mathml:cn>
        </mathml:apply>
        <mathml:apply>
          <mathml:power/>
          <mathml:ci> b </mathml:ci>
          <mathml:cn> 2 </mathml:cn>
        </mathml:apply>        
      </mathml:apply>
    </mathml:apply>
 </mathml:apply>
</mathml:math>
    <xhtml:hr/>
    <xhtml:p>Last Modified January 10, 2002</xhtml:p>    
  </xhtml:body>
</xhtml:html>

Binding namespaces to prefixes with #FIXED ATTLIST declarations

<!ATTLIST svg:svg xmlns:svg (CDATA) #FIXED "http://www.w3.org/2000/svg">

<svg:svg width="12cm" height="10cm">
  <svg:ellipse rx="110" ry="130" />
  <svg:rect x="4cm" y="1cm" width="3cm" height="6cm" />
</svg:svg>

Namespaces and Attributes

An attribute can be placed in a namespace by prefixing its name.
Normally only used for global atributes that can appear on any element such as XLink attributes rather than attributes that are tied to a specific element.
Unprefixed attributes are never in any namespace.
Being an attribute of an element in the http://www.w3.org/1999/xhtml namespace is not sufficient to put the attribute in the http://www.w3.org/1999/xhtml namespace.
The only way an attribute belongs to a namespace is if it has a declared prefix, like xlink:type and xlink:href.

<xhtml:html xmlns:xhtml="http://www.w3.org/1999/xhtml"
            xmlns:xlink="http://www.w3.org/1999/xlink">
  <xhtml:head><xhtml:title>Three Namespaces</xhtml:title></xhtml:head>
  <xhtml:body>
    <xhtml:h1 align="center">An Ellipse and a Rectangle</xhtml:h1>
    <svg:svg xmlns:svg="http://www.w3.org/2000/svg" 
             width="12cm" height="10cm">
      <svg:ellipse rx="110" ry="130" />
      <svg:rect x="4cm" y="1cm" width="3cm" height="6cm" />
    </svg:svg>
    <xhtml:p xlink:type="simple" 
      xlink:href="ellipses.html">
      More about ellipses
    </xhtml:p>
    <xhtml:p xlink:type="simple" xlink:href="rectangles.html">
      More about rectangles
    </xhtml:p>
    <xhtml:hr/>
    <xhtml:p>Last Modified February 13, 2000</xhtml:p>    
  </xhtml:body>
</xhtml:html>

URIs matter, not prefixes

Many XML applications have recommended prefixes. For example, SVG elements often use the prefix svg and Resource Description Framework (RDF) elements often have the prefix rdf. However, these prefixes are simply conventions, and can be changed based on necessity, convenience or whim.
Before a prefix can be used, it must be bound to a URI.
These URIs are standardized, not the prefixes.
The prefix can change as long as the URI stays the same.

The Default Namespace

Indicates that an unprefixed element and all its unprefixed descendant elements belong to a particular namespace by attaching an xmlns attribute with no prefix:

<DATASCHEMA xmlns="http://www.w3.org/2000/P3Pv1">
  <DATA name="vehicle.make" type="text" short="Make" 
        category="preference" size="31"/>
  <DATA name="vehicle.model" type="text" short="Model" 
        category="preference" size="31"/>
  <DATA name="vehicle.year" type="number" short="Year" 
        category="preference" size="4"/>
  <DATA name="vehicle.license.state." type="postal." short="State" 
        category="preference" size="2"/>
  <DATA name="vehicle.license.number" type="text" 
        short="License Plate Number" category="preference" size="12"/>
</DATASCHEMA>

Both the DATASCHEMA and DATA elements are in the http://www.w3.org/2000/P3Pv1 namespace.
Default namespaces apply only to elements, not to attributes. The name, type, short, category, and size attributes are not in any namespace.

Multiple default namespaces

You can change the default namespace within a particular element by adding an xmlns attribute to the element.

