Android Open Source - image-loader Lru Cache

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Java Source Code

/*
 * Copyright (C) 2011 The Android Open Source Project
 *// w  w  w .  j av  a  2 s.c o  m
 * 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 com.novoda.imageloader.core.cache.util;

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

/**
 * A cache that holds strong references to a limited number of values. Each time a value is accessed, it is moved to the head of a queue. When a value is added
 * to a full cache, the value at the end of that queue is evicted and may become eligible for garbage collection.
 * <p/>
 * <p/>
 * If your cached values hold resources that need to be explicitly released, override {@link #entryRemoved}.
 * <p/>
 * <p/>
 * If a cache miss should be computed on demand for the corresponding keys, override {@link #create}. This simplifies the calling code, allowing it to assume a
 * value will always be returned, even when there's a cache miss.
 * <p/>
 * <p/>
 * By default, the cache size is measured in the number of entries. Override {@link #sizeOf} to size the cache in different units. For example, this cache is
 * limited to 4MiB of bitmaps:
 * <p/>
 * <pre>
 * {@code
 *   int cacheSize = 4 * 1024 * 1024; // 4MiB
 *   LruCache<String, Bitmap> bitmapCache = new LruCache<String, Bitmap>(cacheSize) {
 *       protected int sizeOf(String key, Bitmap value) {
 *           return value.getByteCount();
 *       }
 *   }}
 * </pre>
 * <p/>
 * <p/>
 * This class is thread-safe. Perform multiple cache operations atomically by synchronizing on the cache:
 * <p/>
 * <pre>
 * {@code
 *   synchronized (cache) {
 *     if (cache.get(key) == null) {
 *         cache.put(key, value);
 *     }
 *   }}
 * </pre>
 * <p/>
 * <p/>
 * This class does not allow null to be used as a key or value. A return value of null from {@link #get}, {@link #put} or {@link #remove} is unambiguous: the
 * key was not in the cache.
 */
public class LruCache<K, V> {
    private final LinkedHashMap<K, V> map;

    /**
     * Size of this cache in units. Not necessarily the number of elements.
     */
    private int size;
    private final int maxSize;

    private int putCount;
    private int createCount;
    private int evictionCount;
    private int hitCount;
    private int missCount;

    /**
     * @param maxSize for caches that do not override {@link #sizeOf}, this is the maximum number of entries in the cache. For all other caches, this is the maximum
     *                sum of the sizes of the entries in this cache.
     */
    public LruCache(int maxSize) {
        if (maxSize <= 0) {
            throw new IllegalArgumentException("maxSize <= 0");
        }
        this.maxSize = maxSize;
        this.map = new LinkedHashMap<K, V>(0, 0.75f, true);
    }

    /**
     * Returns the value for {@code key} if it exists in the cache or can be created by {@code #create}. If a value was returned, it is moved to the head of the
     * queue. This returns null if a value is not cached and cannot be created.
     */
    public final V get(K key) {
        if (key == null) {
            throw new NullPointerException("key == null");
        }
        V mapValue;
        synchronized (this) {
            mapValue = map.get(key);
            if (mapValue != null) {
                hitCount++;
                return mapValue;
            }
            missCount++;
        }

    /*
         * Attempt to create a value. This may take a long time, and the map may be different when create() returns. If a conflicting value was added to the map
     * while create() was working, we leave that value in the map and release the created value.
     */

        V createdValue = create(key);
        if (createdValue == null) {
            return null;
        }

        synchronized (this) {
            createCount++;
            mapValue = map.put(key, createdValue);

            if (mapValue != null) {
                // There was a conflict so undo that last put
                map.put(key, mapValue);
            } else {
                size += safeSizeOf(key, createdValue);
            }
        }

        if (mapValue != null) {
            entryRemoved(false, key, createdValue, mapValue);
            return mapValue;
        }
        trimToSize(maxSize);
        return createdValue;
    }

    /**
     * Caches {@code value} for {@code key}. The value is moved to the head of the queue.
     *
     * @return the previous value mapped by {@code key}.
     */
    public final V put(K key, V value) {
        if (key == null || value == null) {
            throw new NullPointerException("key == null || value == null");
        }
        V previous;
        synchronized (this) {
            putCount++;
            size += safeSizeOf(key, value);
            previous = map.put(key, value);
            if (previous != null) {
                size -= safeSizeOf(key, previous);
            }
        }

        if (previous != null) {
            entryRemoved(false, key, previous, value);
        }

        trimToSize(maxSize);
        return previous;
    }

    /**
     * @param maxSize the maximum size of the cache before returning. May be -1 to evict even 0-sized elements.
     */
    private void trimToSize(int maxSize) {
        while (true) {
            K key = null;
            V value = null;
            synchronized (this) {
                if (size < 0 || (map.isEmpty() && size != 0)) {
                    throw new IllegalStateException(getClass().getName() + ".sizeOf() is reporting inconsistent results!");
                }

