Android经典面试题之Glide的缓存大揭秘

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Glide缓存

关联类：Engine、LruResourceCache、LruCache、ActiveResources

ActiveResources：弱引用缓存池

@VisibleForTesting final Map<Key, ResourceWeakReference> activeEngineResources = new HashMap<>();
private final ReferenceQueue<EngineResource<?>> resourceReferenceQueue = new ReferenceQueue<>();

LruCache：LinkedHashMap缓存池

private final Map<T, Entry<Y>> cache = new LinkedHashMap<>(100, 0.75f, true);

入口：Engine.load方法

先从缓存中取

--> loadFromMemory

--> loadFromActiveResources(弱引用池) 

--> loadFromCache 
--> getEngineResourceFromCache(LRU缓存池，LinkedHashMap)  

LRU缓存池中取到EngineResource后，会从LRU缓存中删除，然后对它引用计数+1，放入弱引用池

缓存中没有找到，就需要创建任务执行

-->  waitForExistingOrStartNewJob  
--> 如果当前图片任务已经有EngineJob了，就直接加个Callback

---> 没有的话就创建EngineJob和DecodeJob从本地加载或者是网络加载

EngineResource通过引用计数来判断是否需要释放资源，释放的资源会从弱引用池中删除，放入LRU缓存中

缓存大小设置

涉及的类：MemorySizeCalculator

• 首先获取App可用内存大小，Glide的内存大小限制在0.4以下，如果是低内存的系统，则是在0.33

private static int getMaxSize(
	ActivityManager activityManager, float maxSizeMultiplier, float lowMemoryMaxSizeMultiplier) {
	final int memoryClassBytes = activityManager.getMemoryClass() * 1024 * 1024;
	final boolean isLowMemoryDevice = isLowMemoryDevice(activityManager);
	//lowMemoryMaxSizeMultiplier是0.33
	//maxSizeMultiplier是0.4
	return Math.round(
			memoryClassBytes * (isLowMemoryDevice ? lowMemoryMaxSizeMultiplier : maxSizeMultiplier));
}

@TargetApi(Build.VERSION_CODES.KITKAT)
@Synthetic
static boolean isLowMemoryDevice(ActivityManager activityManager) {
	if (Build.VERSION.SDK\_INT >= Build.VERSION\_CODES.KITKAT) {
		return activityManager.isLowRamDevice();
	} else {
		return true;
	}
}

• 图片缓存大小，用几屏来表示，跟屏幕的分辨率有关

//一屏的图片大小 宽*高*4（ARG888图片的像素大小就是4字节）
int screenSize = widthPixels * heightPixels * BYTES_PER_ARGB_8888_PIXEL;

//BitmapPool最新的是API26以上是4，以下的是1
int targetBitmapPoolSize = Math.round(screenSize * builder.bitmapPoolScreens);

//memoryCache是2屏
int targetMemoryCacheSize = Math.round(screenSize * builder.memoryCacheScreens);

• LRU缓存动态限制图片缓存大小

 //在低内存回调，或是put新的图片后，都会进行缓存大小检查，如果超过就移除不太用的
protected synchronized void trimToSize(long size) {
	Map.Entry<T, Entry<Y>> last;
	Iterator<Map.Entry<T, Entry<Y>>> cacheIterator;
	while (currentSize > size) {
		cacheIterator = cache.entrySet().iterator();
		last = cacheIterator.next();
		final Entry\<Y> toRemove = last.getValue();
		currentSize -= toRemove.size;
		final T key = last.getKey();
		cacheIterator.remove();
		onItemEvicted(key, toRemove.value);
	}
}

DiskLruCache中的读写锁

写的时候会加锁，这个锁是自定义的，并且有一个锁的池子

 private static class WriteLock {
   final Lock lock = new ReentrantLock();
   int interestedThreads;

   @Synthetic
   WriteLock() {}
}

每次写的时候会加锁，并且会对这个WriteLock的interestedThreads分别在开始写和结束时进行加减操作

writeLocker.acquire(safeKey);
....
writeLocker.release(safeKey);

writeLocker从锁池子里取，key的话是用请求的key做哈希得到

 //SafeKeyGenerator.class
 private String calculateHexStringDigest(Key key) {
     PoolableDigestContainer container = Preconditions.checkNotNull(digestPool.acquire());
     try {
       key.updateDiskCacheKey(container.messageDigest);
       // calling digest() will automatically reset()
       return Util.sha256BytesToHex(container.messageDigest.digest());
     } finally {
       digestPool.release(container);
     }
}

锁的缓存做了2级，一级是通过上面的key和锁放在一个HashMap中；

//DiskCacheWriteLocker.class

private final Map<String, WriteLock> locks = new HashMap<>();
private final WriteLockPool writeLockPool = new WriteLockPool();

另一级是定义在内部类WriteLockPool的ArrayDeque里面，默认大小是10

 private static class WriteLockPool {
    private static final int MAX_POOL_SIZE = 10;
    private final Queue<WriteLock> pool = new ArrayDeque<>();
    ...
}

取的时候先从HashMap中取，取不到再从WriteLockPool中取

void acquire(String safeKey) {
    WriteLock writeLock;
    synchronized (this) {
      writeLock = locks.get(safeKey);
      if (writeLock == null) {
        writeLock = writeLockPool.obtain();
        locks.put(safeKey, writeLock);
      }
      writeLock.interestedThreads++;
    }

    writeLock.lock.lock();
}

释放锁的时候，会把writeLock的interestedThreads进行减一操作，如果为0了就释放锁，放入WriteLockPool中

void release(String safeKey) {
    WriteLock writeLock;
    synchronized (this) {
      writeLock = Preconditions.checkNotNull(locks.get(safeKey));
      if (writeLock.interestedThreads < 1) {
        ...
      }

      writeLock.interestedThreads--;
      if (writeLock.interestedThreads == 0) {
        WriteLock removed = locks.remove(safeKey);
        ...
        writeLockPool.offer(removed);
      }
    }

    writeLock.lock.unlock();
}

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