DefaultParametersManager

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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.Collection;
import java.util.concurrent.CopyOnWriteArrayList;

/**
 * <p>
 * A class for managing a set of {@link DefaultParametersHandler} objects.
 * </p>
 * <p>
 * This class provides functionality for registering and removing {@code DefaultParametersHandler} objects for arbitrary
 * parameters classes. The handlers registered at an instance can then be applied on a passed in parameters object, so
 * that it gets initialized with the provided default values.
 * </p>
 * <p>
 * Usage of this class is as follows: First the {@code DefaultParametersHandler} objects to be supported must be
 * registered using one of the {@code registerDefaultHandler()} methods. After that arbitrary parameters objects can be
 * passed to the {@code initializeParameters()} method. This causes all {@code DefaultParametersHandler} objects
 * supporting this parameters class to be invoked on this object.
 * </p>
 * <p>
 * Implementation note: This class is thread-safe.
 * </p>
 *
 * @since 2.0
 */
public class DefaultParametersManager {

    /**
     * A data class storing information about {@code DefaultParametersHandler} objects added to a {@code Parameters} object.
     * Using this class it is possible to find out which default handlers apply for a given parameters object and to invoke
     * them.
     */
    private static final class DefaultHandlerData {

        /** The handler object. */
        private final DefaultParametersHandler<?> handler;

        /** The class supported by this handler. */
        private final Class<?> parameterClass;

        /** The start class for applying this handler. */
        private final Class<?> startClass;

        /**
         * Creates a new instance of {@code DefaultHandlerData}.
         *
         * @param h the {@code DefaultParametersHandler}
         * @param cls the handler's data class
         * @param startCls the start class
         */
        public DefaultHandlerData(final DefaultParametersHandler<?> h, final Class<?> cls, final Class<?> startCls) {
            handler = h;
            parameterClass = cls;
            startClass = startCls;
        }

        /**
         * Checks whether the managed {@code DefaultParametersHandler} can be applied to the given parameters object. If this is
         * the case, it is executed on this object and can initialize it with default values.
         *
         * @param obj the parameters object to be initialized
         */
        @SuppressWarnings("unchecked")
        // There are explicit isInstance() checks, so there won't be
        // ClassCastExceptions
        public void applyHandlerIfMatching(final BuilderParameters obj) {
            if (parameterClass.isInstance(obj) && (startClass == null || startClass.isInstance(obj))) {
                @SuppressWarnings("rawtypes")
                final DefaultParametersHandler handlerUntyped = handler;
                handlerUntyped.initializeDefaults(obj);
            }
        }

        /**
         * Tests whether this instance refers to the specified occurrence of a {@code DefaultParametersHandler}.
         *
         * @param h the handler to be checked
         * @param startCls the start class
         * @return <strong>true</strong> if this instance refers to this occurrence, <strong>false</strong> otherwise
         */
        public boolean isOccurrence(final DefaultParametersHandler<?> h, final Class<?> startCls) {
            return h == handler && (startCls == null || startCls.equals(startClass));
        }
    }

    /** A collection with the registered default handlers. */
    private final Collection<DefaultHandlerData> defaultHandlers;

    /**
     * Creates a new instance of {@code DefaultParametersManager}.
     */
    public DefaultParametersManager() {
        defaultHandlers = new CopyOnWriteArrayList<>();
    }

    /**
     * Initializes the passed in {@code BuilderParameters} object by applying all matching {@link DefaultParametersHandler}
     * objects registered at this instance. Using this method the passed in parameters object can be populated with default
     * values.
     *
     * @param params the parameters object to be initialized (may be <strong>null</strong>, then this method has no effect)
     */
    public void initializeParameters(final BuilderParameters params) {
        if (params != null) {
            defaultHandlers.forEach(dhd -> dhd.applyHandlerIfMatching(params));
        }
    }

