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}