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/*
* Copyright 2005-2010 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 org.wamblee.cache;
import java.io.Serializable;
import java.util.logging.Level;
import java.util.logging.Logger;
/**
* Represents a cached object identified by the key it has in a certain
* {@link Cache}. The object is either retrieved from the cache if the cache has
* it, or a call back is invoked to get the object (and put it in the cache).
*
* @author Erik Brakkee
*/
public class CachedObject {
private static final Logger LOGGER = Logger.getLogger(CachedObject.class.getName());
/**
* Cache to use.
*/
private Cache cache;
/**
* Key of the object in the cache.
*/
private KeyType objectKey;
/**
* Last known value. We only use this to return the last known value in case recomputation of the value fails.
*/
private ValueType value;
/**
* Are we now computing the value or not?
*/
private boolean computing;
/**
* Computation used to obtain the object if it is not found in the cache.
*/
private Computation computation;
/**
* Constructs the cached object.
*
* @param aCache Cache to use.
* @param aObjectKey Key of the object in the cache.
* @param aComputation Computation to get the object in case the object is not in the
* cache.
*/
public CachedObject(Cache aCache, KeyType aObjectKey,
Computation aComputation) {
cache = aCache;
objectKey = aObjectKey;
computation = aComputation;
}
/**
* Gets the object. Since the object is cached, different calls to this
* method may return different objects.
*
* If the object is expired from the cache it is recomputed using the
* callback. In case the callback throws an exception the last known value
* is used. In case an exception is thrown, the problem is also logged. In
* case a recomputation is already being done by another thread, the last
* known value is immediately returned.
*
* @return Object.
*/
public ValueType get() {
synchronized (this) {
if (computing) {
// always return old value while computing.
return value;
}
ValueType cachedValue = cache.get(objectKey);
if (cachedValue == null) {
// expired
computing = true;
} else {
// Two different instances of cached object might share the same cache and so it can occur
// that the value in one of the instances it out of date.
value = cachedValue;
return value;
}
}
try {
// we only get here if we are computing
// do the computation without holding the lock.
LOGGER.fine("Refreshing cache for '" + objectKey + "'");
ValueType object = computation.getObject(objectKey);
cache.put(objectKey, object);
synchronized (this) {
value = object;
}
}
catch (Exception e) {
LOGGER.log(Level.INFO, "Recomputation of cached item failed for key '" + objectKey +
"'", e);
} finally {
synchronized (this) {
computing = false;
}
}
synchronized (this) {
return value;
}
}
/**
* Invalidates the cache for the object so that it is recomputed the next
* time it is requested.
*/
public void invalidate() {
cache.remove(objectKey);
}
/**
* Gets the cache.
*
* @return Cache.
*/
public Cache getCache() {
return cache;
}
/**
* Callback invoked to compute an object if it was not found in the cache.
*
* @param Type of the object
*/
public static interface Computation {
/**
* Gets the object. Called when the object is not in the cache. In case
* computation fails, an exception should be thrown to ensure that the
* last known value will be used.
*
* @param aObjectKey Id of the object in the cache.
* @return Object, must be non-null.
*/
Value getObject(Key aObjectKey) throws Exception;
}
}