001/*
002 * Licensed to the Apache Software Foundation (ASF) under one or more
003 * contributor license agreements.  See the NOTICE file distributed with
004 * this work for additional information regarding copyright ownership.
005 * The ASF licenses this file to You under the Apache License, Version 2.0
006 * (the "License"); you may not use this file except in compliance with
007 * the License.  You may obtain a copy of the License at
008 *
009 *      http://www.apache.org/licenses/LICENSE-2.0
010 *
011 * Unless required by applicable law or agreed to in writing, software
012 * distributed under the License is distributed on an "AS IS" BASIS,
013 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
014 * See the License for the specific language governing permissions and
015 * limitations under the License.
016 */
017
018package org.apache.commons.lang3.event;
019
020import java.io.ByteArrayOutputStream;
021import java.io.IOException;
022import java.io.ObjectInputStream;
023import java.io.ObjectOutputStream;
024import java.io.Serializable;
025import java.lang.reflect.InvocationHandler;
026import java.lang.reflect.InvocationTargetException;
027import java.lang.reflect.Method;
028import java.lang.reflect.Proxy;
029import java.util.ArrayList;
030import java.util.List;
031import java.util.Objects;
032import java.util.concurrent.CopyOnWriteArrayList;
033
034import org.apache.commons.lang3.ArrayUtils;
035import org.apache.commons.lang3.Validate;
036
037/**
038 * An EventListenerSupport object can be used to manage a list of event
039 * listeners of a particular type. The class provides
040 * {@link #addListener(Object)} and {@link #removeListener(Object)} methods
041 * for registering listeners, as well as a {@link #fire()} method for firing
042 * events to the listeners.
043 *
044 * <p>
045 * To use this class, suppose you want to support ActionEvents.  You would do:
046 * </p>
047 * <pre><code>
048 * public class MyActionEventSource
049 * {
050 *   private EventListenerSupport&lt;ActionListener&gt; actionListeners =
051 *       EventListenerSupport.create(ActionListener.class);
052 *
053 *   public void someMethodThatFiresAction()
054 *   {
055 *     ActionEvent e = new ActionEvent(this, ActionEvent.ACTION_PERFORMED, "somethingCool");
056 *     actionListeners.fire().actionPerformed(e);
057 *   }
058 * }
059 * </code></pre>
060 *
061 * <p>
062 * Serializing an {@link EventListenerSupport} instance will result in any
063 * non-{@link Serializable} listeners being silently dropped.
064 * </p>
065 *
066 * @param <L> the type of event listener that is supported by this proxy.
067 *
068 * @since 3.0
069 */
070public class EventListenerSupport<L> implements Serializable {
071
072    /** Serialization version */
073    private static final long serialVersionUID = 3593265990380473632L;
074
075    /**
076     * The list used to hold the registered listeners. This list is
077     * intentionally a thread-safe copy-on-write-array so that traversals over
078     * the list of listeners will be atomic.
079     */
080    private List<L> listeners = new CopyOnWriteArrayList<>();
081
082    /**
083     * The proxy representing the collection of listeners. Calls to this proxy
084     * object will be sent to all registered listeners.
085     */
086    private transient L proxy;
087
088    /**
089     * Empty typed array for #getListeners().
090     */
091    private transient L[] prototypeArray;
092
093    /**
094     * Creates an EventListenerSupport object which supports the specified
095     * listener type.
096     *
097     * @param <T> the type of the listener interface
098     * @param listenerInterface the type of listener interface that will receive
099     *        events posted using this class.
100     *
101     * @return an EventListenerSupport object which supports the specified
102     *         listener type.
103     *
104     * @throws NullPointerException if {@code listenerInterface} is
105     *         {@code null}.
106     * @throws IllegalArgumentException if {@code listenerInterface} is
107     *         not an interface.
