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/*
Calimero 2 - A library for KNX network access
Copyright (c) 2006, 2011 B. Malinowsky
This program is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 2 of the License, or
(at your option) any later version.
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
along with this program; if not, write to the Free Software
Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
Linking this library statically or dynamically with other modules is
making a combined work based on this library. Thus, the terms and
conditions of the GNU General Public License cover the whole
combination.
As a special exception, the copyright holders of this library give you
permission to link this library with independent modules to produce an
executable, regardless of the license terms of these independent
modules, and to copy and distribute the resulting executable under terms
of your choice, provided that you also meet, for each linked independent
module, the terms and conditions of the license of that module. An
independent module is a module which is not derived from or based on
this library. If you modify this library, you may extend this exception
to your version of the library, but you are not obligated to do so. If
you do not wish to do so, delete this exception statement from your
version.
*/
package tuwien.auto.calimero.buffer.cache;
import java.util.HashMap;
import java.util.Iterator;
import java.util.LinkedHashMap;
import java.util.Map;
/**
* Implements a cache expiring mechanism for {@link CacheObject}s.
*
* A time span is specified for how long a value is considered valid. After that time for
* expiring, the cache object is removed on the next call of {@link #removeExpired()} by
* an internal {@link CacheSweeper}. If a cache with expiring cache objects is not used
* anymore, invoke {@link Cache#clear()} to quit the running cache sweeping mechanism.
* The timestamp of {@link CacheObject#getTimestamp()} is used to determine if a cache
* object value has expired.
* Note that if the timestamp of a {@link CacheObject} changes after it was put into the
* cache, the object has to be reinserted (see {@link #put(CacheObject)}) to keep the
* cache in a consistent state. If no expiring is used, this might be omitted.
*
* @author B. Malinowsky
*/
public abstract class ExpiringCache implements Cache
{
/**
* Default sweep interval in seconds used for a {@link CacheSweeper}
* ({@value defaultSweepInterval} seconds).
*
*/
protected static final int defaultSweepInterval = 60;
/**
* Sweep interval in seconds used for the {@link CacheSweeper}.
*
* It defaults to {@value #defaultSweepInterval} seconds. For a new value to take
* effect, it has to be assigned either before the first {@link #startSweeper()} call,
* or the cache sweeper has to be restarted.
*/
protected int sweepInterval = defaultSweepInterval;
/**
* The map holding the {@link CacheObject}s.
*
* The map instance itself is not synchronized, synchronization is done using the
* cache object (this).
*/
protected Map map;
private CacheSweeper sweeper;
private final int timeToExpire;
/**
* Creates an {@link ExpiringCache}.
*
* Note that for the actual begin of cache sweeping {@link #startSweeper()} has to be
* invoked.
*
* @param timeToExpire time > 0 in seconds for cache entries to stay valid, on time =
* 0 no expiring of cache entries will occur
*/
public ExpiringCache(final int timeToExpire)
{
if (timeToExpire > 0) {
this.timeToExpire = timeToExpire;
map = new LinkedHashMap();
}
else {
this.timeToExpire = 0;
map = new HashMap();
}
}
/**
* Removes all {@link CacheObject}s, where
* {@link CacheObject#getTimestamp()} + timeToExpire <= now, with
* timeToExpire > 0 and now is the point of time
* {@link #removeExpired()} is invoked.
* If no expiring time was specified at creation of cache, no cache object will be
* expired.
*/
public void removeExpired()
{
if (timeToExpire == 0 || !(map instanceof LinkedHashMap))
return;
final long now = System.currentTimeMillis();
final long duration = timeToExpire * 1000;
CacheObject o = null;
synchronized (this) {
for (final Iterator i = map.values().iterator(); i.hasNext();) {
o = (CacheObject) i.next();
if (now >= o.getTimestamp() + duration) {
i.remove();
notifyRemoved(o);
}
else
break;
}
}
}
/**
* Override this method to get notified when {@link #removeExpired()} removed a
* {@link CacheObject} from the {@link #map}.
*
*
* @param obj removed {@link CacheObject}
*/
protected void notifyRemoved(final CacheObject obj)
{}
/**
* Starts a new {@link CacheSweeper}, if not already running, and if an expiring time
* for {@link CacheObject} was specified.
* If the methods startSweeper and stopSweeper are invoked
* by different threads, they need to be synchronized.
*/
protected final void startSweeper()
{
if (timeToExpire > 0 && sweeper == null)
(sweeper = new CacheSweeper(this, sweepInterval)).start();
}
/**
* Stops the {@link CacheSweeper}, if any.
*/
protected final void stopSweeper()
{
final CacheSweeper s = sweeper;
if (s != null) {
s.stopSweeper();
sweeper = null;
}
}
static void updateAccess(final CacheObject obj)
{
obj.incCount();
obj.setUsage(obj.getCount());
}
}