BaseEventSource.java

/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package org.apache.commons.configuration2.event;

import java.util.Collection;
import java.util.Collections;
import java.util.LinkedList;
import java.util.List;

/**
 * <p>
 * A base class for objects that can generate configuration events.
 * </p>
 * <p>
 * This class implements functionality for managing a set of event listeners that can be notified when an event occurs.
 * It can be extended by configuration classes that support the event mechanism. In this case these classes only need to
 * call the {@code fireEvent()} method when an event is to be delivered to the registered listeners.
 * </p>
 * <p>
 * Adding and removing event listeners can happen concurrently to manipulations on a configuration that cause events.
 * The operations are synchronized.
 * </p>
 * <p>
 * With the {@code detailEvents} property the number of detail events can be controlled. Some methods in configuration
 * classes are implemented in a way that they call other methods that can generate their own events. One example is the
 * {@code setProperty()} method that can be implemented as a combination of {@code clearProperty()} and
 * {@code addProperty()}. With {@code detailEvents} set to <strong>true</strong>, all involved methods will generate events (i.e.
 * listeners will receive property set events, property clear events, and property add events). If this mode is turned
 * off (which is the default), detail events are suppressed, so only property set events will be received. Note that the
 * number of received detail events may differ for different configuration implementations.
 * {@link org.apache.commons.configuration2.BaseHierarchicalConfiguration BaseHierarchicalConfiguration} for instance
 * has a custom implementation of {@code setProperty()}, which does not generate any detail events.
 * </p>
 * <p>
 * In addition to &quot;normal&quot; events, error events are supported. Such events signal an internal problem that
 * occurred during access of properties. They are handled via the regular {@link EventListener} interface, but there are
 * special event types defined by {@link ConfigurationErrorEvent}. The {@code fireError()} method can be used by derived
 * classes to send notifications about errors to registered observers.
 * </p>
 *
 * @since 1.3
 */
public class BaseEventSource implements EventSource {

    /** The list for managing registered event listeners. */
    private EventListenerList eventListeners;

    /** A lock object for guarding access to the detail events counter. */
    private final Object lockDetailEventsCount = new Object();

    /** A counter for the detail events. */
    private int detailEvents;

    /**
     * Creates a new instance of {@code BaseEventSource}.
     */
    public BaseEventSource() {
        initListeners();
    }

    @Override
    public <T extends Event> void addEventListener(final EventType<T> eventType, final EventListener<? super T> listener) {
        eventListeners.addEventListener(eventType, listener);
    }

    /**
     * Helper method for checking the current counter for detail events. This method checks whether the counter is greater
     * than the passed in limit.
     *
     * @param limit the limit to be compared to
     * @return <strong>true</strong> if the counter is greater than the limit, <strong>false</strong> otherwise
     */
    private boolean checkDetailEvents(final int limit) {
        synchronized (lockDetailEventsCount) {
            return detailEvents > limit;
        }
    }

    /**
     * Removes all registered error listeners.
     *
     * @since 1.4
     */
    public void clearErrorListeners() {
        eventListeners.getRegistrationsForSuperType(ConfigurationErrorEvent.ANY).forEach(eventListeners::removeEventListener);
    }

    /**
     * Removes all registered event listeners.
     */
    public void clearEventListeners() {
        eventListeners.clear();
    }

    /**
     * Overrides the {@code clone()} method to correctly handle so far registered event listeners. This implementation ensures that the clone will have empty
     * event listener lists, i.e. the listeners registered at an {@code BaseEventSource} object will not be copied.
     *
     * @return the cloned object
     * @throws CloneNotSupportedException if the object's class does not support the {@code Cloneable} interface. Subclasses that override the {@code clone}
     *                                    method can also throw this exception to indicate that an instance cannot be cloned.
     * @since 1.4
     */
    @Override
    protected Object clone() throws CloneNotSupportedException {
        final BaseEventSource copy = (BaseEventSource) super.clone();
        copy.initListeners();
        return copy;
    }

    /**
     * Copies all event listener registrations maintained by this object to the specified {@code BaseEventSource} object.
     *
     * @param source the target source for the copy operation (must not be <strong>null</strong>)
     * @throws IllegalArgumentException if the target source is <strong>null</strong>
     * @since 2.0
     */
    public void copyEventListeners(final BaseEventSource source) {
        if (source == null) {
            throw new IllegalArgumentException("Target event source must not be null!");
        }
        source.eventListeners.addAll(eventListeners);
    }

    /**
     * Creates a {@code ConfigurationErrorEvent} object based on the passed in parameters. This is called by
     * {@code fireError()} if it decides that an event needs to be generated.
     *
     * @param type the event's type
     * @param opType the operation type related to this error
     * @param propName the name of the affected property (can be <strong>null</strong>)
     * @param propValue the value of the affected property (can be <strong>null</strong>)
     * @param ex the {@code Throwable} object that caused this error event
     * @return the event object
     */
    protected ConfigurationErrorEvent createErrorEvent(final EventType<? extends ConfigurationErrorEvent> type, final EventType<?> opType,
        final String propName, final Object propValue, final Throwable ex) {
        return new ConfigurationErrorEvent(this, type, opType, propName, propValue, ex);
    }

