    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     * Returns 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}