android LRUCache解析

LRU（Least Recently Used）最近最少使用算法

原理

缓存保存了一个强引用（Android 2.3开始，垃圾回收器更倾向于回收弱引用和软引用，软引用和弱引用变得不可靠，Android 3.0中，图片的数据会存储在本地的内存当中，因而无法用一种可预见的方式将其释放）限制值的数量. 每当值被访问的时候，它会被移动到队列的头部. 当缓存已满的时候加入新的值时，队列中最后的值会出队，可能被回收

LRUCache内部维护主要是通过LinkedHashMap实现

这是一个安全的线程，多线程缓存通过同步实现


使用

默认情况下，缓存的大小是由值的数量决定，重写sizeOf计算不同的值

如果你缓存值需要明确释放，重写entryRemoved()

int maxMemory = (int) Runtime.getRuntime().maxMemory();    
int mCacheSize = maxMemory / 8;  
//给LruCache分配1/8 4M 
mMemoryCache = new LruCache<String, Bitmap>(mCacheSize){  

  //必须重写此方法，来测量Bitmap的大小 
  @Override  
  protected int sizeOf(String key, Bitmap value) {  
          return value.getRowBytes() * value.getHeight();  
  }  

};

mMemoryCache.put(key, bitmap)

mMemoryCache.get(key)

这个类不允许有空的键值. get，put，remove 返回空值，key对应的值不在缓存中

源码分析

构造函数，初始化了最大容量和LinkedHashMap

 /** * @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);
    }

这里将LinkedHashMap最后一个参数（accessOrder）设置为true，将accessOrder设置为true时，可以使遍历顺序和访问顺序一致，其内部双向链表将会按照近期最少访问到近期最多访问的顺序排列Entry对象

put方法，首先不允许键值为空，然后是线程安全，put的次数加一，size增加，以键值对的形式存入LinkedHashMap，如果之前已经存在了这个键值对，size减少成原来的大小，如果容量超过maxsize，将会删除最近很少访问的entry

/**
     * 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;
    }

put方法有一个很关键的地方超过最大值是会删除最近最少访问的

trimToSize首先线程安全，检查当前大小是否大于最大值，如果大于最大值，从LinkedHashMap中去除最近最少（循环删除链表首部元素）被访问的元素，获得键值，删除

/** * 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. */
    public 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<K, V> 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);
        }
    }

get方法，首先key不能为空，线程安全，根据key，从LinkedHashMap中获得value，不为空的话返回，为空的话，创建一个key，创建失败返回null，创建成功，在LinkedHashMap中创建键值对，存在就覆盖，不存在size增加，返回value值

/**
     * 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;
        } else {
            trimToSize(maxSize);
            return createdValue;
        }
    }

核心代码分析完毕，想知道LinkedHashMap，请听下回哔哔

注：本文源码来自api 23