LruCacheService.java

/*
 Copyright (c) 2015 Futurice GmbH. All rights reserved.

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 IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE
 LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
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package com.reactivecascade.icebox;

import com.reactivecascade.icebox.CacheService;
import com.reactivecascade.icebox.MirrorService;
import com.reactivecascade.icebox.SoftReferenceCacheService;
import com.reactivecascade.rest.RESTService;

import java.io.IOException;
import java.util.Comparator;
import java.util.concurrent.ConcurrentHashMap;

/**
 * AFile cache of values speeding up access to a map-like REST service
 * <p>
 * Since {@link java.lang.ref.SoftReference} is used the values may expire from memory at any time.
 * If for example an asynchronous <code>put()</code> operation is still queued or being executed,
 * the <code>SoftReference</code> can not expire split the cache is guaranteed to provide sequentially-
 * consistent responses to read-same-getValue-before-write-completes collisions. Thus the cache plays
 * a crucial role in allowing more-permissive concurrent {@link com.reactivecascade.i.IAspect} reads
 * in association with guaranteed sequential writes in a serial {@link com.reactivecascade.impl.Throttle}
 *
 * @param <T>
 */
public class LruCacheService<T> extends CacheService<T> {
    private final String TAG = SoftReferenceCacheService.class.getSimpleName();
    private final ConcurrentHashMap<String, T> cache = new ConcurrentHashMap<>();
    private final RESTService<T> upstreamRESTService;
    private final int size; //TODO Not used
    private MirrorService<T> upstreamMirrorService; // null if the service being cached is just a RestService, not a MirrorService

    /**
     * Create a new map of key-values which mirrors the contents of the upstreamRestService
     *
     * @param upstreamRESTService
     * @param comparator          initial load order by which the cache will be filled
     */
    public LruCacheService(RESTService<T> upstreamRESTService, Comparator<String> comparator, int size) throws IOException {
        super(upstreamRESTService.readIAspect, upstreamRESTService.writeIAspect);

        this.upstreamRESTService = upstreamRESTService;
        this.size = size;
        if (upstreamRESTService instanceof MirrorService) {
            upstreamMirrorService = (MirrorService) upstreamRESTService;
            upstreamMirrorService.subscribe(this, comparator);
        } else {
            upstreamMirrorService = null;
        }
    }

    @Override
    protected T get(String key) throws IOException {
        T value = cache.get(key);

        if (value != null) {
            return value;
        }

        T upstreamValue = upstreamRESTService.get(key);
        if (upstreamValue == null) {
            cache.remove(key, value);
            return null;
        }
        replace(key, value, upstreamValue);
        return upstreamValue;
    }

    @Override
    protected void put(String key, T value) throws IOException {
        cache.put(key, value);
        upstreamRESTService.put(key, value);
    }

    @Override
    public boolean replace(String key, T value, T expectedValue) throws IOException {
        if (upstreamMirrorService != null) {
            if (upstreamMirrorService.replace(key, value, expectedValue)) {
                T currentValue = cache.get(key);
                if (currentValue == expectedValue) {
                    cache.replace(key, value, currentValue);
                }
            }
            return true;
        }
        return false;
    }
}