Android的listview里面有从文件decode的bit地图为imageview的资源的时候加载很卡怎么处理

/** * 先从内存缓存中查找，若没有再开线程， * 从文件缓存中查找都没有，则从指定的url中查找，并对bitmap进行处理，最后对UI进行更新操作 * @author Treasure * */public class ImageLoader {    MemoryCache memoryCache = new MemoryCache();    FileCache fileCache;    private Map<ImageView, String> imageViews = Collections            .synchronizedMap(new WeakHashMap<ImageView, String>());    // 线程池    ExecutorService executorService;        public ImageLoader(Context context)    {        fileCache = new FileCache(context);        executorService = Executors.newFixedThreadPool(5);    }        // 当进入listview时默认的图片，可换成你自己的默认图片    final int stub_id = R.drawable.none_logo;        // 最主要的方法    public void displayImage(String url, ImageView imageView)    {        imageViews.put(imageView, url);                // 先从内存缓存中查找        Bitmap bitmap = memoryCache.get(url);        if (bitmap != null)            imageView.setImageBitmap(bitmap);        else        {            // 若没有的话，则开启新线程加载图片            queuePhoto(url, imageView);            imageView.setImageResource(stub_id);        }    }        private void queuePhoto(String url, ImageView imageView)    {        PhotoToLoad p = new PhotoToLoad(url, imageView);        executorService.submit(new PhotosLoader(p));    }        private Bitmap getBitmap(String url)    {        File f = fileCache.getFile(url);                // 先从文件缓存中查找是否有        Bitmap b = decodeFile(f);        if (b != null)        {            return b;        }                // 最后从指定的url中下载图片        try         {            Bitmap bitmap = null;            URL imageUrl = new URL(url);            HttpURLConnection conn = (HttpURLConnection)imageUrl                    .openConnection();            conn.setConnectTimeout(30 * 1000);            conn.setReadTimeout(30 * 1000);            conn.setInstanceFollowRedirects(true);            InputStream is = conn.getInputStream();            OutputStream os = new FileOutputStream(f);            copyStream(is, os);            os.close();            bitmap = decodeFile(f);            return bitmap;        }        catch (Exception e)         {            e.printStackTrace();            return null;        }    }        /**     * decode 这个图片并且按比例缩放以减少内存消耗，虚拟机对每张图片的缓存大小也是有限制的     * @param f     * @return     */    private Bitmap decodeFile(File f)    {        try         {            // decode image size            BitmapFactory.Options o = new BitmapFactory.Options();            o.inJustDecodeBounds = true;            BitmapFactory.decodeStream(new FileInputStream(f), null, o);                        // Find the correct scale value. It should be the power of 2.            final int REQUIRED_SIZE = 70;            int width_tmp = o.outWidth, height_tmp = o.outHeight;            int scale = 1;            while (true)            {                if (width_tmp / 2 < REQUIRED_SIZE                        || height_tmp /2 < REQUIRED_SIZE)                    break;                width_tmp /= 2;                height_tmp /= 2;                scale *= 2;            }                        // decode with inSampleSize            BitmapFactory.Options o2 = new BitmapFactory.Options();            o2.inSampleSize = scale;            return BitmapFactory.decodeStream(new FileInputStream(f), null, o2);        }        catch (FileNotFoundException e)         {        }                return null;    }        /**     * Task for the queue     * @author Treasure     *     */    private class PhotoToLoad    {        public String url;        public ImageView imageView;                public PhotoToLoad(String u, ImageView i)        {            url = u;            imageView = i;        }    }        class PhotosLoader implements Runnable    {        PhotoToLoad photoToLoad;                PhotosLoader(PhotoToLoad photoToLoad)        {            this.photoToLoad = photoToLoad;        }                @Override        public void run()        {            if (imageViewReused(photoToLoad))                return;            Bitmap bmp = getBitmap(photoToLoad.url);            memoryCache.put(photoToLoad.url, bmp);            if (imageViewReused(photoToLoad))                return;            BitmapDisplayer bd = new BitmapDisplayer(bmp, photoToLoad);            // 更新的操作放在UI线程中            Activity a = (Activity)photoToLoad.imageView.getContext();            a.runOnUiThread(bd);        }    }        /**     * 防止图片错位     * @param photoToLoad     * @return     */    boolean imageViewReused(PhotoToLoad photoToLoad)    {        String tag = imageViews.get(photoToLoad.imageView);        if (tag == null || !tag.equals(photoToLoad.url))            return true;        return false;    }        class BitmapDisplayer implements Runnable    {        Bitmap bitmap;        PhotoToLoad photoToLoad;                public BitmapDisplayer(Bitmap b, PhotoToLoad p)        {            bitmap = b;            photoToLoad = p;        }                @Override        public void run()         {            if (imageViewReused(photoToLoad))                return;            if (bitmap != null)                photoToLoad.imageView.setImageBitmap(bitmap);            else                photoToLoad.imageView.setImageResource(stub_id);        }    }        public void clearCache()    {        memoryCache.clear();        fileCache.clear();    }        public static void copyStream(InputStream is, OutputStream os)    {        final int buffer_size = 1024;        try        {            byte[] bytes = new byte[buffer_size];            for (; ;)            {                int count = is.read(bytes, 0, buffer_size);                if (count == -1)                    break;                os.write(bytes, 0, count);            }        }         catch (Exception e)        {        }    }}