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
018package org.apache.commons.beanutils;
019
020import java.util.Map;
021import 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&lt;String&gt; global
054 *      = new ContextClassLoaderLocal&lt;String&gt;() {
055 *          protected String initialValue() {
056 *              return new String("Initial value");
057 *          };
058 *
059 *    public void testGlobal() {
060 *      String s = 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 * @param <T> the type of data stored in an instance
101 * @version $Id: ContextClassLoaderLocal.java 1540186 2013-11-08 21:08:30Z oheger $
102 * @see java.lang.Thread#getContextClassLoader
103 */
104public class ContextClassLoaderLocal<T> {
105    private final Map<ClassLoader, T> valueByClassLoader = new WeakHashMap<ClassLoader, T>();
106    private boolean globalValueInitialized = false;
107    private T globalValue;
108
109    /**
110     * Construct a context classloader instance
111     */
112    public ContextClassLoaderLocal() {
113        super();
114    }
115
116    /**
117     * Returns the initial value for this ContextClassLoaderLocal
118     * variable. This method will be called once per Context ClassLoader for
119     * each ContextClassLoaderLocal, the first time it is accessed
120     * with get or set.  If the programmer desires ContextClassLoaderLocal variables
121     * to be initialized to some value other than null, ContextClassLoaderLocal must
122     * be subclassed, and this method overridden.  Typically, an anonymous
123     * inner class will be used.  Typical implementations of initialValue
124     * will call an appropriate constructor and return the newly constructed
125     * object.
126     *
127     * @return a new Object to be used as an initial value for this ContextClassLoaderLocal
128     */
129    protected T initialValue() {
130        return null;
131    }
132
133    /**
134     * Gets the instance which provides the functionality for {@link BeanUtils}.
135     * This is a pseudo-singleton - an single instance is provided per (thread) context classloader.
136     * This mechanism provides isolation for web apps deployed in the same container.
137     * @return the object currently associated with the context-classloader of the current thread.
138     */
139    public synchronized T get() {
140        // synchronizing the whole method is a bit slower
141        // but guarantees no subtle threading problems, and there's no
142        // need to synchronize valueByClassLoader
143
144        // make sure that the map is given a change to purge itself
145        valueByClassLoader.isEmpty();
146        try {
147
148            ClassLoader contextClassLoader = Thread.currentThread().getContextClassLoader();
149            if (contextClassLoader != null) {
150
151                T value = valueByClassLoader.get(contextClassLoader);
152                if ((value == null)
153                && !valueByClassLoader.containsKey(contextClassLoader)) {
154                    value = initialValue();
155                    valueByClassLoader.put(contextClassLoader, value);
156                }
157                return value;
158
159            }
160
161        } catch (SecurityException e) { /* SWALLOW - should we log this? */ }
162
163        // if none or exception, return the globalValue
164        if (!globalValueInitialized) {
165            globalValue = initialValue();
166            globalValueInitialized = true;
167        }//else already set
168        return globalValue;
169    }
170
171    /**
172     * Sets the value - a value is provided per (thread) context classloader.
173     * This mechanism provides isolation for web apps deployed in the same container.
174     *
175     * @param value the object to be associated with the entrant thread's context classloader
176     */
177    public synchronized void set(T value) {
178        // synchronizing the whole method is a bit slower
179        // but guarentees no subtle threading problems
180
181        // make sure that the map is given a change to purge itself
182        valueByClassLoader.isEmpty();
183        try {
184
185            ClassLoader contextClassLoader = Thread.currentThread().getContextClassLoader();
186            if (contextClassLoader != null) {
187                valueByClassLoader.put(contextClassLoader, value);
188                return;
189            }
190
191        } catch (SecurityException e) { /* SWALLOW - should we log this? */ }
192
193        // if in doubt, set the global value
194        globalValue = value;
195        globalValueInitialized = true;
196    }
197
198    /**
199     * Unsets the value associated with the current thread's context classloader
200     */
201    public synchronized void unset() {
202        try {
203
204            ClassLoader contextClassLoader = Thread.currentThread().getContextClassLoader();
205            unset(contextClassLoader);
206
207        } catch (SecurityException e) { /* SWALLOW - should we log this? */ }
208    }
209
210    /**
211     * Unsets the value associated with the given classloader
212     * @param classLoader The classloader to <i>unset</i> for
213     */
214    public synchronized void unset(ClassLoader classLoader) {
215        valueByClassLoader.remove(classLoader);
216    }
217}