XML Name spaces

Different people may invent similar tag names

• Here is an XML element describing a piece of furniture:<table> <name>table99</name> <type>dining table</type> <width>80</width> <length>180</length></table>

• Here is an XML element describing a collection of data:<table> <person> <name>Bertie Ahern</name> <type>male</type> </person> <person> <name>Mary Harney</name> <type>female</type> </person></table>

Distinguishing tags with similar names

• Sometimes an application will involve multiple documents, each containing tags that have the same names but different meanings

• We can distinguish between similar tag names by using prefixes

Distinguishing tags with similar names (contd.)

• The XML element describing a piece of furniture:<furniture:table> <furniture:name>table99</furniture:name> <furniture:type>dining table</furniture:type> <furniture:width>80</furniture:width> <furniture:length>180</furniture:length></furniture:table>

• The XML element describing a collection of data:<data:table> <data:person> <data:name>Bertie Ahern</data:name> <data:type>male</data:type> </data:person> <data:person> <data:name>Mary Harney</data:name> <data:type>female</data:type> </data:person></data:table>

Distinguishing tags with similar names (contd.)

• But two different software engineers might even still choose similar prefixes!

• Extract from one application:<furniture:table> <furniture:name>table99</furniture:name> <furniture:type>dining table</furniture:type> <furniture:width>80</furniture:width> <furniture:length>180</furniture:length></furniture:table>

• Extract from another application:<furniture:table> <furniture:price>199</furniture:price> <furniture:status>second-hand</furniture:status></furniture:table>

Distinguishing tags with similar names (contd.)

• We avoid ambiguity by associating each prefix with something that must be unique– a Universal Resource Identifier

• The association between a prefix and a URI is made using a special XML attribute– the XML Namespace (xmlns) Attribute

• Format of the xmlns attribute:

xmlns:prefix="URI”• Example usages:<furniture:table xmlns:furniture=“http://abc.com”><furniture:table xmlns:furniture=“http://abracadabra.co.uk”>

• By the way: never invent your own tag/attribute names starting with “xml…”– these are reserved for W3C usage

Distinguishing tags with similar names (contd.)

• When a namespace is defined in the start tag of an element, all child elements with the same prefix are associated with the same namespace

• Example<furniture:table xmlns:furniture=“http://abc.com” > <furniture:name>table99</furniture:name> <furniture:type>dining table</furniture:type> <furniture:width>80</furniture:width> <furniture:length>180</furniture:length></furniture:table>

• In this case, all the furniture prefixes are associated with the http://abc.com URI

Some more examples• A different furniture example:

<furniture:table xmlns:furniture=“http://abracadabra.co.uk”> <furniture:price>199</furniture:price> <furniture:status>second-hand</furniture:status></furniture:table>

• A personnel data example<data:table xmlns:data=“http://irlgov.ie”> <data:person> <data:name>Bertie Ahern</data:name> <data:type>male</data:type> </data:person> <data:person> <data:name>Mary Harney</data:name> <data:type>female</data:type> </data:person></data:table>

Default Namespaces• We can reduce the need for prefixes if we define a

default namespace for an element• All tag names in descendant elements are then

understood as implicitly having the same namespace as that for the ancestor element

• The format for a default namespace usage is<someTagName xmlns=“someURI” > … </someTagName>

• Example usage:<furniture:table xmlns:furniture=“http://abracadabra.co.uk”> <furniture:price>199</furniture:price> <furniture:status>second-hand</furniture:status></furniture:table>

is equivalent to:<table xmlns=“http://abracadabra.co.uk”> <price>199</price> <status>second-hand</status></table>

Example usage of XML namespaces

• A frequent usage of namespaces is in XSL stylesheets• There is an example on the next slide • The XSL stylesheet transforms a particular form of XML

document into XHTML• Thus many of the tags in the stylesheet are XHTML tags• But many tags are are XSL tags

– they are identified as such by having the xsl prefix

– and the semantics of the xsl prefix are defined by associating it with a W3C URI

<?xml version="1.0"?>

<xsl:transform

xmlns:xsl="http://www.w3.org/1999/XSL/Transform” version="1.0">

<xsl:template match="/">

<html><head><title>Data</title></head>

<body><table rules="all">

<thead><tr><th>Name</th><th>Sex</th></tr></thead>

<tbody>

<xsl:apply-templates/>

</tbody></table></body></html>

</xsl:template>

<xsl:template match="person">

<tr><xsl:apply-templates/></tr>

</xsl:template>

<xsl:template match="male">

<td><xsl:value-of select="."/></td><td>male</td>

</xsl:template>

<xsl:template match="female">

<td> <xsl:value-of select="."/> </td><td>female</td>

</xsl:template>

</xsl:transform>

An XSL application where there would have been tagname clashes

• Consider the following XML fragment:<table xmlns=“http://abc.com” > <name>table99</name> <type>dining table</type> <width>80</width> <length>180</length></table>

• Suppose we wanted to use XSL to transform elements like this into elements like this

<table xmlns=“http://abc.com” > <name>table99</name> <template>dining table</template> <width>80</width> <length>180</length></table>

• That is, we are replacing an application-specific element having the tagname type with one having the tagname template

An XSL application with potential tagname clash (contd.)

• Our XSL stylesheet would have two kinds of template element– one from the XSL language– another from the application-specific furniture language

• We would use the xsl prefix to distinguish one of these from the other

• Here is an extract from the relevant XML stylesheet:

<?xml version="1.0"?>

<xsl:transform xmlns:xsl="http://www.w3.org/1999/XSL/Transform” version="1.0">

...

<xsl:template match="type">

<template><xsl:value-of select="."/></template>

</xsl:template>

...

<xsl:transform>