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     */
017    package org.apache.commons.discovery.resource;
018    
019    import java.util.LinkedList;
020    import java.util.List;
021    
022    import org.apache.commons.discovery.jdk.JDKHooks;
023    
024    /**
025     * There are many different contexts in which
026     * loaders can be used.  This provides a holder
027     * for a set of class loaders, so that they
028     * don't have to be build back up everytime...
029     */
030    public class ClassLoaders {
031    
032        protected List<ClassLoader> classLoaders = new LinkedList<ClassLoader>();
033    
034        /**
035         * Construct a new class loader set.
036         */
037        public ClassLoaders() {
038        }
039    
040        /**
041         * Returns the size of class loaders set.
042         *
043         * @return The size of class loaders set
044         */
045        public int size() {
046            return classLoaders.size();
047        }
048    
049        /**
050         * Returns the class loader positioned at the given index.
051         *
052         * @param idx The index the class loader has to be retrieved from
053         * @return The class loader positioned at the given index
054         */
055        public ClassLoader get(int idx) {
056            return classLoaders.get(idx);
057        }
058    
059        /**
060         * Specify a new class loader to be used in searching.
061         *
062         * The order of loaders determines the order of the result.
063         * It is recommended to add the most specific loaders first;
064         * {@code null} class loaders are discarded.
065         *
066         * @param classLoader The class loader has to added in the set
067         */
068        public void put(ClassLoader classLoader) {
069            if (classLoader != null) {
070                classLoaders.add(classLoader);
071            }
072        }
073    
074        /**
075         * Specify a new class loader to be used in searching.
076         * The order of loaders determines the order of the result.
077         * It is recommended to add the most specific loaders first;
078         * {@code null} class loaders are discarded.
079         *
080         * @param classLoader  The class loader has to added in the set
081         * @param prune if true, verify that the class loader is
082         *              not an Ancestor (@see isAncestor) before
083         *              adding it to our list.
084         */
085        public void put(ClassLoader classLoader, boolean prune) {
086            if (classLoader != null  &&  !(prune && isAncestor(classLoader))) {
087                classLoaders.add(classLoader);
088            }
089        }
090    
091        /**
092         * Check to see if <code>classLoader</code> is an
093         * ancestor of any contained class loader.
094         *
095         * This can be used to eliminate redundant class loaders
096         * IF all class loaders defer to parent class loaders
097         * before resolving a class.
098         *
099         * It may be that this is not always true.  Therefore,
100         * this check is not done internally to eliminate
101         * redundant class loaders, but left to the discretion
102         * of the user.
103         *
104         * @param classLoader The class loader under test
105         * @return true, if the class loader under test is an ancestor
106         *         of any contained class loader, false otherwise
107         */
108        public boolean isAncestor(final ClassLoader classLoader) {
109            /* bootstrap classloader, at root of all trees! */
110            if (classLoader == null) {
111                return true;
112            }
113    
114            for (int idx = 0; idx < size(); idx++) {
115                for (ClassLoader walker = get(idx);
116                        walker != null;
117                        walker = walker.getParent()) {
118                    if (walker == classLoader) {
119                        return true;
120                    }
121                }
122            }
123            return false;
124        }
125    
126        /**
127         * Utility method.  Returns a preloaded ClassLoaders instance
128         * containing the following class loaders, in order:
129         *
130         * <ul>
131         *   <li>spi.getClassLoader</li>
132         *   <li>seeker.getClassLoader</li>
133         *   <li>System Class Loader</li>
134         * </ul>
135         *
136         * Note that the thread context class loader is NOT present.
137         * This is a reasonable set of loaders to try if the resource to be found
138         * should be restricted to a libraries containing the SPI and Factory.
139         *
140         * @param spi WHAT is being looked for (an implementation of this class,
141         *            a default property file related to this class).
142         * @param factory WHO is performing the lookup.
143         * @param prune Determines if ancestors are allowed to be loaded or not.
144         * @return The class loaders holder
145         */    
146        public static ClassLoaders getLibLoaders(Class<?> spi, Class<?> factory, boolean prune) {
147            ClassLoaders loaders = new ClassLoaders();
148    
149            if (spi != null) {
150                loaders.put(spi.getClassLoader());
151            }
152            if (factory != null) {
153                loaders.put(factory.getClassLoader(), prune);
154            }
155            loaders.put(JDKHooks.getJDKHooks().getSystemClassLoader(), prune);
156    
157            return loaders;
158        }
159    
160        /**
161         * Utility method.  Returns a preloaded ClassLoaders instance
162         * containing the following class loaders, in order:
163         *
164         * <ul>
165         *   <li>Thread Context Class Loader</li>
166         *   <li>spi.getClassLoader</li>
167         *   <li>seeker.getClassLoader</li>
168         *   <li>System Class Loader</li>
169         * </ul>
170         *
171         * Note that the thread context class loader IS  present.
172         * This is a reasonable set of loaders to try if the resource to be found
173         * may be provided by an application.
174         *
175         * @param spi WHAT is being looked for (an implementation of this class,
176         *            a default property file related to this class).
177         * @param factory WHO is performing the lookup (factory).
178         * @param prune Determines if ancestors are allowed to be loaded or not.
179         * @return The class loaders holder
180         */
181        public static ClassLoaders getAppLoaders(Class<?> spi, Class<?> factory, boolean prune) {
182            ClassLoaders loaders = new ClassLoaders();
183    
184            loaders.put(JDKHooks.getJDKHooks().getThreadContextClassLoader());
185            if (spi != null) {
186                loaders.put(spi.getClassLoader(), prune);
187            }
188            if (factory != null) {
189                loaders.put(factory.getClassLoader(), prune);
190            }
191            loaders.put(JDKHooks.getJDKHooks().getSystemClassLoader(), prune);
192    
193            return loaders;
194        }
195    
196    }