XML - Java Examples in a Nutshell - page 496

<servlet>
<servlet-name>logout</servlet-name>
<servlet-class>com.davidflanagan.examples.servlet.Logout</servlet-class>
</servlet>

ListServlets1.java includes a main() method that uses the JAXP API to obtain a SAX
parser instance. It then tells the parser what to parse and starts the parser running.
The remaining methods of the class are invoked by the parser. Note that List-
Servlets1 extends the SAX HandlerBase class. This superclass provides dummy
implementations of all the SAX event handler methods. The example simply over-
rides the handlers of interest. The parser calls the startElement() method when it
reads an XML tag; it calls endElement() when it finds a closing tag. characters()
is invoked when the parser reads a string of plain text with no markup. Finally,
the parser calls warning() , error() ,or fatalError() when something goes wrong
in the parsing process. The implementations of these methods are written specifi-
cally to extract the desired information from a web.xml file and are based on a
knowledge of the structure of this type of file.

Note that web.xml files are somewhat unusual in that they don't rely on attributes
for any of the XML tags. That is, servlet names are defined by a <servlet-name>
tag nested within a <servlet> tag, instead of simply using a name attribute of the
<servlet> tag itself. This fact makes the example program slightly more complex
than it would otherwise be. The web.xml file does allow id attributes for all its
tags. Although servlet engines are not expected to use these attributes, they may
be useful to a configuration tool that parses and automatically generates web.xml
files. For completeness, the startElement() method in Example 19-1 looks for an
id attribute of the <servlet> tag. The value of that attribute, if it exists, is reported
in the program's output.

Example 19−1: ListServlets1.java

package com.davidflanagan.examples.xml;
import javax.xml.parsers.*; // The JAXP package

import org.xml.sax.*; // The main SAX package

import java.io.*;

/**
* Parse a web.xml file using JAXP and SAX1. Print out the names
* and class names of all servlets listed in the file.
*
* This class implements the HandlerBase helper class, which means
* that it defines all the "callback" methods that the SAX parser will
* invoke to notify the application. In this example we override the
* methods that we require.
*
* This example uses full package names in places to help keep the JAXP
* and SAX APIs distinct.
**/
public class ListServlets1 extends org.xml.sax.HandlerBase {
/** The main method sets things up for parsing */
public static void main(String[] args)
throws IOException, SAXException, ParserConfigurationException

{

// Create a JAXP "parser factory" for creating SAX parsers
javax.xml.parsers.SAXParserFactory spf=SAXParserFactory.newInstance();