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<h1>XML Namespaces</h1>
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<p class="intro">XML Namespaces provide a method to avoid element name
conflicts.</p>
<hr />
<h2>Name Conflicts</h2>
<p>In XML, element names are defined by the developer. This often results in a
conflict when trying to mix XML documents from different XML applications.</p>
<p>This XML carries HTML table information:</p>
<table border="1" width="100%" class="ex" cellspacing="0"><tr><td>
<pre><table>
<tr>
<td>Apples</td>
<td>Bananas</td>
</tr>
</table></pre>
</td></tr></table>
<p>This XML carries information about a table (a piece of furniture):</p>
<table border="1" width="100%" class="ex" cellspacing="0"><tr><td>
<pre><table>
<name>African Coffee Table</name>
<width>80</width>
<length>120</length>
</table></pre>
</td></tr></table>
<p>If these XML fragments were added together, there would be a name
conflict. Both contain a <table> element, but the elements have different
content and meaning.</p>
<p>An XML parser will not know how to handle these differences.</p>
<hr />
<h2>Solving the Name Conflict Using a Prefix</h2>
<p>Name conflicts in XML can easily be avoided using a name prefix.</p>
<p>This XML carries information about an HTML table, and a piece of furniture:</p>
<table border="1" width="100%" class="ex" cellspacing="0"><tr><td>
<pre><h:table>
<h:tr>
<h:td>Apples</h:td>
<h:td>Bananas</h:td>
</h:tr>
</h:table></pre>
<pre><f:table>
<f:name>African Coffee Table</f:name>
<f:width>80</f:width>
<f:length>120</f:length>
</f:table></pre>
</td></tr></table>
<p>In the example above, there will be no conflict because the two <table> elements
have
different names.</p>
<hr />
<h2>XML Namespaces - The xmlns Attribute</h2>
<p>When using prefixes in XML, a so-called <b>namespace</b> for the prefix must
be defined.</p>
<p>The namespace is defined by the <b>xmlns attribute</b> in the start tag of an element.</p>
<p>The namespace declaration has the following syntax. xmlns:<i>prefix</i>="<i>URI</i>".</p>
<table border="1" width="100%" class="ex" cellspacing="0"><tr><td>
<pre><root></pre>
<pre><h:table <span class="marked">xmlns:h="http://www.w3.org/TR/html4/"</span>>
<h:tr>
<h:td>Apples</h:td>
<h:td>Bananas</h:td>
</h:tr>
</h:table></pre>
<pre><f:table <span class="marked">xmlns:f="http://www.w3schools.com/furniture"</span>>
<f:name>African Coffee Table</f:name>
<f:width>80</f:width>
<f:length>120</f:length>
</f:table></pre>
<pre></root></pre>
</td></tr></table>
<p>In the example above, the xmlns attribute in the <table> tag give the h: and
f: prefixes a qualified namespace.</p>
<p>When a namespace is defined for an element, all child
elements with the same prefix are associated with the same namespace.</p>
<p>Namespaces can be declared in the elements where they are used or in the XML
root element:</p>
<table border="1" width="100%" class="ex" cellspacing="0" id="table1"><tr><td>
<pre><root
<span class="marked">xmlns:h="http://www.w3.org/TR/html4/"
xmlns:f="http://www.w3schools.com/furniture"</span>></pre>
<pre><h:table>
<h:tr>
<h:td>Apples</h:td>
<h:td>Bananas</h:td>
</h:tr>
</h:table></pre>
<pre><f:table>
<f:name>African Coffee Table</f:name>
<f:width>80</f:width>
<f:length>120</f:length>
</f:table></pre>
<pre></root></pre>
</td></tr></table>
<p><b>Note:</b> The namespace URI is not used by the
parser to look up information.</p>
<p>The purpose is to give the namespace a
unique name. However, often companies use the namespace as a pointer to a
web page containing namespace information.</p>
<p>Try to go to <a href="../../www.w3.org/TR/html4/default.htm" target="_blank">http://www.w3.org/TR/html4/</a>.</p>
<hr />
<h2>Uniform Resource Identifier (URI)</h2>
<p>A <b>Uniform Resource Identifier</b> (URI) is a string of characters which
identifies an Internet Resource.</p>
<p>The most common URI is the <b>Uniform Resource
Locator</b> (URL) which identifies an Internet domain address. Another, not so
common type of URI is the <b>Universal Resource Name</b> (URN).</p>
<p>In our examples
we will only use URLs.</p>
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