/*
- * 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.
* 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.
*/
package org.wamblee.cache;
-import org.apache.log4j.Logger;
-
import java.io.Serializable;
-
+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
- *
- * @param <KeyType> DOCUMENT ME!
- * @param <ValueType> DOCUMENT ME!
*/
public class CachedObject<KeyType extends Serializable, ValueType extends Serializable> {
- /**
- * DOCUMENT ME!
- */
- private static final Logger LOGGER = Logger.getLogger(CachedObject.class);
+ private static final Logger LOGGER = Logger.getLogger(CachedObject.class.getName());
/**
* Cache to use.
private KeyType objectKey;
/**
- * Computation used to obtain the object if it is not found in the
- * cache.
+ * 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<KeyType, ValueType> 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.
+ * @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<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.
+ * Gets the object. Since the object is cached, different calls to this
+ * method may return different objects.
+ * <p/>
+ * 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) {
- object = (ValueType) cache.get(objectKey);
+ synchronized (this) {
+ if (computing) {
+ // always return old value while computing.
+ return value;
+ }
- 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);
- }
+ 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 {
- return object;
+ // 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.
+ * Invalidates the cache for the object so that it is recomputed the next
+ * time it is requested.
*/
public void invalidate() {
cache.remove(objectKey);
return cache;
}
-/**
+ /**
* Callback invoked to compute an object if it was not found in the cache.
*
- * @param <T>
- * Type of the object
+ * @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.
+ * 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);
+ Value getObject(Key aObjectKey) throws Exception;
}
}
git://wamblee.org / utils / blobdiff