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.io;
018
019import java.net.URL;
020import java.util.ArrayList;
021import java.util.Collection;
022import java.util.Collections;
023
024/**
025 * <p>
026 * A specialized implementation of a {@code FileLocationStrategy} which
027 * encapsulates an arbitrary number of {@code FileLocationStrategy} objects.
028 * </p>
029 * <p>
030 * A collection with the wrapped {@code FileLocationStrategy} objects is passed
031 * at construction time. During a [{@code locate()} operation the wrapped
032 * strategies are called one after the other until one returns a non <b>null</b>
033 * URL. This URL is returned. If none of the wrapped strategies is able to
034 * resolve the passed in {@link FileLocator}, result is <b>null</b>. This is
035 * similar to the <em>chain of responsibility</em> design pattern.
036 * </p>
037 * <p>
038 * This class, together with the provided concrete {@code FileLocationStrategy}
039 * implementations, offers a convenient way to customize the lookup for
040 * configuration files: Just add the desired concrete strategies to a
041 * {@code CombinedLocationStrategy} object. If necessary, custom strategies can
042 * be implemented if there are specific requirements. Note that the order in
043 * which strategies are added to a {@code CombinedLocationStrategy} matters: sub
044 * strategies are queried in the same order as they appear in the collection
045 * passed to the constructor.
046 * </p>
047 *
048 * @since 2.0
049 */
050public class CombinedLocationStrategy implements FileLocationStrategy
051{
052    /** A collection with all sub strategies managed by this object. */
053    private final Collection<FileLocationStrategy> subStrategies;
054
055    /**
056     * Creates a new instance of {@code CombinedLocationStrategy} and
057     * initializes it with the provided sub strategies. The passed in collection
058     * must not be <b>null</b> or contain <b>null</b> elements.
059     *
060     * @param subs the collection with sub strategies
061     * @throws IllegalArgumentException if the collection is <b>null</b> or has
062     *         <b>null</b> elements
063     */
064    public CombinedLocationStrategy(
065            final Collection<? extends FileLocationStrategy> subs)
066    {
067        if (subs == null)
068        {
069            throw new IllegalArgumentException(
070                    "Collection with sub strategies must not be null!");
071        }
072        subStrategies =
073                Collections
074                        .unmodifiableCollection(new ArrayList<>(
075                                subs));
076        if (subStrategies.contains(null))
077        {
078            throw new IllegalArgumentException(
079                    "Collection with sub strategies contains null entry!");
080        }
081    }
082
083    /**
084     * Returns a (unmodifiable) collection with the sub strategies managed by
085     * this object.
086     *
087     * @return the sub {@code FileLocationStrategy} objects
088     */
089    public Collection<FileLocationStrategy> getSubStrategies()
090    {
091        return subStrategies;
092    }
093
094    /**
095     * {@inheritDoc} This implementation tries to locate the file by delegating
096     * to the managed sub strategies.
097     */
098    @Override
099    public URL locate(final FileSystem fileSystem, final FileLocator locator)
100    {
101        for (final FileLocationStrategy sub : getSubStrategies())
102        {
103            final URL url = sub.locate(fileSystem, locator);
104            if (url != null)
105            {
106                return url;
107            }
108        }
109
110        return null;
111    }
112}