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
019 package org.apache.commons.logging.impl;
020
021 import java.lang.reflect.InvocationTargetException;
022 import java.lang.reflect.Method;
023
024 import javax.servlet.ServletContextEvent;
025 import javax.servlet.ServletContextListener;
026
027 import org.apache.commons.logging.LogFactory;
028
029
030 /**
031 * This class is capable of receiving notifications about the undeployment of
032 * a webapp, and responds by ensuring that commons-logging releases all
033 * memory associated with the undeployed webapp.
034 * <p>
035 * In general, the WeakHashtable support added in commons-logging release 1.1
036 * ensures that logging classes do not hold references that prevent an
037 * undeployed webapp's memory from being garbage-collected even when multiple
038 * copies of commons-logging are deployed via multiple classloaders (a
039 * situation that earlier versions had problems with). However there are
040 * some rare cases where the WeakHashtable approach does not work; in these
041 * situations specifying this class as a listener for the web application will
042 * ensure that all references held by commons-logging are fully released.
043 * <p>
044 * To use this class, configure the webapp deployment descriptor to call
045 * this class on webapp undeploy; the contextDestroyed method will tell
046 * every accessable LogFactory class that the entry in its map for the
047 * current webapp's context classloader should be cleared.
048 *
049 * @since 1.1
050 */
051
052 public class ServletContextCleaner implements ServletContextListener {
053
054 private Class[] RELEASE_SIGNATURE = {ClassLoader.class};
055
056 /**
057 * Invoked when a webapp is undeployed, this tells the LogFactory
058 * class to release any logging information related to the current
059 * contextClassloader.
060 */
061 public void contextDestroyed(ServletContextEvent sce) {
062 ClassLoader tccl = Thread.currentThread().getContextClassLoader();
063
064 Object[] params = new Object[1];
065 params[0] = tccl;
066
067 // Walk up the tree of classloaders, finding all the available
068 // LogFactory classes and releasing any objects associated with
069 // the tccl (ie the webapp).
070 //
071 // When there is only one LogFactory in the classpath, and it
072 // is within the webapp being undeployed then there is no problem;
073 // garbage collection works fine.
074 //
075 // When there are multiple LogFactory classes in the classpath but
076 // parent-first classloading is used everywhere, this loop is really
077 // short. The first instance of LogFactory found will
078 // be the highest in the classpath, and then no more will be found.
079 // This is ok, as with this setup this will be the only LogFactory
080 // holding any data associated with the tccl being released.
081 //
082 // When there are multiple LogFactory classes in the classpath and
083 // child-first classloading is used in any classloader, then multiple
084 // LogFactory instances may hold info about this TCCL; whenever the
085 // webapp makes a call into a class loaded via an ancestor classloader
086 // and that class calls LogFactory the tccl gets registered in
087 // the LogFactory instance that is visible from the ancestor
088 // classloader. However the concrete logging library it points
089 // to is expected to have been loaded via the TCCL, so the
090 // underlying logging lib is only initialised/configured once.
091 // These references from ancestor LogFactory classes down to
092 // TCCL classloaders are held via weak references and so should
093 // be released but there are circumstances where they may not.
094 // Walking up the classloader ancestry ladder releasing
095 // the current tccl at each level tree, though, will definitely
096 // clear any problem references.
097 ClassLoader loader = tccl;
098 while (loader != null) {
099 // Load via the current loader. Note that if the class is not accessable
100 // via this loader, but is accessable via some ancestor then that class
101 // will be returned.
102 try {
103 Class logFactoryClass = loader.loadClass("org.apache.commons.logging.LogFactory");
104 Method releaseMethod = logFactoryClass.getMethod("release", RELEASE_SIGNATURE);
105 releaseMethod.invoke(null, params);
106 loader = logFactoryClass.getClassLoader().getParent();
107 } catch(ClassNotFoundException ex) {
108 // Neither the current classloader nor any of its ancestors could find
109 // the LogFactory class, so we can stop now.
110 loader = null;
111 } catch(NoSuchMethodException ex) {
112 // This is not expected; every version of JCL has this method
113 System.err.println("LogFactory instance found which does not support release method!");
114 loader = null;
115 } catch(IllegalAccessException ex) {
116 // This is not expected; every ancestor class should be accessable
117 System.err.println("LogFactory instance found which is not accessable!");
118 loader = null;
119 } catch(InvocationTargetException ex) {
120 // This is not expected
121 System.err.println("LogFactory instance release method failed!");
122 loader = null;
123 }
124 }
125
126 // Just to be sure, invoke release on the LogFactory that is visible from
127 // this ServletContextCleaner class too. This should already have been caught
128 // by the above loop but just in case...
129 LogFactory.release(tccl);
130 }
131
132 /**
133 * Invoked when a webapp is deployed. Nothing needs to be done here.
134 */
135 public void contextInitialized(ServletContextEvent sce) {
136 // do nothing
137 }
138 }