+/*
+ * Copyright 2005 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 org.apache.log4j.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).
+ *
+ * @author Erik Brakkee
+ */
+public class CachedObject<KeyType extends Serializable, ValueType extends Serializable> {
+
+ private static final Logger LOGGER = Logger.getLogger(CachedObject.class);
+
+ /**
+ * 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.
+ *
+ * @param aObjectKey
+ * Id of the object in the cache.
+ * @return Object, must be non-null.
+ */
+ Value getObject(Key aObjectKey);
+ }
+
+ /**
+ * Cache to use.
+ */
+ private Cache<KeyType, ValueType> _cache;
+
+ /**
+ * Key of the object in the cache.
+ */
+ private KeyType _objectKey;
+
+ /**
+ * 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.
+ */
+ public CachedObject(Cache<KeyType, ValueType> aCache, KeyType aObjectKey,
+ 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.
+ *
+ * @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);
+ 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);
+ }
+ }
+ }
+ return object;
+ }
+
+ /**
+ * 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;
+ }
+}