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.builder;
018
019import java.util.Collection;
020import java.util.concurrent.CopyOnWriteArrayList;
021
022/**
023 * <p>
024 * A class for managing a set of {@link DefaultParametersHandler} objects.
025 * </p>
026 * <p>
027 * This class provides functionality for registering and removing {@code DefaultParametersHandler} objects for arbitrary
028 * parameters classes. The handlers registered at an instance can then be applied on a passed in parameters object, so
029 * that it gets initialized with the provided default values.
030 * </p>
031 * <p>
032 * Usage of this class is as follows: First the {@code DefaultParametersHandler} objects to be supported must be
033 * registered using one of the {@code registerDefaultHandler()} methods. After that arbitrary parameters objects can be
034 * passed to the {@code initializeParameters()} method. This causes all {@code DefaultParametersHandler} objects
035 * supporting this parameters class to be invoked on this object.
036 * </p>
037 * <p>
038 * Implementation note: This class is thread-safe.
039 * </p>
040 *
041 * @since 2.0
042 */
043public class DefaultParametersManager {
044    /** A collection with the registered default handlers. */
045    private final Collection<DefaultHandlerData> defaultHandlers;
046
047    /**
048     * Creates a new instance of {@code DefaultParametersManager}.
049     */
050    public DefaultParametersManager() {
051        defaultHandlers = new CopyOnWriteArrayList<>();
052    }
053
054    /**
055     * Registers the specified {@code DefaultParametersHandler} object for the given parameters class. This means that this
056     * handler object is invoked every time a parameters object of the specified class or one of its subclasses is
057     * initialized. The handler can set arbitrary default values for the properties supported by this parameters object. If
058     * there are multiple handlers registered supporting a specific parameters class, they are invoked in the order in which
059     * they were registered. So handlers registered later may override the values set by handlers registered earlier.
060     *
061     * @param <T> the type of the parameters supported by this handler
062     * @param paramsClass the parameters class supported by this handler (must not be <b>null</b>)
063     * @param handler the {@code DefaultParametersHandler} to be registered (must not be <b>null</b>)
064     * @throws IllegalArgumentException if a required parameter is missing
065     */
066    public <T> void registerDefaultsHandler(final Class<T> paramsClass, final DefaultParametersHandler<? super T> handler) {
067        registerDefaultsHandler(paramsClass, handler, null);
068    }
069
070    /**
071     * Registers the specified {@code DefaultParametersHandler} object for the given parameters class and start class in the
072     * inheritance hierarchy. This method works like {@link #registerDefaultsHandler(Class, DefaultParametersHandler)}, but
073     * the defaults handler is only executed on parameter objects that are instances of the specified start class. Parameter
074     * classes do not stand in a real inheritance hierarchy; however, there is a logic hierarchy defined by the methods
075     * supported by the different parameter objects. A properties parameter object for instance supports all methods defined
076     * for a file-based parameter object. So one can argue that
077     * {@link org.apache.commons.configuration2.builder.fluent.FileBasedBuilderParameters FileBasedBuilderParameters} is a
078     * base interface of {@link org.apache.commons.configuration2.builder.fluent.PropertiesBuilderParameters
079     * PropertiesBuilderParameters} (although, for technical reasons, this relation is not reflected in the Java classes). A
080     * {@link DefaultParametersHandler} object defined for a base interface can also deal with parameter objects "derived"
081     * from this base interface (i.e. supporting a super set of the methods defined by the base interface). Now there may be
082     * the use case that there is an implementation of {@code DefaultParametersHandler} for a base interface (e.g.
083     * {@code FileBasedBuilderParameters}), but it should only process specific derived interfaces (say
084     * {@code PropertiesBuilderParameters}, but not
085     * {@link org.apache.commons.configuration2.builder.fluent.XMLBuilderParameters XMLBuilderParameters}). This can be
086     * achieved by passing in {@code PropertiesBuilderParameters} as start class. In this case,
087     * {@code DefaultParametersManager} ensures that the handler is only called on parameter objects having both the start
088     * class and the actual type supported by the handler as base interfaces. The passed in start class can be <b>null</b>;
089     * then the parameter class supported by the handler is used (which is the default behavior of the
090     * {@link #registerDefaultsHandler(Class, DefaultParametersHandler)} method).
