Android 图片缓存机制的深入理解

private LruCache<String, Bitmap> mMemoryCache; 
 
@Override 
protected void onCreate(Bundle savedInstanceState) { 
  ... 
  // Get max available VM memory, exceeding this amount will throw an 
  // OutOfMemory exception. Stored in kilobytes as LruCache takes an 
  // int in its constructor. 
  final int maxMemory = (int) (Runtime.getRuntime().maxMemory() / 1024); 
 
  // Use 1/8th of the available memory for this memory cache. 
  final int cacheSize = maxMemory / 8; 
 
  mMemoryCache = new LruCache<String, Bitmap>(cacheSize) { 
    @Override 
    protected int sizeOf(String key, Bitmap bitmap) { 
      // The cache size will be measured in kilobytes rather than 
      // number of items. 
      return bitmap.getByteCount() / 1024; 
    } 
  }; 
  ... 
} 
 
public void addBitmapToMemoryCache(String key, Bitmap bitmap) { 
  if (getBitmapFromMemCache(key) == null) { 
    mMemoryCache.put(key, bitmap); 
  } 
} 
 
public Bitmap getBitmapFromMemCache(String key) { 
  return mMemoryCache.get(key); 
} 

public void loadBitmap(int resId, ImageView imageView) { 
  final String imageKey = String.valueOf(resId); 
 
  final Bitmap bitmap = getBitmapFromMemCache(imageKey); 
  if (bitmap != null) { 
    mImageView.setImageBitmap(bitmap); 
  } else { 
    mImageView.setImageResource(R.drawable.image_placeholder); 
    BitmapWorkerTask task = new BitmapWorkerTask(mImageView); 
    task.execute(resId); 
  } 
} 

class BitmapWorkerTask extends AsyncTask<Integer, Void, Bitmap> { 
  ... 
  // Decode image in background. 
  @Override 
  protected Bitmap doInBackground(Integer... params) { 
    final Bitmap bitmap = decodeSampledBitmapFromResource( 
        getResources(), params[0], 100, 100)); 
    addBitmapToMemoryCache(String.valueOf(params[0]), bitmap); 
    return bitmap; 
  } 
  ... 
} 

private DiskLruCache mDiskLruCache; 
private final Object mDiskCacheLock = new Object(); 
private boolean mDiskCacheStarting = true; 
private static final int DISK_CACHE_SIZE = 1024 * 1024 * 10; // 10MB 
private static final String DISK_CACHE_SUBDIR = "thumbnails"; 
 
@Override 
protected void onCreate(Bundle savedInstanceState) { 
  ... 
  // Initialize memory cache 
  ... 
  // Initialize disk cache on background thread 
  File cacheDir = getDiskCacheDir(this, DISK_CACHE_SUBDIR); 
  new InitDiskCacheTask().execute(cacheDir); 
  ... 
} 
 
class InitDiskCacheTask extends AsyncTask<File, Void, Void> { 
  @Override 
  protected Void doInBackground(File... params) { 
    synchronized (mDiskCacheLock) { 
      File cacheDir = params[0]; 
      mDiskLruCache = DiskLruCache.open(cacheDir, DISK_CACHE_SIZE); 
      mDiskCacheStarting = false; // Finished initialization 
      mDiskCacheLock.notifyAll(); // Wake any waiting threads 
    } 
    return null; 
  } 
} 
 
class BitmapWorkerTask extends AsyncTask<Integer, Void, Bitmap> { 
  ... 
  // Decode image in background. 
  @Override 
  protected Bitmap doInBackground(Integer... params) { 
    final String imageKey = String.valueOf(params[0]); 
 
    // Check disk cache in background thread 
    Bitmap bitmap = getBitmapFromDiskCache(imageKey); 
 
    if (bitmap == null) { // Not found in disk cache 
      // Process as normal 
      final Bitmap bitmap = decodeSampledBitmapFromResource( 
          getResources(), params[0], 100, 100)); 
    } 
 
    // Add final bitmap to caches 
    addBitmapToCache(imageKey, bitmap); 
 
    return bitmap; 
  } 
  ... 
} 
 
public void addBitmapToCache(String key, Bitmap bitmap) { 
  // Add to memory cache as before 
  if (getBitmapFromMemCache(key) == null) { 
    mMemoryCache.put(key, bitmap); 
  } 
 
  // Also add to disk cache 
  synchronized (mDiskCacheLock) { 
    if (mDiskLruCache != null && mDiskLruCache.get(key) == null) { 
      mDiskLruCache.put(key, bitmap); 
    } 
  } 
} 
 
public Bitmap getBitmapFromDiskCache(String key) { 
  synchronized (mDiskCacheLock) { 
    // Wait while disk cache is started from background thread 
    while (mDiskCacheStarting) { 
      try { 
        mDiskCacheLock.wait(); 
      } catch (InterruptedException e) {} 
    } 
    if (mDiskLruCache != null) { 
      return mDiskLruCache.get(key); 
    } 
  } 
  return null; 
} 
 
// Creates a unique subdirectory of the designated app cache directory. Tries to use external 
// but if not mounted, falls back on internal storage. 
public static File getDiskCacheDir(Context context, String uniqueName) { 
  // Check if media is mounted or storage is built-in, if so, try and use external cache dir 
  // otherwise use internal cache dir 
  final String cachePath = 
      Environment.MEDIA_MOUNTED.equals(Environment.getExternalStorageState()) || 
          !isExternalStorageRemovable() ? getExternalCacheDir(context).getPath() : 
              context.getCacheDir().getPath(); 
 
  return new File(cachePath + File.separator + uniqueName); 
}