<html xmlns="http://www.w3.org/1999/xhtml"
      xmlns:xlink="http://www.w3.org/1999/xlink">
  <head><title>Three Namespaces</title></head>
  <body>
    <h1 align="center">An Ellipse and a Rectangle</h1>
    <svg xmlns="http://www.w3.org/2000/svg" 
         width="12cm" height="10cm">
      <ellipse rx="110" ry="130" />
      <rect x="4cm" y="1cm" width="3cm" height="6cm" />
    </svg>
    <p xlink:type="simple" xlink:href="ellipses.html">
      More about ellipses
    </p>
    <p xlink:type="simple" xlink:href="rectangles.html">
      More about rectangles
    </p>
    <hr/>
    <p>Last Modified February 13, 2000</p>    
  </body>
</html>

Binding default namespaces to elements with #FIXED ATTLISTs

<!ATTLIST svg xmlns (CDATA) #FIXED "http://www.w3.org/2000/svg">

<svg width="12cm" height="10cm">
  <ellipse rx="110" ry="130" />
  <rect x="4cm" y="1cm" width="3cm" height="6cm" />
</svg>

Namespace URIs do not necessarily point to a document, page, or schema

Purely formal
Can point somewhere but do not have to

Parsers compare namespace URIs character by character

These are three different namespaces:

http://www.w3.org/1999/XSL/Transform
http://www.w3.org/1999/XSL/Transform/
http://www.w3.org/1999/XSL/Transform/index.html

Namespaces and DTDs

DTDs must declare the qualified names
<!ELEMENT svg:text (#PCDATA)>
If the prefix changes, the DTD needs to change too.

Parameter Entity References for Namespace Prefixes

Must use double indirection

<!ENTITY % NS.prefixed     "IGNORE" >
<!ENTITY % MATHML.prefixed "%NS.prefixed;" >

<!ENTITY % MATHML.xmlns    "http://www.w3.org/1998/Math/MathML" >
<!ENTITY % MATHML.prefix   "m" >
<![%MATHML.prefixed;[
  <!ENTITY % MATHML.xmlns.extra.attrib  "" >
]]>
<!ENTITY % MATHML.xmlns.extra.attrib "" >

<![%MATHML.prefixed;[
<!ENTITY % MATHML.pfx  "%MATHML.prefix;:" >
<!ENTITY % MATHML.xmlns.attrib
     "xmlns:%MATHML.prefix;  CDATA   #FIXED '%MATHML.xmlns;'
      %MATHML.xmlns.extra.attrib;"
>
]]>
<!ENTITY % MATHML.pfx  "" >
<!ENTITY % MATHML.xmlns.attrib
     "xmlns CDATA #FIXED '%MATHML.xmlns;'
      %MATHML.xmlns.extra.attrib;"
>

<![%NS.prefixed;[
<!ENTITY % XHTML.xmlns.extra.attrib 
     "%MATHML.xmlns.attrib;" >
]]>
<!ENTITY % XHTML.xmlns.extra.attrib ""
>

<!-- Section B: MathML Qualified Names ::::::::::::::::::::::::::::: -->

<!-- 9. This section declares parameter entities used to provide
        namespace-qualified names for all MathML element types.
-->

<!ENTITY % abs.qname  "%MATHML.pfx;abs" >
<!ENTITY % and.qname  "%MATHML.pfx;and" >

...

Only works in external DTD subset

Overriding the prefix in the internal DTD subset

<?xml version="1.0"?>
<!DOCTYPE math:math 
 PUBLIC "-//W3C//DTD MathML 2.0//EN" "mathml2.dtd" [
  <!ENTITY % NS.prefixed   "INCLUDE" >
  <!ENTITY % MATHML.prefix "math" >
]>
<math:math xmlns="http://www.w3.org/1998/Math/MathML">
  <math:apply>
    <math:eq/>
    <math:cn> 1 </math:cn>
    <math:apply>
      <math:plus/>
      <math:apply>
        <math:divide/>
        <math:apply>
          <math:power/>
          <math:ci> x </math:ci>
          <math:cn> 2 </math:cn>
        </math:apply>
        <math:apply>
          <math:power/>
          <math:ci> a </math:ci>
          <math:cn> 2 </math:cn>
        </math:apply>
      </math:apply>
      <math:apply>
        <math:divide/>
        <math:apply>
          <math:power/>
          <math:ci> y </math:ci>
          <math:cn> 2 </math:cn>
        </math:apply>
        <math:apply>
          <math:power/>
          <math:ci> b </math:ci>
          <math:cn> 2 </math:cn>
        </math:apply>        
      </math:apply>
    </math:apply>
 </math:apply>
</math:math>

