Commons IO example source code file (FileCleaningTracker.java)
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The Commons IO FileCleaningTracker.java source code
/*
* 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.
* <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: FileCleaningTracker.java 1005707 2010-10-08 04:02:57Z niallp $
*/
public class FileCleaningTracker {
/**
* Queue of <code>Tracker instances being watched.
*/
ReferenceQueue<Object> q = new ReferenceQueue();
/**
* Collection of <code>Tracker instances in existence.
*/
final Collection<Tracker> trackers = Collections.synchronizedSet(new HashSet()); // synchronized
/**
* Collection of File paths that failed to delete.
*/
final List<String> 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(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();
}
/**
* Return the file paths that failed to delete.
*
* @return the file paths that failed to delete
* @since Commons IO 2.0
*/
public List<String> getDeleteFailures() {
return deleteFailures;
}
/**
* 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}.
* 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.
Tracker tracker = (Tracker) q.remove(); // cannot return null
trackers.remove(tracker);
if (!tracker.delete()) {
deleteFailures.add(tracker.getPath());
}
tracker.clear();
} catch (InterruptedException e) {
continue;
}
}
}
}
//-----------------------------------------------------------------------
/**
* Inner class which acts as the reference for a file pending deletion.
*/
private static final class Tracker extends PhantomReference<Object> {
/**
* 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<? super Object> 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));
}
}
}
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