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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 1.5 2004/03/19 20:17:55 maartenc 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 
144 // END DISTRIBUTION-SPECIFIC
145 	}
146 	
147 	// do we know the XMLReader implementation class yet?
148 	if (className != null)
149 	    return loadClass (loader, className);
150 
151 	// 4. panic -- adapt any SAX1 parser
152 	try {
153 	    return new ParserAdapter (ParserFactory.makeParser ());
154 	} catch (Exception e) {
155 	    throw new SAXException ("Can't create default XMLReader; "
156 		    + "is system property org.xml.sax.driver set?");
157 	}
158     }
159 
160 
161     /**
162      * Attempt to create an XML reader from a class name.
163      *
164      * <p>Given a class name, this method attempts to load
165      * and instantiate the class as an XML reader.</p>
166      *
167      * <p>Note that this method will not be usable in environments where
168      * the caller (perhaps an applet) is not permitted to load classes
169      * dynamically.</p>
170      *
171      * @return A new XML reader.
172      * @exception org.xml.sax.SAXException If the class cannot be
173      *            loaded, instantiated, and cast to XMLReader.
174      * @see #createXMLReader()
175      */
176     public static XMLReader createXMLReader (String className)
177 	throws SAXException
178     {
179 	return loadClass (NewInstance.getClassLoader (), className);
180     }
181 
182     private static XMLReader loadClass (ClassLoader loader, String className)
183     throws SAXException
184     {
185 	try {
186 	    return (XMLReader) NewInstance.newInstance (loader, className);
187 	} catch (ClassNotFoundException e1) {
188 	    throw new SAXException("SAX2 driver class " + className +
189 				   " not found", e1);
190 	} catch (IllegalAccessException e2) {
191 	    throw new SAXException("SAX2 driver class " + className +
192 				   " found but cannot be loaded", e2);
193 	} catch (InstantiationException e3) {
194 	    throw new SAXException("SAX2 driver class " + className +
195 	   " loaded but cannot be instantiated (no empty public constructor?)",
196 				   e3);
197 	} catch (ClassCastException e4) {
198 	    throw new SAXException("SAX2 driver class " + className +
199 				   " does not implement XMLReader", e4);
200 	}
201     }
202 }