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The AWS SDK for Java with support for OSGi. The AWS 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 2014-2016 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.internal;

import java.util.concurrent.locks.ReentrantReadWriteLock;
import java.util.concurrent.locks.ReentrantReadWriteLock.ReadLock;
import java.util.concurrent.locks.ReentrantReadWriteLock.WriteLock;

import com.amazonaws.annotation.ThreadSafe;

/**
 * A bounded cache that has a FIFO eviction policy when the cache is full.
 *
 * @param 
 *            value type
 */
@ThreadSafe
public final class FIFOCache {
    private final BoundedLinkedHashMap map;
    private final ReadLock rlock;
    private final WriteLock wlock;

    /**
     * @param maxSize
     *            the maximum number of entries of the cache
     */
    public FIFOCache(final int maxSize) {
        if (maxSize < 1) {
            throw new IllegalArgumentException("maxSize " + maxSize
                    + " must be at least 1");
        }
        map = new BoundedLinkedHashMap(maxSize);
        ReentrantReadWriteLock lock = new ReentrantReadWriteLock();
        rlock = lock.readLock();
        wlock = lock.writeLock();
    }

    /**
     * Adds an entry to the cache, evicting the earliest entry if necessary.
     */
    public T add(String key, T value) {
        wlock.lock();
        try {
            return map.put(key, value);
        } finally {
            wlock.unlock();
        }
    }

    /** Returns the value of the given key; or null of no such entry exists. */
    public T get(String key) {
        rlock.lock();
        try {
            return map.get(key);
        } finally {
            rlock.unlock();
        }
    }

    /**
     * Returns the current size of the cache.
     */
    public int size() {
        rlock.lock();
        try {
            return map.size();
        } finally {
            rlock.unlock();
        }
    }

    /**
     * Returns the maximum size of the cache.
     */
    public int getMaxSize() {
        return map.getMaxSize();
    }

    @Override
    public String toString() {
        rlock.lock();
        try {
            return map.toString();
        } finally {
            rlock.unlock();
        }
    }
}