    /**
     * Creates a {@code ConfigurationEvent} object based on the passed in parameters. This method is called by
     * {@code fireEvent()} if it decides that an event needs to be generated.
     *
     * @param type the event's type
     * @param propName the name of the affected property (can be <strong>null</strong>)
     * @param propValue the value of the affected property (can be <strong>null</strong>)
     * @param before the before update flag
     * @param <T> the type of the event to be created
     * @return the newly created event object
     */
    protected <T extends ConfigurationEvent> ConfigurationEvent createEvent(final EventType<T> type, final String propName, final Object propValue,
        final boolean before) {
        return new ConfigurationEvent(this, type, propName, propValue, before);
    }

    /**
     * Creates an error event object and delivers it to all registered error listeners of a matching type.
     *
     * @param eventType the event's type
     * @param operationType the type of the failed operation
     * @param propertyName the name of the affected property (can be <strong>null</strong>)
     * @param propertyValue the value of the affected property (can be <strong>null</strong>)
     * @param cause the {@code Throwable} object that caused this error event
     * @param <T> the event type
     */
    public <T extends ConfigurationErrorEvent> void fireError(final EventType<T> eventType, final EventType<?> operationType, final String propertyName,
        final Object propertyValue, final Throwable cause) {
        final EventListenerList.EventListenerIterator<T> iterator = eventListeners.getEventListenerIterator(eventType);
        if (iterator.hasNext()) {
            final ConfigurationErrorEvent event = createErrorEvent(eventType, operationType, propertyName, propertyValue, cause);
            while (iterator.hasNext()) {
                iterator.invokeNext(event);
            }
        }
    }

    /**
     * Creates an event object and delivers it to all registered event listeners. The method checks first if sending an
     * event is allowed (making use of the {@code detailEvents} property), and if listeners are registered.
     *
     * @param type the event's type
     * @param propName the name of the affected property (can be <strong>null</strong>)
     * @param propValue the value of the affected property (can be <strong>null</strong>)
     * @param before the before update flag
     * @param <T> the type of the event to be fired
     */
    protected <T extends ConfigurationEvent> void fireEvent(final EventType<T> type, final String propName, final Object propValue, final boolean before) {
        if (checkDetailEvents(-1)) {
            final EventListenerList.EventListenerIterator<T> it = eventListeners.getEventListenerIterator(type);
            if (it.hasNext()) {
                final ConfigurationEvent event = createEvent(type, propName, propValue, before);
                while (it.hasNext()) {
                    it.invokeNext(event);
                }
            }
        }
    }

    /**
     * Gets a list with all {@code EventListenerRegistrationData} objects currently contained for this event source. This
     * method allows access to all registered event listeners, independent on their type.
     *
     * @return a list with information about all registered event listeners
     */
    public List<EventListenerRegistrationData<?>> getEventListenerRegistrations() {
        return eventListeners.getRegistrations();
    }

    /**
     * Gets a collection with all event listeners of the specified event type that are currently registered at this
     * object.
     *
     * @param eventType the event type object
     * @param <T> the event type
     * @return a collection with the event listeners of the specified event type (this collection is a snapshot of the
     *         currently registered listeners; it cannot be manipulated)
     */
    public <T extends Event> Collection<EventListener<? super T>> getEventListeners(final EventType<T> eventType) {
        final List<EventListener<? super T>> result = new LinkedList<>();
        eventListeners.getEventListeners(eventType).forEach(result::add);
        return Collections.unmodifiableCollection(result);
    }

    /**
     * Initializes the collections for storing registered event listeners.
     */
    private void initListeners() {
        eventListeners = new EventListenerList();
    }

    /**
     * Returns a flag whether detail events are enabled.
     *
     * @return a flag if detail events are generated
     */
    public boolean isDetailEvents() {
        return checkDetailEvents(0);
    }

    @Override
    public <T extends Event> boolean removeEventListener(final EventType<T> eventType, final EventListener<? super T> listener) {
        return eventListeners.removeEventListener(eventType, listener);
    }

    /**
     * Determines whether detail events should be generated. If enabled, some methods can generate multiple update events.
     * Note that this method records the number of calls, i.e. if for instance {@code setDetailEvents(false)} was called
     * three times, you will have to invoke the method as often to enable the details.
     *
     * @param enable a flag if detail events should be enabled or disabled
     */
    public void setDetailEvents(final boolean enable) {
        synchronized (lockDetailEventsCount) {
            if (enable) {
                detailEvents++;
            } else {
                detailEvents--;
            }
        }
    }
}