    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.Array;
    026import java.lang.reflect.InvocationHandler;
    027import java.lang.reflect.Method;
    028import java.lang.reflect.Proxy;
    029import java.util.ArrayList;
    030import java.util.List;
    031import java.util.concurrent.CopyOnWriteArrayList;
    032
    033import org.apache.commons.lang3.Validate;
    034
    035/**
    036 * <p>An EventListenerSupport object can be used to manage a list of event
    037 * listeners of a particular type. The class provides
    038 * {@link #addListener(Object)} and {@link #removeListener(Object)} methods
    039 * for registering listeners, as well as a {@link #fire()} method for firing
    040 * events to the listeners.
    041 * </p>
    042 *
    043 * <p>
    044 * To use this class, suppose you want to support ActionEvents.  You would do:
    045 * </p>
    046 * <pre><code>
    047 * public class MyActionEventSource
    048 * {
    049 *   private EventListenerSupport&lt;ActionListener&gt; actionListeners =
    050 *       EventListenerSupport.create(ActionListener.class);
    051 *
    052 *   public void someMethodThatFiresAction()
    053 *   {
    054 *     ActionEvent e = new ActionEvent(this, ActionEvent.ACTION_PERFORMED, "somethingCool");
    055 *     actionListeners.fire().actionPerformed(e);
    056 *   }
    057 * }
    058 * </code></pre>
    059 *
    060 * <p>
    061 * Serializing an {@link EventListenerSupport} instance will result in any
    062 * non-{@link Serializable} listeners being silently dropped.
    063 * </p>
    064 *
    065 * @param <L> the type of event listener that is supported by this proxy.
    066 *
    067 * @since 3.0
    068 */
    069public class EventListenerSupport<L> implements Serializable {
    070
    071    /** Serialization version */
    072    private static final long serialVersionUID = 3593265990380473632L;
    073
    074    /**
    075     * The list used to hold the registered listeners. This list is
    076     * intentionally a thread-safe copy-on-write-array so that traversals over
    077     * the list of listeners will be atomic.
    078     */
    079    private List<L> listeners = new CopyOnWriteArrayList<>();
    080
    081    /**
    082     * The proxy representing the collection of listeners. Calls to this proxy
    083     * object will sent to all registered listeners.
    084     */
    085    private transient L proxy;
    086
    087    /**
    088     * Empty typed array for #getListeners().
    089     */
    090    private transient L[] prototypeArray;
    091
    092    /**
    093     * Creates an EventListenerSupport object which supports the specified
    094     * listener type.
    095     *
    096     * @param <T> the type of the listener interface
    097     * @param listenerInterface the type of listener interface that will receive
    098     *        events posted using this class.
    099     *
    100     * @return an EventListenerSupport object which supports the specified
    101     *         listener type.
    102     *
    103     * @throws NullPointerException if {@code listenerInterface} is
    104     *         {@code null}.
    105     * @throws IllegalArgumentException if {@code listenerInterface} is
    106     *         not an interface.
    107     */
    108    public static <T> EventListenerSupport<T> create(final Class<T> listenerInterface) {
    109        return new EventListenerSupport<>(listenerInterface);
    110    }
    111
    112    /**
    113     * Creates an EventListenerSupport object which supports the provided
    114     * listener interface.
    115     *
    116     * @param listenerInterface the type of listener interface that will receive
    117     *        events posted using this class.
    118     *
    119     * @throws NullPointerException if {@code listenerInterface} is
    120     *         {@code null}.
    121     * @throws IllegalArgumentException if {@code listenerInterface} is
    122     *         not an interface.
    123     */
    124    public EventListenerSupport(final Class<L> listenerInterface) {
    125        this(listenerInterface, Thread.currentThread().getContextClassLoader());
    126    }
    127
    128    /**
    129     * Creates an EventListenerSupport object which supports the provided
    130     * listener interface using the specified class loader to create the JDK
    131     * dynamic proxy.
