LruCache 缓存

内存缓存（LruCache）：
系统提供的LruCache类是非常适合用作缓存Bitmap任务的，
它将最近被引用到的对象存储在一个强引用的LinkedHashMap中，并且在缓存超过了指定大小之后将最近不常使用的对象释放掉。
注意：以前有一个非常流行的内存缓存实现是SoftReference(软引用)或者WeakReference(弱引用)的Bitmap缓存方案，
然而现在已经不推荐使用了。自Android2.3版本(API Level 9)开始，垃圾回收器更着重于对软/弱引用的回收，这使得上述的方案相当无效。
http://developer.android.com/reference/android/util/LruCache.html

/*
 * Copyright (C) 2011 The Android Open Source Project
 *
 * 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 android.util;

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

/**
 * 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。
 * 如果你cache的某个值需要明确释放，重写entryRemoved()
 * 如果key相对应的item丢掉啦，重写create().这简化了调用代码，即使丢失了也总会返回
 * 默认cache大小是测量的item的数量，重写sizeof计算不同item的
 *  大小。
 *   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(); 
 *       } 
 *   }}
 *   不允许key或者value为null
 *   当get（），put（），remove（）返回值为null时，key相应的项不在cache中
 */
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; //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;
        this.map = new LinkedHashMap<K, V>(0, 0.75f, true); // LinkedHashMap的初始化放在构造器中,
                                                            //这里将LinkedHashMap的accessOrder设置为true,为插入顺序，默认是访问顺序
    }

    /**
     * Sets the size of the cache.
     * @param maxSize The new maximum size.
     *
     * @hide
     */
    public void resize(int maxSize) {
        if (maxSize <= 0) {
            throw new IllegalArgumentException("maxSize <= 0");
        }

        synchronized (this) {
            this.maxSize = maxSize;
        }
        trimToSize(maxSize);
    }

    /**
     * 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，或者创建返回相应的item。相应的item会移动到队列的头部，
     * 如果item的value没有被cache或者不能被创建，则返回null。
     */
    public final V get(K key) {
        if (key == null) { //不允许空键  
            throw new NullPointerException("key == null");
        }

        V mapValue;
        synchronized (this) {
            mapValue = map.get(key);//调用LinkedHashMap的get方法  
            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.
         * 如果丢失了就试图创建一个item
         */

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

        synchronized (this) {
            createCount++; //如果创建成功，那么create次数加1  
            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;
        } 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) { //键值不允许为空  
            throw new NullPointerException("key == null || value == null");
        }

        V previous;
        synchronized (this) {
            putCount++;
            size += safeSizeOf(key, value);//统计放入的值的大小，然后增加size的记录值  
            previous = map.put(key, value);//把新值放入缓存map中，然后获得旧值  
            if (previous != null) { //旧值不为空,由于替换了旧值，所以需要把缓存数据总大小减去这个旧值的大小    
                size -= safeSizeOf(key, previous);
            }
        }
        //还要调用entryRemoved()方法来让子类去处理不用的旧值previous，可能按照自己的方式去释放它。  
        //当然了，子类也可以不实现这个方法。  
        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.
     把最少访问的老数据删除，直到总数据大小在上限的范围之内. 
     数据上限. 值可能是-1，这样就会删除所有的缓存数据.
     */
    public void trimToSize(int maxSize) {
        while (true) { //不断删除linkedHashMap头部entry，也就是最近最少访问的条目，直到size小于最大容量  
            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) { //直到容量小于最大容量为止  
                    break;
                }

                Map.Entry<K, V> toEvict = map.eldest(); //获取最少访问的entry
                if (toEvict == null) {
                    break;
                }

                key = toEvict.getKey();
                value = toEvict.getValue();
                map.remove(key);//删除最少访问的entry  
                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);//调用LinkedHashMap的remove方法  
            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>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>The method is called without synchronization: other threads may
     * access the cache while this method is executing.
     *
     * <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>An entry's size must not change while it is in the cache.
     * 这个方法用于计算每个条目的大小，子类必须得复写这个类。
     */
    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);
    }
}

总结：
1.LruCache封装了LinkedHashMap，提供了LRU缓存的功能；
2.LruCache通过trimToSize方法自动删除最近最少访问的键值对；
3.LruCache不允许空键值；
4.LruCache线程安全；
5.继承LruCache时，必须要复写sizeof方法，用于计算每个条目的大小。