code: public String getDocumentTypeDeclarationSystemIdentifier()
    {
  
      Document doc;
  
      if (m_root.getNodeType() == Node.DOCUMENT_NODE)
        doc = (Document) m_root;
      else
        doc = m_root.getOwnerDocument();
  
      if (null != doc)
      {
        DocumentType dtd = doc.getDoctype();
  
        if (null != dtd)
        {
          return dtd.getSystemId();
        }
      }
  
      return null;
    }
docs: A document type declaration information item has the following properties:
  
  1. [system identifier] The system identifier of the external subset, if
  it exists. Otherwise this property has no value.
  
  @return the system identifier String object, or null if there is none.