ReloadingFileBasedConfigurationBuilder

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 * 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
 *
 *     https://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 java.util.Map;

import org.apache.commons.configuration2.FileBasedConfiguration;
import org.apache.commons.configuration2.ex.ConfigurationException;
import org.apache.commons.configuration2.io.FileHandler;
import org.apache.commons.configuration2.reloading.ReloadingController;
import org.apache.commons.configuration2.reloading.ReloadingControllerSupport;
import org.apache.commons.configuration2.reloading.ReloadingDetector;

/**
 * <p>
 * A specialized {@code ConfigurationBuilder} implementation which can handle configurations read from a
 * {@link FileHandler} and supports reloading.
 * </p>
 * <p>
 * This builder class exposes a {@link ReloadingController} object controlling reload operations on the file-based
 * configuration produced as result object. For the {@code FileHandler} defining the location of the configuration a
 * configurable {@link ReloadingDetector} is created and associated with the controller. So changes on the source file
 * can be detected. When ever such a change occurs, the result object of this builder is reset. This means that the next
 * time {@code getConfiguration()} is called a new {@code Configuration} object is created which is loaded from the
 * modified file.
 * </p>
 * <p>
 * Client code interested in notifications can register a listener at this builder to receive reset events. When such an
 * event is received the new result object can be requested. This way client applications can be sure to work with an
 * up-to-date configuration. It is also possible to register a listener directly at the {@code ReloadingController}.
 * </p>
 * <p>
 * This builder does not actively trigger the {@code ReloadingController} to perform a reload check. This has to be done
 * by an external component, for example a timer.
 * </p>
 *
 * @param <T> the concrete type of {@code Configuration} objects created by this builder
 * @since 2.0
 */
public class ReloadingFileBasedConfigurationBuilder<T extends FileBasedConfiguration> extends FileBasedConfigurationBuilder<T>
    implements ReloadingControllerSupport {

    /** The default factory for creating reloading detector objects. */
    private static final ReloadingDetectorFactory DEFAULT_DETECTOR_FACTORY = new DefaultReloadingDetectorFactory();

    /**
     * Returns a {@code ReloadingDetectorFactory} either from the passed in parameters or a default factory.
     *
     * @param params the current parameters object
     * @return the {@code ReloadingDetectorFactory} to be used
     */
    private static ReloadingDetectorFactory fetchDetectorFactory(final FileBasedBuilderParametersImpl params) {
        final ReloadingDetectorFactory factory = params.getReloadingDetectorFactory();
        return factory != null ? factory : DEFAULT_DETECTOR_FACTORY;
    }

    /** The reloading controller associated with this object. */
    private final ReloadingController reloadingController;

    /**
     * The reloading detector which does the actual reload check for the current result object. A new instance is created
     * whenever a new result object (and thus a new current file handler) becomes available. The field must be volatile
     * because it is accessed by the reloading controller probably from within another thread.
     */
    private volatile ReloadingDetector resultReloadingDetector;

    /**
     * Creates a new instance of {@code ReloadingFileBasedConfigurationBuilder} which produces result objects of the
     * specified class.
     *
     * @param resCls the result class (must not be <strong>null</strong>
     * @throws IllegalArgumentException if the result class is <strong>null</strong>
     */
    public ReloadingFileBasedConfigurationBuilder(final Class<? extends T> resCls) {
        super(resCls);
        reloadingController = createReloadingController();
    }

    /**
     * Creates a new instance of {@code ReloadingFileBasedConfigurationBuilder} which produces result objects of the
     * specified class and sets initialization parameters.
     *
     * @param resCls the result class (must not be <strong>null</strong>
     * @param params a map with initialization parameters
     * @throws IllegalArgumentException if the result class is <strong>null</strong>
     */
    public ReloadingFileBasedConfigurationBuilder(final Class<? extends T> resCls, final Map<String, Object> params) {
        super(resCls, params);
        reloadingController = createReloadingController();
    }

