Android缓存策略LruCahe源码解析，Lru到底是怎么算的？

看前须知：

本篇文章是基于数据结构系列来写的，也是他们的后续，请至少观看过后两篇文章的读者，再来看这篇文章，推荐阅读顺序就是链接顺序，如果你不看的话，这篇文章对你毫无意义，因为只是粘贴了源码，标明了些注释，没错文章的标题就是为了无耻的导流而存在的，因为大部分人只想了解Lru算法或者LruCahe而进行搜索，而LruCache关键的核心就是使用了LinkedHashMap，你必须先知道LinkedHashMap是怎样的，而想了解LinkedHashMap你就需要了解他的父类Map和Collection接口下的LinkedList，所以请先观看相关文章。

//LinkedList源码解析，推荐阅读，因为和LinkedHashMap息息相关
http://blog.csdn.net/omyrobin/article/details/78602043

//HashMap源码解析
http://blog.csdn.net/omyrobin/article/details/78614460

//LinkedHashMap源码解析，以及怎么知道那个元素是最近最少使用的
http://blog.csdn.net/omyrobin/article/details/78624798

LruCache源码

public class LruCache<K, V> {
    private final LinkedHashMap map;

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

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

    /**
     *构造函数,设置容量大小
     */
    public LruCache(int maxSize) {
        if (maxSize <= 0) {
            throw new IllegalArgumentException("maxSize <= 0");
        }
        this.maxSize = maxSize;
        //创建了一个accessOrder为true的LinkedHashMap
        this.map = new LinkedHashMap(0, 0.75f, true);
    }

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

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

    /**
     *使用的还是map集合的get方法
     */
    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++;
        }

        /*
         *
         */

        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;
        }
    }

    /**
     *放入元素，调用的还是map的put方法
     */
    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);
        }
        //计算容量
        trimToSize(maxSize);
        return previous;
    }

    /**
     *容量的计算方法，通过该方法可以知道容量是否超出，如果超出需要进行删除（最近最少使用）
     */
    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;
                }
                //获取最近最少使用的元素（就是header.nxt的引用）
                Map.Entry toEvict = map.eldest();
                if (toEvict == null) {
                    break;
                }

                key = toEvict.getKey();
                value = toEvict.getValue();
                //删除最近最少使用的元素
                map.remove(key);
                //重新计算size
                size -= safeSizeOf(key, value);
                evictionCount++;
            }

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

    /**
     *
     */
    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;
    }

    /**
     *
     */
    protected void entryRemoved(boolean evicted, K key, V oldValue, V newValue) {}

    /**
     *
     */
    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;
    }

    /**
     *
     */
    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
    }

    /**
     *
     */
    public synchronized final int size() {
        return size;
    }

    /**
     *最大size
     */
    public synchronized final int maxSize() {
        return maxSize;
    }

    /**
     *
     */
    public synchronized final int hitCount() {
        return hitCount;
    }

    /**
     *
     */
    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 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);
    }
}