java - SAXParseException 为 getSystemId() 返回 null

Question

为什么 SAXParseException 对 getSystemId() 返回 null？什么是系统标识符？

    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();
        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><a href="https://stackoverflow.com/cdn-cgi/l/email-protection" class="__cf_email__" data-cfemail="d7bdb8b297aeb8a2a5a4b2a5a1b2a5f9b4b8ba" rel="noreferrer noopener nofollow">[email protected]</a></EMAIL>" +
      "   <TELEPHONE>202-999-9999</TELEPHONE>" +
      "   <WEB>www.java2s.com</WEB>" +
      "  </PERSON>" +
      "  <PERSON>   " +
      "<NAME>Karol</NAE>" +    // error here
      "   <EMAIL><a href="https://stackoverflow.com/cdn-cgi/l/email-protection" class="__cf_email__" data-cfemail="076c6675686b477e6872757462757162752964686a" rel="noreferrer noopener nofollow">[email protected]</a></EMAIL>" +
      "   <TELEPHONE>306-999-9999</TELEPHONE>" +
      "   <WEB>www.java2s.com</WEB>" +
      "  </PERSON>" +
      "  <PERSON>" +
      "   <NAME>Green</NAME>" +
      "   <EMAIL><a href="https://stackoverflow.com/cdn-cgi/l/email-protection" class="__cf_email__" data-cfemail="dcbbaeb9b9b29ca5b3a9aeafb9aeaab9aef2bfb3b1" rel="noreferrer noopener nofollow">[email protected]</a></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);

      }
    }

Answer 1

XML 中的“系统标识符”是您从中获取某些内容的物理位置。当您只是解析内存中的字符串时，它根本没有系统标识符，除非您进行额外的调用来给它一个系统标识符。

在这种情况下，您可以调用InputSource.setSystemId。