如何下载和显示位图与AsyncTask

我是android新手，最近几天我找不到问题的原因:

我有ActivityA与ListView。单击ListView中的每个项目将打开ActivityB，这将显示一些图像下载从网络在一个ImageView。因此，在ActivityB中，我有一个循环，其中包含以下代码来尝试下载图像:

ImageView ivPictureSmall = new ImageView(this);
ImageDownloader ido = new ImageDownloader();
ido.download(this.getResources().getString(R.string.images_uri) + strPictureSmall, ivPictureSmall);
ivPictureSmall.setPadding(3, 5, 3, 5);
linearLayout.addView(ivPictureSmall);

类ImageDownloader

public class ImageDownloader
{
    public void download(String url, ImageView imageView)
    {
            BitmapDownloaderTask task = new BitmapDownloaderTask(imageView);
            task.execute(url);
    }
}

类BitmapDownloaderTask

class BitmapDownloaderTask extends AsyncTask
{
    private String url;
    private final WeakReference imageViewReference;
    public BitmapDownloaderTask(ImageView imageView)
    {
        imageViewReference = new WeakReference(imageView);
    }
    @Override
    // Actual download method, run in the task thread
    protected Bitmap doInBackground(String... params)
    {
        // params comes from the execute() call: params[0] is the url.
        return downloadBitmap(params[0]);
    }
    @Override
    // Once the image is downloaded, associates it to the imageView
    protected void onPostExecute(Bitmap bitmap)
    {
        if (isCancelled())
        {
            bitmap = null;
        }
        if (imageViewReference != null)
        {
            ImageView imageView = imageViewReference.get();
            if (imageView != null) {
                imageView.setImageBitmap(bitmap);
            }
        }
    }
    protected Bitmap downloadBitmap(String url)
    {
        final DefaultHttpClient client = new DefaultHttpClient();
        final HttpGet getRequest = new HttpGet(url);
        try {
            HttpResponse response = client.execute(getRequest);
            final int statusCode = response.getStatusLine().getStatusCode();
            if (statusCode != HttpStatus.SC_OK) {
                Log.w("ImageDownloader", "Error " + statusCode + " while retrieving bitmap from " + url);
                return null;
            }
            final HttpEntity entity = response.getEntity();
            if (entity != null) {
                InputStream inputStream = null;
                try {
                    inputStream = entity.getContent();
                    final Bitmap bitmap = BitmapFactory.decodeStream(new FlushedInputStream(inputStream));
                    return bitmap;
                } finally {
                    if (inputStream != null) {
                        inputStream.close();
                    }
                    entity.consumeContent();
                }
            }
        } catch (Exception e) {
            // Could provide a more explicit error message for IOException or IllegalStateException
            getRequest.abort();
            Log.w("ImageDownloader", "Error while retrieving bitmap from: " + e.toString());
        } finally {
            if (client != null) {
                //client.close();
            }
        }
        return null;
    }

    static class FlushedInputStream extends FilterInputStream
    {
        public FlushedInputStream(InputStream inputStream)
        {
            super(inputStream);
        }
        @Override
        public long skip(long n) throws IOException
        {
            long totalBytesSkipped = 0L;
            while (totalBytesSkipped 

final Bitmap bitmap = BitmapFactory.decodeStream(new FlushedInputStream(inputStream));