108     */
109    public static <T> EventListenerSupport<T> create(final Class<T> listenerInterface) {
110        return new EventListenerSupport<>(listenerInterface);
111    }
112
113    /**
114     * Creates an EventListenerSupport object which supports the provided
115     * listener interface.
116     *
117     * @param listenerInterface the type of listener interface that will receive
118     *        events posted using this class.
119     *
120     * @throws NullPointerException if {@code listenerInterface} is
121     *         {@code null}.
122     * @throws IllegalArgumentException if {@code listenerInterface} is
123     *         not an interface.
124     */
125    public EventListenerSupport(final Class<L> listenerInterface) {
126        this(listenerInterface, Thread.currentThread().getContextClassLoader());
127    }
128
129    /**
130     * Creates an EventListenerSupport object which supports the provided
131     * listener interface using the specified class loader to create the JDK
132     * dynamic proxy.
133     *
134     * @param listenerInterface the listener interface.
135     * @param classLoader       the class loader.
136     *
137     * @throws NullPointerException if {@code listenerInterface} or
138     *         {@code classLoader} is {@code null}.
139     * @throws IllegalArgumentException if {@code listenerInterface} is
140     *         not an interface.
141     */
142    public EventListenerSupport(final Class<L> listenerInterface, final ClassLoader classLoader) {
143        this();
144        Objects.requireNonNull(listenerInterface, "listenerInterface");
145        Objects.requireNonNull(classLoader, "classLoader");
146        Validate.isTrue(listenerInterface.isInterface(), "Class %s is not an interface",
147                listenerInterface.getName());
148        initializeTransientFields(listenerInterface, classLoader);
149    }
150
151    /**
152     * Create a new EventListenerSupport instance.
153     * Serialization-friendly constructor.
154     */
155    private EventListenerSupport() {
156    }
157
158    /**
159     * Returns a proxy object which can be used to call listener methods on all
160     * of the registered event listeners. All calls made to this proxy will be
161     * forwarded to all registered listeners.
162     *
163     * @return a proxy object which can be used to call listener methods on all
164     * of the registered event listeners
165     */
166    public L fire() {
167        return proxy;
168    }
169
170//**********************************************************************************************************************
171// Other Methods
172//**********************************************************************************************************************
173
174    /**
175     * Registers an event listener.
176     *
177     * @param listener the event listener (may not be {@code null}).
178     *
179     * @throws NullPointerException if {@code listener} is
180     *         {@code null}.
181     */
182    public void addListener(final L listener) {
183        addListener(listener, true);
184    }
185
186    /**
187     * Registers an event listener.  Will not add a pre-existing listener
188     * object to the list if {@code allowDuplicate} is false.
189     *
190     * @param listener the event listener (may not be {@code null}).
191     * @param allowDuplicate the flag for determining if duplicate listener
192     * objects are allowed to be registered.
193     *
194     * @throws NullPointerException if {@code listener} is {@code null}.
195     * @since 3.5
196     */
197    public void addListener(final L listener, final boolean allowDuplicate) {
198        Objects.requireNonNull(listener, "listener");
199        if (allowDuplicate || !listeners.contains(listener)) {
200            listeners.add(listener);
201        }
202    }
203
204    /**
205     * Returns the number of registered listeners.
206     *
207     * @return the number of registered listeners.
208     */
209    int getListenerCount() {
210        return listeners.size();
211    }
212
213    /**
214     * Unregisters an event listener.
215     *
216     * @param listener the event listener (may not be {@code null}).
217     *
218     * @throws NullPointerException if {@code listener} is
219     *         {@code null}.
220     */
221    public void removeListener(final L listener) {
222        Objects.requireNonNull(listener, "listener");
223        listeners.remove(listener);
224    }
225
226    /**
227     * Gets an array containing the currently registered listeners.
228     * Modification to this array's elements will have no effect on the
229     * {@link EventListenerSupport} instance.
230     * @return L[]
231     */
232    public L[] getListeners() {
233        return listeners.toArray(prototypeArray);
234    }
235
236    /**
237     * Serialize.
