/*
- * Copyright 2005 the original author or authors.
+ * 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.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
-
package org.wamblee.cache;
import java.io.Serializable;
-
-import org.apache.log4j.Logger;
+import java.util.logging.Level;
+import java.util.logging.Logger;
/**
- * Represents a cached object. 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).
- *
+ * 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<KeyType extends Serializable, ValueType extends Serializable> {
+ private static final Logger LOGGER = Logger.getLogger(CachedObject.class
+ .getName());
- private static final Logger LOGGER = Logger.getLogger(CachedObject.class);
+ /**
+ * Cache to use.
+ */
+ private Cache<KeyType, ValueType> cache;
/**
- * Callback invoked to compute an object if it was not found in the cache.
- *
- * @param <T>
- * Type of the object
+ * Key of the object in the cache.
*/
- public static interface Computation<Key extends Serializable, Value extends Serializable> {
- /**
- * Gets the object. Called when the object is not in the cache.
- *
- * @param aObjectKey
- * Id of the object in the cache.
- * @return Object, must be non-null.
- */
- Value getObject(Key aObjectKey);
- }
+ private KeyType objectKey;
/**
- * Cache to use.
+ * Last known value.
*/
- private Cache<KeyType, ValueType> _cache;
+ private ValueType value;
/**
- * Key of the object in the cache.
+ * Are we now computing the value or not?
*/
- private KeyType _objectKey;
+ private boolean computing;
/**
* Computation used to obtain the object if it is not found in the cache.
*/
- private Computation<KeyType, ValueType> _computation;
+ private Computation<KeyType, ValueType> computation;
/**
* Constructs the cached object.
* cache.
*/
public CachedObject(Cache<KeyType, ValueType> aCache, KeyType aObjectKey,
- Computation<KeyType, ValueType> aComputation) {
- _cache = aCache;
- _objectKey = aObjectKey;
- _computation = aComputation;
+ Computation<KeyType, ValueType> 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() {
- ValueType object = (ValueType) _cache.get(_objectKey); // the used
- // cache is
- // thread safe.
- if (object == null) {
- // synchronize the computation to make sure that the object is only
- // computed
- // once when multiple concurrent threads detect that the entry must
- // be
- // recomputed.
+ synchronized (this) {
+ if (computing) {
+ // always return old value while computing.
+ return value;
+ }
+
+ ValueType cachedValue = cache.get(objectKey);
+ if (cachedValue == null) {
+ // expired
+ computing = true;
+ } else {
+ // no change.
+ 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) {
- object = (ValueType) _cache.get(_objectKey);
- if (object == null) {
- // No other thread did a recomputation so we must do this
- // now.
- LOGGER.debug("Refreshing cache for '" + _objectKey + "'");
- object = _computation.getObject(_objectKey);
- _cache.put(_objectKey, object);
- }
+ computing = false;
}
}
- return object;
+ 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);
+ cache.remove(objectKey);
}
/**
* @return Cache.
*/
public Cache getCache() {
- return _cache;
+ return cache;
+ }
+
+ /**
+ * Callback invoked to compute an object if it was not found in the cache.
+ *
+ * @param <T>
+ * Type of the object
+ */
+ public static interface Computation<Key extends Serializable, Value extends Serializable> {
+ /**
+ * 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;
}
}
git://wamblee.org / utils / blobdiff