View Javadoc

1   /*
2    * Licensed to the Apache Software Foundation (ASF) under one or more
3    * contributor license agreements.  See the NOTICE file distributed with
4    * this work for additional information regarding copyright ownership.
5    * The ASF licenses this file to You under the Apache License, Version 2.0
6    * (the "License"); you may not use this file except in compliance with
7    * the License.  You may obtain a copy of the License at
8    * 
9    *      http://www.apache.org/licenses/LICENSE-2.0
10   * 
11   * Unless required by applicable law or agreed to in writing, software
12   * distributed under the License is distributed on an "AS IS" BASIS,
13   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
14   * See the License for the specific language governing permissions and
15   * limitations under the License.
16   */
17  package org.apache.commons.io;
18  
19  import java.io.File;
20  import java.lang.ref.PhantomReference;
21  import java.lang.ref.ReferenceQueue;
22  import java.util.Collection;
23  import java.util.Vector;
24  
25  /**
26   * Keeps track of files awaiting deletion, and deletes them when an associated
27   * marker object is reclaimed by the garbage collector.
28   * <p>
29   * This utility creates a background thread to handle file deletion.
30   * Each file to be deleted is registered with a handler object.
31   * When the handler object is garbage collected, the file is deleted.
32   * <p>
33   * In an environment with multiple class loaders (a servlet container, for
34   * example), you should consider stopping the background thread if it is no
35   * longer needed. This is done by invoking the method
36   * {@link #exitWhenFinished}, typically in
37   * {@link javax.servlet.ServletContextListener#contextDestroyed} or similar.
38   *
39   * @author Noel Bergman
40   * @author Martin Cooper
41   * @version $Id: FileCleaner.java 490987 2006-12-29 12:11:48Z scolebourne $
42   */
43  public class FileCleaningTracker {
44      /**
45       * Queue of <code>Tracker</code> instances being watched.
46       */
47      ReferenceQueue /* Tracker */ q = new ReferenceQueue();
48      /**
49       * Collection of <code>Tracker</code> instances in existence.
50       */
51      final Collection /* Tracker */ trackers = new Vector();  // synchronized
52      /**
53       * Whether to terminate the thread when the tracking is complete.
54       */
55      volatile boolean exitWhenFinished = false;
56      /**
57       * The thread that will clean up registered files.
58       */
59      Thread reaper;
60  
61      //-----------------------------------------------------------------------
62      /**
63       * Track the specified file, using the provided marker, deleting the file
64       * when the marker instance is garbage collected.
65       * The {@link FileDeleteStrategy#NORMAL normal} deletion strategy will be used.
66       *
67       * @param file  the file to be tracked, not null
68       * @param marker  the marker object used to track the file, not null
69       * @throws NullPointerException if the file is null
70       */
71      public void track(File file, Object marker) {
72          track(file, marker, (FileDeleteStrategy) null);
73      }
74  
75      /**
76       * Track the specified file, using the provided marker, deleting the file
77       * when the marker instance is garbage collected.
78       * The speified deletion strategy is used.
79       *
80       * @param file  the file to be tracked, not null
81       * @param marker  the marker object used to track the file, not null
82       * @param deleteStrategy  the strategy to delete the file, null means normal
83       * @throws NullPointerException if the file is null
84       */
85      public void track(File file, Object marker, FileDeleteStrategy deleteStrategy) {
86          if (file == null) {
87              throw new NullPointerException("The file must not be null");
88          }
89          addTracker(file.getPath(), marker, deleteStrategy);
90      }
91  
92      /**
93       * Track the specified file, using the provided marker, deleting the file
94       * when the marker instance is garbage collected.
95       * The {@link FileDeleteStrategy#NORMAL normal} deletion strategy will be used.
96       *
97       * @param path  the full path to the file to be tracked, not null
98       * @param marker  the marker object used to track the file, not null
99       * @throws NullPointerException if the path is null
100      */
101     public void track(String path, Object marker) {
102         track(path, marker, (FileDeleteStrategy) null);
103     }
104 
105     /**
106      * Track the specified file, using the provided marker, deleting the file
107      * when the marker instance is garbage collected.
108      * The speified deletion strategy is used.
109      *
110      * @param path  the full path to the file to be tracked, not null
111      * @param marker  the marker object used to track the file, not null
112      * @param deleteStrategy  the strategy to delete the file, null means normal
113      * @throws NullPointerException if the path is null
114      */
115     public void track(String path, Object marker, FileDeleteStrategy deleteStrategy) {
116         if (path == null) {
117             throw new NullPointerException("The path must not be null");
118         }
119         addTracker(path, marker, deleteStrategy);
120     }
121 
122     /**
123      * Adds a tracker to the list of trackers.
