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 */
017package org.apache.commons.io;
018
019import java.io.File;
020import java.lang.ref.PhantomReference;
021import java.lang.ref.ReferenceQueue;
022import java.util.ArrayList;
023import java.util.Collection;
024import java.util.Collections;
025import java.util.HashSet;
026import java.util.List;
027import java.util.Objects;
028
029/**
030 * Keeps track of files awaiting deletion, and deletes them when an associated
031 * marker object is reclaimed by the garbage collector.
032 * <p>
033 * This utility creates a background thread to handle file deletion.
034 * Each file to be deleted is registered with a handler object.
035 * When the handler object is garbage collected, the file is deleted.
036 * <p>
037 * In an environment with multiple class loaders (a servlet container, for
038 * example), you should consider stopping the background thread if it is no
039 * longer needed. This is done by invoking the method
040 * {@link #exitWhenFinished}, typically in
041 * {@code javax.servlet.ServletContextListener.contextDestroyed(javax.servlet.ServletContextEvent)} or similar.
042 *
043 */
044public class FileCleaningTracker {
045
046    // Note: fields are package protected to allow use by test cases
047
048    /**
049     * Queue of {@code Tracker} instances being watched.
050     */
051    ReferenceQueue<Object> q = new ReferenceQueue<>();
052    /**
053     * Collection of {@code Tracker} instances in existence.
054     */
055    final Collection<Tracker> trackers = Collections.synchronizedSet(new HashSet<>()); // synchronized
056    /**
057     * Collection of File paths that failed to delete.
058     */
059    final List<String> deleteFailures = Collections.synchronizedList(new ArrayList<>());
060    /**
061     * Whether to terminate the thread when the tracking is complete.
062     */
063    volatile boolean exitWhenFinished;
064    /**
065     * The thread that will clean up registered files.
066     */
067    Thread reaper;
068
069    /**
070     * Track the specified file, using the provided marker, deleting the file
071     * when the marker instance is garbage collected.
072     * The {@link FileDeleteStrategy#NORMAL normal} deletion strategy will be used.
073     *
074     * @param file  the file to be tracked, not null
075     * @param marker  the marker object used to track the file, not null
076     * @throws NullPointerException if the file is null
077     */
078    public void track(final File file, final Object marker) {
079        track(file, marker, null);
080    }
081
082    /**
083     * Track the specified file, using the provided marker, deleting the file
084     * when the marker instance is garbage collected.
085     * The specified deletion strategy is used.
086     *
087     * @param file  the file to be tracked, not null
088     * @param marker  the marker object used to track the file, not null
089     * @param deleteStrategy  the strategy to delete the file, null means normal
090     * @throws NullPointerException if the file is null
091     */
092    public void track(final File file, final Object marker, final FileDeleteStrategy deleteStrategy) {
093        Objects.requireNonNull(file, "file");
094        addTracker(file.getPath(), marker, deleteStrategy);
095    }
096
097    /**
098     * Track the specified file, using the provided marker, deleting the file
099     * when the marker instance is garbage collected.
100     * The {@link FileDeleteStrategy#NORMAL normal} deletion strategy will be used.
101     *
102     * @param path  the full path to the file to be tracked, not null
103     * @param marker  the marker object used to track the file, not null
104     * @throws NullPointerException if the path is null
105     */
106    public void track(final String path, final Object marker) {
107        track(path, marker, null);
108    }
109
110    /**
111     * Track the specified file, using the provided marker, deleting the file
112     * when the marker instance is garbage collected.
113     * The specified deletion strategy is used.
114     *
115     * @param path  the full path to the file to be tracked, not null
116     * @param marker  the marker object used to track the file, not null
117     * @param deleteStrategy  the strategy to delete the file, null means normal
118     * @throws NullPointerException if the path is null
119     */
120    public void track(final String path, final Object marker, final FileDeleteStrategy deleteStrategy) {
121        Objects.requireNonNull(path, "path");
122        addTracker(path, marker, deleteStrategy);
123    }
124
125    /**
126     * Adds a tracker to the list of trackers.
127     *
128     * @param path  the full path to the file to be tracked, not null
129     * @param marker  the marker object used to track the file, not null
130     * @param deleteStrategy  the strategy to delete the file, null means normal
131     */
132    private synchronized void addTracker(final String path, final Object marker, final FileDeleteStrategy
133            deleteStrategy) {
134        // synchronized block protects reaper
135        if (exitWhenFinished) {
136            throw new IllegalStateException("No new trackers can be added once exitWhenFinished() is called");
137        }
138        if (reaper == null) {
139            reaper = new Reaper();
140            reaper.start();
141        }
142        trackers.add(new Tracker(path, deleteStrategy, marker, q));
143    }
144
145    /**
146     * Retrieve the number of files currently being tracked, and therefore
147     * awaiting deletion.