Namespaces and Schemas

URI matters; prefix doesn't
Do not have to declare xmlns and xmlns:prefix attributes
Namespaces are used on elements and attribute values, but not attribute names

A Song with a Namespace

<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<SONG xmlns="http://ibiblio.org/xml/namespace/song"
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
      xsi:schemaLocation = 
       "http://www.cafeconleche.org/namespace/song 
        namespace_song.xsd"
>
  <TITLE>Hot Cop</TITLE>
  <PHOTO ALT="Victor Willis in Cop Outfit" WIDTH="100" HEIGHT="200"/>
  <COMPOSER>Jacques Morali</COMPOSER>
  <COMPOSER>Henri Belolo</COMPOSER>
  <COMPOSER>Victor Willis</COMPOSER>
  <PRODUCER>Jacques Morali</PRODUCER>
  <PUBLISHER>PolyGram Records</PUBLISHER>
  <YEAR>1978</YEAR>
  <ARTIST>Village People</ARTIST>  
</SONG>

A Schema for a Document that Uses the Default Namespace

<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema"
  xmlns="http://ibiblio.org/xml/namespace/song"
  targetNamespace="http://ibiblio.org/xml/namespace/song"
  elementFormDefault="qualified"
  attributeFormDefault="unqualified"
>
 
  <xsd:element name="SONG" type="SongType"/>

  <xsd:complexType name="SongType">
    <xsd:sequence>
      <xsd:element name="TITLE" type="xsd:string"/>
      <xsd:element name="PHOTO" type="PhotoType"  
        minOccurs="0"/>
      <xsd:element name="COMPOSER"  type="xsd:string" 
        maxOccurs="unbounded"/>
      <xsd:element name="PRODUCER"  type="xsd:string" 
        minOccurs="0" maxOccurs="unbounded"/>
      <xsd:element name="PUBLISHER" type="xsd:string" 
        minOccurs="0"/>    
      <xsd:element name="YEAR" type="xsd:gYear"/>
      <xsd:element name="ARTIST" type="xsd:string" 
        minOccurs="0" maxOccurs="unbounded"/>
    </xsd:sequence>
  </xsd:complexType>

  <xsd:complexType name="PhotoType">
    <xsd:complexContent>
       <xsd:restriction base="xsd:anyType">
         <xsd:attribute name="ALT"    type="xsd:string"/>
         <xsd:attribute name="WIDTH"  type="xsd:nonNegativeInteger"/>
         <xsd:attribute name="HEIGHT" type="xsd:nonNegativeInteger"/> 
      </xsd:restriction>
    </xsd:complexContent>
  </xsd:complexType>
  
</xsd:schema>

Reversing the Default Namespace

<schema xmlns="http://www.w3.org/2001/XMLSchema"
  xmlns:song="http://www.cafeconleche.org/namespace/song"
  targetNamespace="http://www.cafeconleche.org/namespace/song"
  elementFormDefault="qualified"
  attributeFormDefault="unqualified"
>
 
  <element name="song:SONG" type="song:SongType"/>

  <complexType name="song:SongType">
    <sequence>
      <element name="song:TITLE" type="string"/>
      <element name="song:PHOTO" type="song:PhotoType"  
        minOccurs="0"/>
      <element name="song:COMPOSER"  type="string" 
        maxOccurs="unbounded"/>
      <element name="song:PRODUCER"  type="string" 
        minOccurs="0" maxOccurs="unbounded"/>
      <element name="song:PUBLISHER" type="string" 
        minOccurs="0"/>    
      <element name="song:YEAR" type="gYear"/>
      <element name="song:ARTIST" type="string" 
        minOccurs="0" maxOccurs="unbounded"/>
    </sequence>
  </complexType>

  <complexType name="song:PhotoType">
    <complexContent>
       <restriction base="anyType">
         <attribute name="ALT"    type="string"/>
         <attribute name="WIDTH"  type="nonNegativeInteger"/>
         <attribute name="HEIGHT" type="nonNegativeInteger"/> 
      </restriction>
    </complexContent>
  </complexType>
  
