Android 实现异步加载图片

/**
 * 异步加载图片，并将图片设置到ImageView控件中
*/
public class ImageDownloader extends AsyncTask<String, Integer, Object> {

    private static final String TAG = "ImageDownloader";
    // 为了加快速度，在内存中开启缓存（主要应用于重复图片较多时，或者同一个图片要多次被访问，比如在ListView时来回滚动）
    private Map<String, SoftReference<Drawable>> imageCache = new HashMap<String, SoftReference<Drawable>>();
    /**
     * 显示图片的控件
     */
    private ImageView mImageView;

    public ImageDownloader(ImageView image) {
        mImageView = image;
    }

    @Override
    protected void onPreExecute() {
        super.onPreExecute();
    }

    @Override
    protected Object doInBackground(String... params) {
        // Log.i("ImageDownloader", "loading image...");
        String url = params[0];
        Drawable drawable = null;
        try {
            if (!"".equals(url) && url != null) {
                String fileName = url.hashCode()+".jpg";
                // 如果缓存过就从缓存中取出数据
                if (imageCache.containsKey(fileName)) {
                    SoftReference<Drawable> softReference = imageCache.get(fileName);
                    drawable = softReference.get();
                    if (drawable != null) {
                        return drawable;
                    }
                }
                File dir = new File(FileConstant.IMAGE_FILE_PATH);
                if (!dir.exists()) {
                    boolean m = dir.mkdirs();
                }
                File file = new File(dir, fileName);
                if (file.exists() && file.length() > 0) {
                    Log.i(TAG, "load image from sd card");
                    // 如果文件存在则直接读取sdcard
                    drawable = readFromSdcard(file);
                } else {
                    //file.createNewFile();
                    Log.i(TAG, "load image from network");
                    URL imageUrl = new URL(url);
                    // 写入sdcard
                    if (Environment.getExternalStorageState().equals(Environment.MEDIA_MOUNTED)) {
                        saveImageFile(imageUrl, file);
                        drawable = Drawable.createFromStream(new FileInputStream(file), fileName);
                    }else{
                        //直接从流读取
                        drawable = Drawable.createFromStream(imageUrl.openStream(), fileName);
                    }
                }
                if(drawable!=null){
                    //保存在缓存中
                    imageCache.put(fileName, new SoftReference<Drawable>(drawable));
                }
            }
        } catch (Exception e) {
            e.printStackTrace();
        } 
        return drawable;
    }
    /**
     * save image
*/
    private void saveImageFile(URL url, File file) {
        FileOutputStream out = null;
        InputStream in = null;
        try {
            file.deleteOnExit();
            out = new FileOutputStream(file);
            in = url.openStream();
            byte[] buf = new byte[1024];
            int len = -1;
            while((len = in.read(buf))!=-1){
                out.write(buf, 0, len);
                out.flush();
            }
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            if(out!=null){
                try {
                    out.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
            if(in!=null){
                try {
                    in.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }
    }

    /**
     * 从sdcard中获取图片
  */
    private Drawable readFromSdcard(File file) throws Exception {
        FileInputStream in = new FileInputStream(file);
        return Drawable.createFromStream(in, file.getName());
    }

    @Override
    protected void onPostExecute(Object result) {
        super.onPostExecute(result);
        Drawable drawable = (Drawable) result;
        if (mImageView != null && drawable != null) {
            mImageView.setBackgroundDrawable(drawable);
        }
    }

    @Override
    protected void onProgressUpdate(Integer... values) {
        super.onProgressUpdate(values);
    }

    @Override
    protected void onCancelled() {
        super.onCancelled();
    }

}

public class ImageUtil {
    /**
     * image loader
     */
    static ImageDownloader2 loader = null;
    
    /**
     * load image
*/
    public static void loadImage(String url,final ImageView imageView){
        if(loader == null){
            loader = new ImageDownloader2();
        }
        loader.loadDrawable(url, new ImageCallback() {
            
            @Override
            public void imageLoaded(Drawable imageDrawable) {
                if(imageDrawable!=null){
                    imageView.setBackgroundDrawable(imageDrawable);
                }
            }
        });
    }
    
}