code: public void setSystemId(String systemId) {
  
          if (null == inputSource) {
              inputSource = new InputSource(systemId);
          } else {
              inputSource.setSystemId(systemId);
          }
      }
docs: 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.
  
  <p>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).</p>
  
  @param systemId The system identifier as a URI string.