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The Amazon Web Services SDK for Java provides Java APIs for building software on AWS' cost-effective, scalable, and reliable infrastructure products. The AWS Java SDK allows developers to code against APIs for all of Amazon's infrastructure web services (Amazon S3, Amazon EC2, Amazon SQS, Amazon Relational Database Service, Amazon AutoScaling, etc).

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/*
 * Copyright 2010-2014 Amazon.com, Inc. or its affiliates. All Rights Reserved.
 *
 * Licensed under the Apache License, Version 2.0 (the "License").
 * You may not use this file except in compliance with the License.
 * A copy of the License is located at
 *
 *  http://aws.amazon.com/apache2.0
 *
 * or in the "license" file accompanying this file. This file 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 com.amazonaws.util;

import java.util.LinkedHashMap;
import java.util.Map.Entry;

import com.amazonaws.ResponseMetadata;

/**
 * Cache of response metadata for recently executed requests for diagnostic
 * purposes. This cache has a max size and as entries are added, the oldest
 * entry is aged out once the max size has been reached.
 */
public class ResponseMetadataCache {
    private final InternalCache internalCache;


    /**
     * Creates a new cache that will contain, at most the specified number of
     * entries.
     *
     * @param maxEntries
     *            The maximum size of this cache.
     */
    public ResponseMetadataCache(int maxEntries) {
        internalCache = new InternalCache(maxEntries);
    }

    /**
     * Adds a new entry to this cache, possibly evicting the oldest entry if the
     * cache is at its size limit.
     *
     * @param obj
     *            The key by which to store the metadata.
     * @param metadata
     *            The metadata for this entry.
     */
    public synchronized void add(Object obj, ResponseMetadata metadata) {
        if (obj == null) return;
        internalCache.put(System.identityHashCode(obj), metadata);
    }

    /**
     * Returns the response metadata associated with the specified object, or
     * null if no metadata is associated with that object.
     *
     * @param obj
     *            The key by which the desired metadata is stored.
     *
     * @return The response metadata associated with the given object key,
     *         otherwise null if no metadata is associated with that object.
     */
    public ResponseMetadata get(Object obj) {
        // System.identityHashCode isn't guaranteed to be unique
        // on all platforms, but should be reasonable enough to use
        // for a few requests at a time.  We can always easily move
        // to our own unique IDs if needed.
        return internalCache.get(System.identityHashCode(obj));
    }


    /**
     * Simple implementation of LinkedHashMap that overrides the
     * removeEldestEntry method to turn LinkedHashMap into a
     * LRU(ish) cache that automatically evicts old entries.
     */
    private static final class InternalCache extends LinkedHashMap {
        private int maxSize;

        public InternalCache(int maxSize) {
            super(maxSize);
            this.maxSize = maxSize;
        }

        @Override
        protected boolean removeEldestEntry(Entry eldest) {
            return size() > maxSize;
        }
    }
}