SAXParseException returns null for getSystemId()

Why SAXParseException returns null for getSystemId()? What is System Identifier?

    import java.io.StringReader;

    import javax.xml.parsers.SAXParser;
    import javax.xml.parsers.SAXParserFactory;

    import org.xml.sax.ErrorHandler;
    import org.xml.sax.InputSource;
    import org.xml.sax.SAXException;
    import org.xml.sax.SAXParseException;
    import org.xml.sax.XMLReader;

    public class MainClass {
      static public void main(String[] arg) throws Exception{
        boolean validate = false;

        SAXParserFactory spf = SAXParserFactory.newInstance()开发者_JAVA技巧;
        spf.setValidating(validate);

        XMLReader reader = null;
          SAXParser parser = spf.newSAXParser();
          reader = parser.getXMLReader();

        reader.setErrorHandler(new MyErrorHandler());
          reader.parse(new InputSource(new StringReader(xmlString)));
    }
      static String xmlString = "<PHONEBOOK>" +
      "  <PERSON>" +
      "   <NAME>Joe Wang</NAME>" +
      "   <EMAIL>joe@yourserver.com</EMAIL>" +
      "   <TELEPHONE>202-999-9999</TELEPHONE>" +
      "   <WEB>www.java2s.com</WEB>" +
      "  </PERSON>" +
      "  <PERSON>   " +
      "<NAME>Karol</NAE>" +    // error here
      "   <EMAIL>karol@yourserver.com</EMAIL>" +
      "   <TELEPHONE>306-999-9999</TELEPHONE>" +
      "   <WEB>www.java2s.com</WEB>" +
      "  </PERSON>" +
      "  <PERSON>" +
      "   <NAME>Green</NAME>" +
      "   <EMAIL>green@yourserver.com</EMAIL>" +
      "   <TELEPHONE>202-414-9999</TELEPHONE>" +
      "   <WEB>www.java2s.com</WEB>" +
      "  </PERSON>" +
      "  </PHONEBOOK>";
    }
    class MyErrorHandler implements ErrorHandler {
      public void warning(SAXParseException e) throws SAXException {
        show("Warning", e);
        throw (e);
      }

      public void error(SAXParseException e) throws SAXException {
        show("Error", e);
        throw (e);
      }

      public void fatalError(SAXParseException e) throws SAXException {
        show("Fatal Error", e);
        throw (e);
      }

      private void show(String type, SAXParseException e) {
        System.out.println(type + ": " + e.getMessage());
        System.out.println("Line " + e.getLineNumber() + " Column "
            + e.getColumnNumber());
        System.out.println("System ID: " + e.getSystemId());
        System.out.println(e);

      }
    }

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The 'system identifier' in XML is the physical location you got something from. When you just parse a string in memory, it has no system identifier at all unless you make an extra call to give it one.

You can, in this case, call InputSource.setSystemId.

---

The System Identifier is a URI you can specify, it's there so it can be used by the EntityResolver to decide how relative paths get resolved during xml parsing. Whether it is a physical location or just a label is up to you. Of course, in your example you don't have anything to resolve so it's not needed.