public class ImageResizer {
 private static final String TAG = "ImageResizer";

 public ImageResizer() {
  super();
  // TODO Auto-generated constructor stub
 }

 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 Bitmap decodeSampledBitmapFromBitmapFileDescriptor(FileDescriptor fd,
   int reqWidth,int reqHeight){
  final BitmapFactory.Options options = new BitmapFactory.Options();
  options.inJustDecodeBounds = true;

  BitmapFactory.decodeFileDescriptor(fd, null, options);

  options.inSampleSize = calculateInSampleSize(options, reqWidth,
    reqHeight);

  options.inJustDecodeBounds = false;
  return BitmapFactory.decodeFileDescriptor(fd, null, options);
 }

 public int calculateInSampleSize(BitmapFactory.Options options,
   int reqWidth, int reqHeight) {

  final int width = options.outWidth;
  final int height = options.outHeight;

  int inSampleSize = 1;
  if (height > reqHeight || width > reqWidth) {
   final int halfHeight = height / 2;
   final int halfWidth = width / 2;
   while ((halfHeight / inSampleSize) > reqHeight
     && (halfWidth / inSampleSize) > halfWidth) {
    inSampleSize *= 2;
   }
  }
  return inSampleSize;

 }

}

private LruCache<String, Bitmap> mMemoryCache;
private DiskLruCache mDiskLruCache;

public ImageLoader(Context context) {
  mContext = context.getApplicationContext();
  //分配内存缓存为当前进程的1/8,磁盘缓存容量为50M
  int maxMemory = (int) (Runtime.getRuntime().maxMemory() * 1024);
  int cacheSize = maxMemory / 8;
  mMemoryCache = new LruCache<String, Bitmap>(cacheSize) {

   @Override
   protected int sizeOf(String key, Bitmap value) {
    return value.getRowBytes() * value.getHeight() / 1024;
   }

  };

  File diskCacheDir = getDiskChaheDir(mContext, "bitmap");
  if (!diskCacheDir.exists()) {
   diskCacheDir.mkdirs();
  }
  if (getUsableSpace(diskCacheDir) > DISK_CACHE_SIZE) {
   try {
    mDiskLruCache = DiskLruCache.open(diskCacheDir, 1, 1,
      DISK_CACHE_SIZE);
    mIsDiskLruCacheCreated = true;
   } catch (IOException e) {
    e.printStackTrace();
   }
  }
 }

private void addBitmapToMemoryCache(String key, Bitmap bitmap) {
  if (getBitmapFromMemCache(key) == null) {
   mMemoryCache.put(key, bitmap);
  }
 }

 private Bitmap getBitmapFromMemCache(String key) {
  return mMemoryCache.get(key);
 }

//将下载的图片写入文件系统，实现磁盘缓存
 private Bitmap loadBitmapFromHttp(String url, int reqWidth, int reqHeight)
   throws IOException {
  if (Looper.myLooper() == Looper.getMainLooper()) {
   throw new RuntimeException("can not visit network from UI Thread.");
  }
  if (mDiskLruCache == null)
   return null;
  String key = hashKeyFormUrl(url);
  DiskLruCache.Editor editor = mDiskLruCache.edit(key);
  if (editor != null) {
   OutputStream outputStream = editor
     .newOutputStream(DISK_CACHE_INDEX);
   if (downloadUrlToStream(url, outputStream)) {
    editor.commit();
   } else {
    editor.abort();
   }

  }
  mDiskLruCache.flush();
  return loadBitmapForDiskCache(url, reqWidth, reqHeight);
 }

public Bitmap loadBitmapForDiskCache(String url, int reqWidth, int reqHeight)
   throws IOException {
  if (Looper.myLooper() == Looper.getMainLooper()) {
   Log.w(TAG, "load bitmap from UI Thread , it's not recommended");
  }
  if (mDiskLruCache == null)
   return null;
  Bitmap bitmap = null;
  String key = hashKeyFormUrl(url);
  DiskLruCache.Snapshot snapshot = mDiskLruCache.get(key);
  if (snapshot != null) {
   FileInputStream fileInputStream = (FileInputStream) snapshot
     .getInputStream(DISK_CACHE_INDEX);
   FileDescriptor fileDescriptor = fileInputStream.getFD();
   bitmap = mImageResizer.decodeSampledBitmapFromBitmapFileDescriptor(
     fileDescriptor, reqWidth, reqHeight);
   if (bitmap != null) {
    addBitmapToMemoryCache(key, bitmap);
   }
  }
  return bitmap;
 }

//同步加载
 public Bitmap loadBitmap(String uri, int reqWidth, int reqHeight) {
  Bitmap bitmap = loadBitmpaFromMemCache(uri);
  if (bitmap != null) {
   return bitmap;
  }
  try {
   bitmap = loadBitmapForDiskCache(uri, reqWidth, reqHeight);
   if (bitmap != null) {
    return bitmap;
   }
   bitmap = loadBitmapFromHttp(uri, reqWidth, reqHeight);

  } catch (IOException e) {
   e.printStackTrace();
  }
  if (bitmap == null && !mIsDiskLruCacheCreated) {
   bitmap = downloadBitmapFromUrl(uri);
  }
  return bitmap;
 }

if (Looper.myLooper() == Looper.getMainLooper()) {
   throw new RuntimeException("can not visit network from UI Thread.");
  }

//异步加载
 public void bindBitmap(final String uri, final ImageView imageView,
   final int reqWidth, final int reqHeight) {

  imageView.setTag(TAG_KEY_URI, uri);
  Bitmap bitmap = loadBitmpaFromMemCache(uri);
  if (bitmap != null) {
   imageView.setImageBitmap(bitmap);
   return;
  }
  Runnable loadBitmapTask = new Runnable() {

   @Override
   public void run() {
    Bitmap bitmap = loadBitmap(uri, reqWidth, reqHeight);
    if (bitmap != null) {
     LoaderResult result = new LoaderResult(imageView, uri,
       bitmap);
     mMainHandler.obtainMessage(MESSAGE_POST_RESULT, result)
       .sendToTarget();

    }
   }
  };
  THREAD_POOL_EXECUTOR.execute(loadBitmapTask);
 }

private static final int CPU_COUNT = Runtime.getRuntime()
   .availableProcessors();
private static final int CORE_POOL_SIZE = CPU_COUNT + 1;
private static final int MAXIMUM_POOL_SIZE = CPU_COUNT * 2 + 1;
private static final long KEEP_ALIVE = 10L;


private static final ThreadFactory sThreadFactory = new ThreadFactory() {
  private final AtomicInteger mCount = new AtomicInteger();

  @Override
  public Thread newThread(Runnable r) {
   // TODO Auto-generated method stub
   return new Thread(r, "ImageLoader#" + mCount.getAndIncrement());
  }
 };


public static final Executor THREAD_POOL_EXECUTOR = new ThreadPoolExecutor(
   CORE_POOL_SIZE, MAXIMUM_POOL_SIZE, KEEP_ALIVE, TimeUnit.SECONDS,
   new LinkedBlockingDeque<Runnable>(), sThreadFactory);

private Handler mMainHandler = new Handler(Looper.getMainLooper()) {

  @Override
  public void handleMessage(Message msg) {
   LoaderResult result = (LoaderResult) msg.obj;
   ImageView imageView = result.imageView;
   imageView.setImageBitmap(result.bitmap);
   String uri = (String) imageView.getTag(TAG_KEY_URI);
   if (uri.equals(result.uri)) {
    imageView.setImageBitmap(result.bitmap);
   } else {
    Log.w(TAG, "set image bitmap,but url has changed , ignored!");
   }
  }

 };