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 *      https://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;
036import org.apache.commons.lang3.exception.ExceptionUtils;
037import org.apache.commons.lang3.function.FailableConsumer;
038
039/**
040 * Manages a list of event listeners of a given generic type. This class provides {@link #addListener(Object)} and {@link #removeListener(Object)} methods for
041 * managing listeners, as well as a {@link #fire()} method for firing events to the listeners.
042 *
043 * <p>
044 * For example, to support ActionEvents:
045 * </p>
046 *
047 * <pre>{@code
048 * public class MyActionEventSource {
049 *
050 *     private EventListenerSupport<ActionListener> actionListeners = EventListenerSupport.create(ActionListener.class);
051 *
052 *     public void someMethodThatFiresAction() {
053 *         ActionEvent e = new ActionEvent(this, ActionEvent.ACTION_PERFORMED, "something");
054 *         actionListeners.fire().actionPerformed(e);
055 *     }
056 * }
057 * }</pre>
058 * <p>
059 * Events are fired
060 * <p>
061 * Serializing an {@link EventListenerSupport} instance will result in any non-{@link Serializable} listeners being silently dropped.
062 * </p>
063 *
064 * @param <L> the type of event listener that is supported by this proxy.
065 * @since 3.0
066 */
067public class EventListenerSupport<L> implements Serializable {
068
069    /**
070     * Invokes listeners through {@link #invoke(Object, Method, Object[])} in the order added to the underlying {@link List}.
071     */
072    protected class ProxyInvocationHandler implements InvocationHandler {
073
074        private final FailableConsumer<Throwable, IllegalAccessException> handler;
075
076        /**
077         * Constructs a new instance.
078         */
079        public ProxyInvocationHandler() {
080            this(ExceptionUtils::rethrow);
081        }
082
083        /**
084         * Constructs a new instance.
085         *
086         * @param handler Handles Throwables.
087         * @since 3.15.0
088         */
089        public ProxyInvocationHandler(final FailableConsumer<Throwable, IllegalAccessException> handler) {
090            this.handler = Objects.requireNonNull(handler);
091        }
092
093        /**
094         * Handles an exception thrown by a listener. By default rethrows the given Throwable.
095         *
096         * @param t The Throwable
097         * @throws IllegalAccessException thrown by the listener.
098         * @throws IllegalArgumentException thrown by the listener.
099         * @throws InvocationTargetException thrown by the listener.
100         * @since 3.15.0
101         */
102        protected void handle(final Throwable t) throws IllegalAccessException, IllegalArgumentException, InvocationTargetException {
103            handler.accept(t);
104        }
105
106        /**
107         * Propagates the method call to all registered listeners in place of the proxy listener object.
108         * <p>
109         * Calls listeners in the order added to the underlying {@link List}.
110         * </p>
111         *
112         * @param unusedProxy the proxy object representing a listener on which the invocation was called; not used
113         * @param method the listener method that will be called on all of the listeners.
114         * @param args event arguments to propagate to the listeners.
115         * @return the result of the method call
116         * @throws InvocationTargetException if an error occurs
117         * @throws IllegalArgumentException if an error occurs
118         * @throws IllegalAccessException if an error occurs
119         */
120        @Override
121        public Object invoke(final Object unusedProxy, final Method method, final Object[] args)
122                throws IllegalAccessException, IllegalArgumentException, InvocationTargetException {
123            for (final L listener : listeners) {
124                try {
125                    method.invoke(listener, args);
126                } catch (final Throwable t) {
127                    handle(t);
128                }
129            }
130            return null;
131        }
132    }
133
134    /** Serialization version */
135    private static final long serialVersionUID = 3593265990380473632L;
136
137    /**
138     * Creates an EventListenerSupport object which supports the specified
139     * listener type.
140     *
141     * @param <T> the type of the listener interface
142     * @param listenerInterface the type of listener interface that will receive
143     *        events posted using this class.
