001 /* java.beans.XMLDecoder --
002 Copyright (C) 2004, 2005, 2006 Free Software Foundation, Inc.
003
004 This file is part of GNU Classpath.
005
006 GNU Classpath is free software; you can redistribute it and/or modify
007 it under the terms of the GNU General Public License as published by
008 the Free Software Foundation; either version 2, or (at your option)
009 any later version.
010
011 GNU Classpath is distributed in the hope that it will be useful, but
012 WITHOUT ANY WARRANTY; without even the implied warranty of
013 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
014 General Public License for more details.
015
016 You should have received a copy of the GNU General Public License
017 along with GNU Classpath; see the file COPYING. If not, write to the
018 Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
019 02110-1301 USA.
020
021 Linking this library statically or dynamically with other modules is
022 making a combined work based on this library. Thus, the terms and
023 conditions of the GNU General Public License cover the whole
024 combination.
025
026 As a special exception, the copyright holders of this library give you
027 permission to link this library with independent modules to produce an
028 executable, regardless of the license terms of these independent
029 modules, and to copy and distribute the resulting executable under
030 terms of your choice, provided that you also meet, for each linked
031 independent module, the terms and conditions of the license of that
032 module. An independent module is a module which is not derived from
033 or based on this library. If you modify this library, you may extend
034 this exception to your version of the library, but you are not
035 obligated to do so. If you do not wish to do so, delete this
036 exception statement from your version. */
037
038
039 package java.beans;
040
041 import gnu.java.beans.DefaultExceptionListener;
042 import gnu.java.beans.decoder.PersistenceParser;
043
044 import java.io.IOException;
045 import java.io.InputStream;
046 import java.util.Iterator;
047 import 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 * <java>
058 * <string>Hello World</string>
059 * <int>200</int>
060 * </java>
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 * <java>
076 * <void method="getOwner">
077 * <void method="setVisible">
078 * <boolean>true<boolean>
079 * </void>
080 * </void>
081 * </java>
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 */
106 public 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 }