Source Home >> Java Source 1.6.0 >> org.xml.sax.helpers.XMLReaderFactory V 0.09

001// XMLReaderFactory.java - factory for creating a new reader.
002// http://www.saxproject.org
003// Written by David Megginson
004// and by David Brownell
005// NO WARRANTY!  This class is in the Public Domain.
006// $Id: XMLReaderFactory.java,v 1.2.2.1 2005/07/31 22:48:08 jeffsuttor Exp $
007
008package org.xml.sax.helpers;
009import java.io.BufferedReader;
010import java.io.InputStream;
011import java.io.InputStreamReader;
012import org.xml.sax.XMLReader;
013import org.xml.sax.SAXException;
014
015
016/**
017 * Factory for creating an XML reader.
018 *
019 * <blockquote>
020 * <em>This module, both source code and documentation, is in the
021 * Public Domain, and comes with <strong>NO WARRANTY</strong>.</em>
022 * See <a href='http://www.saxproject.org'>http://www.saxproject.org</a>
023 * for further information.
024 * </blockquote>
025 *
026 * <p>This class contains static methods for creating an XML reader
027 * from an explicit class name, or based on runtime defaults:</p>
028 *
029 * <pre>
030 * try {
031 *   XMLReader myReader = XMLReaderFactory.createXMLReader();
032 * } catch (SAXException e) {
033 *   System.err.println(e.getMessage());
034 * }
035 * </pre>
036 *
037 * <p><strong>Note to Distributions bundled with parsers:</strong>
038 * You should modify the implementation of the no-arguments
039 * <em>createXMLReader</em> to handle cases where the external
040 * configuration mechanisms aren't set up.  That method should do its
041 * best to return a parser when one is in the class path, even when
042 * nothing bound its class name to <code>org.xml.sax.driver</code> so
043 * those configuration mechanisms would see it.</p>
044 *
045 * @since SAX 2.0
046 * @author David Megginson, David Brownell
047 * @version 2.0.1 (sax2r2)
048 */
049final public class XMLReaderFactory
050{
051    /**
052     * Private constructor.
053     *
054     * <p>This constructor prevents the class from being instantiated.</p>
055     */
056    private XMLReaderFactory ()
057    {
058    }
059
060    private static final String property = "org.xml.sax.driver";
061
062    /**
063     * Attempt to create an XMLReader from system defaults.
064     * In environments which can support it, the name of the XMLReader
065     * class is determined by trying each these options in order, and
066     * using the first one which succeeds:</p> <ul>
067     *
068     * <li>If the system property <code>org.xml.sax.driver</code>
069     * has a value, that is used as an XMLReader class name. </li>
070     *
071     * <li>The JAR "Services API" is used to look for a class name
072     * in the <em>META-INF/services/org.xml.sax.driver</em> file in
073     * jarfiles available to the runtime.</li>
074     *
075     * <li> SAX parser distributions are strongly encouraged to provide
076     * a default XMLReader class name that will take effect only when
077     * previous options (on this list) are not successful.</li>
078     *
079     * <li>Finally, if {@link ParserFactory#makeParser()} can
080     * return a system default SAX1 parser, that parser is wrapped in
081     * a {@link ParserAdapter}.  (This is a migration aid for SAX1
082     * environments, where the <code>org.xml.sax.parser</code> system
083     * property will often be usable.) </li>
084     *
085     * </ul>
086     *
087     * <p> In environments such as small embedded systems, which can not
088     * support that flexibility, other mechanisms to determine the default
089     * may be used. </p>
090     *
091     * <p>Note that many Java environments allow system properties to be
092     * initialized on a command line.  This means that <em>in most cases</em>
093     * setting a good value for that property ensures that calls to this
094     * method will succeed, except when security policies intervene.
095     * This will also maximize application portability to older SAX
096     * environments, with less robust implementations of this method.
097     * </p>
098     *
099     * @return A new XMLReader.
100     * @exception org.xml.sax.SAXException If no default XMLReader class
101     *            can be identified and instantiated.
102     * @see #createXMLReader(java.lang.String)
103     */
104    public static XMLReader createXMLReader ()
105	throws SAXException
106    {
107	String		className = null;
108	ClassLoader	loader = NewInstance.getClassLoader ();
109	
110	// 1. try the JVM-instance-wide system property
111	try { className = System.getProperty (property); }
112	catch (RuntimeException e) { /* normally fails for applets */}
113
114	// 2. if that fails, try META-INF/services/
115	if (className == null) {
116	    try {
117		String		service = "META-INF/services/" + property;
118		InputStream	in;
119		BufferedReader	reader;
120
121		if (loader == null)
122		    in = ClassLoader.getSystemResourceAsStream (service);
123		else
124		    in = loader.getResourceAsStream (service);
125
126		if (in != null) {
127		    reader = new BufferedReader (
128			    new InputStreamReader (in, "UTF8"));
129		    className = reader.readLine ();
130		    in.close ();
131		}
132	    } catch (Exception e) {
133	    }
134	}
135
136	// 3. Distro-specific fallback
137	if (className == null) {
138// BEGIN DISTRIBUTION-SPECIFIC
139
140	    // EXAMPLE:
141	    // className = "com.example.sax.XmlReader";
142	    // or a $JAVA_HOME/jre/lib/*properties setting...
143            className = "com.sun.org.apache.xerces.internal.parsers.SAXParser";
144
145// END DISTRIBUTION-SPECIFIC
146	}
147	
148	// do we know the XMLReader implementation class yet?
149	if (className != null)
150	    return loadClass (loader, className);
151
152	// 4. panic -- adapt any SAX1 parser
153	try {
154	    return new ParserAdapter (ParserFactory.makeParser ());
155	} catch (Exception e) {
156	    throw new SAXException ("Can't create default XMLReader; "
157		    + "is system property org.xml.sax.driver set?");
158	}
159    }
160
161
162    /**
163     * Attempt to create an XML reader from a class name.
164     *
165     * <p>Given a class name, this method attempts to load
166     * and instantiate the class as an XML reader.</p>
167     *
168     * <p>Note that this method will not be usable in environments where
169     * the caller (perhaps an applet) is not permitted to load classes
170     * dynamically.</p>
171     *
172     * @return A new XML reader.
173     * @exception org.xml.sax.SAXException If the class cannot be
174     *            loaded, instantiated, and cast to XMLReader.
175     * @see #createXMLReader()
176     */
177    public static XMLReader createXMLReader (String className)
178	throws SAXException
179    {
180	return loadClass (NewInstance.getClassLoader (), className);
181    }
182
183    private static XMLReader loadClass (ClassLoader loader, String className)
184    throws SAXException
185    {
186	try {
187	    return (XMLReader) NewInstance.newInstance (loader, className);
188	} catch (ClassNotFoundException e1) {
189	    throw new SAXException("SAX2 driver class " + className +
190				   " not found", e1);
191	} catch (IllegalAccessException e2) {
192	    throw new SAXException("SAX2 driver class " + className +
193				   " found but cannot be loaded", e2);
194	} catch (InstantiationException e3) {
195	    throw new SAXException("SAX2 driver class " + className +
196	   " loaded but cannot be instantiated (no empty public constructor?)",
197				   e3);
198	} catch (ClassCastException e4) {
199	    throw new SAXException("SAX2 driver class " + className +
200				   " does not implement XMLReader", e4);
201	}
202    }
203}