Is xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" a special case in XML?

Question

When we use a namespace, we should also indicate where its associated XSD is located at, as can be seen in the following example:

<?xml version="1.0"?>
<Artist BirthYear="1958" 
 xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
 xmlns="http://www.webucator.com/Artist"
 xsi:schemaLocation="http://www.webucator.com/Artist Artist.xsd">
 <Name>
  <Title>Mr.</Title>
  <FirstName>Michael</FirstName>
  <LastName>Jackson</LastName>
 </Name>
</Artist>

Here, we have indicated that Artist.xsd should be used for validating the http://www.webucator.com/Artist namespace. However, we are also using the http://www.w3.org/2001/XMLSchema-instance namespace, but we have not specified where its XSD is located at. How do XML parsers know how to handle this namespace?

Update (in response to the first commenter)

So, can we instead of using:

<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xmlns:ehcache="http://www.springmodules.org/schema/ehcache"
       xsi:schemaLocation="http://www.springframework.org/schema/beans 
            http://www.springframework.org/schema/beans/spring-beans-2.0.xsd
            http://www.springmodules.org/schema/ehcache
            http://www.springmodules.org/schema/cache/springmodules-ehcache.xsd"&gt;
...
</beans>

use

<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xmlns:ehcache="http://www.springmodules.org/schema/ehcache"&gt;
...
</beans>

?

Thanks in advance.

Answer 1

There's no requirement to say where the schema is located. You can do it if you want, but you don't have to.

In this example, all platforms are likely to understand where the schemas for xsi, xml, xsd and soap are all located.

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EDIT: Like I said, all platforms are likely to know where the schemas are for these well-known namespaces. Quite likely, they all have copies of the schemas. I use Visual Studio, and it keeps copies of these schemas online, and refers to them as necessary.

Answer 2

There are four built in declarations for Xml schemata; type, nil, schemaLocation and noNamespaceSchemaLocation as are "present in every schema by definition." You can read about them in the Xml Schema recommendation.