1 /*
2 * Licensed to the Apache Software Foundation (ASF) under one or more
3 * contributor license agreements. See the NOTICE file distributed with
4 * this work for additional information regarding copyright ownership.
5 * The ASF licenses this file to You under the Apache License, Version 2.0
6 * (the "License"); you may not use this file except in compliance with
7 * the License. You may obtain a copy of the License at
8 *
9 * http://www.apache.org/licenses/LICENSE-2.0
10 *
11 * Unless required by applicable law or agreed to in writing, software
12 * distributed under the License is distributed on an "AS IS" BASIS,
13 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
14 * See the License for the specific language governing permissions and
15 * limitations under the License.
16 */
17 package org.apache.commons.configuration2.io;
18
19 import java.net.URL;
20 import java.util.ArrayList;
21 import java.util.Collection;
22 import java.util.Collections;
23
24 /**
25 * <p>
26 * A specialized implementation of a {@code FileLocationStrategy} which encapsulates an arbitrary number of
27 * {@code FileLocationStrategy} objects.
28 * </p>
29 * <p>
30 * A collection with the wrapped {@code FileLocationStrategy} objects is passed at construction time. During a
31 * [{@code locate()} operation the wrapped strategies are called one after the other until one returns a non <b>null</b>
32 * URL. This URL is returned. If none of the wrapped strategies is able to resolve the passed in {@link FileLocator},
33 * result is <b>null</b>. This is similar to the <em>chain of responsibility</em> design pattern.
34 * </p>
35 * <p>
36 * This class, together with the provided concrete {@code FileLocationStrategy} implementations, offers a convenient way
37 * to customize the lookup for configuration files: Just add the desired concrete strategies to a
38 * {@code CombinedLocationStrategy} object. If necessary, custom strategies can be implemented if there are specific
39 * requirements. Note that the order in which strategies are added to a {@code CombinedLocationStrategy} matters: sub
40 * strategies are queried in the same order as they appear in the collection passed to the constructor.
41 * </p>
42 *
43 * @since 2.0
44 */
45 public class CombinedLocationStrategy implements FileLocationStrategy {
46 /** A collection with all sub strategies managed by this object. */
47 private final Collection<FileLocationStrategy> subStrategies;
48
49 /**
50 * Creates a new instance of {@code CombinedLocationStrategy} and initializes it with the provided sub strategies. The
51 * passed in collection must not be <b>null</b> or contain <b>null</b> elements.
52 *
53 * @param subs the collection with sub strategies
54 * @throws IllegalArgumentException if the collection is <b>null</b> or has <b>null</b> elements
55 */
56 public CombinedLocationStrategy(final Collection<? extends FileLocationStrategy> subs) {
57 if (subs == null) {
58 throw new IllegalArgumentException("Collection with sub strategies must not be null!");
59 }
60 subStrategies = Collections.unmodifiableCollection(new ArrayList<>(subs));
61 if (subStrategies.contains(null)) {
62 throw new IllegalArgumentException("Collection with sub strategies contains null entry!");
63 }
64 }
65
66 /**
67 * Gets a (unmodifiable) collection with the sub strategies managed by this object.
68 *
69 * @return the sub {@code FileLocationStrategy} objects
70 */
71 public Collection<FileLocationStrategy> getSubStrategies() {
72 return subStrategies;
73 }
74
75 /**
76 * {@inheritDoc} This implementation tries to locate the file by delegating to the managed sub strategies.
77 */
78 @Override
79 public URL locate(final FileSystem fileSystem, final FileLocator locator) {
80 for (final FileLocationStrategy sub : getSubStrategies()) {
81 final URL url = sub.locate(fileSystem, locator);
82 if (url != null) {
83 return url;
84 }
85 }
86
87 return null;
88 }
89 }