Source Code Cross Referenced for ContextClassLoaderLocal.java in  » Library » Apache-commons-beanutils-1.8.0-BETA-src » org » apache » commons » beanutils » Java Source Code / Java DocumentationJava Source Code and Java Documentation

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Java Source Code / Java Documentation » Library » Apache commons beanutils 1.8.0 BETA src » org.apache.commons.beanutils 
Source Cross Referenced  Class Diagram Java Document (Java Doc) 


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()
148:                            .getContextClassLoader();
149:                    if (contextClassLoader != null) {
150:
151:                        Object value = valueByClassLoader
152:                                .get(contextClassLoader);
153:                        if ((value == null)
154:                                && !valueByClassLoader
155:                                        .containsKey(contextClassLoader)) {
156:                            value = initialValue();
157:                            valueByClassLoader.put(contextClassLoader, value);
158:                        }
159:                        return value;
160:
161:                    }
162:
163:                } catch (SecurityException e) { /* SWALLOW - should we log this? */
164:                }
165:
166:                // if none or exception, return the globalValue 
167:                if (!globalValueInitialized) {
168:                    globalValue = initialValue();
169:                    globalValueInitialized = true;
170:                }//else already set
171:                return globalValue;
172:            }
173:
174:            /** 
175:             * Sets the value - a value is provided per (thread) context classloader.
176:             * This mechanism provides isolation for web apps deployed in the same container. 
177:             * 
178:             * @param value the object to be associated with the entrant thread's context classloader
179:             */
180:            public synchronized void set(Object value) {
181:                // synchronizing the whole method is a bit slower 
182:                // but guarentees no subtle threading problems
183:
184:                // make sure that the map is given a change to purge itself
185:                valueByClassLoader.isEmpty();
186:                try {
187:
188:                    ClassLoader contextClassLoader = Thread.currentThread()
189:                            .getContextClassLoader();
190:                    if (contextClassLoader != null) {
191:                        valueByClassLoader.put(contextClassLoader, value);
192:                        return;
193:                    }
194:
195:                } catch (SecurityException e) { /* SWALLOW - should we log this? */
196:                }
197:
198:                // if in doubt, set the global value
199:                globalValue = value;
200:                globalValueInitialized = true;
201:            }
202:
203:            /** 
204:             * Unsets the value associated with the current thread's context classloader
205:             */
206:            public synchronized void unset() {
207:                try {
208:
209:                    ClassLoader contextClassLoader = Thread.currentThread()
210:                            .getContextClassLoader();
211:                    unset(contextClassLoader);
212:
213:                } catch (SecurityException e) { /* SWALLOW - should we log this? */
214:                }
215:            }
216:
217:            /** 
218:             * Unsets the value associated with the given classloader
219:             * @param classLoader The classloader to <i>unset</i> for
220:             */
221:            public synchronized void unset(ClassLoader classLoader) {
222:                valueByClassLoader.remove(classLoader);
223:            }
224:        }
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