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;
        }
    }
}