android检查位图是否为有效图像

import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.os.AsyncTask;
import android.widget.ImageView;
import java.io.InputStream;

public class ImageViewLoader extends AsyncTask<String, Void, Bitmap>
{
    ImageView bmImage;
    public ImageViewLoader(ImageView bmImage)
    {
        this.bmImage = bmImage;
    }
    protected Bitmap doInBackground(String... urls)
    {
        String urldisplay = urls[0];
        Bitmap bitmap = null;
        try
        {
            InputStream in = new java.net.URL(urldisplay).openStream();
            bitmap = BitmapFactory.decodeStream(in);
        }
        catch (Exception e)
        {
            e.printStackTrace();
        }
        return bitmap;
    }

    protected void onPostExecute(Bitmap result)
    {
        bmImage.setImageBitmap(result);
    }
}

如果我运行这个例子

ImageViewLoader loader=new ImageViewLoader(imageView);
loader.execute("http://localhost:8000/media/12");

图像显示成功,但如果图像不存在或连接错误,我会得到一个空的imageview,我想知道如何检查从doInBackground返回的位图是否是有效的图像,如果不是,那么我将把imageview可见性设置为GONE