为何我的缓存机制无法按预期运行？

概述我对此深感困惑.我有一个静态类,用于管理已下载位图图像的缓存.此类可由多个线程访问,并且有可能进行并发访问.这是实现：publicclassBitmapCache{privatestaticfinalintMAX_NUMBER_BITMAPS_TO_CACHE=30;privatestaticMap<String,Bitmap>bitmapCache=new

我对此深感困惑.我有一个静态类,用于管理已下载位图图像的缓存.此类可由多个线程访问,并且有可能进行并发访问.这是实现：

public class BitmapCache {    private static final int MAX_NUMBER_BITMAPS_TO_CACHE = 30;    private static Map<String, Bitmap> bitmapCache = new HashMap<String, Bitmap>();    private static List<String> cachedBitmapUrlsOrder = new ArrayList<String>();    private BitmapCache(){}    public static synchronized voID addBitmapToCache(String url, Bitmap bitmap) {        if (bitmapCache.size() >= MAX_NUMBER_BITMAPS_TO_CACHE) {            Log.i("MyApp", "Max cache size reached.  Removing oldest bitmap.  Size = " + bitmapCache.size());            String oldestUrl = cachedBitmapUrlsOrder.remove(0);            bitmapCache.remove(oldestUrl);        }        bitmapCache.put(url, bitmap);        cachedBitmapUrlsOrder.add(url);    }    public static int size() {        return bitmapCache.size();    }    public static Bitmap get(String url) {        return bitmapCache.get(url);    }    public synchronized static voID clearCache() {        bitmapCache.clear();    }}

我正在尝试实现滚动缓存,以便在达到最大缓存大小(常见情况)时,从缓存中删除最旧的位图.运行我的应用程序将提供以下输出：

02-29 23:00:26.590: I/MyApp(10789): Max cache size reached.  Removing oldest bitmap.  Size = 3002-29 23:00:26.600: I/MyApp(10789): Max cache size reached.  Removing oldest bitmap.  Size = 3002-29 23:00:26.720: I/MyApp(10789): Max cache size reached.  Removing oldest bitmap.  Size = 3002-29 23:00:26.790: I/MyApp(10789): Max cache size reached.  Removing oldest bitmap.  Size = 3002-29 23:00:26.820: I/MyApp(10789): Max cache size reached.  Removing oldest bitmap.  Size = 3102-29 23:00:26.850: I/MyApp(10789): Max cache size reached.  Removing oldest bitmap.  Size = 3102-29 23:00:27.050: I/MyApp(10789): Max cache size reached.  Removing oldest bitmap.  Size = 3202-29 23:00:27.070: I/MyApp(10789): Max cache size reached.  Removing oldest bitmap.  Size = 3202-29 23:00:27.100: I/MyApp(10789): Max cache size reached.  Removing oldest bitmap.  Size = 3302-29 23:00:27.130: I/MyApp(10789): Max cache size reached.  Removing oldest bitmap.  Size = 3402-29 23:00:27.170: I/MyApp(10789): Max cache size reached.  Removing oldest bitmap.  Size = 3502-29 23:00:27.210: I/MyApp(10789): Max cache size reached.  Removing oldest bitmap.  Size = 3502-29 23:00:27.330: I/MyApp(10789): Max cache size reached.  Removing oldest bitmap.  Size = 3502-29 23:00:27.360: I/MyApp(10789): Max cache size reached.  Removing oldest bitmap.  Size = 35

当缓存大小达到30时,该代码正确地开始记录“已达到最大缓存大小”,并停留在此处几次执行.但随后奇怪的是,它开始增加到35.此时,它将停留在那里,以供数百个输出使用.我无法将其增加到35岁以上.

我的实现有什么问题？鉴于addBitmapToCache方法已同步,我对高速缓存的大小如何超出最大设置感到困惑.

解决方法:

几个问题.

首先,cachedBitmapUrlsOrder是一个列表.如果多次请求同一位图,会发生什么情况？您会在列表中找到一大堆重复的URL.因此,第一次达到限制时,您便从列表和地图中删除了一个URL.但是,相同的URL仍在列表中,没有匹配的映射条目.因此,后续尝试从列表中删除相同的URL不会从地图中删除任何内容,并且地图将会增长.

您可以通过使用bitmapCache.containsKey(url)在addBitmapToCache()开始时检查URL是否已被缓存来避免这种情况.如果是这样,则无需修改地图.只需确保该条目被记住为最新条目即可.

或者,将列表更改为唯一值的集合； URL到时间戳或Set的映射. (我让你决定.)

同样,get()和size()方法也应该同步.

并且get()应该更新检索到的条目的时间戳,以便它成为最新的.

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