/*
    * Licensed to the Apache Software Foundation (ASF) under one or more
    * contributor license agreements.  See the NOTICE file distributed with
    * this work for additional information regarding copyright ownership.
    * The ASF licenses this file to You under the Apache License, Version 2.0
    * (the "License"); you may not use this file except in compliance with
    * the License.  You may obtain a copy of the License at
    *
    *      http://www.apache.org/licenses/LICENSE-2.0
   *
   * Unless required by applicable law or agreed to in writing, software
   * distributed under the License is distributed on an "AS IS" BASIS,
   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   * See the License for the specific language governing permissions and
   * limitations under the License.
   */
  
  package org.apache.commons.logging.impl;
  
  import java.lang.reflect.InvocationTargetException;
  import java.lang.reflect.Method;
  
  import javax.servlet.ServletContextEvent;
  import javax.servlet.ServletContextListener;
  
  import org.apache.commons.logging.LogFactory;
  
  /**
   * This class is capable of receiving notifications about the undeployment of
   * a webapp, and responds by ensuring that commons-logging releases all
   * memory associated with the undeployed webapp.
   * <p>
   * In general, the WeakHashtable support added in commons-logging release 1.1
   * ensures that logging classes do not hold references that prevent an
   * undeployed webapp's memory from being garbage-collected even when multiple
   * copies of commons-logging are deployed via multiple class loaders (a
   * situation that earlier versions had problems with). However there are
   * some rare cases where the WeakHashtable approach does not work; in these
   * situations specifying this class as a listener for the web application will
   * ensure that all references held by commons-logging are fully released.
   * <p>
   * To use this class, configure the webapp deployment descriptor to call
   * this class on webapp undeploy; the contextDestroyed method will tell
   * every accessible LogFactory class that the entry in its map for the
   * current webapp's context class loader should be cleared.
   *
   * @since 1.1
   */
  public class ServletContextCleaner implements ServletContextListener {
  
      private static final Class<?>[] RELEASE_SIGNATURE = { ClassLoader.class };
  
      /**
       * Invoked when a webapp is undeployed, this tells the LogFactory
       * class to release any logging information related to the current
       * contextClassloader.
       */
      @Override
      public void contextDestroyed(final ServletContextEvent sce) {
          final ClassLoader tccl = Thread.currentThread().getContextClassLoader();
  
          final Object[] params = new Object[1];
          params[0] = tccl;
  
          // Walk up the tree of class loaders, finding all the available
          // LogFactory classes and releasing any objects associated with
          // the tccl (ie the webapp).
          //
          // When there is only one LogFactory in the classpath, and it
          // is within the webapp being undeployed then there is no problem;
          // garbage collection works fine.
          //
          // When there are multiple LogFactory classes in the classpath but
          // parent-first classloading is used everywhere, this loop is really
          // short. The first instance of LogFactory found will
          // be the highest in the classpath, and then no more will be found.
          // This is ok, as with this setup this will be the only LogFactory
          // holding any data associated with the tccl being released.
          //
          // When there are multiple LogFactory classes in the classpath and
          // child-first classloading is used in any class loader, then multiple
          // LogFactory instances may hold info about this TCCL; whenever the
          // webapp makes a call into a class loaded via an ancestor class loader
          // and that class calls LogFactory the tccl gets registered in
          // the LogFactory instance that is visible from the ancestor
          // class loader. However the concrete logging library it points
          // to is expected to have been loaded via the TCCL, so the
          // underlying logging lib is only initialized/configured once.
          // These references from ancestor LogFactory classes down to
          // TCCL class loaders are held via weak references and so should
          // be released but there are circumstances where they may not.
          // Walking up the class loader ancestry ladder releasing
          // the current tccl at each level tree, though, will definitely
          // clear any problem references.
          ClassLoader loader = tccl;
          while (loader != null) {
              // Load via the current loader. Note that if the class is not accessible
              // via this loader, but is accessible via some ancestor then that class
              // will be returned.
             try {
                 @SuppressWarnings("unchecked")
                 final Class<LogFactory> logFactoryClass = (Class<LogFactory>) loader.loadClass("org.apache.commons.logging.LogFactory");
                 final Method releaseMethod = logFactoryClass.getMethod("release", RELEASE_SIGNATURE);
                 releaseMethod.invoke(null, params);
                 loader = logFactoryClass.getClassLoader().getParent();
             } catch (final ClassNotFoundException ex) {
                 // Neither the current class loader nor any of its ancestors could find
                 // the LogFactory class, so we can stop now.
                 loader = null;
             } catch (final NoSuchMethodException ex) {
                 // This is not expected; every version of JCL has this method
                 System.err.println("LogFactory instance found which does not support release method!");
                 loader = null;
             } catch (final IllegalAccessException ex) {
                 // This is not expected; every ancestor class should be accessible
                 System.err.println("LogFactory instance found which is not accessible!");
                 loader = null;
             } catch (final InvocationTargetException ex) {
                 // This is not expected
                 System.err.println("LogFactory instance release method failed!");
                 loader = null;
             }
         }
 
         // Just to be sure, invoke release on the LogFactory that is visible from
         // this ServletContextCleaner class too. This should already have been caught
         // by the above loop but just in case...
         LogFactory.release(tccl);
     }
 
     /**
      * Invoked when a webapp is deployed. Nothing needs to be done here.
      */
     @Override
     public void contextInitialized(final ServletContextEvent sce) {
         // do nothing
     }
 }