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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.discovery.resource;
18  
19  import java.util.LinkedList;
20  import java.util.List;
21  
22  import org.apache.commons.discovery.jdk.JDKHooks;
23  
24  /**
25   * There are many different contexts in which
26   * loaders can be used.  This provides a holder
27   * for a set of class loaders, so that they
28   * don't have to be build back up everytime...
29   */
30  public class ClassLoaders {
31  
32      protected List<ClassLoader> classLoaders = new LinkedList<ClassLoader>();
33  
34      /**
35       * Construct a new class loader set.
36       */
37      public ClassLoaders() {
38      }
39  
40      /**
41       * Returns the size of class loaders set.
42       *
43       * @return The size of class loaders set
44       */
45      public int size() {
46          return classLoaders.size();
47      }
48  
49      /**
50       * Returns the class loader positioned at the given index.
51       *
52       * @param idx The index the class loader has to be retrieved from
53       * @return The class loader positioned at the given index
54       */
55      public ClassLoader get(int idx) {
56          return classLoaders.get(idx);
57      }
58  
59      /**
60       * Specify a new class loader to be used in searching.
61       *
62       * The order of loaders determines the order of the result.
63       * It is recommended to add the most specific loaders first;
64       * {@code null} class loaders are discarded.
65       *
66       * @param classLoader The class loader has to added in the set
67       */
68      public void put(ClassLoader classLoader) {
69          if (classLoader != null) {
70              classLoaders.add(classLoader);
71          }
72      }
73  
74      /**
75       * Specify a new class loader to be used in searching.
76       * The order of loaders determines the order of the result.
77       * It is recommended to add the most specific loaders first;
78       * {@code null} class loaders are discarded.
79       *
80       * @param classLoader  The class loader has to added in the set
81       * @param prune if true, verify that the class loader is
82       *              not an Ancestor (@see isAncestor) before
83       *              adding it to our list.
84       */
85      public void put(ClassLoader classLoader, boolean prune) {
86          if (classLoader != null  &&  !(prune && isAncestor(classLoader))) {
87              classLoaders.add(classLoader);
88          }
89      }
90  
91      /**
92       * Check to see if <code>classLoader</code> is an
93       * ancestor of any contained class loader.
94       *
95       * This can be used to eliminate redundant class loaders
96       * IF all class loaders defer to parent class loaders
97       * before resolving a class.
98       *
99       * 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 }