XML解析如何在SAX解析器内部工作？

Question

I'm trying to parse XML using SAX. Below is a code snippet:

public class ReadXML {

   public static void main(String argv[]) {

    try {

    SAXParserFactory factory = SAXParserFactory.newInstance();
    SAXParser saxParser = factory.newSAXParser();

    DefaultHandler handler = new DefaultHandler() {

    boolean bfname = false;
    boolean blname = false;
    boolean bnname = false;
    boolean bsalary = false;

    public void startElement(String uri, String localName,String qName, 
                Attributes attributes) throws SAXException {

        System.out.println("Parameters :" + uri +":"+ localName +":"+ qName +":"+ attributes);
        System.out.println("Start Element :" + qName);

        if (qName.equalsIgnoreCase("FIRSTNAME")) {
            bfname = true;
        }

        if (qName.equalsIgnoreCase("LASTNAME")) {
            blname = true;
        }

        if (qName.equalsIgnoreCase("NICKNAME")) {
            bnname = true;
        }

        if (qName.equalsIgnoreCase("SALARY")) {
            bsalary = true;
        }

    }

    public void endElement(String uri, String localName,
        String qName) throws SAXException {

        System.out.println("End Element :" + qName);

    }

    public void characters(char[] ch, int start, int length) throws SAXException {

        System.out.println("Im here:"+Arrays.toString(ch));
        if (bfname) {
            System.out.println("First Name : " + new String(ch, start, length));
            bfname = false;
        }

        if (blname) {
            System.out.println("Last Name : " + new String(ch, start, length));
            blname = false;
        }

        if (bnname) {
            System.out.println("Nick Name : " + new String(ch, start, length));
            bnname = false;
        }

        if (bsalary) {
            System.out.println("Salary : " + new String(ch, start, length));
            bsalary = false;
        }

    }

     };

       saxParser.parse("C:\\Lenny\\Work\\XML\\SaxParsing_01.xml", handler); --(1)


     } catch (Exception e) {
       e.printStackTrace();
     }

   }

}

My first question is, When the code reaches at saxParser.parse("C:\\\\Ashish\\\\Work\\\\XML\\\\SaxParsing_01.xml", handler); , below two methods gets called..!

    public void parse(File f, HandlerBase hb)
            throws SAXException, IOException {
            if (f == null) {
                throw new IllegalArgumentException("File cannot be null");
            }

            String escapedURI = FilePathToURI.filepath2URI(f.getAbsolutePath());
            if (DEBUG) {
                System.out.println("Escaped URI = " + escapedURI);
            }
            InputSource input = new InputSource(escapedURI);
            this.parse(input, hb);
        }

public void parse(InputSource is, DefaultHandler dh)
        throws SAXException, IOException {
        if (is == null) {
            throw new IllegalArgumentException("InputSource cannot be null");
        }
        XMLReader reader = this.getXMLReader();
        if (dh != null) {
            reader.setContentHandler(dh);
            reader.setEntityResolver(dh);
            reader.setErrorHandler(dh);
            reader.setDTDHandler(dh);
        }
        reader.parse(is);
    }

Am curious to know, What happens inside when reader.parse(is) is called ? The only thing I'm assuming is, reader is reading XML and putting into DefautHandler's data structure created in above code and producing output accordingly.

I've tried alot to find out the source code of parse(is) method, but couldn't find it. In SAXParser class, parse is an abstract method, so not able to find the implementation class where I could check the source code an understand further.

Second but silly question, May I know please, when we'r creating the DefautHandler instance, are methods inside that block are overridden ? And In constructor's block, Are we allowed to create variables like we have created four Boolean variables ? Never saw this kind of approach in Java.

Can anyone help me for the same ....?

Thanks

Answer 1

SAXParser is an interface and there are many XML parsers that implement this interface. If you want to know how it works you will need to choose one of these XML parsers - the most accessible is the Apache Xerces parser. You could start here, but be warned that it's not easy reading:

https://apache.googlesource.com/xerces2-j/+/f59f47412e404f4984480a45a99957ac07d287d4/src/org/apache/xerces/parsers/AbstractSAXParser.java

In very simple terms, the parser is going to look for a "<", and when it finds one, it's going to call the supplied ContentHandler 's startElement() method with appropriate parameters.

You don't actually need to understand how it works internally in order to successfully make use of a SAX parser, though well done for trying.

You're right that writing a SAX ContentHandler (perhaps as an extension of DefaultHandler) involves a rather different style of Java programming than you may be used to. Because your code is processing events through callbacks, you can't maintain the current state on the stack in the way that you would if you owned the main control loop. Rather you have to think how each call on a method such as startElement() or characters() affects the current state that your application needs to maintain, and work out how to modify the data structure that holds this state. It's a rather different way of programming, and is one of the reasons why some people say that "pull" parsing interfaces are easier to use than "push" interfaces.