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.reloading;
018
019import org.apache.commons.configuration2.event.Event;
020import org.apache.commons.configuration2.event.EventListener;
021import org.apache.commons.configuration2.event.EventListenerList;
022import org.apache.commons.configuration2.event.EventSource;
023import org.apache.commons.configuration2.event.EventType;
024
025/**
026 * <p>
027 * A class for adding support for reload operations in a generic way.
028 * </p>
029 * <p>
030 * A {@code ReloadingController} monitors a specific source and triggers reloading events if necessary. So it does not
031 * perform reloading itself, but only sends out notifications when it thinks that this should be done. This allows for a
032 * very generic setup in which different components involved in reloading are loosely coupled via events.
033 * </p>
034 * <p>
035 * A typical usage scenario is as follows:
036 * </p>
037 * <ul>
038 * <li>A {@code ReloadingController} instance is created and initialized with a {@link ReloadingDetector} object.</li>
039 * <li>A number of {@link EventListener} objects for reloading events can be registered at the controller.</li>
040 * <li>Now the controller's {@code checkForReloading()} method is called whenever a check is to be performed. This could
041 * be done for instance by a timer in regular intervals or by any other means appropriate for a specific
042 * application.</li>
043 * <li>When a check reveals that a reload operation is necessary all registered event listeners are notified.</li>
044 * <li>Typically one of the listeners is responsible to perform the actual reload operation. (How this is done is not in
045 * the scope of the controller object.) After this has been done, the controller's {@code resetReloadingState()} method
046 * must be called. It tells the controller that the last notification has been processed and that new checks are
047 * possible again. It is important that this method is called. Otherwise, {@code checkForReloading()} will not do any
048 * new checks or send out event notifications any more.</li>
049 * </ul>
050 * <p>
051 * This class can be accessed from multiple threads concurrently. It shields the associated {@link ReloadingDetector}
052 * object for concurrent access, so that a concrete detector implementation does not have to be thread-safe.
053 * </p>
054 *
055 * @since 2.0
056 */
057public class ReloadingController implements EventSource {
058    /** Stores a reference to the reloading detector. */
059    private final ReloadingDetector detector;
060
061    /** The helper object which manages the registered event listeners. */
062    private final EventListenerList listeners;
063
064    /** A flag whether this controller is in reloading state. */
065    private boolean reloadingState;
066
067    /**
068     * Creates a new instance of {@code ReloadingController} and associates it with the given {@code ReloadingDetector}
069     * object.
070     *
071     * @param detect the {@code ReloadingDetector} (must not be <b>null</b>)
072     * @throws IllegalArgumentException if the detector is undefined
073     */
074    public ReloadingController(final ReloadingDetector detect) {
075        if (detect == null) {
076            throw new IllegalArgumentException("ReloadingDetector must not be null!");
077        }
078
079        detector = detect;
080        listeners = new EventListenerList();
081    }
082
083    /**
084     * Gets the {@code ReloadingDetector} used by this controller.
085     *
086     * @return the {@code ReloadingDetector}
087     */
088    public ReloadingDetector getDetector() {
089        return detector;
090    }
091
092    /**
093     * {@inheritDoc} This class generates events of type {@code ReloadingEvent}.
094     */
095    @Override
096    public <T extends Event> void addEventListener(final EventType<T> eventType, final EventListener<? super T> listener) {
097        listeners.addEventListener(eventType, listener);
098    }
099
100    @Override
101    public <T extends Event> boolean removeEventListener(final EventType<T> eventType, final EventListener<? super T> listener) {
102        return listeners.removeEventListener(eventType, listener);
103    }
104
105    /**
106     * Tests whether this controller is in <em>reloading state</em>. A return value of <b>true</b> means that a previous
107     * invocation of {@code checkForReloading()} has detected the necessity for a reload operation, but
108     * {@code resetReloadingState()} has not been called yet. In this state no further reloading checks are possible.
109     *
110     * @return a flag whether this controller is in reloading state
111     */
112    public synchronized boolean isInReloadingState() {
113        return reloadingState;
114    }
115
116    /**
117     * Performs a check whether a reload operation is necessary. This method has to be called to trigger the generation of
118     * reloading events. It delegates to the associated {@link ReloadingDetector} and sends out notifications if necessary.
119     * The argument can be an arbitrary data object; it will be part of the event notification sent out when a reload
120     * operation should be performed. The return value indicates whether a change was detected and an event was sent. Once a
121     * need for a reload is detected, this controller is in <em>reloading state</em>. Until this state is reset (by calling
122     * {@link #resetReloadingState()}), no further reloading checks are performed by this method, and no events are fired;
123     * it then returns always <b>true</b>.
124     *
125     * @param data additional data for an event notification
126     * @return a flag whether a reload operation is necessary
127     */
128    public boolean checkForReloading(final Object data) {
129        boolean sendEvent = false;
130        synchronized (this) {
131            if (isInReloadingState()) {
132                return true;
133            }
134            if (getDetector().isReloadingRequired()) {
135                sendEvent = true;
136                reloadingState = true;
137            }
138        }
139
140        if (sendEvent) {
141            listeners.fire(new ReloadingEvent(this, data));
142            return true;
143        }
144        return false;
145    }
146
147    /**
148     * Resets the reloading state. This tells the controller that reloading has been performed and new checks are possible
149     * again. If this controller is not in reloading state, this method has no effect.
150     */
151    public synchronized void resetReloadingState() {
152        if (isInReloadingState()) {
153            getDetector().reloadingPerformed();
154            reloadingState = false;
155        }
156    }
157}