LruCache详解之 Android 内存优化

public class LruCache<K, V> {
    //缓存 map 集合，为什么要用LinkedHashMap
    //因为没错取了缓存值之后，都要进行排序，以确保
    //下次移除的是最少使用的值
    private final LinkedHashMap<K, V> map;
    //当前缓存的值
    private int size;
    //最大值
    private int maxSize;
    //添加到缓存中的个数
    private int putCount;
    //创建的个数
    private int createCount;
    //被移除的个数
    private int evictionCount;
    //命中个数
    private int hitCount;
    //丢失个数
    private int missCount;

    //实例化 Lru，需要传入缓存的最大值
    //这个最大值可以是个数，比如对象的个数，也可以是内存的大小
    //比如，最大内存只能缓存5兆
    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);
    }

    //重置最大缓存的值
    public void resize(int maxSize) {
        if (maxSize <= 0) {
            throw new IllegalArgumentException("maxSize <= 0");
        }

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

    //通过 key 获取缓存值
    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++;
        }


        //如果没有，用户可以去创建
        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 {
                //缓存的大小改变
                size += safeSizeOf(key, createdValue);
            }
        }
        //这里没有移除，只是改变了位置
        if (mapValue != null) {
            entryRemoved(false, key, createdValue, mapValue);
            return mapValue;
        } else {
            //判断缓存是否越界
            trimToSize(maxSize);
            return createdValue;
        }
    }

    //添加缓存，跟上面这个方法的 create 之后的代码一样的
    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;
    }

    //检测缓存是否越界
    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!");
                }
                //如果没有，则返回
                if (size <= maxSize) {
                    break;
                }
                //以下代码表示已经超出了最大范围
                Map.Entry<K, V> toEvict = null;
                for (Map.Entry<K, V> entry : map.entrySet()) {
                    toEvict = entry;
                }

                if (toEvict == null) {
                    break;
                }
                //移除最后一个，也就是最少使用的缓存
                key = toEvict.getKey();
                value = toEvict.getValue();
                map.remove(key);
                size -= safeSizeOf(key, value);
                evictionCount++;
            }

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

    //手动移除，用户调用
    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;
    }
    //这里用户可以重写它，实现数据和内存回收操作
    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;
    }

        //这个方法要特别注意，跟我们实例化 LruCache 的 maxSize 要呼应，怎么做到呼应呢，比如 maxSize 的大小为缓存的个数，这里就是 return 1就 ok，如果是内存的大小，如果5M，这个就不能是个数 了，这是应该是每个缓存 value 的 size 大小，如果是 Bitmap，这应该是 bitmap.getByteCount();
    protected int sizeOf(K key, V value) {
        return 1;
    }

    //清空缓存
    public final void evictAll() {
        trimToSize(-1); // -1 will evict 0-sized elements
    }


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


    public synchronized final int maxSize() {
        return maxSize;
    }


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


    public synchronized final int missCount() {
        return missCount;
    }


    public synchronized final int createCount() {
        return createCount;
    }


    public synchronized final int putCount() {
        return putCount;
    }


    public synchronized final int evictionCount() {
        return evictionCount;
    }


    public synchronized final Map<K, V> snapshot() {
        return new LinkedHashMap<K, V>(map);
    }
}

/**
 * Created by gyzhong on 15/4/5.
 */
public class LruPageAdapter extends PagerAdapter {

    private List<String> mData ;
    private LruCache<String,Bitmap> mLruCache ;
    private int mTotalSize = (int) Runtime.getRuntime().totalMemory();
    private ViewPager mViewPager ;

    public LruPageAdapter(ViewPager viewPager ,List<String> data){
        mData = data ;
        mViewPager = viewPager ;
        /*实例化LruCache*/
        mLruCache = new LruCache<String,Bitmap>(mTotalSize/5){

            /*当缓存大于我们设定的最大值时，会调用这个方法，我们可以用来做内存释放操作*/
            @Override
            protected void entryRemoved(boolean evicted, String key, Bitmap oldValue, Bitmap newValue) {
                super.entryRemoved(evicted, key, oldValue, newValue);
                if (evicted && oldValue != null){
                    oldValue.recycle();
                }
            }
            /*创建 bitmap*/
            @Override
            protected Bitmap create(String key) {
                final int resId = mViewPager.getResources().getIdentifier(key,"drawable",
                        mViewPager.getContext().getPackageName()) ;
                return BitmapFactory.decodeResource(mViewPager.getResources(),resId) ;
            }
            /*获取每个 value 的大小*/
            @Override
            protected int sizeOf(String key, Bitmap value) {
                return value.getByteCount();
            }
        } ;
    }


    @Override
    public Object instantiateItem(ViewGroup container, int position) {
        View view = LayoutInflater.from(container.getContext()).inflate(R.layout.view_pager_item, null) ;
        ImageView imageView = (ImageView) view.findViewById(R.id.id_view_pager_item);
        Bitmap bitmap = mLruCache.get(mData.get(position));
        imageView.setImageBitmap(bitmap);
        container.addView(view);
        return view;
    }

    @Override
    public void destroyItem(ViewGroup container, int position, Object object) {
        container.removeView((View) object);
    }

    @Override
    public int getCount() {
        return mData.size();
    }



    @Override
    public boolean isViewFromObject(View view, Object object) {
        return view == object;
    }
}