Set the system identifier for this Source. If an input source
has already been set, it will set the system ID or that
input source, otherwise it will create a new input source.
The system identifier is optional if there is a byte stream
or a character stream, but it is still useful to provide one,
since the application can use it to resolve relative URIs
and can include it in error messages and warnings (the parser
will attempt to open a connection to the URI only if
no byte stream or character stream is specified).
Parameters:
-
systemId - The system identifier as a URI string.