Java tutorial
/* * 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.io; import java.io.File; import java.lang.ref.PhantomReference; import java.lang.ref.ReferenceQueue; import java.util.Collection; import java.util.Vector; /** * Keeps track of files awaiting deletion, and deletes them when an associated * marker object is reclaimed by the garbage collector. * <p> * This utility creates a background thread to handle file deletion. * Each file to be deleted is registered with a handler object. * When the handler object is garbage collected, the file is deleted. * <p> * In an environment with multiple class loaders (a servlet container, for * example), you should consider stopping the background thread if it is no * longer needed. This is done by invoking the method * {@link #exitWhenFinished}, typically in * {@link javax.servlet.ServletContextListener#contextDestroyed} or similar. * * @author Noel Bergman * @author Martin Cooper * @version $Id: FileCleaner.java 490987 2006-12-29 12:11:48Z scolebourne $ */ public class FileCleaningTracker { /** * Queue of <code>Tracker</code> instances being watched. */ ReferenceQueue /* Tracker */ q = new ReferenceQueue(); /** * Collection of <code>Tracker</code> instances in existence. */ final Collection /* Tracker */ trackers = new Vector(); // synchronized /** * Whether to terminate the thread when the tracking is complete. */ volatile boolean exitWhenFinished = false; /** * The thread that will clean up registered files. */ Thread reaper; //----------------------------------------------------------------------- /** * Track the specified file, using the provided marker, deleting the file * when the marker instance is garbage collected. * The {@link FileDeleteStrategy#NORMAL normal} deletion strategy will be used. * * @param file the file to be tracked, not null * @param marker the marker object used to track the file, not null * @throws NullPointerException if the file is null */ public void track(File file, Object marker) { track(file, marker, (FileDeleteStrategy) null); } /** * Track the specified file, using the provided marker, deleting the file * when the marker instance is garbage collected. * The speified deletion strategy is used. * * @param file the file to be tracked, not null * @param marker the marker object used to track the file, not null * @param deleteStrategy the strategy to delete the file, null means normal * @throws NullPointerException if the file is null */ public void track(File file, Object marker, FileDeleteStrategy deleteStrategy) { if (file == null) { throw new NullPointerException("The file must not be null"); } addTracker(file.getPath(), marker, deleteStrategy); } /** * Track the specified file, using the provided marker, deleting the file * when the marker instance is garbage collected. * The {@link FileDeleteStrategy#NORMAL normal} deletion strategy will be used. * * @param path the full path to the file to be tracked, not null * @param marker the marker object used to track the file, not null * @throws NullPointerException if the path is null */ public void track(String path, Object marker) { track(path, marker, (FileDeleteStrategy) null); } /** * Track the specified file, using the provided marker, deleting the file * when the marker instance is garbage collected. * The speified deletion strategy is used. * * @param path the full path to the file to be tracked, not null * @param marker the marker object used to track the file, not null * @param deleteStrategy the strategy to delete the file, null means normal * @throws NullPointerException if the path is null */ public void track(String path, Object marker, FileDeleteStrategy deleteStrategy) { if (path == null) { throw new NullPointerException("The path must not be null"); } addTracker(path, marker, deleteStrategy); } /** * Adds a tracker to the list of trackers. * * @param path the full path to the file to be tracked, not null * @param marker the marker object used to track the file, not null * @param deleteStrategy the strategy to delete the file, null means normal */ private synchronized void addTracker(String path, Object marker, FileDeleteStrategy deleteStrategy) { // synchronized block protects reaper if (exitWhenFinished) { throw new IllegalStateException("No new trackers can be added once exitWhenFinished() is called"); } if (reaper == null) { reaper = new Reaper(); reaper.start(); } trackers.add(new Tracker(path, deleteStrategy, marker, q)); } //----------------------------------------------------------------------- /** * Retrieve the number of files currently being tracked, and therefore * awaiting deletion. * * @return the number of files being tracked */ public int getTrackCount() { return trackers.size(); } /** * Call this method to cause the file cleaner thread to terminate when * there are no more objects being tracked for deletion. * <p> * In a simple environment, you don't need this method as the file cleaner * thread will simply exit when the JVM exits. In a more complex environment, * with multiple class loaders (such as an application server), you should be * aware that the file cleaner thread will continue running even if the class * loader it was started from terminates. This can consitute a memory leak. * <p> * For example, suppose that you have developed a web application, which * contains the commons-io jar file in your WEB-INF/lib directory. In other * words, the FileCleaner class is loaded through the class loader of your * web application. If the web application is terminated, but the servlet * container is still running, then the file cleaner thread will still exist, * posing a memory leak. * <p> * This method allows the thread to be terminated. Simply call this method * in the resource cleanup code, such as {@link javax.servlet.ServletContextListener#contextDestroyed}. * One called, no new objects can be tracked by the file cleaner. */ public synchronized void exitWhenFinished() { // synchronized block protects reaper exitWhenFinished = true; if (reaper != null) { synchronized (reaper) { reaper.interrupt(); } } } //----------------------------------------------------------------------- /** * The reaper thread. */ private final class Reaper extends Thread { /** Construct a new Reaper */ Reaper() { super("File Reaper"); setPriority(Thread.MAX_PRIORITY); setDaemon(true); } /** * Run the reaper thread that will delete files as their associated * marker objects are reclaimed by the garbage collector. */ public void run() { // thread exits when exitWhenFinished is true and there are no more tracked objects while (exitWhenFinished == false || trackers.size() > 0) { Tracker tracker = null; try { // Wait for a tracker to remove. tracker = (Tracker) q.remove(); } catch (Exception e) { continue; } if (tracker != null) { tracker.delete(); tracker.clear(); trackers.remove(tracker); } } } } //----------------------------------------------------------------------- /** * Inner class which acts as the reference for a file pending deletion. */ private static final class Tracker extends PhantomReference { /** * The full path to the file being tracked. */ private final String path; /** * The strategy for deleting files. */ private final FileDeleteStrategy deleteStrategy; /** * Constructs an instance of this class from the supplied parameters. * * @param path the full path to the file to be tracked, not null * @param deleteStrategy the strategy to delete the file, null means normal * @param marker the marker object used to track the file, not null * @param queue the queue on to which the tracker will be pushed, not null */ Tracker(String path, FileDeleteStrategy deleteStrategy, Object marker, ReferenceQueue queue) { super(marker, queue); this.path = path; this.deleteStrategy = (deleteStrategy == null ? FileDeleteStrategy.NORMAL : deleteStrategy); } /** * Deletes the file associated with this tracker instance. * * @return <code>true</code> if the file was deleted successfully; * <code>false</code> otherwise. */ public boolean delete() { return deleteStrategy.deleteQuietly(new File(path)); } } }