src/main/java/org/apache/commons/configuration2/builder/ReloadingBuilderSupportListener.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.builder;

import org.apache.commons.configuration2.event.Event;
import org.apache.commons.configuration2.event.EventListener;
import org.apache.commons.configuration2.reloading.ReloadingController;
import org.apache.commons.configuration2.reloading.ReloadingEvent;

/**
 * <p>
 * An internally used helper class for adding reloading support to an arbitrary {@link ConfigurationBuilder}.
 * </p>
 * <p>
 * This class connects a configuration builder with a {@link ReloadingController}. This is done in the following way:
 * <ul>
 * <li>An instance is registered as listener at a {@code ReloadingController}. Whenever the controller indicates that a
 * reload should happen, the associated configuration builder's {@link BasicConfigurationBuilder#resetResult()} method
 * is called.</li>
 * <li>When the builder fires a {@link ConfigurationBuilderResultCreatedEvent} event the reloading controller's
 * reloading state is reset. At that time the reload has actually happened, and the controller is prepared to observe
 * new changes.</li>
 * </ul>
 * </p>
 *
 * @since 2.0
 */
final class ReloadingBuilderSupportListener implements EventListener<Event> {
    /**
     * Creates a new instance of {@code ReloadingBuilderSupportListener} which connects the specified
     * {@code ConfigurationBuilder} with the given {@code ReloadingController}. Listeners are registered to react on
     * notifications and implement a reloading protocol as described in the class comment.
     *
     * @param configBuilder the {@code ConfigurationBuilder}
     * @param controller the {@code ReloadingController}
     * @return the newly created listener object
     */
    public static ReloadingBuilderSupportListener connect(final BasicConfigurationBuilder<?> configBuilder, final ReloadingController controller) {
        final ReloadingBuilderSupportListener listener = new ReloadingBuilderSupportListener(configBuilder, controller);
        controller.addEventListener(ReloadingEvent.ANY, listener);
        configBuilder.installEventListener(ConfigurationBuilderResultCreatedEvent.RESULT_CREATED, listener);
        return listener;
    }

    /** Stores the associated configuration builder. */
    private final BasicConfigurationBuilder<?> builder;

    /** Stores the associated reloading controller. */
    private final ReloadingController reloadingController;

    /**
     * Creates a new instance of {@code ReloadingBuilderSupportListener} and initializes it with the associated objects.
     *
     * @param configBuilder the configuration builder
     * @param controller the {@code ReloadingController}
     */
    private ReloadingBuilderSupportListener(final BasicConfigurationBuilder<?> configBuilder, final ReloadingController controller) {
        builder = configBuilder;
        reloadingController = controller;
    }

    /**
     * {@inheritDoc} This implementation resets the controller's reloading state if an event about a newly created result
     * was received. Otherwise, in case of a reloading event, the builder's result object is reset.
     */
    @Override
    public void onEvent(final Event event) {
        if (ConfigurationBuilderResultCreatedEvent.RESULT_CREATED.equals(event.getEventType())) {
            reloadingController.resetReloadingState();
        } else {
            builder.resetResult();
        }
    }
}