android中内存缓存是如何实现的

public class LruCache<K, V> {
    //map用来存储外界的缓存对象
    private final LinkedHashMap<K, V> map;

    // 构造函数
    public LruCache(int maxSize) {
        if (maxSize <= 0) {
            throw new IllegalArgumentException("maxSize <= 0");
        }
        this.maxSize = maxSize;
        //设置accessOrder为true，按照读取顺序来存储缓存
        this.map = new LinkedHashMap<K, V>(0, 0.75f, true);
    }

    //获取一个缓存对象
    public final V get(K key) {
        if (key == null) {
            throw new NullPointerException("key == null");
        }

        V mapValue;
        synchronized (this) {
            //设置为true了，还要将这个节点放到双向链表的最后一位
            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++;
            //试着添加一个新值
            //如果是要添加数据的，mapValue=null，size扩大然后trimToSize
            //如果是替换数据，mapValue！=null
            mapValue = map.put(key, createdValue);
            if (mapValue != null) {
                map.put(key, mapValue);
            } else {
                size += safeSizeOf(key, createdValue);
            }
        }

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

    //添加一个缓存对象
    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);
            // previous = null表示新添加的缓存之前未存在过
            // previous != null表示之前已存在数据
            if (previous != null) {
                // 之前已有数据，那么size再减回去
                size -= safeSizeOf(key, previous);
            }
        }

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

        trimToSize(maxSize);
        return previous;
    }

    //重点在这里****************************************
    //如果超出最大容量，那就去掉header.next
    //由于新添加的数据已经跑到header.prv去了，所以删除的必定是最近最少使用的
    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 || map.isEmpty()) {
                    break;
                }

                Map.Entry<K, V> toEvict = map.entrySet().iterator().next();
                key = toEvict.getKey();
                value = toEvict.getValue();
                map.remove(key);
                size -= safeSizeOf(key, value);
                evictionCount++;
            }

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