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// XMLReaderFactory.java - factory for creating a new reader.

// http://www.saxproject.org

// Written by David Megginson

// and by David Brownell

// NO WARRANTY!  This class is in the Public Domain.

// $Id: XMLReaderFactory.java,v 1.1 2004/12/23 22:38:42 mark Exp $

package org.xml.sax.helpers;

import java.io.BufferedReader;

import java.io.InputStream;

import java.io.InputStreamReader;

import org.xml.sax.XMLReader;

import org.xml.sax.SAXException;

/**

 * Factory for creating an XML reader.

 *

 * <blockquote>

 * <em>This module, both source code and documentation, is in the

 * Public Domain, and comes with <strong>NO WARRANTY</strong>.</em>

 * See <a href='http://www.saxproject.org'>http://www.saxproject.org</a>

 * for further information.

 * </blockquote>

 *

 * <p>This class contains static methods for creating an XML reader

 * from an explicit class name, or based on runtime defaults:</p>

 *

 * <pre>

 * try {

 *   XMLReader myReader = XMLReaderFactory.createXMLReader();

 * } catch (SAXException e) {

 *   System.err.println(e.getMessage());

 * }

 * </pre>

 *

 * <p><strong>Note to Distributions bundled with parsers:</strong>

 * You should modify the implementation of the no-arguments

 * <em>createXMLReader</em> to handle cases where the external

 * configuration mechanisms aren't set up.  That method should do its

 * best to return a parser when one is in the class path, even when

 * nothing bound its class name to <code>org.xml.sax.driver</code> so

 * those configuration mechanisms would see it.</p>

 *

 * @since SAX 2.0

 * @author David Megginson, David Brownell

 * @version 2.0.1 (sax2r2)

 */

final public class XMLReaderFactory

{

    /**

     * Private constructor.

     *

     * <p>This constructor prevents the class from being instantiated.</p>

     */

    private XMLReaderFactory ()

    {

    }

    private static final String property = "org.xml.sax.driver";

    /**

     * Attempt to create an XMLReader from system defaults.

     * In environments which can support it, the name of the XMLReader

     * class is determined by trying each these options in order, and

     * using the first one which succeeds:</p> <ul>

     *

     * <li>If the system property <code>org.xml.sax.driver</code>

     * has a value, that is used as an XMLReader class name. </li>

     *

     * <li>The JAR "Services API" is used to look for a class name

     * in the <em>META-INF/services/org.xml.sax.driver</em> file in

     * jarfiles available to the runtime.</li>

     *

     * <li> SAX parser distributions are strongly encouraged to provide

     * a default XMLReader class name that will take effect only when

     * previous options (on this list) are not successful.</li>

     *

     * <li>Finally, if {@link ParserFactory#makeParser()} can

     * return a system default SAX1 parser, that parser is wrapped in

     * a {@link ParserAdapter}.  (This is a migration aid for SAX1

     * environments, where the <code>org.xml.sax.parser</code> system

     * property will often be usable.) </li>

     *

     * </ul>

     *

     * <p> In environments such as small embedded systems, which can not

     * support that flexibility, other mechanisms to determine the default

     * may be used. </p>

     *

     * <p>Note that many Java environments allow system properties to be

     * initialized on a command line.  This means that <em>in most cases</em>

     * setting a good value for that property ensures that calls to this

     * method will succeed, except when security policies intervene.

     * This will also maximize application portability to older SAX

     * environments, with less robust implementations of this method.

     * </p>

     *

     * @return A new XMLReader.

     * @exception org.xml.sax.SAXException If no default XMLReader class

     *            can be identified and instantiated.

     * @see #createXMLReader(java.lang.String)

     */

    public static XMLReader createXMLReader ()

        throws SAXException

    {

        String          className = null;

        ClassLoader     loader = NewInstance.getClassLoader ();

        // 1. try the JVM-instance-wide system property

        try { className = System.getProperty (property); }

        catch (RuntimeException e) { /* normally fails for applets */ }

        // 2. if that fails, try META-INF/services/

        if (className == null) {

            try {

                String          service = "META-INF/services/" + property;

                InputStream     in;

                BufferedReader  reader;

                if (loader == null)

                    in = ClassLoader.getSystemResourceAsStream (service);

                else

                    in = loader.getResourceAsStream (service);

                if (in != null) {

                    reader = new BufferedReader (

                            new InputStreamReader (in, "UTF8"));

                    className = reader.readLine ();

                    in.close ();

                }

            } catch (Exception e) {

            }

        }

        // 3. Distro-specific fallback

        if (className == null) {

// BEGIN DISTRIBUTION-SPECIFIC

            // CLASSPATH LOCAL: have to code in the backup.

            // Among other things, see PR 31303, and this thread:

            // http://gcc.gnu.org/ml/java-patches/2007-q1/msg00661.html

            className = "gnu.xml.stream.SAXParser";

            // EXAMPLE:

            // className = "com.example.sax.XmlReader";

            // or a $JAVA_HOME/jre/lib/*properties setting...

// END DISTRIBUTION-SPECIFIC

        }

        // do we know the XMLReader implementation class yet?

        if (className != null)

            return loadClass (loader, className);

        // 4. panic -- adapt any SAX1 parser

        try {

            return new ParserAdapter (ParserFactory.makeParser ());

        } catch (Exception e) {

            throw new SAXException ("Can't create default XMLReader; "

                    + "is system property org.xml.sax.driver set?");

        }

    }

    /**

     * Attempt to create an XML reader from a class name.

     *

     * <p>Given a class name, this method attempts to load

     * and instantiate the class as an XML reader.</p>

     *

     * <p>Note that this method will not be usable in environments where

     * the caller (perhaps an applet) is not permitted to load classes

     * dynamically.</p>

     *

     * @return A new XML reader.

     * @exception org.xml.sax.SAXException If the class cannot be

     *            loaded, instantiated, and cast to XMLReader.

     * @see #createXMLReader()

     */

    public static XMLReader createXMLReader (String className)

        throws SAXException

    {

        return loadClass (NewInstance.getClassLoader (), className);

    }

    private static XMLReader loadClass (ClassLoader loader, String className)

    throws SAXException

    {

        try {

            return (XMLReader) NewInstance.newInstance (loader, className);

        } catch (ClassNotFoundException e1) {

            throw new SAXException("SAX2 driver class " + className +

                                   " not found", e1);

        } catch (IllegalAccessException e2) {

            throw new SAXException("SAX2 driver class " + className +

                                   " found but cannot be loaded", e2);

        } catch (InstantiationException e3) {

            throw new SAXException("SAX2 driver class " + className +

           " loaded but cannot be instantiated (no empty public constructor?)",

                                   e3);

        } catch (ClassCastException e4) {

            throw new SAXException("SAX2 driver class " + className +

                                   " does not implement XMLReader", e4);

        }

    }

}