/*
 * 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 org.apache.log4j.Logger;

import java.io.Serializable;


/**
 * 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
 *
 * @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);

    /**
     * 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;
    }

/**
     * 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);
    }
}