/*
* Copyright (C) 2009 eXo Platform SAS.
*
* This is free software; you can redistribute it and/or modify it
* under the terms of the GNU Lesser General Public License as
* published by the Free Software Foundation; either version 2.1 of
* the License, or (at your option) any later version.
*
* This software 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
* Lesser General Public License for more details.
*
* You should have received a copy of the GNU Lesser General Public
* License along with this software; if not, write to the Free
* Software Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA
* 02110-1301 USA, or see the FSF site: http://www.fsf.org.
*/
package org.exoplatform.commons.cache.future;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ConcurrentMap;
import java.util.concurrent.ExecutionException;
import org.gatein.common.logging.Logger;
import org.gatein.common.logging.LoggerFactory;
/**
* <p>
* A future cache that prevents the loading of the same resource twice. This should be used when the resource to load is very
* expensive or cannot be concurrently retrieved (like a classloading).
* </p>
*
* <p>
* The future cache should be used with the {@link #get(Object, Object)} method, that retrieves an object from the cache. When
* the object is not found, then the {@link Loader#retrieve(Object, Object)} method is used to retrieve the data and then this
* data is inserted in the cache.
* </p>
*
* <p>
* The class is abstract and does not implement a cache technology by itself, the cache implementation is delegated to the
* contractual methods {@link #get(Object)} and {@link #put(Object, Object)}. Those methods are intended to be used by the
* future cache only.
* </p>
*
* <p>
* The {@link Loader} interface provides a source to retrieve objects to put in the cache. The goal to maintain this interface
* is to decouple the cache from the object source.
* </p>
*
* @author <a href="mailto:julien.viet@exoplatform.com">Julien Viet</a>
* @version $Revision$
* @param <K> the key type parameter
* @param <V> the value type parameter
* @param <C> the context type parameter
*/
public abstract class FutureCache<K, V, C> {
/** . */
final Loader<K, V, C> loader;
/** . */
private final ConcurrentMap<K, Retrieval<K, V, C>> futureEntries;
/** . */
private final Logger log = LoggerFactory.getLogger(FutureCache.class);
public FutureCache(Loader<K, V, C> loader) {
this.loader = loader;
this.futureEntries = new ConcurrentHashMap<K, Retrieval<K, V, C>>();
}
/**
* Retrieves the cached value corresponding to the specified key from the cache, it must returns null when the key does not
* exist. This method is intended for internal use by the future cache only.
*
* @param key the key
* @return the cache value
*/
protected abstract V get(K key);
/**
* Updates the cache with a new key/value pair. This method is intended for internal use by the future cache only.
*
* @param key the key
* @param value the cache value
*/
protected abstract void put(K key, V value);
/**
* Perform a cache lookup for the specified key within the specified context. When the value cannot be loaded (because it
* does not exist or it failed or anything else that does not come to my mind), the value null is returned.
*
* @param context the context in which the resource is accessed
* @param key the key identifying the resource
* @return the value
*/
public final V get(final C context, final K key) {
// First we try a simple cache get
V value = get(key);
// If it does not succeed then we go through a process that will avoid to load
// the same resource concurrently
if (value == null) {
// Create our future
Retrieval<K, V, C> retrieval = new Retrieval<K, V, C>(context, key, this);
// This boolean means we inserted in the local
boolean inserted = true;
//
try {
Retrieval<K, V, C> phantom = futureEntries.putIfAbsent(key, retrieval);
// Use the value that could have been inserted by another thread
if (phantom != null) {
retrieval = phantom;
inserted = false;
} else {
try {
retrieval.current = Thread.currentThread();
retrieval.future.run();
} catch (Exception e) {
log.error("Retrieval of resource " + key + " threw an exception", e);
} finally {
retrieval.current = null;
}
}
// Returns the value
if (retrieval.current == Thread.currentThread()) {
throw new IllegalStateException("Reentrancy detected when obtaining key " + key + " with context "
+ context + " detected");
} else {
try {
value = retrieval.future.get();
} catch (ExecutionException e) {
log.error("Computing of resource " + key + " threw an exception", e.getCause());
} catch (InterruptedException e) {
// We should handle interruped exception in some manner
log.error("Retrieval of resource " + key + " threw an exception", e);
}
}
} finally {
// Clean up the per key map but only if our insertion succeeded and with our future
if (inserted) {
futureEntries.remove(key, retrieval);
}
}
}
//
return value;
}
}