Namespaces

Namespaces are created using the standard methods available in the DOM Core specification, for creating attributes and elements. See Document.createElementNS, Document.createAttributeNS, and Element.setAttributeNS. A convenience method is available for creating attributes to define a namespace; Element:defineNamespace.

In this Lua implementation the namespaceURI, localName, and prefix properties are tracked on the Nodes. Not in Attribute nodes in Elements. This means that there will be no attributes in the document tree for defining namespaces.

Example with 2 namespaces defined on the root element:

 <node xmlns='default_ns' xmlns:lua='https://lua.org' just_attribute='value'>
     <lua:source>print [[hello world]]</lua:source>
 </node>

The top-level "node" element defines 3 attributes, of which 2 are namespace declarations. Yet in the document object there will only be 2 attributes; xmlns:lua, and just_attribute since the definition of the default namespace is reflected in the properties of the node Element. It is 'implicit' in the document.

Implicit and explicit namespace definitions

If the document would be parsed into a doc variable then;

local elem = doc.documentElement
print(elem.localName)            --> "node"
print(elem.prefix)               --> nil
print(elem.namespaceURI)         --> "default_ns"
print(elem.attributes.length)    --> 1
local source = elem.childNodes[1]
print(source.localName)          --> "source"
print(source.prefix)             --> "lua"
print(source.namespaceURI)       --> "https://lua.org"
local source = elem.childNodes[2]
print(source.localName)          --> "lua"
print(source.prefix)             --> "xmlns"
print(source.namespaceURI)       --> "http://www.w3.org/2000/xmlns/"

The attribute xmlns:lua defines the 'explicit' namespace for 'lua'. This namespace is implictly defined on the 'lua:source' Element, but since it is defined ahead of use on the 'node' Element, it is explicit.

When writing an Xml document, the namespace attributes will implicitly be added to the elements where they are needed. To explicitly define namespaces they can be added as namespace definition attributes to an Element in the same way.

As an example, here's the same document created from code;

local DOM = require("expadom.DOMImplementation")()
doc = DOM:createDocument("default_ns", "node")
doc.documentElement:setAttribute("just_attribute", "value")
local source = doc:createElementNS("https://lua.org", "lua:source")
doc.documentElement:appendChild(doc:createTextNode("\n    "))
doc.documentElement:appendChild(source)
doc.documentElement:appendChild(doc:createTextNode("\n"))
source:appendChild(doc:createTextNode("print [[hello world]]"))
print(table.concat(doc:write()))

Output:

 <?xml version="1.0" encoding="UTF-8" ?>
 <node xmlns="default_ns" just_attribute="value">
     <lua:source xmlns:lua="https://lua.org">print [[hello world]]</lua:source>
 </node>

The output however has the namespace declaration moved to the "lua:source" element, since that is where it is first used. So it is defined implicitly.

If an 'explicit' declaration is added to the root element (called "node"), like so:

doc.documentElement:defineNamespace("https://lua.org", "lua")
print(table.concat(doc:write()))

Then the output becomes:

 <?xml version="1.0" encoding="UTF-8" ?>
 <node xmlns='default_ns' xmlns:lua='https://lua.org' just_attribute='value'>
     <lua:source>print [[hello world]]</lua:source>
 </node>

Caveats

When adding namespaces either explicit or implicit, there are no checks to see if they collide with existing definitions. So it can happen that the same prefix has 2 different namespaceURI's set. In that case, writing the document will fail with an error.

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Contents

Topics

Modules

Classes

Namespaces

Implicit and explicit namespace definitions

Caveats