LruCache详解

LruCache是什么

LruCache是Android中实现LRU(Least recently used，最近最少算法)的一个类，在android-support-v4中提供，在API12以下可以使用suppoort包中的该类。LruCache把最近使用过的对象储存在LinkedHashMap中，并在缓存达到设定缓存最大值之前将最近最少使用的对象从内存中移除。它的核心思想是“如果数据最近被访问过，那么接下来被访问的几率会更高”。

LruCache的使用

LruCache在创建时要指定最大的缓存容量，默认缓存容量的值代表缓存对象的数量，可以通过重写sizeOf计算每个对象的大小。使用get、put、remove进行操作

LruCache的实现

LruCache内部采用一个LinkedHashMap来保存缓存对象，LinkedHashMap的实现见我的另一篇博客 ，不同于LinkedHashMap，LruCache是线程安全的，以满足并发写入缓存的情况，详细实现如下：

/**
 * 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.
 * Cache保存一个强引用来限制内容数量，每当Item被访问的时候，此Item就会移动到队列的头部。
 * 当cache已满的时候加入新的item时，在队列尾部的item会被回收。
 * 

 * If your cached values hold resources that need to be explicitly released,
 * override {@link #entryRemoved}. 如果cache的某个值需要明确释放，重写entryRemoved()
 * 

 * 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.
 * 如果key相对应的item丢失，重写create()初始化丢失的对象，即使丢失了也总会返回。
 * 

 * 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: 默认cache大小是测量的item的数量，重写sizeOf计算不同item的 大小。
 * 
 * 
 * {@code
 * 这里的示例代码即可实现对图片的总大小进行限制，而不是对数量进行限制
 * int cacheSize = 4 * 1024 * 1024; // 4MiB
 * LruCache bitmapCache = new LruCache(cacheSize) {
 * protected int sizeOf(String key, Bitmap value) {
 * return value.getByteCount();
 * }
 * }}
 * 
 *
 * 

 * This class is thread-safe. Perform multiple cache operations atomically by
 * synchronizing on the cache:
 * 线程安全
 * 
 * {@code
 * synchronized (cache) {
 * if (cache.get(key) == null) {
 * cache.put(key, value);
 * }
 * }}
 * 
 *
 * 

 * 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. 不允许key或者value为null
 * 当get（），put（），remove（）返回值为null时，表示key相应的项不在cache中
 */
public class LruCache<K, V> {
    private final LinkedHashMap map;

    /** Size of this cache in units. Not necessarily the number of elements. */
    private int size; // 已经存储的大小
    private int maxSize; // 规定的最大存储空间

    private int putCount; // put的次数
    private int createCount; // create的次数
    private int evictionCount; // 回收的次数
    private int hitCount; // 命中的次数
    private int missCount; // 丢失的次数

    /**
     * @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;
        //创建了内部存储缓存的LinkedHashMap
        this.map = new LinkedHashMap(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. 通过key返回相应的item，或者使用create返回相应的item。相
     * 应的item会移动到队列的头部，
     * 如果item的value在cache中不存在或者create没有重写，则返回null。
     */
    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++; // 命中
                return mapValue;
            }
            missCount++; // 丢失
        }

        /*
         * 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. 如果丢失试图创建一个item，在create()创建时如果一个冲突的值被添加，那么不采用create()创建的值
         */

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

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

            if (mapValue != null) {
                // There was a conflict so undo that last put
                //在create执行的过程中添加了值，撤销put（）
                map.put(key, mapValue);
            } else {
                size += safeSizeOf(key, createdValue);
            }
        }

        if (mapValue != null) {
            entryRemoved(false, key, createdValue, mapValue);
            return mapValue;
        } else {
            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) { // 返回的先前的value值
                size -= safeSizeOf(key, previous);
            }
        }

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

        trimToSize(maxSize);
        return previous;
    }

     /**
     * Remove the eldest entries until the total of remaining entries is at or
     * below the requested size.
     *
     * @param maxSize the maximum size of the cache before returning. May be -1
     *            to evict even 0-sized elements.
     * 执行修剪操作，如果在添加对象后map中对象的容量大于等于maxSize，删除最近最少
     * 的元素
     */     
    private void trimToSize(int maxSize) {
        while (true) {
            K key;
            V value;
            synchronized (this) {
                if (size < 0 || (map.isEmpty() && size != 0)) {
                    throw new IllegalStateException(getClass().getName() + ".sizeOf() is reporting inconsistent results!");
                }

                if (size <= maxSize) {
                    break;
                }

                Map.Entry toEvict =  map.eldest();
                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. 
     * 删除key相应的cache项，返回相应的value
     * 
     * @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.
     * 当item被回收或者删掉时调用。该方法当value被回收释放存储空间时、remove调
     * 用和替换item值put时调用
     * 

     * 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}.
     *            true---为释放空间被删除；false---put或remove导致
     * @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. 当某Item丢失时会调用到，返回计算的相应
     *     的value或者null
     * 

     * The method is called without synchronization: other threads may access
     * the cache while this method is executing.
     *
     * 

     * 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.
     * 返回用户定义的item的大小，默认返回1代表item的数量，最大size就是最大item值
     * 

     * 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.
     * 清空cache
     */
    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. 返回被回收的数量
     */
    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. 返回当前cache的副本，从最近最少访问到最多访问
     */
    public synchronized final Map snapshot() {
        return new LinkedHashMap(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);
    }
}