Hervian · December 13, 2016 21:04
diff --git a/PeriodicEventHelper.java b/PeriodicEventHelper.java
 package com.github.hervian.util;

 import java.lang.ref.WeakReference;
 import java.util.Hashtable;
 import java.util.Map;
 import java.util.concurrent.Executors;
 import java.util.concurrent.ScheduledFuture;
 import java.util.concurrent.ScheduledThreadPoolExecutor;
 import java.util.concurrent.ThreadFactory;
 import java.util.concurrent.TimeUnit;

 public enum PeriodicEventHelper {
 	INSTANCE;

 	private static final int CORE_THREAD_POOL_SIZE = 1;
 	private ScheduledThreadPoolExecutor daemonThreadBasedScheduler;
 	private final Map<WeakReference<PeriodicEventDefiner>, ScheduledFuture<?>> mapFromPeriodicEventsToScheduledFutures = new Hashtable<>();

 	/**
 	 * This service wraps the argument provided {@link PeriodicEventDefiner} in a
 	 * WeakReference, and starts a background job, as specified by the definer.
 	 * The job will be terminated/removed once the PeriodicDefiner has been
 	 * garbage collected. A typical job is one that refreshes a cache every X
 	 * minutes. <br>
 	 * Using the {@link PeriodicEventHelper} offers the following advantage over
 	 * manually creating a new Thread for each job:
 	 * <ul>
 	 * <li>In essence, we only have one Thread which handles our background jobs.
 	 * If need be (if the jobs are scheduled to run at the same time) new Threads
 	 * are created on the fly, and destroyed afterwards.
 	 * </ul>
 	 * 
 	 * @param periodicEventDefiner
 	 *          A bean that defines a task, and an interval. The task is used in a
 	 *          recurring job and the time between each execution is defined by
 	 *          the interval. The interval is described as a unit type and a
 	 *          number of units.
 	 */
 	public void register(PeriodicEventDefiner periodicEventDefiner) {
 		if (periodicEventDefiner != null && periodicEventDefiner.getPeriodEventInterval() != null) {
 			WeakReference<PeriodicEventDefiner> weakReferenceToPeriodicEvent = new WeakReference<PeriodicEventDefiner>(
 					periodicEventDefiner);
 			ScheduledFuture<?> scheduledFuture = createAndScheduleRunnable(periodicEventDefiner,
 					weakReferenceToPeriodicEvent);
 			mapFromPeriodicEventsToScheduledFutures.put(weakReferenceToPeriodicEvent, scheduledFuture);
 		}
 	}

 	private ScheduledFuture<?> createAndScheduleRunnable(PeriodicEventDefiner periodicEventDefiner,
 			WeakReference<PeriodicEventDefiner> weakReferenceToPeriodicEvent) {
 		Runnable command = wrapEventInRunnable(weakReferenceToPeriodicEvent);
 		PeriodEventInterval interval = periodicEventDefiner.getPeriodEventInterval();
 		return scheduleRunnable(command, interval);
 	}

 	private ScheduledFuture<?> scheduleRunnable(Runnable command, PeriodEventInterval interval) {
 		ScheduledThreadPoolExecutor scheduler = getScheduler();
 		return scheduler.scheduleWithFixedDelay(command, interval.getInitialDelay(), interval.getDelay(),
 				interval.getTimeUnit());
 	}

 	private Runnable wrapEventInRunnable(final WeakReference<PeriodicEventDefiner> weakReferenceToPeriodicEvent) {
 		return new Runnable() {
 			@Override
 			public void run() {
 				PeriodicEventDefiner periodicEventDefiner = weakReferenceToPeriodicEvent.get();
 				if (periodicEventDefiner != null) {
 					periodicEventDefiner.executePeriodicEvent();
 				} else {
 					cancelAndRemovePeriodicEvent(weakReferenceToPeriodicEvent); //PeriodicEventDefiner has been garbage collected. Cancel thread and remove map entry.
 				}
 			}
 		};
 	}

 	private void cancelAndRemovePeriodicEvent(final WeakReference<PeriodicEventDefiner> weakReferenceToPeriodicEvent) {
 		ScheduledFuture<?> scheduledFuture = mapFromPeriodicEventsToScheduledFutures.get(weakReferenceToPeriodicEvent);
 		if (scheduledFuture != null) {
 			scheduledFuture.cancel(true);
 			mapFromPeriodicEventsToScheduledFutures.remove(weakReferenceToPeriodicEvent);
 		}
 	}

 	private ScheduledThreadPoolExecutor getScheduler() {
 		if (daemonThreadBasedScheduler == null) {
 			daemonThreadBasedScheduler = new ScheduledThreadPoolExecutor(CORE_THREAD_POOL_SIZE, new ThreadFactory() {
 				@Override
 				public Thread newThread(Runnable runnable) {
 					Thread daemonThread = Executors.defaultThreadFactory().newThread(runnable);
 					daemonThread.setDaemon(true);
 					return daemonThread;
 				}
 			});
 			daemonThreadBasedScheduler.setRemoveOnCancelPolicy(true);
 		}
 		return daemonThreadBasedScheduler;
 	}

 	public static interface PeriodicEventDefiner {

 		void executePeriodicEvent();

 		PeriodEventInterval getPeriodEventInterval();