091     *
092     * @param <T> the type of the parameters supported by this handler
093     * @param paramsClass the parameters class supported by this handler (must not be <b>null</b>)
094     * @param handler the {@code DefaultParametersHandler} to be registered (must not be <b>null</b>)
095     * @param startClass an optional start class in the hierarchy of parameter objects for which this handler should be
096     *        applied
097     * @throws IllegalArgumentException if a required parameter is missing
098     */
099    public <T> void registerDefaultsHandler(final Class<T> paramsClass, final DefaultParametersHandler<? super T> handler, final Class<?> startClass) {
100        if (paramsClass == null) {
101            throw new IllegalArgumentException("Parameters class must not be null!");
102        }
103        if (handler == null) {
104            throw new IllegalArgumentException("DefaultParametersHandler must not be null!");
105        }
106        defaultHandlers.add(new DefaultHandlerData(handler, paramsClass, startClass));
107    }
108
109    /**
110     * Removes the specified {@code DefaultParametersHandler} from this instance. If this handler has been registered
111     * multiple times for different start classes, all occurrences are removed.
112     *
113     * @param handler the {@code DefaultParametersHandler} to be removed
114     */
115    public void unregisterDefaultsHandler(final DefaultParametersHandler<?> handler) {
116        unregisterDefaultsHandler(handler, null);
117    }
118
119    /**
120     * Removes the specified {@code DefaultParametersHandler} from this instance if it is in combination with the given
121     * start class. If this handler has been registered multiple times for different start classes, only occurrences for the
122     * given start class are removed. The {@code startClass} parameter can be <b>null</b>, then all occurrences of the
123     * handler are removed.
124     *
125     * @param handler the {@code DefaultParametersHandler} to be removed
126     * @param startClass the start class for which this handler is to be removed
127     */
128    public void unregisterDefaultsHandler(final DefaultParametersHandler<?> handler, final Class<?> startClass) {
129        defaultHandlers.removeIf(dhd -> dhd.isOccurrence(handler, startClass));
130    }
131
132    /**
133     * Initializes the passed in {@code BuilderParameters} object by applying all matching {@link DefaultParametersHandler}
134     * objects registered at this instance. Using this method the passed in parameters object can be populated with default
135     * values.
136     *
137     * @param params the parameters object to be initialized (may be <b>null</b>, then this method has no effect)
138     */
139    public void initializeParameters(final BuilderParameters params) {
140        if (params != null) {
141            defaultHandlers.forEach(dhd -> dhd.applyHandlerIfMatching(params));
142        }
143    }
144
145    /**
146     * A data class storing information about {@code DefaultParametersHandler} objects added to a {@code Parameters} object.
147     * Using this class it is possible to find out which default handlers apply for a given parameters object and to invoke
148     * them.
149     */
150    private static final class DefaultHandlerData {
151        /** The handler object. */
152        private final DefaultParametersHandler<?> handler;
153
154        /** The class supported by this handler. */
155        private final Class<?> parameterClass;
156
157        /** The start class for applying this handler. */
158        private final Class<?> startClass;
159
160        /**
161         * Creates a new instance of {@code DefaultHandlerData}.
162         *
163         * @param h the {@code DefaultParametersHandler}
164         * @param cls the handler's data class
165         * @param startCls the start class
166         */
167        public DefaultHandlerData(final DefaultParametersHandler<?> h, final Class<?> cls, final Class<?> startCls) {
168            handler = h;
169            parameterClass = cls;
170            startClass = startCls;
171        }
172
173        /**
174         * Checks whether the managed {@code DefaultParametersHandler} can be applied to the given parameters object. If this is
175         * the case, it is executed on this object and can initialize it with default values.
176         *
177         * @param obj the parameters object to be initialized
178         */
179        @SuppressWarnings("unchecked")
180        // There are explicit isInstance() checks, so there won't be
181        // ClassCastExceptions
182        public void applyHandlerIfMatching(final BuilderParameters obj) {
183            if (parameterClass.isInstance(obj) && (startClass == null || startClass.isInstance(obj))) {
184                @SuppressWarnings("rawtypes")
185                final DefaultParametersHandler handlerUntyped = handler;
186                handlerUntyped.initializeDefaults(obj);
187            }
188        }
189
190        /**
191         * Tests whether this instance refers to the specified occurrence of a {@code DefaultParametersHandler}.
192         *
193         * @param h the handler to be checked
194         * @param startCls the start class
195         * @return <b>true</b> if this instance refers to this occurrence, <b>false</b> otherwise
196         */
197        public boolean isOccurrence(final DefaultParametersHandler<?> h, final Class<?> startCls) {
198            return h == handler && (startCls == null || startCls.equals(startClass));
199        }
200    }
201}