modificationwatcher.java

Wicket一个开发Java Web应用程序框架。它使得开发web应用程序变得容易而轻松。 Wicket利用一个POJO data beans组件使得它可以与任何持久层技术相结合。

/*
 * 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.wicket.util.watch;

import java.util.ArrayList;
import java.util.Iterator;
import java.util.Map;
import java.util.Set;

import org.apache.wicket.util.concurrent.ConcurrentHashMap;
import org.apache.wicket.util.listener.ChangeListenerSet;
import org.apache.wicket.util.listener.IChangeListener;
import org.apache.wicket.util.thread.ICode;
import org.apache.wicket.util.thread.Task;
import org.apache.wicket.util.time.Duration;
import org.apache.wicket.util.time.Time;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;


/**
 * Monitors one or more <code>IModifiable</code> objects, calling a
 * {@link IChangeListener IChangeListener} when a given object's modification time changes.
 * 
 * @author Jonathan Locke
 * @since 1.2.6
 */
public final class ModificationWatcher
{
	/** logger */
	private static final Logger log = LoggerFactory.getLogger(ModificationWatcher.class);

	/** maps <code>IModifiable</code> objects to <code>Entry</code> objects */
	private final Map modifiableToEntry = new ConcurrentHashMap();

	/** the <code>Task</code> to run */
	private Task task;

	/**
	 * Container class for holding modifiable entries to watch.
	 */
	private static final class Entry
	{
		// The most recent lastModificationTime polled on the object
		Time lastModifiedTime;

		// The set of listeners to call when the modifiable changes
		final ChangeListenerSet listeners = new ChangeListenerSet();

		// The modifiable thing
		IModifiable modifiable;
	}

	/**
	 * Default constructor for two-phase construction.
	 */
	public ModificationWatcher()
	{
	}

	/**
	 * Constructor that accepts a <code>Duration</code> argument representing the poll frequency.
	 * 
	 * @param pollFrequency
	 *            how often to check on <code>IModifiable</code>s
	 */
	public ModificationWatcher(final Duration pollFrequency)
	{
		start(pollFrequency);
	}

	/**
	 * Adds an <code>IModifiable</code> object and an <code>IChangeListener</code> object to
	 * call when the modifiable object is modified.
	 * 
	 * @param modifiable
	 *            an <code>IModifiable</code> object to monitor
	 * @param listener
	 *            an <code>IChangeListener</code> to call if the <code>IModifiable</code> object
	 *            is modified
	 * @return <code>true</code> if the set did not already contain the specified element
	 */
	public final boolean add(final IModifiable modifiable, final IChangeListener listener)
	{
		// Look up entry for modifiable
		final Entry entry = (Entry)modifiableToEntry.get(modifiable);

		// Found it?
		if (entry == null)
		{
			if (modifiable.lastModifiedTime() != null)
			{
				// Construct new entry
				final Entry newEntry = new Entry();

				newEntry.modifiable = modifiable;
				newEntry.lastModifiedTime = modifiable.lastModifiedTime();
				newEntry.listeners.add(listener);

				// Put in map
				modifiableToEntry.put(modifiable, newEntry);
			}
			else
			{
				// The IModifiable is not returning a valid lastModifiedTime
				log.info("Cannot track modifications to resource " + modifiable);
			}

			return true;
		}
		else
		{
			// Add listener to existing entry
			return entry.listeners.add(listener);
		}
	}

	/**
	 * Removes all entries associated with an <code>IModifiable</code> object.
	 * 
	 * @param modifiable
	 *            an <code>IModifiable</code> object
	 * @return the <code>IModifiable</code> object that was removed, else <code>null</code>
	 */
	public IModifiable remove(final IModifiable modifiable)
	{
		final Entry entry = (Entry)modifiableToEntry.remove(modifiable);
		if (entry != null)
		{
			return entry.modifiable;
		}
		return null;
	}

	/**
	 * Starts watching at a given <code>Duration</code> polling rate.
	 * 
	 * @param pollFrequency
	 *            the polling rate <code>Duration</code>
	 */
	public void start(final Duration pollFrequency)
	{
		// Construct task with the given polling frequency
		task = new Task("ModificationWatcher");

		task.run(pollFrequency, new ICode()
		{
			public void run(final Logger log)
			{
				// Iterate over a copy of the list of entries to avoid
				// concurrent
				// modification problems without the associated liveness issues
				// of holding a lock while potentially polling file times!
				for (final Iterator iterator = new ArrayList(modifiableToEntry.values()).iterator(); iterator
					.hasNext();)
				{
					// Get next entry
					final Entry entry = (Entry)iterator.next();

					// If the modifiable has been modified after the last known
					// modification time
					final Time modifiableLastModified = entry.modifiable.lastModifiedTime();

					if (modifiableLastModified.after(entry.lastModifiedTime))
					{
						// Notify all listeners that the modifiable was modified
						entry.listeners.notifyListeners();

						// Update timestamp
						entry.lastModifiedTime = modifiableLastModified;
					}
				}
			}
		});
	}

	/**
	 * Stops this <code>ModificationWatcher</code>.
	 */
	public void destroy()
	{
		if (task != null)
		{
			// task.stop();
			task.interrupt();
		}
	}

	/**
	 * Retrieves a key set of all <code>IModifiable</code> objects currently being monitored.
	 * 
	 * @return a <code>Set</code> of all <code>IModifiable</code> entries currently maintained
	 */
	public final Set getEntries()
	{
		return modifiableToEntry.keySet();
	}
}