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
031 * reloading events if necessary. So it does not perform reloading itself, but
032 * only sends out notifications when it thinks that this should be done. This
033 * allows for a very generic setup in which different components involved in
034 * reloading are loosely coupled via events.
035 * </p>
036 * <p>
037 * A typical usage scenario is as follows:
038 * </p>
039 * <ul>
040 * <li>A {@code ReloadingController} instance is created and initialized with a
041 * {@link ReloadingDetector} object.</li>
042 * <li>A number of {@link EventListener} objects for reloading events can be
043 * registered at the controller.</li>
044 * <li>Now the controller's {@code checkForReloading()} method is called
045 * whenever a check is to be performed. This could be done for instance by a
046 * timer in regular intervals or by any other means appropriate for a specific
047 * application.</li>
048 * <li>When a check reveals that a reload operation is necessary all registered
049 * event listeners are notified.</li>
050 * <li>Typically one of the listeners is responsible to perform the actual
051 * reload operation. (How this is done is not in the scope of the controller
052 * object.) After this has been done, the controller's
053 * {@code resetReloadingState()} method must be called. It tells the controller
054 * that the last notification has been processed and that new checks are
055 * possible again. It is important that this method is called. Otherwise,
056 * {@code checkForReloading()} will not do any new checks or send out event
057 * notifications any more.</li>
058 * </ul>
059 * <p>
060 * This class can be accessed from multiple threads concurrently. It shields the
061 * associated {@link ReloadingDetector} object for concurrent access, so that a
062 * concrete detector implementation does not have to be thread-safe.
063 * </p>
064 *
065 * @since 2.0
066 */
067public class ReloadingController implements EventSource
068{
069    /** Stores a reference to the reloading detector. */
070    private final ReloadingDetector detector;
071
072    /** The helper object which manages the registered event listeners. */
073    private final EventListenerList listeners;
074
075    /** A flag whether this controller is in reloading state. */
076    private boolean reloadingState;
077
078    /**
079     * Creates a new instance of {@code ReloadingController} and associates it
080     * with the given {@code ReloadingDetector} object.
081     *
082     * @param detect the {@code ReloadingDetector} (must not be <b>null</b>)
083     * @throws IllegalArgumentException if the detector is undefined
084     */
085    public ReloadingController(final ReloadingDetector detect)
086    {
087        if (detect == null)
088        {
089            throw new IllegalArgumentException(
090                    "ReloadingDetector must not be null!");
091        }
092
093        detector = detect;
094        listeners = new EventListenerList();
095    }
096
097    /**
098     * Returns the {@code ReloadingDetector} used by this controller.
099     *
100     * @return the {@code ReloadingDetector}
101     */
102    public ReloadingDetector getDetector()
103    {
104        return detector;
105    }
106
107    /**
108     * {@inheritDoc} This class generates events of type {@code ReloadingEvent}.
109     */
110    @Override
111    public <T extends Event> void addEventListener(
112            final EventType<T> eventType, final EventListener<? super T> listener)
113    {
114        listeners.addEventListener(eventType, listener);
115    }
116
117    @Override
118    public <T extends Event> boolean removeEventListener(
119            final EventType<T> eventType, final EventListener<? super T> listener)
120    {
121        return listeners.removeEventListener(eventType, listener);
122    }
123
124    /**
125     * Tests whether this controller is in <em>reloading state</em>. A return
126     * value of <b>true</b> means that a previous invocation of
127     * {@code checkForReloading()} has detected the necessity for a reload
128     * operation, but {@code resetReloadingState()} has not been called yet. In
129     * this state no further reloading checks are possible.
130     *
131     * @return a flag whether this controller is in reloading state
132     */
133    public synchronized boolean isInReloadingState()
134    {
135        return reloadingState;
136    }
137
138    /**
139     * Performs a check whether a reload operation is necessary. This method has
140     * to be called to trigger the generation of reloading events. It delegates
141     * to the associated {@link ReloadingDetector} and sends out notifications
142     * if necessary. The argument can be an arbitrary data object; it will be
143     * part of the event notification sent out when a reload operation should be
144     * performed. The return value indicates whether a change was detected and
145     * an event was sent. Once a need for a reload is detected, this controller
146     * is in <em>reloading state</em>. Until this state is reset (by calling
147     * {@link #resetReloadingState()}), no further reloading checks are
148     * performed by this method, and no events are fired; it then returns always
149     * <b>true</b>.
150     *
151     * @param data additional data for an event notification
152     * @return a flag whether a reload operation is necessary
153     */
154    public boolean checkForReloading(final Object data)
155    {
156        boolean sendEvent = false;
157        synchronized (this)
158        {
159            if (isInReloadingState())
160            {
161                return true;
162            }
163            if (getDetector().isReloadingRequired())
164            {
165                sendEvent = true;
166                reloadingState = true;
167            }
168        }
169
170        if (sendEvent)
171        {
172            listeners.fire(new ReloadingEvent(this, data));
173            return true;
174        }
175        return false;
176    }
177
178    /**
179     * Resets the reloading state. This tells the controller that reloading has
180     * been performed and new checks are possible again. If this controller is
181     * not in reloading state, this method has no effect.
182     */
183    public synchronized void resetReloadingState()
184    {
185        if (isInReloadingState())
186        {
187            getDetector().reloadingPerformed();
188            reloadingState = false;
189        }
190    }
191}