</schema>

Multiple Namespaces, Multiple Schemas

<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<SONG xmlns="http://www.cafeconleche.org/namespace/song"
      xmlns:xlink="http://www.w3.org/1999/xlink"
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
      xsi:schemaLocation = 
       "http://www.cafeconleche.org/namespace/song xlink_song.xsd
        http://www.w3.org/1999/xlink xlink.xsd"
>
  <TITLE>Hot Cop</TITLE>
  <PHOTO xlink:type="simple" xlink:href="hotcop.jpg"
         xlink:actuate="onLoad" xlink:show="embed"
    ALT="Victor Willis in Cop Outfit" WIDTH="100" HEIGHT="200"/>
  <COMPOSER>Jacques Morali</COMPOSER>
  <COMPOSER>Henri Belolo</COMPOSER>
  <COMPOSER>Victor Willis</COMPOSER>
  <PRODUCER>Jacques Morali</PRODUCER>
  <PUBLISHER>PolyGram Records</PUBLISHER>
  <YEAR>1978</YEAR>
  <ARTIST>Village People</ARTIST>  
</SONG>

XLink Schema

<?xml version="1.0"?>
<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema"
  xmlns="http://www.w3.org/1999/xlink"
  targetNamespace="http://www.w3.org/1999/xlink"
>

  <xsd:attribute name="type" type="xsd:string" 
                 fixed="simple" />
  <xsd:attribute name="href" type="xsd:anyURI" />
  <xsd:attribute name="actuate" type="xsd:string"
                 fixed="onLoad" />
  <xsd:attribute name="show" type="xsd:string"
                 fixed="embed" />

</xsd:schema>

Song Schema with XLink Support

<?xml version="1.0"?>
<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema"
  xmlns="http://www.cafeconleche.org/namespace/song"
  xmlns:xlink="http://www.w3.org/1999/xlink"
  targetNamespace="http://www.cafeconleche.org/namespace/song"
  elementFormDefault="qualified"
  attributeFormDefault="unqualified"
>

  <xsd:import namespace="http://www.w3.org/1999/xlink"
              schemaLocation="xlink.xsd"/>

  <xsd:element name="SONG" type="SongType"/>

  <xsd:complexType name="PhotoType">
    <xsd:attribute name="WIDTH"  type="xsd:positiveInteger"
                   use="required" />
    <xsd:attribute name="HEIGHT" type="xsd:positiveInteger"
                   use="required" />
    <xsd:attribute name="ALT"    type="xsd:string"
                   use="required" />
    <xsd:attribute ref="xlink:type"/>
    <xsd:attribute ref="xlink:href" use="required"/>
    <xsd:attribute ref="xlink:actuate"/>
    <xsd:attribute ref="xlink:show"/>                 
  </xsd:complexType>

  <xsd:complexType name="SongType">
    <xsd:sequence>
      <xsd:element name="TITLE"     type="xsd:string"/>
      <xsd:element name="PHOTO"     type="PhotoType"/>
      <xsd:element name="COMPOSER"  type="xsd:string"
                   maxOccurs="unbounded"/>
      <xsd:element name="PRODUCER"  type="xsd:string"
                   minOccurs="0" maxOccurs="unbounded"/>
      <xsd:element name="PUBLISHER" type="xsd:string"
                   minOccurs="0"/>
      <xsd:element name="YEAR"      type="xsd:gYear"/>
      <xsd:element name="ARTIST"    type="xsd:string"
                   maxOccurs="unbounded"/>
    </xsd:sequence>
  </xsd:complexType>

</xsd:schema>

elementFormDefault and attributeFormDefault

Specifies whether locally declared elements and attributes are namespace qualified

In a schema, a locally declared element is an element declared directly inside a complexType (not by reference)

<complexType name="CarType">
  <sequence>
    <element name="steering_wheel" type="SteeringWheelType"/>
    <element name="wheel" type="WheelType"/>
  </sequence>
</complexType>

In an instance document, this is a locally declared element:

<vg:car xmlns:vg="http://www.vg.org/autos/" 
  width="12cm" height="10cm">
    <steering_wheel diameter="24cm" />
    <wheel number="4" />
</vg:car>

Does not apply to globally declared items
Default is unqualified for both elements and attributes. This is correct for attributes but backwards from what it should be for elements.
Always set elementFormDefault to qualified and don't use local elements.