 	}

 	public static class PeriodEventInterval {

 		private long initialDelay;
 		private TimeUnit timeUnit;
 		private long delay;

 		public PeriodEventInterval(long initialDelay, TimeUnit timeUnit, long delay) {
 			this.initialDelay = initialDelay;
 			this.timeUnit = timeUnit;
 			this.delay = delay;
 		}

 		public long getInitialDelay() {
 			return initialDelay;
 		}

 		public TimeUnit getTimeUnit() {
 			return timeUnit;
 		}

 		public long getDelay() {
 			return delay;
 		}

 	}

 }
	package com.github.hervian.util;

	import java.lang.ref.WeakReference;
	import java.util.Hashtable;
	import java.util.Map;
	import java.util.concurrent.Executors;
	import java.util.concurrent.ScheduledFuture;
	import java.util.concurrent.ScheduledThreadPoolExecutor;
	import java.util.concurrent.ThreadFactory;
	import java.util.concurrent.TimeUnit;

	public enum PeriodicEventHelper {
	INSTANCE;

	private static final int CORE_THREAD_POOL_SIZE = 1;
	private ScheduledThreadPoolExecutor daemonThreadBasedScheduler;
	private final Map<WeakReference<PeriodicEventDefiner>, ScheduledFuture<?>> mapFromPeriodicEventsToScheduledFutures = new Hashtable<>();

	/**
	* This service wraps the argument provided {@link PeriodicEventDefiner} in a
	* WeakReference, and starts a background job, as specified by the definer.
	* The job will be terminated/removed once the PeriodicDefiner has been
	* garbage collected. A typical job is one that refreshes a cache every X
	* minutes. <br>
	* Using the {@link PeriodicEventHelper} offers the following advantage over
	* manually creating a new Thread for each job:
	* <ul>
	* <li>In essence, we only have one Thread which handles our background jobs.
	* If need be (if the jobs are scheduled to run at the same time) new Threads
	* are created on the fly, and destroyed afterwards.
	* </ul>
	*
	* @param periodicEventDefiner
	* A bean that defines a task, and an interval. The task is used in a
	* recurring job and the time between each execution is defined by
	* the interval. The interval is described as a unit type and a
	* number of units.
	*/
	public void register(PeriodicEventDefiner periodicEventDefiner) {
	if (periodicEventDefiner != null && periodicEventDefiner.getPeriodEventInterval() != null) {
	WeakReference<PeriodicEventDefiner> weakReferenceToPeriodicEvent = new WeakReference<PeriodicEventDefiner>(
	periodicEventDefiner);
	ScheduledFuture<?> scheduledFuture = createAndScheduleRunnable(periodicEventDefiner,
	weakReferenceToPeriodicEvent);
	mapFromPeriodicEventsToScheduledFutures.put(weakReferenceToPeriodicEvent, scheduledFuture);
	}
	}

	private ScheduledFuture<?> createAndScheduleRunnable(PeriodicEventDefiner periodicEventDefiner,
	WeakReference<PeriodicEventDefiner> weakReferenceToPeriodicEvent) {
	Runnable command = wrapEventInRunnable(weakReferenceToPeriodicEvent);
	PeriodEventInterval interval = periodicEventDefiner.getPeriodEventInterval();
	return scheduleRunnable(command, interval);
	}

	private ScheduledFuture<?> scheduleRunnable(Runnable command, PeriodEventInterval interval) {
	ScheduledThreadPoolExecutor scheduler = getScheduler();
	return scheduler.scheduleWithFixedDelay(command, interval.getInitialDelay(), interval.getDelay(),
	interval.getTimeUnit());
	}

	private Runnable wrapEventInRunnable(final WeakReference<PeriodicEventDefiner> weakReferenceToPeriodicEvent) {
	return new Runnable() {
	@Override
	public void run() {
	PeriodicEventDefiner periodicEventDefiner = weakReferenceToPeriodicEvent.get();
	if (periodicEventDefiner != null) {
	periodicEventDefiner.executePeriodicEvent();
	} else {
	cancelAndRemovePeriodicEvent(weakReferenceToPeriodicEvent); //PeriodicEventDefiner has been garbage collected. Cancel thread and remove map entry.
	}
	}
	};
	}

	private void cancelAndRemovePeriodicEvent(final WeakReference<PeriodicEventDefiner> weakReferenceToPeriodicEvent) {
	ScheduledFuture<?> scheduledFuture = mapFromPeriodicEventsToScheduledFutures.get(weakReferenceToPeriodicEvent);
	if (scheduledFuture != null) {
	scheduledFuture.cancel(true);
	mapFromPeriodicEventsToScheduledFutures.remove(weakReferenceToPeriodicEvent);
	}
	}

	private ScheduledThreadPoolExecutor getScheduler() {
	if (daemonThreadBasedScheduler == null) {
	daemonThreadBasedScheduler = new ScheduledThreadPoolExecutor(CORE_THREAD_POOL_SIZE, new ThreadFactory() {
	@Override
	public Thread newThread(Runnable runnable) {
	Thread daemonThread = Executors.defaultThreadFactory().newThread(runnable);
	daemonThread.setDaemon(true);
	return daemonThread;
	}
	});
	daemonThreadBasedScheduler.setRemoveOnCancelPolicy(true);
	}
	return daemonThreadBasedScheduler;
	}

	public static interface PeriodicEventDefiner {

	void executePeriodicEvent();

	PeriodEventInterval getPeriodEventInterval();

	}

	public static class PeriodEventInterval {

	private long initialDelay;
	private TimeUnit timeUnit;
	private long delay;

	public PeriodEventInterval(long initialDelay, TimeUnit timeUnit, long delay) {
	this.initialDelay = initialDelay;
	this.timeUnit = timeUnit;
	this.delay = delay;
	}

	public long getInitialDelay() {
	return initialDelay;
	}

	public TimeUnit getTimeUnit() {
	return timeUnit;
	}

	public long getDelay() {
	return delay;
	}

	}

	}