LruCache的使用及原理

采用LRU算法实现的话就是将最老的数据删掉。利用LRU缓存，我们能够提高系统的性能.

 

一，是它本身已经实现了按照访问顺序的存储，也就是说，最近读取的会放在最前面，最不常读取的会放在最后（当然，它也可以实现按照插入顺序存储）。

二，LinkedHashMap本身有一个方法用于判断是否需要移除最不常读取的数，但是，原始方法默认不需要移除（这是，LinkedHashMap相当于一个linkedlist），所以，我们需要override这样一个方法，使得当缓存里存放的数据个数超过规定个数后，就把最不常用的移除掉。

源码分析：

/**
 * Static library version of {@link android.util.LruCache}. Used to write apps
 * that run on API levels prior to 12. When running on API level 12 or above,
 * this implementation is still used; it does not try to switch to the
 * framework's implementation. See the framework SDK documentation for a class
 * overview.
 */
public class LruCache<K, V> {
    /**缓存 map 集合，要用LinkedHashMap  */
    private final LinkedHashMap<K, V> map;

    /**缓存大小 */
    private int size;
    /**最大缓存大小*/
    private int maxSize;
    /**put的次数*/
    private int putCount;
    /**create的次数*/
    private int createCount;
    /**回收的次数*/
    private int evictionCount;
    /**命中的次数*/
    private int hitCount;
    /**丢失的次数*/
    private int missCount;

    /**
     *构造方法，maxSize最大缓存大小，初始化LinkedHashMap
     */
    public LruCache(int maxSize) {
        if (maxSize <= 0) {
            throw new IllegalArgumentException("maxSize <= 0");
        }
        this.maxSize = maxSize;
         //将LinkedHashMap的accessOrder设置为true来实现LRU
         //false 插入顺序
         //true  访问顺序
        this.map = new LinkedHashMap<K, V>(0, 0.75f, true);
    }

    /**
     * 重新设置最大缓存大小
     * @param maxSize 最大缓存大小.
     */
    public void resize(int maxSize) {
        if (maxSize <= 0) {
            throw new IllegalArgumentException("maxSize <= 0");
        }

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

    /**
     如果缓存中存在或者被创建过，返回该值，如果已经返回了，会被移动到队列头部，如果没有被缓存和创建，会被返回null
     */
    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++;
        }

        /*
         *如果丢失了就试图创建一个item
         */

        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 {
            //每次新加入对象都需要调用trimToSize方法看是否需要回收
            trimToSize(maxSize);
            return createdValue;
        }
    }

    /**
     *会被移动到队列头部
     * @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加上预put对象的大小
            size += safeSizeOf(key, value);
            previous = map.put(key, value);
            if (previous != null) {
                //如果之前存在键为key的对象，则size应该减去原来对象的大小
                size -= safeSizeOf(key, previous);
            }
        }

        if (previous != null) {
            entryRemoved(false, key, previous, value);
        }
        //每次新加入对象都需要调用trimToSize方法看是否需要回收
        trimToSize(maxSize);
        return previous;
    }

    /**
     * 删除最老的条目，直到其余条目的总数达到或低于要求的大小。
     */
    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!");
                }
                //如果当前size小于maxSize或者map没有任何对象,则结束循环
                if (size <= maxSize || map.isEmpty()) {
                    break;
                }
                //移除链表头部的元素，并进入下一次循环
                Map.Entry<K, V> toEvict = map.entrySet().iterator().next();
                key = toEvict.getKey();
                value = toEvict.getValue();
                map.remove(key);
                size -= safeSizeOf(key, value);
                 //回收次数+1
                evictionCount++;
            }

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

    /**
     *从内存缓存中根据key值移除某个对象并返回该对象
     */
    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;
    }

    /**
     * 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.
     *当item被回收或者删掉时调用。改方法当value被回收释放存储空间时被remove调用，
     * 或者替换item值时put调用，默认实现什么都没做
     * <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}.
     * true---为释放空间被删除；false---put或remove导致
     * @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.
     * 当某Item丢失时会调用到，返回计算的相应的value或者null
     */
    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，就需要覆盖这个方法，返回每个缓存 
     *value的size大小，如果是Bitmap，这应该是bitmap.getByteCount();
     */
    protected int sizeOf(K key, V value) {
        return 1;
    }

    /**
     * Clear the cache, calling {@link #entryRemoved} on each removed entry.
     * 清理缓存
     */
    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.
     * 缓存大小
     */
    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.
     * 缓存最大值
     */
    public synchronized final int maxSize() {
        return maxSize;
    }

    /**
     * Returns the number of times {@link #get} returned a value that was
     * already present in the cache.
     *返回{@link#get}返回值的次数，该值为
     *已存在于缓存中。
     */
    public synchronized final int hitCount() {
        return hitCount;
    }

    /**
     * Returns the number of times {@link #get} returned null or required a new
     * value to be created.
     *返回创建或者返回null的次数
     */
    public synchronized final int missCount() {
        return missCount;
    }