How Parsers Handle Namespaces

Namespaces were added to XML 1.0 after the fact, but care was taken to ensure backwards compatibility.
An XML 1.0 parser that does not know about namespaces will most likely not have any troubles reading a document that uses namespaces.
A namespace aware parser also checks to see that all prefixes are mapped to URIs. Otherwise it behaves almost exactly like a non-namespace aware parser.
Other software that sits on top of the raw XML parser, an XSLT engine for example, may treat elements differently depending on what namespace they belong to. However, the XML parser itself mostly doesn't care as long as all well-formedness and namespace constraints are met.
A possible exception occurs in the unlikely event that elements with different prefixes belong to the same namespace or elements with the same prefix belong to different namespaces
Many parsers have the option of whether to report namespace violations so that you can turn namespace processing on or off as you see fit.

Namespaces in SAX

SAX2 is fully namespace aware:

public void startElement(String namespaceURI, String localName, String qualifiedName, Attributes atts)
 throws SAXException
public void endElement(String namespaceURI, String localName, String qualifiedName)
 throws SAXException
public void startPrefixMapping(String prefix, String uri)
 throws SAXException
public void endPrefixMapping(String prefix)
 throws SAXException

Namespace handling is controlled by these two properties:
- http://xml.org/sax/features/namespaces
- http://xml.org/sax/features/namespace-prefixes
http://xml.org/sax/features/namespaces feature determines whether namespace URIs and local names are passed to startElement() and endElement(). The default, true, passes both namespace URIs and local names. However, if http://xml.org/sax/features/namespaces is false, then the parser may pass the namespace URI and the local name, or it may just pass empty strings for these two arguments.
http://xml.org/sax/features/namespace-prefixes feature determines two things:
- Whether or not namespace declaration xmlns and xmlns:prefix attributes are included in the Attributes object passed to startElement(). The default, false, is not to include them.
- Whether or not the qualified names should be passed as the third argument to the startElement() method. The default, false, is, not to require qualified names. However, even if http://xml.org/sax/features/namespace-prefixes is false, parsers are allowed to report the qualified name, and most do so.
In other words,
- The parser is only guaranteed to provide the namespace URIs and local names of elements and attributes if http://xml.org/sax/features/namespaces is true (which it is by default).
- The parser is only guaranteed to provide the qualified names of elements and attributes if http://xml.org/sax/features/namespace-prefixes is true (which it is not by default).
- The parser provides namespace declaration attributes if and only if http://xml.org/sax/features/namespace-prefixes is true (which it is not by default).
- The parser always has the option to provide the namespace URI, local name, and qualified name, regardless of the values of http://xml.org/sax/features/namespaces and http://xml.org/sax/features/namespace-prefixes. However, you should not rely on this behavior.
Bottom line: the defaults are fine as long as you don’t care about namespace prefixes, only local names and URIs.

Prefix Mappings

By default namespace declaration attributes such as xmlns="http://ns.cafeconleche.org/Orders/" and xmlns:xlink="http://www.w3.org/1999/xlink" are not included in the list of attributes passed to startElement().
Instead each such namespace declaration attribute is signaled by a call to startPrefixMapping() immediately before the startElement() call corresponding to the start tag of the element where the declaration appears. Furthermore, the endElement() call corresponding to the end tag of that element is immediately followed by an endPrefixMapping() call.
public void startPrefixMapping(String prefix, String uri) throws SAXException public void endPrefixMapping(String prefix) throws SAXException
Most of the time you can ignore these events. As long as you only care about URIs and prefixes on element and attribute names, then there’s really no need to pay attention to these methods. You simply inspect the URIs and prefixes passed in the arguments to the startElement() method.
However, in a few applications, including XSLT and schemas, prefixes appear in attribute values and even element content. If so, you need to keep track of which URIs particular prefixes are bound to where.
Keep the prefixes and URIs on a per-document stack. startPrefixMapping() pushes the pair onto the stack. endPrefixMapping() pops them off. If at any time while parsing you need to know the current binding of a prefix you’ve encountered, simply search down the stack starting with the top. The first binding you encounter for the prefix is the one you want.

org.xml.sax.helpers.NamespaceSupport implements this logic.

package org.xml.sax.helpers;

public class NamespaceSupport {

  public final static String XMLNS 
   = "http://www.w3.org/XML/1998/namespace";

  public NamespaceSupport();
  
  public void        reset();
  public void        pushContext();
  public void        popContext();
  public boolean     declarePrefix(String prefix, String uri);
  public String[]    processName(String qualifedName, 
   String parts[], boolean isAttribute);
  public String      getURI(String prefix);
  public Enumeration getPrefixes();
  public String      getPrefix(String uri);
  public Enumeration getPrefixes(String uri);
  public Enumeration getDeclaredPrefixes();

}

Call pushContext() in the first startPrefixMapping() event before each startElement(), declarePrefix() every startPrefixMapping() event, and popContext() in the first endPrefixMapping() event after each endElement() event.