    132     *
    133     * @param listenerInterface the listener interface.
    134     * @param classLoader       the class loader.
    135     *
    136     * @throws NullPointerException if {@code listenerInterface} or
    137     *         {@code classLoader} is {@code null}.
    138     * @throws IllegalArgumentException if {@code listenerInterface} is
    139     *         not an interface.
    140     */
    141    public EventListenerSupport(final Class<L> listenerInterface, final ClassLoader classLoader) {
    142        this();
    143        Validate.notNull(listenerInterface, "Listener interface cannot be null.");
    144        Validate.notNull(classLoader, "ClassLoader cannot be null.");
    145        Validate.isTrue(listenerInterface.isInterface(), "Class %s is not an interface",
    146                listenerInterface.getName());
    147        initializeTransientFields(listenerInterface, classLoader);
    148    }
    149
    150    /**
    151     * Create a new EventListenerSupport instance.
    152     * Serialization-friendly constructor.
    153     */
    154    private EventListenerSupport() {
    155    }
    156
    157    /**
    158     * Returns a proxy object which can be used to call listener methods on all
    159     * of the registered event listeners. All calls made to this proxy will be
    160     * forwarded to all registered listeners.
    161     *
    162     * @return a proxy object which can be used to call listener methods on all
    163     * of the registered event listeners
    164     */
    165    public L fire() {
    166        return proxy;
    167    }
    168
    169//**********************************************************************************************************************
    170// Other Methods
    171//**********************************************************************************************************************
    172
    173    /**
    174     * Registers an event listener.
    175     *
    176     * @param listener the event listener (may not be {@code null}).
    177     *
    178     * @throws NullPointerException if {@code listener} is
    179     *         {@code null}.
    180     */
    181    public void addListener(final L listener) {
    182        addListener(listener, true);
    183    }
    184
    185    /**
    186     * Registers an event listener.  Will not add a pre-existing listener
    187     * object to the list if {@code allowDuplicate} is false.
    188     *
    189     * @param listener the event listener (may not be {@code null}).
    190     * @param allowDuplicate the flag for determining if duplicate listener
    191     * objects are allowed to be registered.
    192     *
    193     * @throws NullPointerException if {@code listener} is {@code null}.
    194     * @since 3.5
    195     */
    196    public void addListener(final L listener, final boolean allowDuplicate) {
    197        Validate.notNull(listener, "Listener object cannot be null.");
    198        if (allowDuplicate) {
    199            listeners.add(listener);
    200        } else if (!listeners.contains(listener)) {
    201            listeners.add(listener);
    202        }
    203    }
    204
    205    /**
    206     * Returns the number of registered listeners.
    207     *
    208     * @return the number of registered listeners.
    209     */
    210    int getListenerCount() {
    211        return listeners.size();
    212    }
    213
    214    /**
    215     * Unregisters an event listener.
    216     *
    217     * @param listener the event listener (may not be {@code null}).
    218     *
    219     * @throws NullPointerException if {@code listener} is
    220     *         {@code null}.
    221     */
    222    public void removeListener(final L listener) {
    223        Validate.notNull(listener, "Listener object cannot be null.");
    224        listeners.remove(listener);
    225    }
    226
    227    /**
    228     * Gets an array containing the currently registered listeners.
    229     * Modification to this array's elements will have no effect on the
    230     * {@link EventListenerSupport} instance.
    231     * @return L[]
    232     */
    233    public L[] getListeners() {
    234        return listeners.toArray(prototypeArray);
    235    }
    236
    237    /**
    238     * Serialize.