    /**
     * Registers the specified {@code DefaultParametersHandler} object for the given parameters class. This means that this
     * handler object is invoked every time a parameters object of the specified class or one of its subclasses is
     * initialized. The handler can set arbitrary default values for the properties supported by this parameters object. If
     * there are multiple handlers registered supporting a specific parameters class, they are invoked in the order in which
     * they were registered. So handlers registered later may override the values set by handlers registered earlier.
     *
     * @param <T> the type of the parameters supported by this handler
     * @param paramsClass the parameters class supported by this handler (must not be <strong>null</strong>)
     * @param handler the {@code DefaultParametersHandler} to be registered (must not be <strong>null</strong>)
     * @throws IllegalArgumentException if a required parameter is missing
     */
    public <T> void registerDefaultsHandler(final Class<T> paramsClass, final DefaultParametersHandler<? super T> handler) {
        registerDefaultsHandler(paramsClass, handler, null);
    }

    /**
     * Registers the specified {@code DefaultParametersHandler} object for the given parameters class and start class in the
     * inheritance hierarchy. This method works like {@link #registerDefaultsHandler(Class, DefaultParametersHandler)}, but
     * the defaults handler is only executed on parameter objects that are instances of the specified start class. Parameter
     * classes do not stand in a real inheritance hierarchy; however, there is a logic hierarchy defined by the methods
     * supported by the different parameter objects. A properties parameter object for instance supports all methods defined
     * for a file-based parameter object. So one can argue that
     * {@link org.apache.commons.configuration2.builder.fluent.FileBasedBuilderParameters FileBasedBuilderParameters} is a
     * base interface of {@link org.apache.commons.configuration2.builder.fluent.PropertiesBuilderParameters
     * PropertiesBuilderParameters} (although, for technical reasons, this relation is not reflected in the Java classes). A
     * {@link DefaultParametersHandler} object defined for a base interface can also deal with parameter objects "derived"
     * from this base interface (i.e. supporting a super set of the methods defined by the base interface). Now there may be
     * the use case that there is an implementation of {@code DefaultParametersHandler} for a base interface (for example
     * {@code FileBasedBuilderParameters}), but it should only process specific derived interfaces (say
     * {@code PropertiesBuilderParameters}, but not
     * {@link org.apache.commons.configuration2.builder.fluent.XMLBuilderParameters XMLBuilderParameters}). This can be
     * achieved by passing in {@code PropertiesBuilderParameters} as start class. In this case,
     * {@code DefaultParametersManager} ensures that the handler is only called on parameter objects having both the start
     * class and the actual type supported by the handler as base interfaces. The passed in start class can be <strong>null</strong>;
     * then the parameter class supported by the handler is used (which is the default behavior of the
     * {@link #registerDefaultsHandler(Class, DefaultParametersHandler)} method).
     *
     * @param <T> the type of the parameters supported by this handler
     * @param paramsClass the parameters class supported by this handler (must not be <strong>null</strong>)
     * @param handler the {@code DefaultParametersHandler} to be registered (must not be <strong>null</strong>)
     * @param startClass an optional start class in the hierarchy of parameter objects for which this handler should be
     *        applied
     * @throws IllegalArgumentException if a required parameter is missing
     */
    public <T> void registerDefaultsHandler(final Class<T> paramsClass, final DefaultParametersHandler<? super T> handler, final Class<?> startClass) {
        if (paramsClass == null) {
            throw new IllegalArgumentException("Parameters class must not be null.");
        }
        if (handler == null) {
            throw new IllegalArgumentException("DefaultParametersHandler must not be null.");
        }
        defaultHandlers.add(new DefaultHandlerData(handler, paramsClass, startClass));
    }

    /**
     * Removes the specified {@code DefaultParametersHandler} from this instance. If this handler has been registered
     * multiple times for different start classes, all occurrences are removed.
     *
     * @param handler the {@code DefaultParametersHandler} to be removed
     */
    public void unregisterDefaultsHandler(final DefaultParametersHandler<?> handler) {
        unregisterDefaultsHandler(handler, null);
    }

    /**
     * Removes the specified {@code DefaultParametersHandler} from this instance if it is in combination with the given
     * start class. If this handler has been registered multiple times for different start classes, only occurrences for the
     * given start class are removed. The {@code startClass} parameter can be <strong>null</strong>, then all occurrences of the
     * handler are removed.
     *
     * @param handler the {@code DefaultParametersHandler} to be removed
     * @param startClass the start class for which this handler is to be removed
     */
    public void unregisterDefaultsHandler(final DefaultParametersHandler<?> handler, final Class<?> startClass) {
        defaultHandlers.removeIf(dhd -> dhd.isOccurrence(handler, startClass));
    }
}