commons-io源码阅读心得

  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 }