001/* java.beans.XMLDecoder --
002   Copyright (C) 2004, 2005, 2006  Free Software Foundation, Inc.
003
004This file is part of GNU Classpath.
005
006GNU Classpath is free software; you can redistribute it and/or modify
007it under the terms of the GNU General Public License as published by
008the Free Software Foundation; either version 2, or (at your option)
009any later version.
010
011GNU Classpath is distributed in the hope that it will be useful, but
012WITHOUT ANY WARRANTY; without even the implied warranty of
013MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
014General Public License for more details.
015
016You should have received a copy of the GNU General Public License
017along with GNU Classpath; see the file COPYING.  If not, write to the
018Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
01902110-1301 USA.
020
021Linking this library statically or dynamically with other modules is
022making a combined work based on this library.  Thus, the terms and
023conditions of the GNU General Public License cover the whole
024combination.
025
026As a special exception, the copyright holders of this library give you
027permission to link this library with independent modules to produce an
028executable, regardless of the license terms of these independent
029modules, and to copy and distribute the resulting executable under
030terms of your choice, provided that you also meet, for each linked
031independent module, the terms and conditions of the license of that
032module.  An independent module is a module which is not derived from
033or based on this library.  If you modify this library, you may extend
034this exception to your version of the library, but you are not
035obligated to do so.  If you do not wish to do so, delete this
036exception statement from your version. */
037
038
039package java.beans;
040
041import gnu.java.beans.DefaultExceptionListener;
042import gnu.java.beans.decoder.PersistenceParser;
043
044import java.io.IOException;
045import java.io.InputStream;
046import java.util.Iterator;
047import java.util.NoSuchElementException;
048
049/**
050 * The XMLDecoder reads XML data that is structured according to
051 * <a href="http://java.sun.com/products/jfc/tsc/articles/persistence3/javabeans.dtd">this</a> DTD
052 * and creates objects according to the content. Usually such data is generated using the
053 * {@link XMLEncoder} class.
054 * <p>
055 * An example XML document might look like this:
056 * <code>
057 * &lt;java&gt;
058 *      &lt;string&gt;Hello World&lt;/string&gt;
059 *      &lt;int&gt;200&lt;/int&gt;
060 * &lt;/java&gt;
061 * </code>
062 * <p>To read the <code>String</code> and the <code>Integer</code> instance the following can be used (assume
063 * the XML data can be obtained from the InputStream):</p>
064 * <code>
065 * XMLDecoder decoder = new XMLDecoder(inputStreamContainingXMLData);
066 * String message = (String) decoder.readObject();
067 * Integer number = (Integer) decoder.readObject();
068 * </code>
069 * <p>Besides this basic functionality the <code>XMLDecoder</code> has some more features that might come
070 * handy in certain situations:</p>
071 * <p>An owner object can be set using the <code>setOwner</code> method which can then be accessed when
072 * decoding. This feature is only useful if the XML data is aware of the owner object. Such data may
073 * look like this (assume that the owner object is a JFrame instance):</p>
074 * <code> 
075 * &lt;java&gt;
076 *      &lt;void method="getOwner"&gt;
077 *              &lt;void method="setVisible"&gt;
078 *                      &lt;boolean&gt;true&lt;boolean&gt;
079 *              &lt;/void&gt;
080 *  &lt;/void&gt;
081 * &lt;/java&gt;
082 * </code>
083 * This accesses the <code>JFrame</code> and makes it visible using the <code>setVisible</code> method.
084 * <p>Please note that changing the owner <b>after</b> the having read the first object has no effect,
085 * because all object have been decoded then.</p>
086 * <p>If the <code>XMLDecoder</code> is created with no {@link ExceptionListener} instance a default one
087 * is used that prints an error message to <code>System.err</code> whenever a recoverable exception
088 * is thrown. Recovarable exceptions occur when the XML data cannot be interpreted correctly (e.g
089 * unknown classes or methods, invocation on null, ...). In general be very careful when the
090 * <code>XMLDecoder</code> provoked such exceptions because the resulting object(s) may be in an
091 * undesirable state.</p>
092 * <p>Note that changing the ExceptionListener instance after <code>readObject</code> has been called
093 * once has no effect because the decoding is completed then.</p>
094 * <p>At last one can provide a specific <code>ClassLoader</code> which is then used when <code>Class</code>
095 * objects are accessed. See {@link java.lang.Class#forName(String, boolean, ClassLoader)} for details
096 * on this.</p>
097 * <p>Note: If the <code>InputStream</code> instance given to any of the constructors is <code>null</code>
098 * the resulting <code>XMLDecoder</code> will be silently (without any exception) useless. Each call
099 * to <code>readObject</code> will return <code>null</code> and never throws an
100 * <code>ArrayIndexOutOfBoundsException</code>.</p>
101 *  
102 * @author Robert Schuster
103 * @since 1.4
104 * @status updated to 1.5
105 */
106public class XMLDecoder
107{
108        private Object owner;
109
110        private ExceptionListener exceptionListener;
111
112        private InputStream inputStream;
113
114        private boolean isStreamClosed;
115
116        private ClassLoader classLoader;
117
118        private Iterator iterator;
119
120        /** Creates a XMLDecoder instance that parses the XML data of the given input stream.
121         * Using this constructor no special ClassLoader, a default ExceptionListener
122         * and no owner object is used.
123         * 
124         * @param in InputStream to read XML data from.
125         */
126        public XMLDecoder(InputStream in)
127        {
128                this(in, null);
129        }
130
131        /** Creates a XMLDecoder instance that parses the XML data of the given input stream.
