我已经阅读了有关 stackoverflow 中延迟列表加载的几个主题，并且我正在尝试了解如何在 android 中处理不同的缓存级别。
如此处所述：
我用
Multithreading For Performance，Gilles Debunne 的教程。
例子。我对其进行了修改，以便以正确的方式工作并且也可以使用 1.6。这是代码：
/*
* Copyright (C) 2010 The Android Open Source Project
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
public class ImageDownloader
{
private static final String TAG = "ImageDownloader";
private static final int HARD_CACHE_CAPACITY = 40;
private static final int DELAY_BEFORE_PURGE = 10 * 1000; // in milliseconds
private final static ConcurrentHashMap<String, SoftReference<Bitmap>> sSoftBitmapCache = new ConcurrentHashMap<String, SoftReference<Bitmap>>(HARD_CACHE_CAPACITY / 2);
/* Download the specified image from the Internet and binds it to the provided ImageView. The
* binding is immediate if the image is found in the cache and will be done asynchronously
* otherwise. A null bitmap will be associated to the ImageView if an error occurs.
*
* @param url The URL of the image to download.
* @param imageView The ImageView to bind the downloaded image to.
*/
public void download(String url, ImageView imageView) {
Log.d(TAG, "download(String url, ImageView imageView)");
resetPurgeTimer();
Bitmap bitmap = getBitmapFromCache(url);
if (bitmap == null) {
Log.d(TAG, "(bitmap == null)");
forceDownload(url, imageView);
} else {
Log.d(TAG, "(bitmap != null) ");
cancelPotentialDownload(url, imageView);
imageView.setImageBitmap(bitmap);
}
}
/*
* Same as download but the image is always downloaded and the cache is not used.
* Kept private at the moment as its interest is not clear.
private void forceDownload(String url, ImageView view) {
forceDownload(url, view, null);
}
*/
/**
* Same as download but the image is always downloaded and the cache is not used.
* Kept private at the moment as its interest is not clear.
*/
private void forceDownload(String url, ImageView imageView) {
Log.d(TAG, "forceDownload(String url, ImageView imageView)");
// State sanity: url is guaranteed to never be null in DownloadedDrawable and cache keys.
if (url == null) {
Log.d(TAG, "(url == null)");
imageView.setImageDrawable(null);
return;
}
Bitmap bitmap = null;
BitmapDownloaderTask task = null;
if (cancelPotentialDownload(url, imageView)) {
Log.d(TAG, "(cancelPotentialDownload(url, imageView))");
task = new BitmapDownloaderTask(imageView);
DownloadedDrawable downloadedDrawable = new DownloadedDrawable(task);
imageView.setImageDrawable(downloadedDrawable);
imageView.setMinimumHeight(156);
task.execute(url);
}
}
/**
* Returns true if the current download has been canceled or if there was no download in
* progress on this image view.
* Returns false if the download in progress deals with the same url. The download is not
* stopped in that case.
*/
private static boolean cancelPotentialDownload(String url, ImageView imageView) {
Log.d(TAG, "---cancelPotentialDownload(String url, ImageView imageView)----)");
BitmapDownloaderTask bitmapDownloaderTask = getBitmapDownloaderTask(imageView);
if (bitmapDownloaderTask != null) {
Log.d(TAG, "(bitmapDownloaderTask != null)");
String bitmapUrl = bitmapDownloaderTask.url;
if ((bitmapUrl == null) || (!bitmapUrl.equals(url))) {
Log.d(TAG, "(((bitmapUrl == null) || (!bitmapUrl.equals(url)))");
bitmapDownloaderTask.cancel(true);
} else {
Log.d(TAG, "The same URL is already being downloaded.");
// The same URL is already being downloaded.
return false;
}
}
return true;
}
/**
* @param imageView Any imageView
* @return Retrieve the currently active download task (if any) associated with this imageView.
* null if there is no such task.
