    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}