7. XML Namespaces

What you will learn

• Why name clashes happen
• How to declare a namespace with xmlns
• Prefixes and default namespaces
• How namespaces are used in real files

The problem: the same name, two meanings

Because you invent your own tag names, two different documents may use the same name for different things. When you combine them, a program cannot tell them apart.

<table>
  <tr><td>Apples</td><td>Bananas</td></tr>
</table>

<table>
  <name>Coffee table</name>
  <width>80</width>
  <length>120</length>
</table>

The first table is an HTML-style table of data. The second is a piece of furniture. If both appear in one file, software has no way to know which is which.

The solution: namespaces

A namespace is a label that says which vocabulary a name belongs to. It is a web address (a URI) that acts purely as a unique identifier. You declare it with the xmlns attribute.

Using a prefix

Write xmlns:prefix="address", then put the prefix and a colon in front of each name from that vocabulary.

<?xml version="1.0" encoding="UTF-8"?>
<root xmlns:h="http://www.w3.org/1999/xhtml"
      xmlns:f="https://example.com/furniture">

  <h:table>
    <h:tr><h:td>Apples</h:td><h:td>Bananas</h:td></h:tr>
  </h:table>

  <f:table>
    <f:name>Coffee table</f:name>
    <f:width>80</f:width>
  </f:table>

</root>

The address is just an identifier

Two addresses are only the same if they match exactly, letter for letter. http://example.com/a and https://example.com/a are different namespaces.

A default namespace

Write xmlns="address" with no prefix to give a namespace to every unprefixed element inside. That saves typing prefixes.

<table xmlns="https://example.com/furniture">
  <name>Coffee table</name>
  <width>80</width>
</table>

Here table, name, and width all belong to the furniture namespace, with no prefixes needed.

Scope

A namespace declaration applies to the element that carries it and everything inside it. It is common to declare all namespaces once on the root element. A declaration deeper in the tree can add or override one for that part only.

<page xmlns="http://www.w3.org/1999/xhtml">
  <p>An XHTML paragraph.</p>
  <figure>
    <svg xmlns="http://www.w3.org/2000/svg" width="40" height="40">
      <circle cx="20" cy="20" r="18"/>
    </svg>
  </figure>
</page>

Real namespaces you will meet

Check each against the official standard before relying on it in a project. You will see the XSD and XSLT ones in later lessons.

Why namespaces matter in practice

• RSS and Atom feeds add extra fields with namespaced elements, such as podcast details.
• Sitemaps declare their namespace on the root element.
• Android layout files use xmlns:android.
• Office documents and many web service formats rely on them.

When a tool does not find your elements, a missing or mistyped namespace is one of the most common causes.

Common mistakes

• Using a prefix that was never declared.
• Mistyping the namespace address by one character, which makes it a different namespace.
• Believing the address must be a working web page.
• Expecting a default namespace to apply to attributes.
• Forgetting that when a default namespace exists, tools such as XPath need it taken into account.

<catalog xmlns:b="https://example.com/books">
  <b:book>
    <b:title>Learning XML</b:title>
  </b:book>
</catalog>

Summary

• A namespace is a unique address that says which vocabulary an element belongs to.
• Use a prefix (xmlns:f) or a default namespace (xmlns), and declare it on the root when you can.
• The address is only an identifier, and it must match exactly.