*/
private static BitmapDownloaderTask getBitmapDownloaderTask(ImageView imageView) {
Log.d(TAG, "getBitmapDownloaderTask(ImageView imageView)");
if (imageView != null) {
Log.d(TAG, "(imageView != null) )");
Drawable drawable = imageView.getDrawable();
if (drawable instanceof DownloadedDrawable) {
Log.d(TAG, "(drawable instanceof DownloadedDrawable) ");
DownloadedDrawable downloadedDrawable = (DownloadedDrawable)drawable;
return downloadedDrawable.getBitmapDownloaderTask();
}
}
return null;
}
Bitmap downloadBitmap(String stringUrl) {
Log.d(TAG, "(downloadBitmap(String stringUrl)");
URL url = null;
HttpURLConnection connection = null;
InputStream inputStream = null;
try {
url = new URL(stringUrl);
connection = (HttpURLConnection) url.openConnection();
connection.setUseCaches(true);
inputStream = connection.getInputStream();
return BitmapFactory.decodeStream(new FlushedInputStream(inputStream));
/*
BitmapFactory.Options bfOptions=new BitmapFactory.Options();
bfOptions.inDither=false; //Disable Dithering mode
bfOptions.inPurgeable=true; //Tell to gc that whether it needs free memory, the Bitmap can be cleared
bfOptions.inInputShareable=true; //Which kind of reference will be used to recover the Bitmap data after being clear, when it will be used in the future
bfOptions.inTempStorage=new byte[32 * 1024];
Bitmap b=BitmapFactory.decodeStream(new FlushedInputStream(inputStream), null,bfOptions );
return b;
*/
} catch (IOException e) {
//getRequest.abort();
Log.w(TAG, "I/O error while retrieving bitmap from " + url, e);
} catch (IllegalStateException e) {
// getRequest.abort();
Log.w(TAG, "Incorrect URL: " + url);
} catch (Exception e) {
//getRequest.abort();
Log.w(TAG, "Error while retrieving bitmap from " + url, e);
} finally {
if (connection != null) {
connection.disconnect();
}
}
return null;
}
/*
* An InputStream that skips the exact number of bytes provided, unless it reaches EOF.
*/
static class FlushedInputStream extends FilterInputStream {
public FlushedInputStream(InputStream inputStream) {
super(inputStream);
}
@Override
public long skip(long n) throws IOException {
long totalBytesSkipped = 0L;
while (totalBytesSkipped < n) {
long bytesSkipped = in.skip(n - totalBytesSkipped);
if (bytesSkipped == 0L) {
int b = read();
if (b < 0) {
break; // we reached EOF
} else {
bytesSkipped = 1; // we read one byte
}
}
totalBytesSkipped += bytesSkipped;
}
return totalBytesSkipped;
}
}
/**
* The actual AsyncTask that will asynchronously download the image.
*/
class BitmapDownloaderTask extends AsyncTask<String, Void, Bitmap> {
private String url;
private final WeakReference<ImageView> imageViewReference;
public BitmapDownloaderTask(ImageView imageView) {
Log.w(TAG, "BitmapDownloaderTask(ImageView imageView)");
imageViewReference = new WeakReference<ImageView>(imageView);
}
/**
* Actual download method.
*/
@Override
protected Bitmap doInBackground(String... params) {
Log.w(TAG, "doInBackground");
url = params[0];
return downloadBitmap(url);
}
/**
* Once the image is downloaded, associates it to the imageView
*/
@Override
protected void onPostExecute(Bitmap bitmap) {
Log.w(TAG, "onPostExecute");
if (isCancelled()) {
bitmap = null;
}
addBitmapToCache(url, bitmap);
if (imageViewReference != null) {
Log.w(TAG, "(imageViewReference != null)");
ImageView imageView = imageViewReference.get();
BitmapDownloaderTask bitmapDownloaderTask = getBitmapDownloaderTask(imageView);
// Change bitmap only if this process is still associated with it
// Or if we don't use any bitmap to task association (NO_DOWNLOADED_DRAWABLE mode)
if ((this == bitmapDownloaderTask)) {
Log.w(TAG, " if ((this == bitmapDownloaderTask)");
imageView.setImageBitmap(bitmap);
}
}
}
}
/**
* A fake Drawable that will be attached to the imageView while the download is in progress.
*
* <p>Contains a reference to the actual download task, so that a download task can be stopped
* if a new binding is required, and makes sure that only the last started download process can
* bind its result, independently of the download finish order.</p>
*/
static class DownloadedDrawable extends ColorDrawable {
private final WeakReference<BitmapDownloaderTask> bitmapDownloaderTaskReference;
public DownloadedDrawable(BitmapDownloaderTask bitmapDownloaderTask) {
super(Color.BLACK);
Log.w(TAG, "DownloadedDrawable(BitmapDownloaderTask bitmapDownloaderTask) ");
bitmapDownloaderTaskReference = new WeakReference<BitmapDownloaderTask>(bitmapDownloaderTask);
}
public BitmapDownloaderTask getBitmapDownloaderTask() {
Log.w(TAG, "BitmapDownloaderTask getBitmapDownloaderTask() ");
return bitmapDownloaderTaskReference.get();
}
}
/*
public void setMode(Mode mode) {
this.mode = mode;
clearCache();
}
*/
/*
* Cache-related fields and methods.
*
* We use a hard and a soft cache. A soft reference cache is too aggressively cleared by the
* Garbage Collector.