148     *
149     * @return the number of files being tracked
150     */
151    public int getTrackCount() {
152        return trackers.size();
153    }
154
155    /**
156     * Return the file paths that failed to delete.
157     *
158     * @return the file paths that failed to delete
159     * @since 2.0
160     */
161    public List<String> getDeleteFailures() {
162        return deleteFailures;
163    }
164
165    /**
166     * Call this method to cause the file cleaner thread to terminate when
167     * there are no more objects being tracked for deletion.
168     * <p>
169     * In a simple environment, you don't need this method as the file cleaner
170     * thread will simply exit when the JVM exits. In a more complex environment,
171     * with multiple class loaders (such as an application server), you should be
172     * aware that the file cleaner thread will continue running even if the class
173     * loader it was started from terminates. This can constitute a memory leak.
174     * <p>
175     * For example, suppose that you have developed a web application, which
176     * contains the commons-io jar file in your WEB-INF/lib directory. In other
177     * words, the FileCleaner class is loaded through the class loader of your
178     * web application. If the web application is terminated, but the servlet
179     * container is still running, then the file cleaner thread will still exist,
180     * posing a memory leak.
181     * <p>
182     * This method allows the thread to be terminated. Simply call this method
183     * in the resource cleanup code, such as
184     * {@code javax.servlet.ServletContextListener.contextDestroyed(javax.servlet.ServletContextEvent)}.
185     * Once called, no new objects can be tracked by the file cleaner.
186     */
187    public synchronized void exitWhenFinished() {
188        // synchronized block protects reaper
189        exitWhenFinished = true;
190        if (reaper != null) {
191            synchronized (reaper) {
192                reaper.interrupt();
193            }
194        }
195    }
196
197    /**
198     * The reaper thread.
199     */
200    private final class Reaper extends Thread {
201        /** Construct a new Reaper */
202        Reaper() {
203            super("File Reaper");
204            setPriority(Thread.MAX_PRIORITY);
205            setDaemon(true);
206        }
207
208        /**
209         * Run the reaper thread that will delete files as their associated
210         * marker objects are reclaimed by the garbage collector.
211         */
212        @Override
213        public void run() {
214            // thread exits when exitWhenFinished is true and there are no more tracked objects
215            while (!exitWhenFinished || !trackers.isEmpty()) {
216                try {
217                    // Wait for a tracker to remove.
218                    final Tracker tracker = (Tracker) q.remove(); // cannot return null
219                    trackers.remove(tracker);
220                    if (!tracker.delete()) {
221                        deleteFailures.add(tracker.getPath());
222                    }
223                    tracker.clear();
224                } catch (final InterruptedException e) {
225                    continue;
226                }
227            }
228        }
229    }
230
231    /**
232     * Inner class which acts as the reference for a file pending deletion.
233     */
234    private static final class Tracker extends PhantomReference<Object> {
235
236        /**
237         * The full path to the file being tracked.
238         */
239        private final String path;
240        /**
241         * The strategy for deleting files.
242         */
243        private final FileDeleteStrategy deleteStrategy;
244
245        /**
246         * Constructs an instance of this class from the supplied parameters.
247         *
248         * @param path  the full path to the file to be tracked, not null
249         * @param deleteStrategy  the strategy to delete the file, null means normal
250         * @param marker  the marker object used to track the file, not null
251         * @param queue  the queue on to which the tracker will be pushed, not null
252         */
253        Tracker(final String path, final FileDeleteStrategy deleteStrategy, final Object marker,
254                final ReferenceQueue<? super Object> queue) {
255            super(marker, queue);
256            this.path = path;
257            this.deleteStrategy = deleteStrategy == null ? FileDeleteStrategy.NORMAL : deleteStrategy;
258        }
259
260        /**
261         * Return the path.
262         *
263         * @return the path
264         */
265        public String getPath() {
266            return path;
267        }
268
269        /**
270         * Deletes the file associated with this tracker instance.
271         *
272         * @return {@code true} if the file was deleted successfully;
273         *         {@code false} otherwise.
274         */
275        public boolean delete() {
276            return deleteStrategy.deleteQuietly(new File(path));
277        }
278    }
279
280}