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 */
017package org.apache.commons.configuration2.event;
018
019import java.util.Collection;
020import java.util.Collections;
021import java.util.LinkedList;
022import java.util.List;
023
024/**
025 * <p>
026 * A base class for objects that can generate configuration events.
027 * </p>
028 * <p>
029 * This class implements functionality for managing a set of event listeners that can be notified when an event occurs.
030 * It can be extended by configuration classes that support the event mechanism. In this case these classes only need to
031 * call the {@code fireEvent()} method when an event is to be delivered to the registered listeners.
032 * </p>
033 * <p>
034 * Adding and removing event listeners can happen concurrently to manipulations on a configuration that cause events.
035 * The operations are synchronized.
036 * </p>
037 * <p>
038 * With the {@code detailEvents} property the number of detail events can be controlled. Some methods in configuration
039 * classes are implemented in a way that they call other methods that can generate their own events. One example is the
040 * {@code setProperty()} method that can be implemented as a combination of {@code clearProperty()} and
041 * {@code addProperty()}. With {@code detailEvents} set to <b>true</b>, all involved methods will generate events (i.e.
042 * listeners will receive property set events, property clear events, and property add events). If this mode is turned
043 * off (which is the default), detail events are suppressed, so only property set events will be received. Note that the
044 * number of received detail events may differ for different configuration implementations.
045 * {@link org.apache.commons.configuration2.BaseHierarchicalConfiguration BaseHierarchicalConfiguration} for instance
046 * has a custom implementation of {@code setProperty()}, which does not generate any detail events.
047 * </p>
048 * <p>
049 * In addition to &quot;normal&quot; events, error events are supported. Such events signal an internal problem that
050 * occurred during access of properties. They are handled via the regular {@link EventListener} interface, but there are
051 * special event types defined by {@link ConfigurationErrorEvent}. The {@code fireError()} method can be used by derived
052 * classes to send notifications about errors to registered observers.
053 * </p>
054 *
055 * @since 1.3
056 */
057public class BaseEventSource implements EventSource {
058    /** The list for managing registered event listeners. */
059    private EventListenerList eventListeners;
060
061    /** A lock object for guarding access to the detail events counter. */
062    private final Object lockDetailEventsCount = new Object();
063
064    /** A counter for the detail events. */
065    private int detailEvents;
066
067    /**
068     * Creates a new instance of {@code BaseEventSource}.
069     */
070    public BaseEventSource() {
071        initListeners();
072    }
073
074    /**
075     * Gets a collection with all event listeners of the specified event type that are currently registered at this
076     * object.
077     *
078     * @param eventType the event type object
079     * @param <T> the event type
080     * @return a collection with the event listeners of the specified event type (this collection is a snapshot of the
081     *         currently registered listeners; it cannot be manipulated)
082     */
083    public <T extends Event> Collection<EventListener<? super T>> getEventListeners(final EventType<T> eventType) {
084        final List<EventListener<? super T>> result = new LinkedList<>();
085        eventListeners.getEventListeners(eventType).forEach(result::add);
086        return Collections.unmodifiableCollection(result);
087    }
088
089    /**
090     * Gets a list with all {@code EventListenerRegistrationData} objects currently contained for this event source. This
091     * method allows access to all registered event listeners, independent on their type.
092     *
093     * @return a list with information about all registered event listeners
094     */
095    public List<EventListenerRegistrationData<?>> getEventListenerRegistrations() {
096        return eventListeners.getRegistrations();
097    }
098
099    /**
100     * Returns a flag whether detail events are enabled.
101     *
102     * @return a flag if detail events are generated
103     */
104    public boolean isDetailEvents() {
105        return checkDetailEvents(0);
106    }
107
108    /**
109     * Determines whether detail events should be generated. If enabled, some methods can generate multiple update events.
110     * Note that this method records the number of calls, i.e. if for instance {@code setDetailEvents(false)} was called
111     * three times, you will have to invoke the method as often to enable the details.
112     *
113     * @param enable a flag if detail events should be enabled or disabled
114     */
115    public void setDetailEvents(final boolean enable) {
116        synchronized (lockDetailEventsCount) {
117            if (enable) {
118                detailEvents++;
119            } else {
120                detailEvents--;
121            }
122        }
123    }
124
125    @Override
126    public <T extends Event> void addEventListener(final EventType<T> eventType, final EventListener<? super T> listener) {
127        eventListeners.addEventListener(eventType, listener);
128    }
129
130    @Override
131    public <T extends Event> boolean removeEventListener(final EventType<T> eventType, final EventListener<? super T> listener) {
132        return eventListeners.removeEventListener(eventType, listener);
133    }
134
135    /**
136     * Removes all registered event listeners.
137     */
138    public void clearEventListeners() {
139        eventListeners.clear();
140    }
141
142    /**
143     * Removes all registered error listeners.
144     *
145     * @since 1.4
146     */
147    public void clearErrorListeners() {
148        eventListeners.getRegistrationsForSuperType(ConfigurationErrorEvent.ANY).forEach(eventListeners::removeEventListener);
149    }
150
151    /**
152     * Copies all event listener registrations maintained by this object to the specified {@code BaseEventSource} object.