238     * @param objectOutputStream the output stream
239     * @throws IOException if an IO error occurs
240     */
241    private void writeObject(final ObjectOutputStream objectOutputStream) throws IOException {
242        final ArrayList<L> serializableListeners = new ArrayList<>();
243
244        // don't just rely on instanceof Serializable:
245        ObjectOutputStream testObjectOutputStream = new ObjectOutputStream(new ByteArrayOutputStream());
246        for (final L listener : listeners) {
247            try {
248                testObjectOutputStream.writeObject(listener);
249                serializableListeners.add(listener);
250            } catch (final IOException exception) {
251                //recreate test stream in case of indeterminate state
252                testObjectOutputStream = new ObjectOutputStream(new ByteArrayOutputStream());
253            }
254        }
255        /*
256         * we can reconstitute everything we need from an array of our listeners,
257         * which has the additional advantage of typically requiring less storage than a list:
258         */
259        objectOutputStream.writeObject(serializableListeners.toArray(prototypeArray));
260    }
261
262    /**
263     * Deserialize.
264     * @param objectInputStream the input stream
265     * @throws IOException if an IO error occurs
266     * @throws ClassNotFoundException if the class cannot be resolved
267     */
268    private void readObject(final ObjectInputStream objectInputStream) throws IOException, ClassNotFoundException {
269        @SuppressWarnings("unchecked") // Will throw CCE here if not correct
270        final L[] srcListeners = (L[]) objectInputStream.readObject();
271
272        this.listeners = new CopyOnWriteArrayList<>(srcListeners);
273
274        final Class<L> listenerInterface = ArrayUtils.getComponentType(srcListeners);
275
276        initializeTransientFields(listenerInterface, Thread.currentThread().getContextClassLoader());
277    }
278
279    /**
280     * Initialize transient fields.
281     * @param listenerInterface the class of the listener interface
282     * @param classLoader the class loader to be used
283     */
284    private void initializeTransientFields(final Class<L> listenerInterface, final ClassLoader classLoader) {
285        // Will throw CCE here if not correct
286        this.prototypeArray = ArrayUtils.newInstance(listenerInterface, 0);
287        createProxy(listenerInterface, classLoader);
288    }
289
290    /**
291     * Create the proxy object.
292     * @param listenerInterface the class of the listener interface
293     * @param classLoader the class loader to be used
294     */
295    private void createProxy(final Class<L> listenerInterface, final ClassLoader classLoader) {
296        proxy = listenerInterface.cast(Proxy.newProxyInstance(classLoader,
297                new Class[] { listenerInterface }, createInvocationHandler()));
298    }
299
300    /**
301     * Create the {@link InvocationHandler} responsible for broadcasting calls
302     * to the managed listeners.  Subclasses can override to provide custom behavior.
303     * @return ProxyInvocationHandler
304     */
305    protected InvocationHandler createInvocationHandler() {
306        return new ProxyInvocationHandler();
307    }
308
309    /**
310     * An invocation handler used to dispatch the event(s) to all the listeners.
311     */
312    protected class ProxyInvocationHandler implements InvocationHandler {
313
314        /**
315         * Propagates the method call to all registered listeners in place of the proxy listener object.
316         *
317         * @param unusedProxy the proxy object representing a listener on which the invocation was called; not used
318         * @param method the listener method that will be called on all of the listeners.
319         * @param args event arguments to propagate to the listeners.
320         * @return the result of the method call
321         * @throws InvocationTargetException if an error occurs
322         * @throws IllegalArgumentException if an error occurs
323         * @throws IllegalAccessException if an error occurs
324         */
325        @Override
326        public Object invoke(final Object unusedProxy, final Method method, final Object[] args)
327                throws IllegalAccessException, IllegalArgumentException, InvocationTargetException {
328            for (final L listener : listeners) {
329                method.invoke(listener, args);
330            }
331            return null;
332        }
333    }
334}