Namespace related attributes in XML and XML Schema (XSD)
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xmlns
is part of the W3C Namespaces in XML Recommendation:The prefix xmlns is used only to declare namespace bindings and
is by definition bound to the namespace name
http://www.w3.org/2000/xmlns/.In your example, it declares that
http://maven.apache.org/POM/4.0.0
is the default namespace for the elements in your Maven project. -
xmlns:xsi
declares a standard namespace prefix (xsi
) for a core namespace used in XSD:http://www.w3.org/2001/XMLSchema-instance
XML Schema: Structures also defines several attributes for direct use
in any XML documents. These attributes are in a different namespace,
which has the namespace name
http://www.w3.org/2001/XMLSchema-instance. For brevity, the text and
examples in this specification use the prefix xsi: to stand for this
latter namespace; in practice, any prefix can be used.In your example, it declares the conventional binding of the
xsi
namespace prefix tohttp://www.w3.org/2001/XMLSchema-instance
, which properly sets up the use of the following attributes:-
xsi:type
allows an XML instance to associate element type information directly rather than through an XSD. See How to restrict the value of an XML element using xsi:type in XSD?In your example,
xsi:type
is not used; included here for completeness regardingxsi
. -
xsi:nil
allows an empty element to be considered to be valid when the XSD might not otherwise have allowed it.In your example,
xsi:nil
is not used; included here for completeness regardingxsi
. -
xsi:schemaLocation
andxsi:noNamespaceSchemaLocation
provide hints to the XML processor as to how to associate an XSD with an XML document. Usexsi:schemaLocation
when there is a namespace; usexsi:noNamespaceSchemaLocation
when there is no namespace.In your example, there is a namespace, so you properly use
xsi:schemaLocation
, whose values are space-separated pairs of namespace and XSD-location-URI. Your example uses the namespace,http://maven.apache.org/POM/4.0.0
, and namespaces are lexical naming constructs that need not be retrivable. Your example also uses the XSD-location-URI,http://maven.apache.org/xsd/maven-4.0.0.xsd
, which is retrivable as it should be.If your example did not use a namespace, you would use
xsi:noNamespaceSchemaLocation
, whose value is a single XSD-location-URI that hints to the location of the intended XSD and which should be retrievable.
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targetNamespace
is an attribute on thexs:schema
root
element of an XSD which specifies the namespace of the root element
of the XML document instances the XSD is intended to govern. It must
match the default or explicit namespace of those XML documents’ root
elements.