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
018 package org.apache.commons.beanutils;
019
020 import java.util.Map;
021 import java.util.WeakHashMap;
022
023 /**
024 * An instance of this class represents a value that is provided per (thread)
025 * context classloader.
026 *
027 * <p>Occasionally it is necessary to store data in "global" variables
028 * (including uses of the Singleton pattern). In applications which have only
029 * a single classloader such data can simply be stored as "static" members on
030 * some class. When multiple classloaders are involved, however, this approach
031 * can fail; in particular, this doesn't work when the code may be run within a
032 * servlet container or a j2ee container, and the class on which the static
033 * member is defined is loaded via a "shared" classloader that is visible to all
034 * components running within the container. This class provides a mechanism for
035 * associating data with a ClassLoader instance, which ensures that when the
036 * code runs in such a container each component gets its own copy of the
037 * "global" variable rather than unexpectedly sharing a single copy of the
038 * variable with other components that happen to be running in the same
039 * container at the same time (eg servlets or EJBs.)</p>
040 *
041 * <p>This class is strongly patterned after the java.lang.ThreadLocal
042 * class, which performs a similar task in allowing data to be associated
043 * with a particular thread.</p>
044 *
045 * <p>When code that uses this class is run as a "normal" application, ie
046 * not within a container, the effect is identical to just using a static
047 * member variable to store the data, because Thread.getContextClassLoader
048 * always returns the same classloader (the system classloader).</p>
049 *
050 * <p>Expected usage is as follows:<br>
051 * <pre>
052 * public class SomeClass {
053 * private static final ContextClassLoaderLocal global
054 * = new ContextClassLoaderLocal() {
055 * protected Object initialValue() {
056 * return new String("Initial value");
057 * };
058 *
059 * public void testGlobal() {
060 * String s = (String) global.get();
061 * System.out.println("global value:" + s);
062 * buf.set("New Value");
063 * }
064 * </pre>
065 * </p>
066 *
067 * <p><strong>Note:</strong> This class takes some care to ensure that when
068 * a component which uses this class is "undeployed" by a container the
069 * component-specific classloader and all its associated classes (and their
070 * static variables) are garbage-collected. Unfortunately there is one
071 * scenario in which this does <i>not</i> work correctly and there
072 * is unfortunately no known workaround other than ensuring that the
073 * component (or its container) calls the "unset" method on this class for
074 * each instance of this class when the component is undeployed. The problem
075 * occurs if:
076 * <ul>
077 * <li>the class containing a static instance of this class was loaded via
078 * a shared classloader, and</li>
079 * <li>the value stored in the instance is an object whose class was loaded
080 * via the component-specific classloader (or any of the objects it refers
081 * to were loaded via that classloader).</li>
082 * </ul>
083 * The result is that the map managed by this object still contains a strong
084 * reference to the stored object, which contains a strong reference to the
085 * classloader that loaded it, meaning that although the container has
086 * "undeployed" the component the component-specific classloader and all the
087 * related classes and static variables cannot be garbage-collected. This is
088 * not expected to be an issue with the commons-beanutils library as the only
089 * classes which use this class are BeanUtilsBean and ConvertUtilsBean and
090 * there is no obvious reason for a user of the beanutils library to subclass
091 * either of those classes.</p>
092 *
093 * <p><strong>Note:</strong> A WeakHashMap bug in several 1.3 JVMs results in
094 * a memory leak for those JVMs.</p>
095 *
096 * <p><strong>Note:</strong> Of course all of this would be unnecessary if
097 * containers required each component to load the full set of classes it
098 * needs, ie avoided providing classes loaded via a "shared" classloader.</p>
099 *
100 * @see java.lang.Thread#getContextClassLoader
101 * @author Eric Pabst
102 */
103 public class ContextClassLoaderLocal {
104 private Map valueByClassLoader = new WeakHashMap();
105 private boolean globalValueInitialized = false;
106 private Object globalValue;
107
108 /**
109 * Construct a context classloader instance
110 */
111 public ContextClassLoaderLocal() {
112 super();
113 }
114
115 /**
116 * Returns the initial value for this ContextClassLoaderLocal
117 * variable. This method will be called once per Context ClassLoader for
118 * each ContextClassLoaderLocal, the first time it is accessed
119 * with get or set. If the programmer desires ContextClassLoaderLocal variables
120 * to be initialized to some value other than null, ContextClassLoaderLocal must
121 * be subclassed, and this method overridden. Typically, an anonymous
122 * inner class will be used. Typical implementations of initialValue
123 * will call an appropriate constructor and return the newly constructed
124 * object.
125 *
126 * @return a new Object to be used as an initial value for this ContextClassLoaderLocal
127 */
128 protected Object initialValue() {
129 return null;
130 }
131
132 /**
133 * Gets the instance which provides the functionality for {@link BeanUtils}.
134 * This is a pseudo-singleton - an single instance is provided per (thread) context classloader.
135 * This mechanism provides isolation for web apps deployed in the same container.
136 * @return the object currently associated with the context-classloader of the current thread.
137 */
138 public synchronized Object get() {
139 // synchronizing the whole method is a bit slower
140 // but guarantees no subtle threading problems, and there's no
141 // need to synchronize valueByClassLoader
142
143 // make sure that the map is given a change to purge itself
144 valueByClassLoader.isEmpty();
145 try {
146
147 ClassLoader contextClassLoader = Thread.currentThread().getContextClassLoader();
148 if (contextClassLoader != null) {
149
150 Object value = valueByClassLoader.get(contextClassLoader);
151 if ((value == null)
152 && !valueByClassLoader.containsKey(contextClassLoader)) {
153 value = initialValue();
154 valueByClassLoader.put(contextClassLoader, value);
155 }
156 return value;
157
158 }
159
160 } catch (SecurityException e) { /* SWALLOW - should we log this? */ }
161
162 // if none or exception, return the globalValue
163 if (!globalValueInitialized) {
164 globalValue = initialValue();
165 globalValueInitialized = true;
166 }//else already set
167 return globalValue;
168 }
169
170 /**
171 * Sets the value - a value is provided per (thread) context classloader.
172 * This mechanism provides isolation for web apps deployed in the same container.
173 *
174 * @param value the object to be associated with the entrant thread's context classloader
175 */
176 public synchronized void set(Object value) {
177 // synchronizing the whole method is a bit slower
178 // but guarentees no subtle threading problems
179
180 // make sure that the map is given a change to purge itself
181 valueByClassLoader.isEmpty();
182 try {
183
184 ClassLoader contextClassLoader = Thread.currentThread().getContextClassLoader();
185 if (contextClassLoader != null) {
186 valueByClassLoader.put(contextClassLoader, value);
187 return;
188 }
189
190 } catch (SecurityException e) { /* SWALLOW - should we log this? */ }
191
192 // if in doubt, set the global value
193 globalValue = value;
194 globalValueInitialized = true;
195 }
196
197 /**
198 * Unsets the value associated with the current thread's context classloader
199 */
200 public synchronized void unset() {
201 try {
202
203 ClassLoader contextClassLoader = Thread.currentThread().getContextClassLoader();
204 unset(contextClassLoader);
205
206 } catch (SecurityException e) { /* SWALLOW - should we log this? */ }
207 }
208
209 /**
210 * Unsets the value associated with the given classloader
211 * @param classLoader The classloader to <i>unset</i> for
212 */
213 public synchronized void unset(ClassLoader classLoader) {
214 valueByClassLoader.remove(classLoader);
215 }
216 }