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The xsi:schemaLocation attribute locates schemas for elements and attributes that are in a specified namespace. Its value is a namespace URI followed by a relative or absolute URL where the schema for that namespace can be found. It is most commonly attached to the root element but can appear further down the tree.
An XML Instance document is a file (generally has an . xml extension) that contains the information, or data, of what you are trying to describe with your schema. The instance document will refer to the schema design (DTD, BizTalk) that provides its structure.
The TargetNamespace is the namespace of all schema components in this schema as well as any schema included using the include element. Included schemas must either have the same target namespace as the containing schema or have no target namespace specified.
The Java XML parser that spring uses will read the schemaLocation values and try to load them from the internet, in order to validate the XML file. Spring, in turn, intercepts those load requests and serves up versions from inside its own JAR files.
If you omit the schemaLocation, then the XML parser won't know where to get the schema in order to validate the config.
An xmlns is a unique identifier within the document - it doesn't have to be a URI to the schema:
XML namespaces provide a simple method for qualifying element and attribute names used in Extensible Markup Language documents by associating them with namespaces identified by URI references.
xsi:schemaLocation is supposed to give a hint as to the actual schema location:
can be used in a document to provide hints as to the physical location of schema documents which may be used for assessment.
According to the spec for locating Schemas
there may or may not be a schema retrievable via the namespace name... User community and/or consumer/provider agreements may establish circumstances in which [trying to retrieve an xsd from the namespace url] is a sensible default strategy
(thanks for being unambiguous, spec!)
and
in case a document author (human or not) created a document with a particular schema in view, and warrants that some or all of the document conforms to that schema, the schemaLocation and noNamespaceSchemaLocation [attributes] are provided.
So basically with specifying just a namespace, your XML "might" be attempted to be validated against an xsd at that location (even if it lacks a schemaLocation attribute), depending on your "community." If you specify a specific schemaLocation, then it basically is implying that the xml document "should" be conformant to said xsd, so "please validate it" (as I read it). My guess is that if you don't do a schemaLocation or noNamespaceSchemaLocation attribute it just "isn't validated" most of the time (based on the other answers, appears java does it this way).
Another wrinkle here is that typically, with xsd validation in java libraries [ex: spring config xml files], if your XML files specifies a particular schemaLocation xsd url in an XML file, like xsi:schemaLocation="http://somewhere http://somewhere/something.xsd" typically within one of your dependency jars it will contain a copy of that xsd file, in its resources section, and spring has a "mapping" capability saying to treat that xsd file as if it maps to the url http://somewhere/something.xsd (so you never end up going to web and downloading the file, it just exists locally). See also https://stackoverflow.com/a/41225329/32453 for slightly more info.
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