Java tutorial
/* * Copyright 2002-2012 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 com.dianping.resource.io.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, com.dianping.resource.io.util.WeakReferenceMonitor.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 { @Override 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(); } }