/*
* Copyright 2002-2010 the original author or authors.
*
* Licensed 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.springframework.util;
import java.lang.ref.Reference;
import java.lang.ref.ReferenceQueue;
import java.lang.ref.WeakReference;
import java.util.HashMap;
import java.util.Map;
import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;
/**
* Track references to arbitrary objects using proxy and weak references. To
* monitor a handle, one should call {@link #monitor(Object, ReleaseListener)},
* with the given handle object usually being a holder that uses the target
* object underneath, and the release listener performing cleanup of the
* target object once the handle is not strongly referenced anymore.
*
* <p>When a given handle becomes weakly reachable, the specified listener
* will be called by a background thread. This thread will only be started
* lazily and will be stopped once no handles are registered for monitoring
* anymore, to be restarted if further handles are added.
*
* <p>Thanks to Tomasz Wysocki for the suggestion and the original
* implementation of this class!
*
* @author Colin Sampaleanu
* @author Juergen Hoeller
* @since 1.2
* @see #monitor
*/
public class WeakReferenceMonitor {
private static final Log logger = LogFactory.getLog(WeakReferenceMonitor.class);
// Queue receiving reachability events
private static final ReferenceQueue<Object> handleQueue = new ReferenceQueue<Object>();
// All tracked entries (WeakReference => ReleaseListener)
private static final Map<Reference, ReleaseListener> trackedEntries = new HashMap<Reference, ReleaseListener>();
// Thread polling handleQueue, lazy initialized
private static Thread monitoringThread = null;
/**
* Start to monitor given handle object for becoming weakly reachable.
* When the handle isn't used anymore, the given listener will be called.
* @param handle the object that will be monitored
* @param listener the listener that will be called upon release of the handle
*/
public static void monitor(Object handle, ReleaseListener listener) {
if (logger.isDebugEnabled()) {
logger.debug("Monitoring handle [" + handle + "] with release listener [" + listener + "]");
}
// Make weak reference to this handle, so we can say when
// handle is not used any more by polling on handleQueue.
WeakReference<Object> weakRef = new WeakReference<Object>(handle, handleQueue);
// Add monitored entry to internal map of all monitored entries.
addEntry(weakRef, listener);
}
/**
* Add entry to internal map of tracked entries.
* Internal monitoring thread is started if not already running.
* @param ref reference to tracked handle
* @param entry the associated entry
*/
private static void addEntry(Reference ref, ReleaseListener entry) {
synchronized (WeakReferenceMonitor.class) {
// Add entry, the key is given reference.
trackedEntries.put(ref, entry);
// Start monitoring thread lazily.
if (monitoringThread == null) {
monitoringThread = new Thread(new MonitoringProcess(), WeakReferenceMonitor.class.getName());
monitoringThread.setDaemon(true);
monitoringThread.start();
}
}
}
/**
* Remove entry from internal map of tracked entries.
* @param reference the reference that should be removed
* @return entry object associated with given reference
*/
private static ReleaseListener removeEntry(Reference reference) {
synchronized (WeakReferenceMonitor.class) {
return trackedEntries.remove(reference);
}
}
/**
* Check whether to keep the monitoring thread alive,
* i.e. whether there are still entries being tracked.
*/
private static boolean keepMonitoringThreadAlive() {
synchronized (WeakReferenceMonitor.class) {
if (!trackedEntries.isEmpty()) {
return true;
}
else {
logger.debug("No entries left to track - stopping reference monitor thread");
monitoringThread = null;
return false;
}
}
}
/**
* Thread implementation that performs the actual monitoring.
*/
private static class MonitoringProcess implements Runnable {
public void run() {
logger.debug("Starting reference monitor thread");
// Check if there are any tracked entries left.
while (keepMonitoringThreadAlive()) {
try {
Reference reference = handleQueue.remove();
// Stop tracking this reference.
ReleaseListener entry = removeEntry(reference);
if (entry != null) {
// Invoke listener callback.
try {
entry.released();
}
catch (Throwable ex) {
logger.warn("Reference release listener threw exception", ex);
}
}
}
catch (InterruptedException ex) {
synchronized (WeakReferenceMonitor.class) {
monitoringThread = null;
}
logger.debug("Reference monitor thread interrupted", ex);
break;
}
}
}
}
/**
* Listener that is notified when the handle is being released.
* To be implemented by users of this reference monitor.
*/
public static interface ReleaseListener {
/**
* This callback method is invoked once the associated handle has been released,
* i.e. once there are no monitored strong references to the handle anymore.
*/
void released();
}
}
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