144     *
145     * @return an EventListenerSupport object which supports the specified
146     *         listener type.
147     *
148     * @throws NullPointerException if {@code listenerInterface} is
149     *         {@code null}.
150     * @throws IllegalArgumentException if {@code listenerInterface} is
151     *         not an interface.
152     */
153    public static <T> EventListenerSupport<T> create(final Class<T> listenerInterface) {
154        return new EventListenerSupport<>(listenerInterface);
155    }
156
157    /**
158     * Hold the registered listeners. This list is intentionally a thread-safe copy-on-write-array so that traversals over the list of listeners will be atomic.
159     */
160    private List<L> listeners = new CopyOnWriteArrayList<>();
161
162    /**
163     * The proxy representing the collection of listeners. Calls to this proxy object will be sent to all registered listeners.
164     */
165    private transient L proxy;
166    /**
167     * Empty typed array for #getListeners().
168     */
169    private transient L[] prototypeArray;
170
171    /**
172     * Constructs a new EventListenerSupport instance.
173     * <p>
174     * This constructor is needed for serialization.
175     * </p>
176     */
177    private EventListenerSupport() {
178    }
179
180    /**
181     * Constructs an EventListenerSupport object which supports the provided
182     * listener interface.
183     *
184     * @param listenerInterface the type of listener interface that will receive
185     *        events posted using this class.
186     *
187     * @throws NullPointerException if {@code listenerInterface} is
188     *         {@code null}.
189     * @throws IllegalArgumentException if {@code listenerInterface} is
190     *         not an interface.
191     */
192    public EventListenerSupport(final Class<L> listenerInterface) {
193        this(listenerInterface, Thread.currentThread().getContextClassLoader());
194    }
195
196    /**
197     * Constructs an EventListenerSupport object which supports the provided
198     * listener interface using the specified class loader to create the JDK
199     * dynamic proxy.
200     *
201     * @param listenerInterface the listener interface.
202     * @param classLoader       the class loader.
203     * @throws NullPointerException if {@code listenerInterface} or
204     *         {@code classLoader} is {@code null}.
205     * @throws IllegalArgumentException if {@code listenerInterface} is
206     *         not an interface.
207     */
208    public EventListenerSupport(final Class<L> listenerInterface, final ClassLoader classLoader) {
209        this();
210        Objects.requireNonNull(listenerInterface, "listenerInterface");
211        Objects.requireNonNull(classLoader, "classLoader");
212        Validate.isTrue(listenerInterface.isInterface(), "Class %s is not an interface", listenerInterface.getName());
213        initializeTransientFields(listenerInterface, classLoader);
214    }
215
216    /**
217     * Adds an event listener.
218     * <p>
219     * Listeners are called in the order added.
220     * </p>
221     *
222     * @param listener the event listener (may not be {@code null}).
223     * @throws NullPointerException if {@code listener} is {@code null}.
224     */
225    public void addListener(final L listener) {
226        addListener(listener, true);
227    }
228
229    /**
230     * Adds an event listener. Will not add a pre-existing listener object to the list if {@code allowDuplicate} is false.
231     * <p>
232     * Listeners are called in the order added.
233     * </p>
234     *
235     * @param listener       the event listener (may not be {@code null}).
236     * @param allowDuplicate the flag for determining if duplicate listener objects are allowed to be registered.
237     *
238     * @throws NullPointerException if {@code listener} is {@code null}.
239     * @since 3.5
240     */
241    public void addListener(final L listener, final boolean allowDuplicate) {
242        Objects.requireNonNull(listener, "listener");
243        if (allowDuplicate || !listeners.contains(listener)) {
244            listeners.add(listener);
245        }
246    }
247
248    /**
249     * Creates the {@link InvocationHandler} responsible for calling
250     * to the managed listeners. Subclasses can override to provide custom behavior.
