Android:下载图片并将其保存为缓存

Question

我有一个从网络下载图片的类，我需要修改我的代码，以便将下载的图片保存在 SDCard 上。由于我是从互联网上获取这段代码的，而且我是初学者，所以我对针对缓存内容进行优化一无所知。

我愿意接受任何一种解决方案。

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

    // Global cache of images.
    // Using SoftReference to allow garbage collector to clean cache if needed
    private final HashMap<String, SoftReference<Bitmap>> Cache = new HashMap<String,  SoftReference<Bitmap>>();

    private final class QueueItem {
        public URL url;
        public ImageLoadedListener listener;
    }
    private final ArrayList<QueueItem> Queue = new ArrayList<QueueItem>();

    private final Handler handler = new Handler();  // Assumes that this is started from the main (UI) thread
    private Thread thread;
    private QueueRunner runner = new QueueRunner();;

    /** Creates a new instance of the ImageThreadLoader */
    public ImageThreadLoader() {
        thread = new Thread(runner);
    }

    /**
     * Defines an interface for a callback that will handle
     * responses from the thread loader when an image is done
     * being loaded.
     */
    public interface ImageLoadedListener {
        public void imageLoaded(Bitmap imageBitmap );
    }

    /**
     * Provides a Runnable class to handle loading
     * the image from the URL and settings the
     * ImageView on the UI thread.
     */
    private class QueueRunner implements Runnable {
        public void run() {
            synchronized(this) {
                while(Queue.size() > 0) {
                    final QueueItem item = Queue.remove(0);

                    // If in the cache, return that copy and be done
                    if( Cache.containsKey(item.url.toString()) && Cache.get(item.url.toString()) != null) {
                        // Use a handler to get back onto the UI thread for the update
                        handler.post(new Runnable() {
                            public void run() {
                                if( item.listener != null ) {
                                    // NB: There's a potential race condition here where the cache item could get
                                    //     garbage collected between when we post the runnable and it's executed.
                                    //     Ideally we would re-run the network load or something.
                                    SoftReference<Bitmap> ref = Cache.get(item.url.toString());
                                    if( ref != null ) {
                                        item.listener.imageLoaded(ref.get());
                                    }
                                }
                            }
                        });
                    } else {
                        final Bitmap bmp = readBitmapFromNetwork(item.url);
                        if( bmp != null ) {
                            Cache.put(item.url.toString(), new SoftReference<Bitmap>(bmp));

                            // Use a handler to get back onto the UI thread for the update
                            handler.post(new Runnable() {
                                public void run() {
                                    if( item.listener != null ) {
                                        item.listener.imageLoaded(bmp);
                                    }
                                }
                            });
                        }

                    }

                }
            }
        }
    }

    /**
     * Queues up a URI to load an image from for a given image view.
     *
     * @param uri   The URI source of the image
     * @param callback  The listener class to call when the image is loaded
     * @throws MalformedURLException If the provided uri cannot be parsed
     * @return A Bitmap image if the image is in the cache, else null.
     */
    public Bitmap loadImage( final String uri, final ImageLoadedListener listener) throws MalformedURLException {
        // If it's in the cache, just get it and quit it
        if( Cache.containsKey(uri)) {
            SoftReference<Bitmap> ref = Cache.get(uri);
            if( ref != null ) {
                return ref.get();
            }
        }

        QueueItem item = new QueueItem();
        item.url = new URL(uri);
        item.listener = listener;
        Queue.add(item);

        // start the thread if needed
        if( thread.getState() == State.NEW) {
            thread.start();
        } else if( thread.getState() == State.TERMINATED) {
            thread = new Thread(runner);
            thread.start();
        }
        return null;
    }

    /**
     * Convenience method to retrieve a bitmap image from
     * a URL over the network. The built-in methods do
     * not seem to work, as they return a FileNotFound
     * exception.
     *
     * Note that this does not perform any threading --
     * it blocks the call while retrieving the data.
     *
     * @param url The URL to read the bitmap from.
     * @return A Bitmap image or null if an error occurs.
     */
    public static Bitmap readBitmapFromNetwork( URL url ) {
        InputStream is = null;
        BufferedInputStream bis = null;
        Bitmap bmp = null;
        try {
            URLConnection conn = url.openConnection();
            conn.connect();
            is = conn.getInputStream();
            bis = new BufferedInputStream(is);
            bmp = BitmapFactory.decodeStream(bis);
        } catch (MalformedURLException e) {
            Log.e(TAG, "Bad ad URL", e);
        } catch (IOException e) {
            Log.e(TAG, "Could not get remote ad image", e);
        } finally {
            try {
                if( is != null )
                    is.close();
                if( bis != null )
                    bis.close();
            } catch (IOException e) {
                Log.w(TAG, "Error closing stream.");
            }
        }
        return bmp;
    }

}

Answer 1

您可以使用位图类的压缩方法存储位图:here

我自己的代码是:

FileOutputStream os = null;
try {
    os = new FileOutputStream(
            new File(   
                   android.os.Environment.getExternalStorageDirectory().getAbsolutePath(),
                fileName));

    bitmap.compress(Bitmap.CompressFormat.JPEG, 90, os);
} catch (FileNotFoundException e) {
    e.printStackTrace();
}
finally {
    if (os != null) {
        try {
            os.close();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

它允许您在 SD 卡上写入位图。

要打开存储在 SD 卡上的位图，请使用 BitmapFactory 类中的 decodeFile() 方法:here

您还应该验证您是否可以使用以下方法在 SD 卡上读写:android.os.environment.getExternalStorageDirectory().canRead()和android.os.environment.getExternalStorageDirectory().canWrite()方法。