java - 结合使用 LRU 图像缓存和 HTTPResponseCache 进行磁盘和内存缓存

Question

最初的目标是同时使用磁盘和内存缓存。这需要实现 LRU 缓存和 DiskLruCache。但是，由于 HTTPResponse 缓存使用磁盘空间，我选择使用 LRU 缓存并执行 con.setUseCaches(true);

问题是我不太明白先实现什么。 对于 LRU 和 DiskLru 缓存，这是算法:

即

首先检查图像的内存缓存

如果有图片，返回它并更新缓存

否则检查磁盘缓存

如果磁盘缓存有图像，返回它并更新缓存

否则从互联网下载图像，返回它并更新缓存

现在使用下面的代码:

public class DownloadImageTask extends AsyncTask<String, Void, Bitmap> {
    private int inSampleSize = 0;

    private String imageUrl;

    private BaseAdapter adapter;

    private ImagesCache cache;

    private int desiredWidth, desiredHeight;

    private Bitmap image = null;

    private ImageView ivImageView;

    Context mContext;

    public DownloadImageTask(BaseAdapter adapter, int desiredWidth, int desiredHeight) {
        this.adapter = adapter;

        this.cache = ImagesCache.getInstance();

        this.desiredWidth = desiredWidth;

        this.desiredHeight = desiredHeight;
    }

    public DownloadImageTask(Context mContext, ImagesCache cache, ImageView ivImageView, int desireWidth, int desireHeight) {
        this.cache = cache;

        this.ivImageView = ivImageView;

        this.desiredHeight = desireHeight;

        this.desiredWidth = desireWidth;

        this.mContext = mContext;
    }

    @Override
    protected Bitmap doInBackground(String... params) {
        imageUrl = params[0];

        return getImage(imageUrl);
    }

    @Override
    protected void onPostExecute(Bitmap result) {
        super.onPostExecute(result);
        if (result != null) {
            cache.addImageToWarehouse(imageUrl, result);
            if (ivImageView != null) {
                ivImageView.setImageBitmap(result);
            }
            else {
            }
            if (adapter != null) {
                adapter.notifyDataSetChanged();
            }
        }
                /*
        * IMPORTANT:
        * This enables your retrieval from the cache when working offline
        */
        /***
         * Force buffered operations to the filesystem. This ensures that responses
         * written to the cache will be available the next time the cache is opened,
         * even if this process is killed.
         */
        HttpResponseCache cache = HttpResponseCache.getInstalled();
        if(cache != null) {
            //the number of HTTP requests issued since this cache was created.
            Log.e("total num HTTP requests", String.valueOf(cache.getHitCount()));
            //the number of those requests that required network use.
            Log.e("num req network", String.valueOf(cache.getNetworkCount()));
            //the number of those requests whose responses were served by the cache.
            Log.e("num use cache", String.valueOf(cache.getHitCount()));
            //   If cache is present, flush it to the filesystem.
            //   Will be used when activity starts again.
            cache.flush();
            /***
             * Uninstalls the cache and releases any active resources. Stored contents
             * will remain on the filesystem.
             */
            //UNCOMMENTING THIS PRODUCES A java.lang.IllegalStateException: cache is closedtry {
              //  cache.close();
            //}
            //catch(IOException e){
              //  e.printStackTrace();
            //}
        }
    }

    private Bitmap getImage(String imageUrl) {
        if (cache.getImageFromWarehouse(imageUrl) == null) {
            BitmapFactory.Options options = new BitmapFactory.Options();
            options.inJustDecodeBounds = true;
            options.inSampleSize = inSampleSize;
            //installing the cache at application startup.
            try {
                HttpResponseCache cache = HttpResponseCache.getInstalled();
                if (cache == null) {
                    File httpCacheDir = new File(mContext.getCacheDir(), "http");
                    long HTTP_CACHE_SIZE_IN_BYTES = 1024 * 1024 * 1024; // 1GB
                    HttpResponseCache.install(httpCacheDir, HTTP_CACHE_SIZE_IN_BYTES);
                    //Log.e("Max DiskLRUCache Size", String.valueOf(DiskLruCache.getMaxSize());
                }
            }
            catch (IOException e) {
                e.printStackTrace();
            }
            try {
                int readTimeout = 300000;
                int connectTimeout = 300000;
                URL url = new URL(imageUrl);
                HttpURLConnection connection = (HttpURLConnection) url.openConnection();
                connection.setUseCaches(true);
                connection.addRequestProperty("Cache-Control", "max-stale=" + maxStale);
                connection.setConnectTimeout(connectTimeout);
                connection.setReadTimeout(readTimeout);
                InputStream stream = connection.getInputStream();
                image = BitmapFactory.decodeStream(stream, null, options);
                int imageWidth = options.outWidth;
                int imageHeight = options.outHeight;
                if (imageWidth > desiredWidth || imageHeight > desiredHeight) {
                    System.out.println("imageWidth:" + imageWidth + ", imageHeight:" + imageHeight);
                    inSampleSize = inSampleSize + 2;
                    getImage(imageUrl);
                }
                else {
                    options.inJustDecodeBounds = false;
                    connection = (HttpURLConnection) url.openConnection();
                    connection.setUseCaches(true);
                    connection.addRequestProperty("Cache-Control", "only-if-cached");
                    connection.setConnectTimeout(connectTimeout);
                    connection.setReadTimeout(readTimeout);
                    stream = connection.getInputStream();
                    image = BitmapFactory.decodeStream(stream, null, options);
                    return image;
                }
            }
            catch (Exception e) {
                Log.e("getImage", e.toString());
            }
        }
        return image;
    }
}

似乎在 doInBackground() 中我将保存到 HttpResponseCache，而在 onPostExecute() 中我将相同的图像保存到 LRUCache。我想做的是:

如果图片没有被缓存，保存到HttpResponseCache 如果 HttpResponseCache 已满，则保存到 LRUCache。 如果两者都已满，请使用默认删除机制删除旧的未使用图像。 问题是保存到两个缓存而不是任何一个

编辑重述问题

**

因为两者都缓存相同的图像，两次并将缓存保存到同一个文件系统，也许问题应该是使用哪个，因为似乎没有必要同时使用两者......

**

Answer 1

如果有人想重复使用上述任何代码，我只会使用 http 响应缓存而不使用 LRU 缓存，因为特别是如果您正在缓存 web 服务响应，例如JSON，XML。为什么？

由于上面的 LRU 缓存实现，我曾经丢失了 200MB 的设备存储空间。

HTTPResponseCache 的优点:

• 将 HTTP 和 HTTPS 响应缓存到文件系统，以便它们可以 重用，节省时间和带宽
• HttpUrlConnection 的作用:自动处理缓存机制，
• 在以下方面的帮助下加快应用程序的响应时间 HttpResponseCache
• 它从 API 级别 1 开始可用=> 它经受住了时间的考验并且很健壮

另一方面:

虽然 LRUCache 相对于 DiskLRUCache 有其优势:

• 您必须实现该类(和其他辅助类)，这意味着如果 android 开发人员的代码发生变化，您将不得不在您的应用程序中断后不断下载和编辑您的本地版本，而之前的实现将被弃用。
• 删除图像后，您可能会发现您的磁盘空间仍在使用，因为图像将位于您设备中的某个位置(就像我的情况一样)。

结论就是...