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[utils] / support / src / org / wamblee / cache / CachedObject.java

diff --git a/support/src/org/wamblee/cache/CachedObject.java b/support/src/org/wamblee/cache/CachedObject.java
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+/*
+ * 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). 
+ */
+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; 
+    }
+}