Indicates that this namespace (specified by {@link #namespace()})
has a schema already available exeternally, available at this location.
This instructs the JAXB schema generators to simply refer to
the pointed schema, as opposed to generating components into the schema.
This schema is assumed to match what would be otherwise produced
by the schema generator (same element names, same type names...)
This feature is intended to be used when a set of the Java classes
is originally generated from an existing schema, hand-written to
match externally defined schema, or the generated schema is modified
manually.
Value could be any absolute URI, like http://example.org/some.xsd.
It is also possible to specify the empty string, to indicate
that the schema is externally available but the location is
unspecified (and thus it's the responsibility of the reader of the generate
schema to locate it.) Finally, the default value of this property
"##generate" indicates that the schema generator is going
to generate components for this namespace (as it did in JAXB 2.0.)
Multiple {@link XmlSchema} annotations on multiple packages are allowed
to govern the same {@link #namespace()}. In such case, all of them
must have the same {@link #location()} values.
Note to implementor
More precisely, the value must be either "", "##generate", or
a valid lexical representation of xs:anyURI that begins
with <scheme>:.
A schema generator is expected to generate a corresponding
<xs:import namespace="..." schemaLocation="..."/> (or
no schemaLocation attribute at all if the empty string is specified.)
However, the schema generator is allowed to use a different value in
the schemaLocation attribute (including not generating
such attribute), for example so that the user can specify a local
copy of the resource through the command line interface.