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The Apache Commons IO library contains utility classes, stream implementations, file filters, file comparators, endian transformation classes, and much more.

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/*
 * 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.ArrayList;
import java.util.Collection;
import java.util.Collections;
import java.util.HashSet;
import java.util.List;

/**
 * Keeps track of files awaiting deletion, and deletes them when an associated
 * marker object is reclaimed by the garbage collector.
 * 

* 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. *

* 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 * {@code javax.servlet.ServletContextListener.contextDestroyed(javax.servlet.ServletContextEvent)} or similar. * */ public class FileCleaningTracker { // Note: fields are package protected to allow use by test cases /** * Queue of Tracker instances being watched. */ ReferenceQueue q = new ReferenceQueue<>(); /** * Collection of Tracker instances in existence. */ final Collection trackers = Collections.synchronizedSet(new HashSet()); // synchronized /** * Collection of File paths that failed to delete. */ final List deleteFailures = Collections.synchronizedList(new ArrayList()); /** * 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(final File file, final Object marker) { track(file, marker, null); } /** * Track the specified file, using the provided marker, deleting the file * when the marker instance is garbage collected. * The specified 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(final File file, final Object marker, final 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(final String path, final Object marker) { track(path, marker, null); } /** * Track the specified file, using the provided marker, deleting the file * when the marker instance is garbage collected. * The specified 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(final String path, final Object marker, final 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(final String path, final Object marker, final 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(); } /** * Return the file paths that failed to delete. * * @return the file paths that failed to delete * @since 2.0 */ public List getDeleteFailures() { return deleteFailures; } /** * Call this method to cause the file cleaner thread to terminate when * there are no more objects being tracked for deletion. *

* 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 constitute a memory leak. *

* 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. *

* This method allows the thread to be terminated. Simply call this method * in the resource cleanup code, such as * {@code javax.servlet.ServletContextListener.contextDestroyed(javax.servlet.ServletContextEvent)}. * Once 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. */ @Override public void run() { // thread exits when exitWhenFinished is true and there are no more tracked objects while (exitWhenFinished == false || trackers.size() > 0) { try { // Wait for a tracker to remove. final Tracker tracker = (Tracker) q.remove(); // cannot return null trackers.remove(tracker); if (!tracker.delete()) { deleteFailures.add(tracker.getPath()); } tracker.clear(); } catch (final InterruptedException e) { continue; } } } } //----------------------------------------------------------------------- /** * 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(final String path, final FileDeleteStrategy deleteStrategy, final Object marker, final ReferenceQueue queue) { super(marker, queue); this.path = path; this.deleteStrategy = deleteStrategy == null ? FileDeleteStrategy.NORMAL : deleteStrategy; } /** * Return the path. * * @return the path */ public String getPath() { return path; } /** * Deletes the file associated with this tracker instance. * * @return {@code true} if the file was deleted successfully; * {@code false} otherwise. */ public boolean delete() { return deleteStrategy.deleteQuietly(new File(path)); } } }