153     *
154     * @param source the target source for the copy operation (must not be <b>null</b>)
155     * @throws IllegalArgumentException if the target source is <b>null</b>
156     * @since 2.0
157     */
158    public void copyEventListeners(final BaseEventSource source) {
159        if (source == null) {
160            throw new IllegalArgumentException("Target event source must not be null!");
161        }
162        source.eventListeners.addAll(eventListeners);
163    }
164
165    /**
166     * Creates an event object and delivers it to all registered event listeners. The method checks first if sending an
167     * event is allowed (making use of the {@code detailEvents} property), and if listeners are registered.
168     *
169     * @param type the event's type
170     * @param propName the name of the affected property (can be <b>null</b>)
171     * @param propValue the value of the affected property (can be <b>null</b>)
172     * @param before the before update flag
173     * @param <T> the type of the event to be fired
174     */
175    protected <T extends ConfigurationEvent> void fireEvent(final EventType<T> type, final String propName, final Object propValue, final boolean before) {
176        if (checkDetailEvents(-1)) {
177            final EventListenerList.EventListenerIterator<T> it = eventListeners.getEventListenerIterator(type);
178            if (it.hasNext()) {
179                final ConfigurationEvent event = createEvent(type, propName, propValue, before);
180                while (it.hasNext()) {
181                    it.invokeNext(event);
182                }
183            }
184        }
185    }
186
187    /**
188     * Creates a {@code ConfigurationEvent} object based on the passed in parameters. This method is called by
189     * {@code fireEvent()} if it decides that an event needs to be generated.
190     *
191     * @param type the event's type
192     * @param propName the name of the affected property (can be <b>null</b>)
193     * @param propValue the value of the affected property (can be <b>null</b>)
194     * @param before the before update flag
195     * @param <T> the type of the event to be created
196     * @return the newly created event object
197     */
198    protected <T extends ConfigurationEvent> ConfigurationEvent createEvent(final EventType<T> type, final String propName, final Object propValue,
199        final boolean before) {
200        return new ConfigurationEvent(this, type, propName, propValue, before);
201    }
202
203    /**
204     * Creates an error event object and delivers it to all registered error listeners of a matching type.
205     *
206     * @param eventType the event's type
207     * @param operationType the type of the failed operation
208     * @param propertyName the name of the affected property (can be <b>null</b>)
209     * @param propertyValue the value of the affected property (can be <b>null</b>)
210     * @param cause the {@code Throwable} object that caused this error event
211     * @param <T> the event type
212     */
213    public <T extends ConfigurationErrorEvent> void fireError(final EventType<T> eventType, final EventType<?> operationType, final String propertyName,
214        final Object propertyValue, final Throwable cause) {
215        final EventListenerList.EventListenerIterator<T> iterator = eventListeners.getEventListenerIterator(eventType);
216        if (iterator.hasNext()) {
217            final ConfigurationErrorEvent event = createErrorEvent(eventType, operationType, propertyName, propertyValue, cause);
218            while (iterator.hasNext()) {
219                iterator.invokeNext(event);
220            }
221        }
222    }
223
224    /**
225     * Creates a {@code ConfigurationErrorEvent} object based on the passed in parameters. This is called by
226     * {@code fireError()} if it decides that an event needs to be generated.
227     *
228     * @param type the event's type
229     * @param opType the operation type related to this error
230     * @param propName the name of the affected property (can be <b>null</b>)
231     * @param propValue the value of the affected property (can be <b>null</b>)
232     * @param ex the {@code Throwable} object that caused this error event
233     * @return the event object
234     */
235    protected ConfigurationErrorEvent createErrorEvent(final EventType<? extends ConfigurationErrorEvent> type, final EventType<?> opType,
236        final String propName, final Object propValue, final Throwable ex) {
237        return new ConfigurationErrorEvent(this, type, opType, propName, propValue, ex);
238    }
239
240    /**
241     * Overrides the {@code clone()} method to correctly handle so far registered event listeners. This implementation
242     * ensures that the clone will have empty event listener lists, i.e. the listeners registered at an
243     * {@code BaseEventSource} object will not be copied.
244     *
245     * @return the cloned object
246     * @throws CloneNotSupportedException if cloning is not allowed
247     * @since 1.4
248     */
249    @Override
250    protected Object clone() throws CloneNotSupportedException {
251        final BaseEventSource copy = (BaseEventSource) super.clone();
252        copy.initListeners();
253        return copy;
254    }
255
256    /**
257     * Initializes the collections for storing registered event listeners.
258     */
259    private void initListeners() {
260        eventListeners = new EventListenerList();
261    }
262
263    /**
264     * Helper method for checking the current counter for detail events. This method checks whether the counter is greater
265     * than the passed in limit.
266     *
267     * @param limit the limit to be compared to
268     * @return <b>true</b> if the counter is greater than the limit, <b>false</b> otherwise
269     */
270    private boolean checkDetailEvents(final int limit) {
271        synchronized (lockDetailEventsCount) {
272            return detailEvents > limit;
273        }
274    }
275}