124      * 
125      * @param path  the full path to the file to be tracked, not null
126      * @param marker  the marker object used to track the file, not null
127      * @param deleteStrategy  the strategy to delete the file, null means normal
128      */
129     private synchronized void addTracker(String path, Object marker, FileDeleteStrategy deleteStrategy) {
130         // synchronized block protects reaper
131         if (exitWhenFinished) {
132             throw new IllegalStateException("No new trackers can be added once exitWhenFinished() is called");
133         }
134         if (reaper == null) {
135             reaper = new Reaper();
136             reaper.start();
137         }
138         trackers.add(new Tracker(path, deleteStrategy, marker, q));
139     }
140 
141     //-----------------------------------------------------------------------
142     /**
143      * Retrieve the number of files currently being tracked, and therefore
144      * awaiting deletion.
145      *
146      * @return the number of files being tracked
147      */
148     public int getTrackCount() {
149         return trackers.size();
150     }
151 
152     /**
153      * Call this method to cause the file cleaner thread to terminate when
154      * there are no more objects being tracked for deletion.
155      * <p>
156      * In a simple environment, you don't need this method as the file cleaner
157      * thread will simply exit when the JVM exits. In a more complex environment,
158      * with multiple class loaders (such as an application server), you should be
159      * aware that the file cleaner thread will continue running even if the class
160      * loader it was started from terminates. This can consitute a memory leak.
161      * <p>
162      * For example, suppose that you have developed a web application, which
163      * contains the commons-io jar file in your WEB-INF/lib directory. In other
164      * words, the FileCleaner class is loaded through the class loader of your
165      * web application. If the web application is terminated, but the servlet
166      * container is still running, then the file cleaner thread will still exist,
167      * posing a memory leak.
168      * <p>
169      * This method allows the thread to be terminated. Simply call this method
170      * in the resource cleanup code, such as {@link javax.servlet.ServletContextListener#contextDestroyed}.
171      * One called, no new objects can be tracked by the file cleaner.
172      */
173     public synchronized void exitWhenFinished() {
174         // synchronized block protects reaper
175         exitWhenFinished = true;
176         if (reaper != null) {
177             synchronized (reaper) {
178                 reaper.interrupt();
179             }
180         }
181     }
182 
183     //-----------------------------------------------------------------------
184     /**
185      * The reaper thread.
186      */
187     private final class Reaper extends Thread {
188         /** Construct a new Reaper */
189         Reaper() {
190             super("File Reaper");
191             setPriority(Thread.MAX_PRIORITY);
192             setDaemon(true);
193         }
194 
195         /**
196          * Run the reaper thread that will delete files as their associated
197          * marker objects are reclaimed by the garbage collector.
198          */
199         public void run() {
200             // thread exits when exitWhenFinished is true and there are no more tracked objects
201             while (exitWhenFinished == false || trackers.size() > 0) {
202                 Tracker tracker = null;
203                 try {
204                     // Wait for a tracker to remove.
205                     tracker = (Tracker) q.remove();
206                 } catch (Exception e) {
207                     continue;
208                 }
209                 if (tracker != null) {
210                     tracker.delete();
211                     tracker.clear();
212                     trackers.remove(tracker);
213                 }
214             }
215         }
216     }
217 
218     //-----------------------------------------------------------------------
219     /**
220      * Inner class which acts as the reference for a file pending deletion.
221      */
222     private static final class Tracker extends PhantomReference {
223 
224         /**
225          * The full path to the file being tracked.
226          */
227         private final String path;
228         /**
229          * The strategy for deleting files.
230          */
231         private final FileDeleteStrategy deleteStrategy;
232 
233         /**
234          * Constructs an instance of this class from the supplied parameters.
235          *
236          * @param path  the full path to the file to be tracked, not null
237          * @param deleteStrategy  the strategy to delete the file, null means normal
238          * @param marker  the marker object used to track the file, not null
239          * @param queue  the queue on to which the tracker will be pushed, not null
240          */
241         Tracker(String path, FileDeleteStrategy deleteStrategy, Object marker, ReferenceQueue queue) {
242             super(marker, queue);
243             this.path = path;
244             this.deleteStrategy = (deleteStrategy == null ? FileDeleteStrategy.NORMAL : deleteStrategy);
245         }
246 
247         /**
248          * Deletes the file associated with this tracker instance.
249          *
250          * @return <code>true</code> if the file was deleted successfully;
251          *         <code>false</code> otherwise.
252          */
253         public boolean delete() {
254             return deleteStrategy.deleteQuietly(new File(path));
255         }
256     }
257 
258 }