/*
 * 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; 
    }
}