*/
// Hard cache, with a fixed maximum capacity and a life duration
private final HashMap<String, Bitmap> sHardBitmapCache = new LinkedHashMap<String, Bitmap>(HARD_CACHE_CAPACITY / 2, 0.75f, true)
// private final HashMap<String, Bitmap> sHardBitmapCache = new LinkedHashMap<String, Bitmap>()
{
// private static final long serialVersionUID = -695136752926135717L;
@Override
protected boolean removeEldestEntry(LinkedHashMap.Entry<String, Bitmap> eldest) {
Log.w(TAG, "removeEldestEntry(LinkedHashMap.Entry<String, Bitmap> eldest) ");
Log.w(TAG, "size() : " + size());
Log.w(TAG, "HARD_CACHE_CAPACITY : " + HARD_CACHE_CAPACITY);
if (size() > HARD_CACHE_CAPACITY) {
Log.w(TAG, "(size() > HARD_CACHE_CAPACITY) ");
Log.e(TAG, "sSoftBitmapCache --> sSoftBitmapCache.put ");
// Entries push-out of hard reference cache are transferred to soft reference cache
sSoftBitmapCache.put(eldest.getKey(), new SoftReference<Bitmap>(eldest.getValue()));
return true;
} else
return false;
}
};
//HARD_CACHE_CAPACITY / 2
// Soft cache for bitmaps kicked out of hard cache
// private final static ConcurrentHashMap<String, SoftReference<Bitmap>> sSoftBitmapCache = new ConcurrentHashMap<String, SoftReference<Bitmap>>();
private final Handler purgeHandler = new Handler();
private final Runnable purger = new Runnable() {
public void run() {
Log.e(TAG, "purger run");
// clearCache();
}
};
/**
* Adds this bitmap to the cache.
* @param bitmap The newly downloaded bitmap.
*/
private void addBitmapToCache(String url, Bitmap bitmap) {
Log.w(TAG, "--------addBitmapToCache-----------");
if (bitmap != null) {
Log.w(TAG, "(bitmap != null) ");
synchronized (sHardBitmapCache) {
//final Bitmap tryData = sHardBitmapCache.get(url);
// if (tryData != null) {
// sHardBitmapCache.remove(url);
// }
Log.w(TAG, " sHardBitmapCache.put(url, bitmap);");
sHardBitmapCache.put(url, bitmap);
}
}
}
/**
* @param url The URL of the image that will be retrieved from the cache.
* @return The cached bitmap or null if it was not found.
*/
private Bitmap getBitmapFromCache(String url) {
Log.e(TAG, " --------getBitmapFromCache(String url)----------------");
// First try the hard reference cache
synchronized (sHardBitmapCache) {
final Bitmap bitmap = sHardBitmapCache.get(url);
if (bitmap != null) {
Log.e(TAG, " getBitmapFromCache ---> Bitmap found in Hard cache!!!!!");
// Bitmap found in hard cache
// Move element to first position, so that it is removed last
sHardBitmapCache.remove(url);
sHardBitmapCache.put(url, bitmap);
return bitmap;
}else{
Log.e(TAG, " getBitmapFromCache ---> Bitmap not found in Hard cache");
}
}
// Then try the soft reference cache
SoftReference<Bitmap> bitmapReference = sSoftBitmapCache.get(url);
if (bitmapReference != null) {
Log.e(TAG, " getBitmapFromCache ---> bitmapReference found in SoftReference cache!");
final Bitmap bitmap = bitmapReference.get();
if (bitmap != null) {
Log.e(TAG, "Bitmap found in SoftReference cache!!!!!!!");
// Bitmap found in soft cache
return bitmap;
} else {
Log.e(TAG, "oooooooooooooooooooo --> Soft reference has been Garbage Collected");
// Soft reference has been Garbage Collected
sSoftBitmapCache.remove(url);
}
}
return null;
}
/**
* Clears the image cache used internally to improve performance. Note that for memory
* efficiency reasons, the cache will automatically be cleared after a certain inactivity delay.
*/
public void clearCache() {
Log.e(TAG, "----------------clearCache() ------------------");
synchronized (sHardBitmapCache) {
//sHardBitmapCache.clear();
}
//sSoftBitmapCache.clear();
sHardBitmapCache.clear();
sSoftBitmapCache.clear();
}
/**
* Allow a new delay before the automatic cache clear is done.
*/
private void resetPurgeTimer() {
purgeHandler.removeCallbacks(purger);
purgeHandler.postDelayed(purger, DELAY_BEFORE_PURGE);
}
}
我完全理解这个例子背后的逻辑，但是我很困惑为什么它不能正常工作。
我们有两级缓存：硬缓存和软引用缓存。我们将位图保存在硬缓存中，然后当填充时我们保存在 SoftReference 中并重新排序硬缓存..
然后我关闭应用程序，关闭互联网并重新启动它。在构造函数中，我们调用 resetPurgeTimer() 来清除缓存，因此缓存中没有任何内容。为了避免这种情况，我注释了该行以便不清除缓存。我重新启动我的应用程序，我可以看到 SoftReference 缓存中存在 6-7 个图像（根据 LogCat）。
我知道当调用 GC 时 SoftReference 缓存可能会变空，但即使在关闭应用程序或禁用 wifi 之前它也几乎是空的……SoftReferences 有一个众所周知的错误，即使内存没有变低也会被垃圾收集。
这个广泛使用的例子有问题吗？
提前致谢，
安德烈亚斯