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Reaching For The Nailgun - XML

So you've already seen how Perl and PHP handle XML data. But you're a Real Programmer, and Real Programmers don't waste time with scripting languages. Nope, you need something a little more powerful, something with more horsepower under the hood. Something written in Java. Something like Xerces.

TABLE OF CONTENTS:
  1. XML Parsing With SAX and Xerces (part 1)
  2. Playing The SAX
  3. Reaching For The Nailgun
  4. Under The Microscope
  5. Sweeping Up The Mess
  6. Diving Deeper
By: icarus, (c) Melonfire
Rating: starstarstarstarstar / 16
January 28, 2002

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Let's begin with a simple XML file, which displays the marked-up inventory statement for a business selling equipment for Quake enthusiasts:


<?xml version="1.0"?> <inventory> <item> <id>758</id> <name>Rusty, jagged nails for nailgun</name> <supplier>NailBarn, Inc.</supplier> <cost>2.99</cost> <quantity>10000</quantity> </item> <item> <id>6273</id> <name>Power pack for death ray</name> <supplier>QuakePower.domain.com</supplier> <cost>9.99</cost> <quantity>10</quantity> </item> </inventory>
Now, we need a simple Java application that will initialize the SAX parser, read and parse the XML file, and fire the callback functions as it encounters tags.

import org.apache.xerces.parsers.SAXParser; import org.xml.sax.*; import java.io.*; // the ContentHandler interface handles all the callbacks public class MyFirstSaxApp implements ContentHandler { // constructor public MyFirstSaxApp (String xmlFile) { // create a Xerces SAX parser SAXParser parser = new SAXParser(); // set the content handler parser.setContentHandler(this); // parse the document try { parser.parse(xmlFile); } catch (SAXException e) { System.err.println (e); } catch (IOException e) { System.err.println (e); } } // call this when a start tag is found public void startElement (String uri, String local, String qName, Attributes atts) { System.out.println ("Found element: " + local); } // the remaining callback handlers // they don't do anything right now...but keep reading! public void setDocumentLocator(Locator locator) {} public void startDocument() {} public void endDocument() {} public void characters(char[] text, int start, int length){} public void startPrefixMapping(String prefix, String uri) {} public void endPrefixMapping(String prefix) {} public void endElement(String namespaceURI, String localName, String qualifiedName) {} public void ignorableWhitespace(char[] text, int start, int length) throws SAXException {} public void processingInstruction(String target, String data){} public void skippedEntity(String name) {} // everything starts here public static void main (String[] args) { MyFirstSaxApp myFirstExample = new MyFirstSaxApp(args[0]); } }
Sure, it looks a little intimidating - but fear not, all will be explained shortly. Before I get to that, though, it's instructive to see what the output of this looks like. So, how about we compile it and run it?

$ javac MyFirstSaxApp.java
Assuming that all goes well, you should now have a class file named "MyFirstSaxApp.class". Copy this class file to your Java CLASSPATH, and then execute it, with the name of the XML file as argument.

$ java MyFirstSaxApp /home/me/sax/inventory.xml
And here's what you should see:

Found element: inventory Found element: item Found element: id Found element: name Found element: supplier Found element: cost Found element: quantity Found element: item Found element: id Found element: name Found element: supplier Found element: cost Found element: quantity
What's happening here? Every time the parser encounters a start tag within the XML document, it calls the startElement()function, which prints the name of the tag to the standard output device. The parser then moves on to the next construct within the document, calling the appropriate callback function to handle it. This process continues until the entire XML document has been processed.

 
 
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