Add namespace declarations to a context by passing their prefix and URI as strings to declarePrefix() inside each startPrefixMapping() call. Use the empty string as the prefix to declare the default namespace:

  private NamespaceSupport namespaces;
  private boolean needNewContext = true;

  public void startDocument() {
    namespaces = new NamespaceSupport(); 
  }
  
  public void startPrefixMapping(String prefix, String uri)
   throws SAXException {
   
    if (needNewContext) {
      namespaces.pushContext();
      needNewContext = false;
    }
    namespaces.declarePrefix(prefix, uri);
    
  }
   
  public void endPrefixMapping(String prefix) 
   throws SAXException {
   
    if (!needNewContext) {
      namespaces.popContext();
      needNewContext = true;
    }
   
  }

The getDeclaredPrefixes(), getPrefix(), getPrefixes(), and getURI() methods all return various information about the namespaces in scope in the current context. For example, this startElement() method prints the namespace URI for the values of type attributes, such as might be found in a schema:

 public void startElement(String namespaceURI, String localName,
   String qualifiedName, Attributes atts) throws SAXException {
  
    String value = atts.getValue("type");
    if (value != null) {
      String prefix = "";
      if (value.indexOf(':') >= 0) {
        prefix = value.substring(0, value.indexOf(':')); 
      }
      String uri = namespaces.getURI(prefix);
    }
   
  }

To reuse a NamespaceSupport for a new document, call reset(), probably from the startDocument() method.

Namespaces in DOM

DOM Level 2 methods are namespace aware:

public Element createElementNS(String namespaceURI, String qualifiedName)
 throws DOMException
public Attr createAttributeNS(String namespaceURI, String qualifiedName)
 throws DOMException
public NodeList getElementsByTagNameNS(String namespaceURI, String localName)

These set all four properties: node name, local name, namespace URI, and prefix.

DOM Level 1 Methods are not namespace aware:

public Element createElement(String tagName) throws DOMException;
public Attr createAttribute(String name) throws DOMException;
public NodeList getElementsByTagName(String tagname);

These set the node name only. The local name, namespace URI, and prefix are all null.

Unlike SAX, DOM does include namespace declaration attributes such as xmlns="http://ns.cafeconleche.org/Orders/" and xmlns:xlink="http://www.w3.org/1999/xlink" in lists of attributes.
Provide a namespace URI and full prefixed name when creating elements and attributes in a namespace
Use local name and URI when searching for an element or attribute
Serialization is tricky, and not all implementations get the namespace declarations in the right place.

Namespaces in JDOM

JDOM is fully namespace aware
A namespace URL (which may be the empty string) is attached to each element and attribute.
Additional namespace declarations (beyond the element's own namespace) can be attached to any element:
public void addNamespaceDeclaration(Namespace additionalNamespace)
If you need to know what URI is mapped to a given prefix or what the default namespace is, you ask the nearest ancestor-or-self element:
public Namespace getNamespace(String prefix)
The Namespace class uses the flyweight design pattern so that multiple elements in the same namespace don't eat huge amounts of memory.
Serialization puts namespace declarations in the right places.

What Goes at the end of a Namespace URI?

The flaw is the conflation of name, location and identity but that flaw is the basic feature by which the WWW runs so we are stuck there. All the handwaving about URN/URI/URL doesn't avoid the simple fact that if one puts http:// anywhere in browser display space, the system colors it blue and puts up a finger.

The monkey expects a resource and when it doesn't get one, it shocks the monkey. Monkeys don't read specs to find out why they should be shocked. They turn red and put up a finger.

