Java SAX Parser
Jakob Jenkov |
SAX is an abbreviation and means "Simple API for XML". A Java SAX XML parser is a stream oriented XML parser. It works by iterating over the XML and call certain methods on a "listener" object when it meets certain structural elements of the XML. For instance, it will call the listener object for the following "events":
- startDocument - startElement - characters - comments - processing instructions - endElement - endDocument
This list is probably not complete, but it is long enough to give you an idea of how it works. Let's move on to see how you create and use a Java SAX Parser.
SAXParserFactory factory = SAXParserFactory.newInstance();
try {
InputStream xmlInput = new FileInputStream("theFile.xml");
SAXParser saxParser = factory.newSAXParser();
DefaultHandler handler = new SaxHandler();
saxParser.parse(xmlInput, handler);
} catch (Throwable err) {
err.printStackTrace ();
}
When you call the SAXParser.parse() method the SAX parser starts the
XML processing. The xmlInput InputStream passed as parameter
to the parse() method is where the XML is read from.
Notice the SaxHandler instance being created, and passed as parameter
to the parse() method. The SaxHandler
class is a subclass of the class org.xml.sax.helpers.DefaultHandler.
The DefaultHandler class comes with the JDK.
While processing the XML the SAXParser calls methods in the DefaultHandler
subclass (here, the SaxHandler) instance corresponding to what the parser
finds in the XML file. To react to those method calls you override the corresponding methods
in the DefaultHandler subclass. Here is an example:
public class SaxHandler extends DefaultHandler {
public void startDocument() throws SAXException {
}
public void endDocument() throws SAXException {
}
public void startElement(String uri, String localName,
String qName, Attributes attributes)
throws SAXException {
}
public void endElement(String uri, String localName, String qName)
throws SAXException {
}
public void characters(char ch[], int start, int length)
throws SAXException {
}
public void ignorableWhitespace(char ch[], int start, int length)
throws SAXException {
}
}
It is the responsibility of the DefaultHandler subclass to extract any necessary
information from the XML via these methods. If you need to build an object graph
based on an XML file, you will have to build that object graph inside the DefaultHandler
subclass.
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Jakob Jenkov | |
