XMLReaderFactory


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   // $Id: XMLReaderFactory.java,v 1.3.14.1.2.1 2004/06/12 02:22:29 rameshm Exp $
7   
8   package org.xml.sax.helpers;
9   import java.io.BufferedReader  ;
10  import java.io.InputStream  ;
11  import java.io.InputStreamReader  ;
12  import org.xml.sax.XMLReader  ;
13  import org.xml.sax.SAXException  ;
14  
15  
16  /**
17   * Factory for creating an XML reader.
18   *
19   * <blockquote>
20   * <em>This module, both source code and documentation, is in the
21   * Public Domain, and comes with <strong>NO WARRANTY</strong>.</em>
22   * See <a HREF='http://www.saxproject.org'>http://www.saxproject.org</a>
23   * for further information.
24   * </blockquote>
25   *
26   * <p>This class contains static methods for creating an XML reader
27   * from an explicit class name, or based on runtime defaults:</p>
28   *
29   * <pre>
30   * try {
31   *   XMLReader myReader = XMLReaderFactory.createXMLReader();
32   * } catch (SAXException e) {
33   *   System.err.println(e.getMessage());
34   * }
35   * </pre>
36   *
37   * <p><strong>Note to Distributions bundled with parsers:</strong>
38   * You should modify the implementation of the no-arguments
39   * <em>createXMLReader</em> to handle cases where the external
40   * configuration mechanisms aren't set up.  That method should do its
41   * best to return a parser when one is in the class path, even when
42   * nothing bound its class name to <code>org.xml.sax.driver</code> so
43   * those configuration mechanisms would see it.</p>
44   *
45   * @since SAX 2.0
46   * @author David Megginson, David Brownell
47   * @version 2.0.1 (sax2r2)
48   */
49  final public class XMLReaderFactory
50  {
51      /**
52       * Private constructor.
53       *
54       * <p>This constructor prevents the class from being instantiated.</p>
55       */
56      private XMLReaderFactory ()
57      {
58      }
59  
60      private static final String   property = "org.xml.sax.driver";
61  
62      /**
63       * Attempt to create an XMLReader from system defaults.
64       * In environments which can support it, the name of the XMLReader
65       * class is determined by trying each these options in order, and
66       * using the first one which succeeds:</p> <ul>
67       *
68       * <li>If the system property <code>org.xml.sax.driver</code>
69       * has a value, that is used as an XMLReader class name. </li>
70       *
71       * <li>The JAR "Services API" is used to look for a class name
72       * in the <em>META-INF/services/org.xml.sax.driver</em> file in
73       * jarfiles available to the runtime.</li>
74       *
75       * <li> SAX parser distributions are strongly encouraged to provide
76       * a default XMLReader class name that will take effect only when
77       * previous options (on this list) are not successful.</li>
78       *
79       * <li>Finally, if {@link ParserFactory#makeParser()} can
80       * return a system default SAX1 parser, that parser is wrapped in
81       * a {@link ParserAdapter}.  (This is a migration aid for SAX1
82       * environments, where the <code>org.xml.sax.parser</code> system
83       * property will often be usable.) </li>
84       *
85       * </ul>
86       *
87       * <p> In environments such as small embedded systems, which can not
88       * support that flexibility, other mechanisms to determine the default
89       * may be used. </p>
90       *
91       * <p>Note that many Java environments allow system properties to be
92       * initialized on a command line.  This means that <em>in most cases</em>
93       * setting a good value for that property ensures that calls to this
94       * method will succeed, except when security policies intervene.
95       * This will also maximize application portability to older SAX
96       * environments, with less robust implementations of this method.
97       * </p>
98       *
99       * @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 }
204
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