    /**
     * Creates a new instance of {@code ReloadingFileBasedConfigurationBuilder} which produces result objects of the
     * specified class and sets initialization parameters and the <em>allowFailOnInit</em> flag.
     *
     * @param resCls the result class (must not be <strong>null</strong>
     * @param params a map with initialization parameters
     * @param allowFailOnInit the <em>allowFailOnInit</em> flag
     * @throws IllegalArgumentException if the result class is <strong>null</strong>
     */
    public ReloadingFileBasedConfigurationBuilder(final Class<? extends T> resCls, final Map<String, Object> params, final boolean allowFailOnInit) {
        super(resCls, params, allowFailOnInit);
        reloadingController = createReloadingController();
    }

    /**
     * {@inheritDoc} This method is overridden here to change the result type.
     */
    @Override
    public ReloadingFileBasedConfigurationBuilder<T> configure(final BuilderParameters... params) {
        super.configure(params);
        return this;
    }

    /**
     * Creates the {@code ReloadingController} associated with this object. The controller is assigned a specialized
     * reloading detector which delegates to the detector for the current result object. (
     * {@code FileHandlerReloadingDetector} does not support changing the file handler, and {@code ReloadingController} does
     * not support changing the reloading detector; therefore, this level of indirection is needed to change the monitored
     * file dynamically.)
     *
     * @return the new {@code ReloadingController}
     */
    private ReloadingController createReloadingController() {
        final ReloadingDetector ctrlDetector = createReloadingDetectorForController();
        final ReloadingController ctrl = new ReloadingController(ctrlDetector);
        connectToReloadingController(ctrl);
        return ctrl;
    }

    /**
     * Creates a {@code ReloadingDetector} which monitors the passed in {@code FileHandler}. This method is called each time
     * a new result object is created with the current {@code FileHandler}. This implementation checks whether a
     * {@code ReloadingDetectorFactory} is specified in the current parameters. If this is the case, it is invoked.
     * Otherwise, a default factory is used to create a {@code FileHandlerReloadingDetector} object. Note: This method is
     * called from a synchronized block.
     *
     * @param handler the current {@code FileHandler}
     * @param fbparams the object with parameters related to file-based builders
     * @return a {@code ReloadingDetector} for this {@code FileHandler}
     * @throws ConfigurationException if an error occurs
     */
    protected ReloadingDetector createReloadingDetector(final FileHandler handler, final FileBasedBuilderParametersImpl fbparams)
        throws ConfigurationException {
        return fetchDetectorFactory(fbparams).createReloadingDetector(handler, fbparams);
    }

    /**
     * Creates a {@code ReloadingDetector} wrapper to be passed to the associated {@code ReloadingController}. This detector
     * wrapper simply delegates to the current {@code ReloadingDetector} if it is available.
     *
     * @return the wrapper {@code ReloadingDetector}
     */
    private ReloadingDetector createReloadingDetectorForController() {
        return new ReloadingDetector() {
            @Override
            public boolean isReloadingRequired() {
                final ReloadingDetector detector = resultReloadingDetector;
                return detector != null && detector.isReloadingRequired();
            }

            @Override
            public void reloadingPerformed() {
                final ReloadingDetector detector = resultReloadingDetector;
                if (detector != null) {
                    detector.reloadingPerformed();
                }
            }
        };
    }

    /**
     * Gets the {@code ReloadingController} associated with this builder. This controller is directly created. However,
     * it becomes active (i.e. associated with a meaningful reloading detector) not before a result object was created.
     *
     * @return the {@code ReloadingController}
     */
    @Override
    public ReloadingController getReloadingController() {
        return reloadingController;
    }

    /**
     * {@inheritDoc} This implementation also takes care that a new {@code ReloadingDetector} for the new current
     * {@code FileHandler} is created. Also, the reloading controller's reloading state has to be reset; after the creation
     * of a new result object changes in the underlying configuration source have to be monitored again.
     */
    @Override
    protected void initFileHandler(final FileHandler handler) throws ConfigurationException {
        super.initFileHandler(handler);

        resultReloadingDetector = createReloadingDetector(handler, FileBasedBuilderParametersImpl.fromParameters(getParameters(), true));
    }
}