    239     * @param objectOutputStream the output stream
    240     * @throws IOException if an IO error occurs
    241     */
    242    private void writeObject(final ObjectOutputStream objectOutputStream) throws IOException {
    243        final ArrayList<L> serializableListeners = new ArrayList<>();
    244
    245        // don't just rely on instanceof Serializable:
    246        ObjectOutputStream testObjectOutputStream = new ObjectOutputStream(new ByteArrayOutputStream());
    247        for (final L listener : listeners) {
    248            try {
    249                testObjectOutputStream.writeObject(listener);
    250                serializableListeners.add(listener);
    251            } catch (final IOException exception) {
    252                //recreate test stream in case of indeterminate state
    253                testObjectOutputStream = new ObjectOutputStream(new ByteArrayOutputStream());
    254            }
    255        }
    256        /*
    257         * we can reconstitute everything we need from an array of our listeners,
    258         * which has the additional advantage of typically requiring less storage than a list:
    259         */
    260        objectOutputStream.writeObject(serializableListeners.toArray(prototypeArray));
    261    }
    262
    263    /**
    264     * Deserialize.
    265     * @param objectInputStream the input stream
    266     * @throws IOException if an IO error occurs
    267     * @throws ClassNotFoundException if the class cannot be resolved
    268     */
    269    private void readObject(final ObjectInputStream objectInputStream) throws IOException, ClassNotFoundException {
    270        @SuppressWarnings("unchecked") // Will throw CCE here if not correct
    271        final
    272        L[] srcListeners = (L[]) objectInputStream.readObject();
    273
    274        this.listeners = new CopyOnWriteArrayList<>(srcListeners);
    275
    276        @SuppressWarnings("unchecked") // Will throw CCE here if not correct
    277        final
    278        Class<L> listenerInterface = (Class<L>) srcListeners.getClass().getComponentType();
    279
    280        initializeTransientFields(listenerInterface, Thread.currentThread().getContextClassLoader());
    281    }
    282
    283    /**
    284     * Initialize transient fields.
    285     * @param listenerInterface the class of the listener interface
    286     * @param classLoader the class loader to be used
    287     */
    288    private void initializeTransientFields(final Class<L> listenerInterface, final ClassLoader classLoader) {
    289        @SuppressWarnings("unchecked") // Will throw CCE here if not correct
    290        final
    291        L[] array = (L[]) Array.newInstance(listenerInterface, 0);
    292        this.prototypeArray = array;
    293        createProxy(listenerInterface, classLoader);
    294    }
    295
    296    /**
    297     * Create the proxy object.
    298     * @param listenerInterface the class of the listener interface
    299     * @param classLoader the class loader to be used
    300     */
    301    private void createProxy(final Class<L> listenerInterface, final ClassLoader classLoader) {
    302        proxy = listenerInterface.cast(Proxy.newProxyInstance(classLoader,
    303                new Class[] { listenerInterface }, createInvocationHandler()));
    304    }
    305
    306    /**
    307     * Create the {@link InvocationHandler} responsible for broadcasting calls
    308     * to the managed listeners.  Subclasses can override to provide custom behavior.
    309     * @return ProxyInvocationHandler
    310     */
    311    protected InvocationHandler createInvocationHandler() {
    312        return new ProxyInvocationHandler();
    313    }
    314
    315    /**
    316     * An invocation handler used to dispatch the event(s) to all the listeners.
    317     */
    318    protected class ProxyInvocationHandler implements InvocationHandler {
    319
    320        /**
    321         * Propagates the method call to all registered listeners in place of
    322         * the proxy listener object.
    323         *
    324         * @param unusedProxy the proxy object representing a listener on which the
    325         *        invocation was called; not used
    326         * @param method the listener method that will be called on all of the
    327         *        listeners.
    328         * @param args event arguments to propagate to the listeners.
    329         * @return the result of the method call
    330         * @throws Throwable if an error occurs
    331         */
    332        @Override
    333        public Object invoke(final Object unusedProxy, final Method method, final Object[] args) throws Throwable {
    334            for (final L listener : listeners) {
    335                method.invoke(listener, args);
    336            }
    337            return null;
    338        }
    339    }
    340}