为什么xmlns：xsi，而不仅仅是XML文档中的xsi？

Question

I understand that, by convention, the default namespace is named xmlns attribute in the root element of an XML document. Other namespaces are named other names. Now, I wonder why is XML Scheme Instance namespace always named xmlns:xsi and not simply xsi ? Would xsi be the same as xmlns:xsi ? If yes, why yes? If no, why no?

Elements and attribute names not coming from the default namespace are always preceded by name of the namespace. Having xmlns:xsi tells me that xsi is an attribute defined in namespace xmlns , which is conflicting, since the attribute names from the default namespace shouldn't be preceded by the namespace.

Answer 1

The default namespace is not named xmlns .

xmlns is the way to declare a namespace prefix (ie short name, handle). The syntax is xmlns[:prefix]="namepace-uri" .

There can be exactly one namespace declaration per XML element where you are allowed to leave off the prefix ( xmlns="namespace-uri" ), and if it is declared that way it is called "default namespace".

xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" declares that the URI http://www.w3.org/2001/XMLSchema-instance shall be known as xsi inside this element.

You are free to choose any prefix you like, you can declare xmlns:bob="http://www.w3.org/2001/XMLSchema-instance" and that would mean that the URI http://www.w3.org/2001/XMLSchema-instance shall be known as bob inside this element.

By convention, many widely-used XML namespace URIs get the same prefix everywhere, but that's not a technical requirement.