XML Namespaces
Sometimes there can be a name conflict when two elements share a same name. It happens normally when two applications that have an element with the same name are merged together. The XML parser won't know that these are separate entities. Hence, a solution to name conflicts is using namespaces. Consider for example the two XML fragments given below:<book> <name>XML for dummies</name> </book>
When the above two fragments are merged together, there will be a name collision since both of them contain the element "name" that holds different data. To handle this difference, namespaces are used. Therefore element names are prefixed by a label that is unique for a namespace. The namespace is defined by using the attribute "xmlns". It is defined in the start tag of the concerned element. The syntax of the namespace declaration is given below:<author> <name> <first_name>Lalit</first_name> <last_name>Pawar</last_name> </name> </author>
Now, let's have a look at the above syntax. What is a URI? URI stands for Uniform Resource Identifier, a string of characters which identifies an internet resource. The only purpose of using a URI is to have a unique prefix. In fact, the URI may not exist. The parser does not validates it, nor does it look it up for some information. The sole purpose is to give the namespace a unique name.xmlns:prefix="URI"
The most common URI is the Uniform Resource Locator (URL) which identifies an Internet domain address. Another, not so common type of URI is the Universal Resource Name (URN).
Ways to Specify Namespaces
Inside the XML file
1) Specify a Namespace for every element
2) Specify all Namespaces within the root element<catalog> <b:book xmlns:b = “http://website/b.html”> <b:name xmlns:b = “http://website/b.html”>XML for dummies </b:name> </b:book> <a:author xmlns:a = “http://website/a.html”> <a:name xmlns:a = “http://website/a.html”> <a:first_name xmlns:a = “http://website/a.html”>Lalit</a:first_name> <a:last_name xmlns:a = “http://website/a.html”>Pawar</a:last_name> </a:name> </a:author> </catalog>
The specification given for the that element is also valid for all elements occurring inside it.<catalog xmlns:b = “http://website/b.html” xmlns:a = “http://website/a.html”> <b:book> <b:name>XML for dummies </b:name> </b:book> <a:author> <a:name> <a:first_name>Lalit</a:first_name> <a:last_name>Pawar</a:last_name> </a:name> </a:author> </catalog>
3) Default namespace
Namespaces do not have to be specified explicitly with prefixes.
4) Attributes can be explicitly assigned a value and be associated with a namespace.<catalog> <book> <name xmlns = “http://website/b.html”>XML for dummies </name> </book> <author> <name xmlns = “http://website/a.html”> <first_name>Lalit</first_name> <last_name>Pawar</last_name> </name> </author> </catalog>
If nothing is prefixed with the attribute name, it does not belong to any namespace.<catalog xmlns:b = “http://website/b.html” xmlns:a = “http://website/a.html”> <b:book b:category="computer"> <b:name>XML for dummies </b:name> </b:book> <a:author> <a:name> <a:first_name>Lalit</a:first_name> <a:last_name>Pawar</a:last_name> </a:name> </a:author> </catalog>
5) A namespace specification can be overridden.
<catalog xmlns:b = “http://website/b.html”> <b:book> <b:name>XML for dummies </b:name> </b:book> <b:author xmlns:b = "http://website/test.html"> <b:name> <b:first_name>Lalit</b:first_name> <b:last_name>Pawar</b:last_name> </b:name> </b:author> </catalog>