                if (size <= maxSize) {
                    break;
                }
                // Change

                if (map.entrySet().iterator().hasNext()) {
                    Map.Entry<K, V> toEvict = map.entrySet().iterator().next();
                    if (toEvict == null) {
                        break;
                    }

                    key = toEvict.getKey();
                    value = toEvict.getValue();
                    map.remove(key);
                    size -= safeSizeOf(key, value);
                    evictionCount++;
                }
            }

            entryRemoved(true, key, value, null);
        }
    }

    /**
     * Removes the entry for {@code key} if it exists.
     *
     * @return the previous value mapped by {@code key}.
     */
    public final V remove(K key) {
        if (key == null) {
            throw new NullPointerException("key == null");
        }

        V previous;
        synchronized (this) {
            previous = map.remove(key);
            if (previous != null) {
                size -= safeSizeOf(key, previous);
            }
        }

        if (previous != null) {
            entryRemoved(false, key, previous, null);
        }

        return previous;
    }

    /**
     * Called for entries that have been evicted or removed. This method is invoked when a value is evicted to make space, removed by a call to {@link #remove},
     * or replaced by a call to {@link #put}. The default implementation does nothing.
     * <p/>
     * <p/>
     * The method is called without synchronization: other threads may access the cache while this method is executing.
     *
     * @param evicted  true if the entry is being removed to make space, false if the removal was caused by a {@link #put} or {@link #remove}.
     * @param newValue the new value for {@code key}, if it exists. If non-null, this removal was caused by a {@link #put}. Otherwise it was caused by an eviction or
     *                 a {@link #remove}.
     */
    protected void entryRemoved(boolean evicted, K key, V oldValue, V newValue) {
    }

    /**
     * Called after a cache miss to compute a value for the corresponding key. Returns the computed value or null if no value can be computed. The default
     * implementation returns null.
     * <p/>
     * <p/>
     * The method is called without synchronization: other threads may access the cache while this method is executing.
     * <p/>
     * <p/>
     * If a value for {@code key} exists in the cache when this method returns, the created value will be released with {@link #entryRemoved} and discarded.
     * This can occur when multiple threads request the same key at the same time (causing multiple values to be created), or when one thread calls {@link #put}
     * while another is creating a value for the same key.
     */
    protected V create(K key) {
        return null;
    }

    private int safeSizeOf(K key, V value) {
        int result = sizeOf(key, value);
        if (result < 0) {
            throw new IllegalStateException("Negative size: " + key + "=" + value);
        }
        return result;
    }

    /**
     * Returns the size of the entry for {@code key} and {@code value} in user-defined units. The default implementation returns 1 so that size is the number of
     * entries and max size is the maximum number of entries.
     * <p/>
     * <p/>
     * An entry's size must not change while it is in the cache.
     */
    protected int sizeOf(K key, V value) {
        return 1;
    }

    /**
     * Clear the cache, calling {@link #entryRemoved} on each removed entry.
     */
    public final void evictAll() {
        trimToSize(-1); // -1 will evict 0-sized elements
    }

    /**
     * For caches that do not override {@link #sizeOf}, this returns the number of entries in the cache. For all other caches, this returns the sum of the sizes
     * of the entries in this cache.
     */
    public synchronized final int size() {
        return size;
    }

    /**
     * For caches that do not override {@link #sizeOf}, this returns the maximum number of entries in the cache. For all other caches, this returns the maximum
     * sum of the sizes of the entries in this cache.
     */
    public synchronized final int maxSize() {
        return maxSize;
    }

    /**
     * Returns the number of times {@link #get} returned a value that was already present in the cache.
     */
    public synchronized final int hitCount() {
        return hitCount;
    }

    /**
     * Returns the number of times {@link #get} returned null or required a new value to be created.
     */
    public synchronized final int missCount() {
        return missCount;
    }

    /**
     * Returns the number of times {@link #create(Object)} returned a value.
     */
    public synchronized final int createCount() {
        return createCount;
    }

    /**
     * Returns the number of times {@link #put} was called.
     */
    public synchronized final int putCount() {
        return putCount;
    }

    /**
     * Returns the number of values that have been evicted.
     */
    public synchronized final int evictionCount() {
        return evictionCount;
    }

    /**
     * Returns a copy of the current contents of the cache, ordered from least recently accessed to most recently accessed.
     */
    public synchronized final Map<K, V> snapshot() {
        return new LinkedHashMap<K, V>(map);
    }

    @Override
    public synchronized final String toString() {
        int accesses = hitCount + missCount;
        int hitPercent = accesses != 0 ? (100 * hitCount / accesses) : 0;
        return String.format("LruCache[maxSize=%d,hits=%d,misses=%d,hitRate=%d%%]", maxSize, hitCount, missCount, hitPercent);
    }
}
Java Source Code List
