How to retrieve xsi:schemaLocation with xercesc::SAX2XMLReader

I use a SAX2XMLReader to parse an XML file that specifies an XSD schema by having the attribute xsi:schemaLocation in its root element.

How can I retrieve the string value of this attribute?

I tried

str parser->getProperty(XMLUni::fgXercesSchemaExternalSchemaLocation)

but the result is a nullptr.