XML and Java: SAX: Simple API for XML (Parsers)

URL: http://www.megginson.com/SAX/

Quickstart: http://www.megginson.com/SAX/quickstart.html

Java Roadmap: http://www.megginson.com/SAX/roadmap.html

Javadoc Documentation: http://www.megginson.com/SAX/javadoc/packages.html

SAX stands for "Simple API for XML"; it is an extremely popular (almost ubiquitous) event-based interface for any XML parser. SAX is somewhat of a grass-roots effort, spearheaded by David Megginson, of the xml-dev mailing list. As you can tell from the rest of this page, SAX is the basis for quite a few Java APIs. Megginson sees SAX addressing a need much like JDBC fills for SQL. The idea behind SAX is to provide an interface by which any Java application can access any XML parser, provided the parser has a SAX driver. Virtually every major XML parser either supports the SAX interface directly or indirectly via third-party drivers. SAX comes with two sample drivers (Lark and MSXML).

SAX uses an event-driven model: the parser identifies an element, resulting in that element's event handler being invoked. It is optimized for applet/browser use and can plug-in to any particular parser. We next present two lists taken from Parsers and Applications Supporting SAX, which contains links to all of the parsers and applications. Most of these are discussed later in this WDVL article.

SAX Parsers (only those written in Java are listed):

• IBM's XML for Java
• James Clark's XP
• DataChannel's DXP
• Microstar's AElfred
• Silfide's SXP
• Sun's XML Library

SAX Applications (only those written in Java are listed):

• James Clark's XMLTest
• Peter Murray-Rust's JUMBO
• Michael Kay's SAXON
• Megginson Technologies' XAF
• Docuverse's DOM SDK
• JXML's Coins
• W3C's SiRPAC
• John Cowan's SAX ParserFilters and DOMParser
• WebEasy's Weasel
• Koala Project's KOML (Koala Object Markup Language)
• Koala Project's XSL Engine

The SAX 1.0 release consists of two packages, org.xml.sax and org.xml.sax.helpers, which the roadmap divides into five groups of classes:

1. interfaces implemented by SAX-conformant parsers (e.g., a SAXDriver);
2. interfaces implemented by applications (e.g., DocumentHandler and ErrorHandler);
3. standard SAX classes (e.g., InputSource and SAXException);
4. optional Java-specific helper (convenience) classes (e.g., ParserFactory); and
5. Java demonstration classes in the nul package.

The core SAX packages can be downloaded separately from the sample SAXDrivers.

The code examples below from David Megginson's Quick Start page illustrate how easy it is to use SAX. The first file, MyHandler.java, subclasses HandlerBase (itself a subclass of DocumentHandler) to define a class that will handle events. In this basic example, only the startElement and EndElement methods are defined, but there are others to choose from and certainly more interesting things can be done with the AttributeList that is sent to the startElement method. Interested readers should read Megginson's javadoc for AttributeList.

import org.xml.sax.HandlerBase;
import org.xml.sax.AttributeList;

   public class MyHandler extends HandlerBase {

     public void startElement (String name, AttributeList atts)
     {
       System.out.println("Start element: " + name);
     }

     public void endElement (String name)
     {
       System.out.println("End element: " + name);
     }
   }

The main program, SAXApp.java, defines the parserClass (Java classpath for the parser of choice -- Microstar's AElred in this example), calls ParserFactory.makeParser to instantiate a parser, instantiates MyHandler, and associates the handler as the DocumentHandler. The beauty of SAX is that to switch to a different parser, the only line in both files that must change is the one that defines the parserClass. (You might want to switch to a different parser if, for example, a faster parser becomes available.) Therefore, by using SAX, your application code remains the same across different SAX-conformant parsers.

import org.xml.sax.Parser;
import org.xml.sax.DocumentHandler;
import org.xml.sax.helpers.ParserFactory;

   public class SAXApp {

     static final String parserClass = "com.microstar.xml.SAXDriver";

     public static void main (String args[])
       throws Exception
     {
       Parser parser = ParserFactory.makeParser(parserClass);
       DocumentHandler handler = new MyHandler();
       parser.setDocumentHandler(handler);
       // Parse all files given on the command line.
       for (int i = 0; i < args.length; i++) {
         parser.parse(args[i]);
       }
     }
   }

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