    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}