--Claude L. Bullard on the xml-dev mailing list

The Resource Directory Description Language

An XML application for pages at the end of a namespace URI.
Invented by Jonathan Borden, Tim Bray, and others on the xml-dev mailing list
A Resource Directory provides a text description of some class of resources and of other resources related to that class. It also contains a directory of links to these related resources.
An XML namespace is one possible kind of resource. Related resources might include schemas, stylesheets, Java content handlers, browser plug-ins, and more.
The RDDL DTD is an extension of XHTML Basic 1.0 using XHTML Modularization.
RDDL adds one new element to XHTML Basic 1.0: resource.

The resource Element

The resource element is in the http://www.rddl.org/ namespace.
The rddl prefix is customary.
The rddl:resource element is a simple XLink; that is, it has an xlink:type="simple" attribute
The rddl:resource element can be placed anywhere in HTML where a p element may appear.
The rddl:resource can contain may of the XHTML Basic elements (headings, paragraphs, lists, hyperlinks, forms, tables, images, meta information ) that the body element may contain.
The content of the rddl:resource element should describe the associated resource.
Resource elements can contain other resource elements.

Example RDDL resource Elements

For the http://www.cafeconleche.org/baseball/ namespace:

<rddl:resource xlink:href="baseball.css" 
    xlink:role="http://www.rddl.org/arcrole.htm#CSS">
  <div id="CSS" class="resource">
    <h3>CSS Stylesheet</h3>
    <p>A <a href="baseball.css">CSS stylesheet</a> 
    for baseball statistics documents.</p> 
  </div>
</rddl:resource>

<rddl:resource xlink:href="baseball.dtd"
    xlink:role="http://www.rddl.org/arcrole.htm#DTD">
  <div id="DTD" class="resource">
    <h3>DTD</h3>
    <p>A <a href="baseball.dtd">DTD</a> for , 
    baseball statistics</p>
  </div>
</rddl:resource>

An Example RDDL Document

<!DOCTYPE html PUBLIC "-//XML-DEV//DTD XHTML RDDL 1.0//EN" 
                      "http://www.openhealth.org/RDDL/rddl-xhtml.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" 
      xmlns:xlink="http://www.w3.org/1999/xlink"
      xmlns:rddl="http://www.rddl.org/">
<head>
<title>RDDL Resources for Baseball Statistics</title>
</head>
<body>
<h1>RDDL Resources for Baseball Statistics</h1>

<p>This is a sample RDDL document 
used in Elliotte Rusty Harold's seminars
including</p>

<ul>
  <li>XLinks and XPointers</li>
  <li>Namespaces in XML</li>
</ul>

<p>It describes resources related to
the baseball statistics DTD developed in the XML
Bible and located at the namespace 
<code>http://www.cafeconleche.org/baseball/</code>.</p>

<p>
<img src="../smallbiblecover.jpg" width="127" height='156'
  alt="Cover of the XMl Bible"/>
</p>

<rddl:resource xlink:href="baseball.css" 
    xlink:role="http://www.rddl.org/arcrole.htm#CSS">
  <div id="CSS" class="resource">
    <h3>CSS Stylesheet</h3>
    <p>A <a href="baseball.css">CSS stylesheet</a> 
    for baseball statistics documents.</p> 
  </div>
</rddl:resource>

<rddl:resource xlink:href="baseball.dtd"
    xlink:role="http://www.rddl.org/arcrole.htm#DTD">
  <div id="DTD" class="resource">
    <h3>DTD</h3>
    <p>A <a href="baseball.dtd">DTD</a> for , 
    baseball statistics</p>
  </div>
</rddl:resource>

<p>
Copyright 2001 <a href="http://www.macfaq.com/personal.html">Elliotte Rusty Harold</a><br class="empty"/>
<a href="mailto:elharo@metalab.unc.edu">elharo@metalab.unc.edu</a><br class="empty"/>
Last Modified Sunday, January 14, 2001
</p>