132         * Using this constructor no special ClassLoader and a default ExceptionListener
133         * is used.
134         * 
135         * @param in InputStream to read XML data from.
136         * @param owner Owner object which can be accessed and modified while parsing.
137         */
138        public XMLDecoder(InputStream in, Object owner)
139        {
140                this(in, owner, null);
141        }
142
143        /** Creates a XMLDecoder instance that parses the XML data of the given input stream.
144         * If the ExceptionListener argument is null a default implementation is used.
145         * 
146         * @param in InputStream to read XML data from.
147         * @param owner Owner object which can be accessed and modified while parsing.
148         * @param exceptionListener ExceptionListener instance to which exception notifications are send.
149         */
150        public XMLDecoder(
151                InputStream in,
152                Object owner,
153                ExceptionListener exceptionListener)
154        {
155                this(
156                        in,
157                        owner,
158                        exceptionListener,
159                        Thread.currentThread().getContextClassLoader());
160        }
161
162        /** Creates a XMLDecoder instance that parses the XML data of the given input stream.
163         * If the ExceptionListener argument is null a default implementation is used.
164         * 
165         * @param in InputStream to read XML data from.
166         * @param owner Owner object which can be accessed and modified while parsing.
167         * @param listener ExceptionListener instance to which exception notifications are send.
168         * @param cl ClassLoader instance that is used for calls to <code>Class.forName(String, boolean, ClassLoader)</code>
169         * @since 1.5
170         */
171        public XMLDecoder(
172                InputStream in,
173                Object owner,
174                ExceptionListener listener,
175                ClassLoader cl)
176        {
177                // initially here was a check for the validity of the InputStream argument but some
178                // great engineers decided that this API should silently discard this and behave rather
179                // odd: readObject will always return null ...
180                inputStream = in;
181
182                setExceptionListener(listener);
183
184                // validity of this object is checked in Class.forName() and therefore may be null
185                classLoader = cl;
186
187                this.owner = owner;
188        }
189
190        /** Closes the stream associated with this decoder. This should be done after having read all 
191         * decoded objects.
192         * <p>See the description of the {@link #readObject()} for the effect caused by <code>close</code>.</p> 
193         */
194        public void close()
195        {
196                if (isStreamClosed)
197                {
198                        return;
199                }
200
201                try
202                {
203                        inputStream.close();
204                        isStreamClosed = true;
205                }
206                catch (IOException e)
207                {
208                        // bad style forced by original API design ... 
209                }
210        }
211
212        /** Returns the ExceptionListener instance associated with this decoder.
213         * <p>See the description of {@link XMLDecoder} class for more information on the ExceptionListener.</p>
214         * 
215         * @return Current ExceptionListener of the decoder.
216         */
217        public ExceptionListener getExceptionListener()
218        {
219                return exceptionListener;
220        }
221
222        /** Returns the owner object of the decoder. This method is usually called
223         * from within the parsed XML data.
224         * <p>See the description of {@link XMLDecoder} class for more information on the owner object.</p>
225         * 
226         * @return The owner object of this decoder.
227         */
228        public Object getOwner()
229        {
230                return owner;
231        }
232
233        /** Returns the next available decoded object.
234         * <p>Note that the actual decoding takes place when the method is called for the first time.</p>
235         * <p>If the <code>close</code> method was already called a <code>NoSuchElementException</code>
236         * is thrown.</p>
237         * <p>If the InputStream instance used in the constructors was <code>null</code> this method
238         * will always return <code>null</code> itself.</p>
239         * 
240         * @return The next object in a sequence decoded from XML data.
241         * @throws ArrayIndexOutOfBoundsException When no more objects are available.
242         */
243        public Object readObject() throws ArrayIndexOutOfBoundsException
244        {
245                // note: the RI does it this way ...
246                if(inputStream == null) {
247                        return null;
248                }
249                
250                // note: the original API documentation says nothing on what to do
251                // when the stream was closed before readObject is called but it actually
252                // throws a NoSuchElementException - this behaviour is imitated here
253                if (isStreamClosed)
254                {
255                        throw new NoSuchElementException("Cannot read any objects - XMLDecoder was already closed.");
256                }
257
258                // creates the PersistenceParser (doing the parsing and decoding) and returns its
259                // Iterator on first invocation
260                if (iterator == null)
261                {
262                        iterator =
263                                new PersistenceParser(
264                                        inputStream,
265                                        exceptionListener,
266                                        classLoader,
267                                        this)
268                                        .iterator();
269                }
270
271                // note: done according to the official documentation
272                if (!iterator.hasNext())
273                {
274                        throw new ArrayIndexOutOfBoundsException("No more objects available from this XMLDecoder.");
275                }
276
277                // returns just the next object if there was no problem
278                return iterator.next();
279        }
280
281        /** Sets the ExceptionListener instance to which notifications of exceptions are send
282         * while parsing the XML data.
283         * <p>See the description of {@link XMLDecoder} class for more information on the ExceptionListener.</p>
284         *
285         * @param listener
286         */
287        public void setExceptionListener(ExceptionListener listener)
288        {
289                // uses a default implementation when null 
290                if (listener == null)
291                {
292                        listener = DefaultExceptionListener.INSTANCE;
293                }
294                exceptionListener = listener;
295        }
296
297        /** Sets the owner object which can be accessed from the parsed XML data.
298         * <p>See the description of {@link XMLDecoder} class for more information on the owner object.</p>
299         * 
300         * @param newOwner
301         */
302        public void setOwner(Object newOwner)
303        {
304                owner = newOwner;
305        }
306
307}