251     *
252     * @return ProxyInvocationHandler
253     */
254    protected InvocationHandler createInvocationHandler() {
255        return new ProxyInvocationHandler();
256    }
257
258    /**
259     * Creates the proxy object.
260     *
261     * @param listenerInterface the class of the listener interface
262     * @param classLoader the class loader to be used
263     */
264    private void createProxy(final Class<L> listenerInterface, final ClassLoader classLoader) {
265        proxy = listenerInterface.cast(Proxy.newProxyInstance(classLoader, new Class[] { listenerInterface }, createInvocationHandler()));
266    }
267
268    /**
269     * Returns a proxy object which can be used to call listener methods on all
270     * of the registered event listeners. All calls made to this proxy will be
271     * forwarded to all registered listeners.
272     *
273     * @return a proxy object which can be used to call listener methods on all
274     * of the registered event listeners
275     */
276    public L fire() {
277        return proxy;
278    }
279
280    /**
281     * Gets the number of registered listeners.
282     *
283     * @return the number of registered listeners.
284     */
285    int getListenerCount() {
286        return listeners.size();
287    }
288
289    /**
290     * Gets an array containing the currently registered listeners.
291     * Modification to this array's elements will have no effect on the
292     * {@link EventListenerSupport} instance.
293     * @return L[]
294     */
295    public L[] getListeners() {
296        return listeners.toArray(prototypeArray);
297    }
298
299    /**
300     * Initializes transient fields.
301     *
302     * @param listenerInterface the class of the listener interface
303     * @param classLoader the class loader to be used
304     */
305    private void initializeTransientFields(final Class<L> listenerInterface, final ClassLoader classLoader) {
306        // Will throw CCE here if not correct
307        this.prototypeArray = ArrayUtils.newInstance(listenerInterface, 0);
308        createProxy(listenerInterface, classLoader);
309    }
310
311    /**
312     * Deserializes the next object into this instance.
313     *
314     * @param objectInputStream the input stream
315     * @throws IOException if an IO error occurs
316     * @throws ClassNotFoundException if the class cannot be resolved
317     */
318    private void readObject(final ObjectInputStream objectInputStream) throws IOException, ClassNotFoundException {
319        @SuppressWarnings("unchecked") // Will throw CCE here if not correct
320        final L[] srcListeners = (L[]) objectInputStream.readObject();
321        this.listeners = new CopyOnWriteArrayList<>(srcListeners);
322        final Class<L> listenerInterface = ArrayUtils.getComponentType(srcListeners);
323        initializeTransientFields(listenerInterface, Thread.currentThread().getContextClassLoader());
324    }
325
326    /**
327     * Removes an event listener.
328     *
329     * @param listener the event listener (may not be {@code null}).
330     * @throws NullPointerException if {@code listener} is
331     *         {@code null}.
332     */
333    public void removeListener(final L listener) {
334        listeners.remove(Objects.requireNonNull(listener, "listener"));
335    }
336
337    /**
338     * Serializes this instance onto the given ObjectOutputStream.
339     *
340     * @param objectOutputStream the output stream
341     * @throws IOException if an IO error occurs
342     */
343    private void writeObject(final ObjectOutputStream objectOutputStream) throws IOException {
344        final ArrayList<L> serializableListeners = new ArrayList<>();
345        // Don't just rely on instanceof Serializable:
346        ObjectOutputStream testObjectOutputStream = new ObjectOutputStream(new ByteArrayOutputStream());
347        for (final L listener : listeners) {
348            try {
349                testObjectOutputStream.writeObject(listener);
350                serializableListeners.add(listener);
351            } catch (final IOException exception) {
352                //recreate test stream in case of indeterminate state
353                testObjectOutputStream = new ObjectOutputStream(new ByteArrayOutputStream());
354            }
355        }
356        // We can reconstitute everything we need from an array of our listeners,
357        // which has the additional advantage of typically requiring less storage than a list:
358        objectOutputStream.writeObject(serializableListeners.toArray(prototypeArray));
359    }
360}