    /**
     * Returns the number of times {@link #create(Object)} returned a value.
     返回创建一个元素的次数
     */
    public synchronized final int createCount() {
        return createCount;
    }

    /**
     * Returns the number of times {@link #put} was called.
     调用put返回次数
     */
    public synchronized final int putCount() {
        return putCount;
    }

    /**
     * Returns the number of values that have been evicted.
     * 返回已被逐出的值的数目。
     */
    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.
     * 返回缓存拷贝，排序规则为最近最多访问
     */
    public synchronized final Map<K, V> snapshot() {
        return new LinkedHashMap<K, V>(map);
    }
    
    /**
     * 返回maxSize hits misses hitRate值
     * LruCache[maxSize=%d,hits=%d,misses=%d,hitRate=%d%%
     */
    @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);
    }
}

实际使用：

public class BitmapCacheActivity extends Activity {
    private ImageView iv_picture;
    private BitmapCache<String, Bitmap> mMemoryCache;
    private BitmapCache.BitmapRemovedCallBack<String> mEnteryRemovedCallBack =
            new BitmapCache.BitmapRemovedCallBack<String>() {
                @Override
                public void onBitmapRemoved(String key) {
                    //处理回收bitmap前，清空相关view的bitmap操作
                    mMemoryCache.remove(key);
                }
            };

    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_bitmapcache);
        iv_picture = findViewById(R.id.iv_picture);
// 获取到可用内存的最大值，使用内存超出这个值会引起OutOfMemory异常。
        // BitmapCache通过构造函数传入缓存值，以bit为单位。
        int memClass = ((ActivityManager) this.getSystemService(Context.ACTIVITY_SERVICE)).getMemoryClass();
        // 使用单个应用最大可用内存值的1/8作为缓存的大小。
        int cacheSize = 1024 * 1024 * memClass / 8;
        mMemoryCache = new BitmapCache<>(cacheSize, mEnteryRemovedCallBack);
        loadBitmap(R.mipmap.ic_launcher_round, iv_picture);
    }

    /**
     * bitmap添加到缓存中去
     * @param key
     * @param bitmap
     */
    public void addBitmapToMemoryCache(String key, Bitmap bitmap) {
        if (getBitmapFromMemCache(key) == null) {
            mMemoryCache.put(key, bitmap);
        }
    }

    /**
     * 从缓存中获取bitmap
     * @param key
     * @return
     */
    public Bitmap getBitmapFromMemCache(String key) {
        return mMemoryCache.get(key);
    }

    /**
     * 加载bitmap
     * @param resId
     * @param imageView
     */
    public void loadBitmap(int resId, ImageView imageView) {
        final String imageKey = String.valueOf(resId);
        final Bitmap bitmap = getBitmapFromMemCache(imageKey);
        //从缓存里面获取，没有设置默认的，然后在进行一步缓存操作
        if (bitmap != null) {
            imageView.setImageBitmap(bitmap);
        } else {
            imageView.setImageResource(R.drawable.person_image_empty);
            BitmapLoadingTask task = new BitmapLoadingTask(imageView);
            task.execute(resId);
        }
    }

    class BitmapLoadingTask extends AsyncTask<Integer, Void, Bitmap> {
        private ImageView imageView;

        public BitmapLoadingTask(ImageView imageView) {
            this.imageView = imageView;
        }

        // 在后台加载图片。
        @Override
        protected Bitmap doInBackground(Integer... params) {
            final Bitmap bitmap = decodeSampledBitmapFromResource(
                    getResources(), params[0], 100, 100);
            addBitmapToMemoryCache(String.valueOf(params[0]), bitmap);
            return bitmap;
        }

        @Override
        protected void onPostExecute(Bitmap bitmap) {
            super.onPostExecute(bitmap);
            //显示在bitmap上
            imageView.setImageBitmap(bitmap);
        }

        public Bitmap decodeSampledBitmapFromResource(Resources res, int resId,
                                                      int reqWidth, int reqHeight) {

            final BitmapFactory.Options options = new BitmapFactory.Options();
            options.inJustDecodeBounds = true;
            BitmapFactory.decodeResource(res, resId, options);
            options.inSampleSize = calculateInSampleSize(options, reqWidth, reqHeight);
            options.inJustDecodeBounds = false;
            return BitmapFactory.decodeResource(res, resId, options);
        }

        public int calculateInSampleSize(
                BitmapFactory.Options options, int reqWidth, int reqHeight) {
            final int height = options.outHeight;
            final int width = options.outWidth;
            int inSampleSize = 1;
            if (height > reqHeight || width > reqWidth) {
                final int heightRatio = Math.round((float) height / (float) reqHeight);
                final int widthRatio = Math.round((float) width / (float) reqWidth);
                inSampleSize = heightRatio < widthRatio ? heightRatio : widthRatio;
            }

            return inSampleSize;
        }
    }
}