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 * @version $Id: CombinedLocationStrategy.java 1790899 2017-04-10 21:56:46Z ggregory $
049 * @since 2.0
050 */
051public class CombinedLocationStrategy implements FileLocationStrategy
052{
053    /** A collection with all sub strategies managed by this object. */
054    private final Collection<FileLocationStrategy> subStrategies;
055
056    /**
057     * Creates a new instance of {@code CombinedLocationStrategy} and
058     * initializes it with the provided sub strategies. The passed in collection
059     * must not be <b>null</b> or contain <b>null</b> elements.
060     *
061     * @param subs the collection with sub strategies
062     * @throws IllegalArgumentException if the collection is <b>null</b> or has
063     *         <b>null</b> elements
064     */
065    public CombinedLocationStrategy(
066            Collection<? extends FileLocationStrategy> subs)
067    {
068        if (subs == null)
069        {
070            throw new IllegalArgumentException(
071                    "Collection with sub strategies must not be null!");
072        }
073        subStrategies =
074                Collections
075                        .unmodifiableCollection(new ArrayList<>(
076                                subs));
077        if (subStrategies.contains(null))
078        {
079            throw new IllegalArgumentException(
080                    "Collection with sub strategies contains null entry!");
081        }
082    }
083
084    /**
085     * Returns a (unmodifiable) collection with the sub strategies managed by
086     * this object.
087     *
088     * @return the sub {@code FileLocationStrategy} objects
089     */
090    public Collection<FileLocationStrategy> getSubStrategies()
091    {
092        return subStrategies;
093    }
094
095    /**
096     * {@inheritDoc} This implementation tries to locate the file by delegating
097     * to the managed sub strategies.
098     */
099    @Override
100    public URL locate(FileSystem fileSystem, FileLocator locator)
101    {
102        for (FileLocationStrategy sub : getSubStrategies())
103        {
104            URL url = sub.locate(fileSystem, locator);
105            if (url != null)
106            {
107                return url;
108            }
109        }
110
111        return null;
112    }
113}