ReloadingController.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.reloading;

import org.apache.commons.configuration2.event.Event;
import org.apache.commons.configuration2.event.EventListener;
import org.apache.commons.configuration2.event.EventListenerList;
import org.apache.commons.configuration2.event.EventSource;
import org.apache.commons.configuration2.event.EventType;

/**
 * <p>
 * A class for adding support for reload operations in a generic way.
 * </p>
 * <p>
 * A {@code ReloadingController} monitors a specific source and triggers reloading events if necessary. So it does not
 * perform reloading itself, but only sends out notifications when it thinks that this should be done. This allows for a
 * very generic setup in which different components involved in reloading are loosely coupled via events.
 * </p>
 * <p>
 * A typical usage scenario is as follows:
 * </p>
 * <ul>
 * <li>A {@code ReloadingController} instance is created and initialized with a {@link ReloadingDetector} object.</li>
 * <li>A number of {@link EventListener} objects for reloading events can be registered at the controller.</li>
 * <li>Now the controller's {@code checkForReloading()} method is called whenever a check is to be performed. This could
 * be done for instance by a timer in regular intervals or by any other means appropriate for a specific
 * application.</li>
 * <li>When a check reveals that a reload operation is necessary all registered event listeners are notified.</li>
 * <li>Typically one of the listeners is responsible to perform the actual reload operation. (How this is done is not in
 * the scope of the controller object.) After this has been done, the controller's {@code resetReloadingState()} method
 * must be called. It tells the controller that the last notification has been processed and that new checks are
 * possible again. It is important that this method is called. Otherwise, {@code checkForReloading()} will not do any
 * new checks or send out event notifications any more.</li>
 * </ul>
 * <p>
 * This class can be accessed from multiple threads concurrently. It shields the associated {@link ReloadingDetector}
 * object for concurrent access, so that a concrete detector implementation does not have to be thread-safe.
 * </p>
 *
 * @since 2.0
 */
public class ReloadingController implements EventSource {
    /** Stores a reference to the reloading detector. */
    private final ReloadingDetector detector;

    /** The helper object which manages the registered event listeners. */
    private final EventListenerList listeners;

    /** A flag whether this controller is in reloading state. */
    private boolean reloadingState;

    /**
     * Creates a new instance of {@code ReloadingController} and associates it with the given {@code ReloadingDetector}
     * object.
     *
     * @param detect the {@code ReloadingDetector} (must not be <strong>null</strong>)
     * @throws IllegalArgumentException if the detector is undefined
     */
    public ReloadingController(final ReloadingDetector detect) {
        if (detect == null) {
            throw new IllegalArgumentException("ReloadingDetector must not be null!");
        }

        detector = detect;
        listeners = new EventListenerList();
    }

    /**
     * {@inheritDoc} This class generates events of type {@code ReloadingEvent}.
     */
    @Override
    public <T extends Event> void addEventListener(final EventType<T> eventType, final EventListener<? super T> listener) {
        listeners.addEventListener(eventType, listener);
    }

    /**
     * Performs a check whether a reload operation is necessary. This method has to be called to trigger the generation of
     * reloading events. It delegates to the associated {@link ReloadingDetector} and sends out notifications if necessary.
     * The argument can be an arbitrary data object; it will be part of the event notification sent out when a reload
     * operation should be performed. The return value indicates whether a change was detected and an event was sent. Once a
     * need for a reload is detected, this controller is in <em>reloading state</em>. Until this state is reset (by calling
     * {@link #resetReloadingState()}), no further reloading checks are performed by this method, and no events are fired;
     * it then returns always <strong>true</strong>.
     *
     * @param data additional data for an event notification
     * @return a flag whether a reload operation is necessary
     */
    public boolean checkForReloading(final Object data) {
        boolean sendEvent = false;
        synchronized (this) {
            if (isInReloadingState()) {
                return true;
            }
            if (getDetector().isReloadingRequired()) {
                sendEvent = true;
                reloadingState = true;
            }
        }

        if (sendEvent) {
            listeners.fire(new ReloadingEvent(this, data));
            return true;
        }
        return false;
    }

    /**
     * Gets the {@code ReloadingDetector} used by this controller.
     *
     * @return the {@code ReloadingDetector}
     */
    public ReloadingDetector getDetector() {
        return detector;
    }

    /**
     * Tests whether this controller is in <em>reloading state</em>. A return value of <strong>true</strong> means that a previous
     * invocation of {@code checkForReloading()} has detected the necessity for a reload operation, but
     * {@code resetReloadingState()} has not been called yet. In this state no further reloading checks are possible.
     *
     * @return a flag whether this controller is in reloading state
     */
    public synchronized boolean isInReloadingState() {
        return reloadingState;
    }

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

    /**
     * Resets the reloading state. This tells the controller that reloading has been performed and new checks are possible
     * again. If this controller is not in reloading state, this method has no effect.
     */
    public synchronized void resetReloadingState() {
        if (isInReloadingState()) {
            getDetector().reloadingPerformed();
            reloadingState = false;
        }
    }
}