libjava/org/xml/sax/helpers/XMLReaderFactory.java

   1 // XMLReaderFactory.java - factory for creating a new reader.
   2 // http://www.saxproject.org
   3 // Written by David Megginson
   4 // and by David Brownell
   5 // NO WARRANTY!  This class is in the Public Domain.
   6
   7 // $Id: XMLReaderFactory.java,v 1.5.2.4 2002/01/29 21:34:15 dbrownell Exp $
   8
   9 package org.xml.sax.helpers;
  10 import java.io.BufferedReader;
  11 import java.io.InputStream;
  12 import java.io.InputStreamReader;
  13 import org.xml.sax.XMLReader;
  14 import org.xml.sax.SAXException;
  15
  16
  17 /**
  18  * Factory for creating an XML reader.
  19  *
  20  * <blockquote>
  21  * <em>This module, both source code and documentation, is in the
  22  * Public Domain, and comes with <strong>NO WARRANTY</strong>.</em>
  23  * See <a href='http://www.saxproject.org'>http://www.saxproject.org</a>
  24  * for further information.
  25  * </blockquote>
  26  *
  27  * <p>This class contains static methods for creating an XML reader
  28  * from an explicit class name, or based on runtime defaults:</p>
  29  *
  30  * <pre>
  31  * try {
  32  *   XMLReader myReader = XMLReaderFactory.createXMLReader();
  33  * } catch (SAXException e) {
  34  *   System.err.println(e.getMessage());
  35  * }
  36  * </pre>
  37  *
  38  * <p><strong>Note to Distributions bundled with parsers:</strong>
  39  * You should modify the implementation of the no-arguments
  40  * <em>createXMLReader</em> to handle cases where the external
  41  * configuration mechanisms aren't set up.  That method should do its
  42  * best to return a parser when one is in the class path, even when
  43  * nothing bound its class name to <code>org.xml.sax.driver</code> so
  44  * those configuration mechanisms would see it.</p>
  45  *
  46  * @since SAX 2.0
  47  * @author David Megginson, David Brownell
  48  * @version 2.0.1 (sax2r2)
  49  */
  50 final public class XMLReaderFactory
  51 {
  52     /**
  53      * Private constructor.
  54      *
  55      * <p>This constructor prevents the class from being instantiated.</p>
  56      */
  57     private XMLReaderFactory ()
  58     {
  59     }
  60
  61     private static final String property = "org.xml.sax.driver";
  62
  63     /**
  64      * Attempt to create an XMLReader from system defaults.
  65      * In environments which can support it, the name of the XMLReader
  66      * class is determined by trying each these options in order, and
  67      * using the first one which succeeds:</p> <ul>
  68      *
  69      * <li>If the system property <code>org.xml.sax.driver</code>
  70      * has a value, that is used as an XMLReader class name. </li>
  71      *
  72      * <li>The JAR "Services API" is used to look for a class name
  73      * in the <em>META-INF/services/org.xml.sax.driver</em> file in
  74      * jarfiles available to the runtime.</li>
  75      *
  76      * <li> SAX parser distributions are strongly encouraged to provide
  77      * a default XMLReader class name that will take effect only when
  78      * previous options (on this list) are not successful.</li>
  79      *
  80      * <li>Finally, if {@link ParserFactory#makeParser()} can
  81      * return a system default SAX1 parser, that parser is wrapped in
  82      * a {@link ParserAdapter}.  (This is a migration aid for SAX1
  83      * environments, where the <code>org.xml.sax.parser</code> system
  84      * property will often be usable.) </li>
  85      *
  86      * </ul>
  87      *
  88      * <p> In environments such as small embedded systems, which can not
  89      * support that flexibility, other mechanisms to determine the default
  90      * may be used. </p>
  91      *
  92      * <p>Note that many Java environments allow system properties to be
  93      * initialized on a command line.  This means that <em>in most cases</em>
  94      * setting a good value for that property ensures that calls to this
  95      * method will succeed, except when security policies intervene.
  96      * This will also maximize application portability to older SAX
  97      * environments, with less robust implementations of this method.
  98      * </p>
  99      *
 100      * @return A new XMLReader.
 101      * @exception org.xml.sax.SAXException If no default XMLReader class
 102      *            can be identified and instantiated.
 103      * @see #createXMLReader(java.lang.String)
 104      */
 105     public static XMLReader createXMLReader ()
 106         throws SAXException
 107     {
 108         String          className = null;
 109         ClassLoader     loader = NewInstance.getClassLoader ();
 110
 111         // 1. try the JVM-instance-wide system property
 112         try { className = System.getProperty (property); }
 113         catch (Exception e) { /* normally fails for applets */ }
 114
 115         // 2. if that fails, try META-INF/services/
 116         if (className == null) {
 117             try {
 118                 String          service = "META-INF/services/" + property;
 119                 InputStream     in;
 120                 BufferedReader  reader;
 121
 122                 if (loader == null)
 123                     in = ClassLoader.getSystemResourceAsStream (service);
 124                 else
 125                     in = loader.getResourceAsStream (service);
 126
 127                 if (in != null) {
 128                     reader = new BufferedReader (
 129                             new InputStreamReader (in, "UTF8"));
 130                     className = reader.readLine ();
 131                     in.close ();
 132                 }
 133             } catch (Exception e) {
 134             }
 135         }
 136
 137         // 3. Distro-specific fallback
 138         if (className == null) {
 139 // BEGIN DISTRIBUTION-SPECIFIC
 140
 141             // EXAMPLE:
 142             // className = "com.example.sax.XmlReader";
 143             // or a $JAVA_HOME/jre/lib/*properties setting...
 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 }