</body>
</html>

View in Browser

The xlink:href attribute

Contains a URI, possibly relative, pointing to the related resource

The xlink:role attribute

Identifies the type of the related resource with an absolute URI
Can be the namespace URI of the root element:

http://www.w3.org/2000/10/XMLSchema

XML Schema

http://www.rddl.org/arcrole.htm#SOCAT

OASIS Open Catalog

http://www.ascc.net/xml/schematron

Schematron

http://www.xml.gr.jp/xmlns/relaxCore

RELAX schema

http://www.w3.org/2000/01/rdf-schema#

RDF Schema
Can be a MIME media type URI at http://www.isi.edu/in-notes/iana/assignments/media-types/

http://www.isi.edu/in-notes/iana/assignments/media-types/application/pdf

Acrobat PDF version of the document
Can be a spec-defined URI:

http://www.rddl.org/natures#SOCAT

OASIS Open Catalog

The xlink:arcrole attribute

For situations where the nature/role alone isn't enough, this offers an additional URI defining the purpose of the link.
Well-known purposes include:

http://www.rddl.org/purposes#validation
This resource should be used for classic XML or SGML DTD validation before the document is parsed.

http://www.rddl.org/purposes#schema-validation
This resource should be used for validation via some sort of schema after the document is parsed.; the The type of schema would normally be identified by the nature.

http://www.rddl.org/purposes#module
A file which that is only part of a complete DTD; and that is typically used in modularized DTDs like such as XHTML 1.1 and SMIL 2.0

http://www.rddl.org/purposes#schema-module
A module used in a schema

http://www.rddl.org/purposes#entities
A DTD fragment containing only entity definitions such as xhtml-special.ent in modular XHTML

http://www.rddl.org/purposes#notations
A DTD fragment containing only notation declarations such as xhtml-notations-1.mod in modular XHTML

http://www.rddl.org/purposes/software#xslt-extension
Software implementing an extension function or element for XSLT

http://www.rddl.org/purposes/software#software-package
A grouping of software resources

http://www.rddl.org/purposes/software#software-project
A collection of resources related to a software package

http://www.rddl.org/purposes#JAR

A zip file with the extension .jar containing Java classes

http://www.rddl.org/purposes/software#reference
Documentation for the resource

http://www.rddl.org/purposes/software#normative-reference
the definitive specification of the resource's syntax and semantics

http://www.rddl.org/purposes/software#non-normative-reference
A useful but non-authoritative description of the resource's syntax and semantics

http://www.rddl.org/purposes#prior-version
Documentation for a previous version of the resource's vocabulary; for example, the XHTML 1.0 specification relative to XHTML 1.1

http://www.rddl.org/purposes#definition
The definition of a term

http://www.rddl.org/purposes#icon
An image that represents the resource

http://www.rddl.org/purposes#directory
Another RDDL document whose resources should be merged with this document's resources

http://www.rddl.org/purposes#alternate
An alternative for a resource with the same nature as this one

The xlink:title attribute

May contain a short, human readable description of the resource as plain, unmarked text.
Should not be used as a substitute for a more complete, HTML-marked-up description of the related resource in the contents of the rddl:resource element.

The xlink:show and xlink:actuate attributes

These two attributes are not used on rddl:resource. They must have the value none.

Declarations for rddl:resource

<!ELEMENT rddl:resource (#PCDATA | %Flow.mix;)*>
<!ATTLIST rddl:resource
          id            ID       #IMPLIED
          xlink:type    (simple) #FIXED "simple"
          xmlns:rddl    CDATA    #FIXED 'http://www.rddl.org/'
          xml:lang      NMTOKEN  #IMPLIED
          xlink:arcrole CDATA    #IMPLIED
          xlink:href    CDATA    #IMPLIED
          xlink:role    CDATA    'http://www.rddl.org/#resource'
          xlink:title   CDATA    #IMPLIED
          xlink:show    (none)   #FIXED "none"
          xlink:actuate (none)   #FIXED "none"
>

To Learn More

The RDDL Specification: http://www.rddl.org/ (Written in RDDL)

To Learn More

This presentation: http://www.cafeconleche.org/slides/xmlone/london2002/namespaces/
XML in a Nutshell
- Elliotte Rusty Harold and W. Scott Means
- O'Reilly & Associates, 2001
- ISBN 0-596-00058-8
XML Bible, Gold edition
- Elliotte Rusty Harold
- Hungry Minds, 2001
- ISBN 0-7645-4819-0
- Chapter 13: Namespaces
- Chapter 28: RDDL
Who's Jon Johansen? http://www.eff.org/IP/DeCSS_prosecutions/Johansen_DeCSS_case/

Index | Cafe con Leche