图片缓存

如果每次加载同一张图片都要从网络获取，那代价实在太大了。所以同一张图片只要从网络获取一次就够了，然后在本地缓存起来，之后加载同一张图片时就从缓存中加载就可以了。从内存缓存读取图片是最快的，但是因为内存容量有限，所以最好再加上文件缓存。文件缓存空间也不是无限大的，容量越大读取效率越低，因此可以设置一个限定大小比如10M，或者限定保存时间比如一天。

因此，加载图片的流程应该是：

1、先从内存缓存中获取，取到则返回，取不到则进行下一步；

2、从文件缓存中获取，取到则返回并更新到内存缓存，取不到则进行下一步；

3、从网络下载图片，并更新到内存缓存和文件缓存。

接下来看内存缓存类：ImageMemoryCache

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public class ImageMemoryCache { /** * 从内存读取数据速度是最快的，为了更大限度使用内存，这里使用了两层缓存。 * 硬引用缓存不会轻易被回收，用来保存常用数据，不常用的转入软引用缓存。 */ private static final int SOFT_CACHE_SIZE = 15; //软引用缓存容量 private static LruCache<String, Bitmap> mLruCache; //硬引用缓存 private static LinkedHashMap<String, SoftReference<Bitmap>> mSoftCache; //软引用缓存 public ImageMemoryCache(Context context) { int memClass = ((ActivityManager)context.getSystemService(Context.ACTIVITY_SERVICE)).getMemoryClass(); int cacheSize = 1024 * 1024 * memClass / 4; //硬引用缓存容量，为系统可用内存的1/4 mLruCache = new LruCache<String, Bitmap>(cacheSize) { @Override protected int sizeOf(String key, Bitmap value) { if (value != null) return value.getRowBytes() * value.getHeight(); else return 0; } @Override protected void entryRemoved(boolean evicted, String key, Bitmap oldValue, Bitmap newValue) { if (oldValue != null) // 硬引用缓存容量满的时候，会根据LRU算法把最近没有被使用的图片转入此软引用缓存 mSoftCache.put(key, new SoftReference<Bitmap>(oldValue)); } }; mSoftCache = new LinkedHashMap<String, SoftReference<Bitmap>>(SOFT_CACHE_SIZE, 0.75f, true) { private static final long serialVersionUID = 6040103833179403725L; @Override protected boolean removeEldestEntry(Entry<String, SoftReference<Bitmap>> eldest) { if (size() > SOFT_CACHE_SIZE){ return true; } return false; } }; } /** * 从缓存中获取图片 */ public Bitmap getBitmapFromCache(String url) { Bitmap bitmap; //先从硬引用缓存中获取 synchronized (mLruCache) { bitmap = mLruCache.get(url); if (bitmap != null) { //如果找到的话，把元素移到LinkedHashMap的最前面，从而保证在LRU算法中是最后被删除 mLruCache.remove(url); mLruCache.put(url, bitmap); return bitmap; } } //如果硬引用缓存中找不到，到软引用缓存中找 synchronized (mSoftCache) { SoftReference<Bitmap> bitmapReference = mSoftCache.get(url); if (bitmapReference != null) { bitmap = bitmapReference.get(); if (bitmap != null) { //将图片移回硬缓存 mLruCache.put(url, bitmap); mSoftCache.remove(url); return bitmap; } else { mSoftCache.remove(url); } } } return null; } /** * 添加图片到缓存 */ public void addBitmapToCache(String url, Bitmap bitmap) { if (bitmap != null) { synchronized (mLruCache) { mLruCache.put(url, bitmap); } } } public void clearCache() { mSoftCache.clear(); } }

文件缓存类：ImageFileCache

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public class ImageFileCache { private static final String CACHDIR = "ImgCach"; private static final String WHOLESALE_CONV = ".cach"; private static final int MB = 1024*1024; private static final int CACHE_SIZE = 10; private static final int FREE_SD_SPACE_NEEDED_TO_CACHE = 10; public ImageFileCache() { //清理文件缓存 removeCache(getDirectory()); } /** 从缓存中获取图片 **/ public Bitmap getImage(final String url) { final String path = getDirectory() + "/" + convertUrlToFileName(url); File file = new File(path); if (file.exists()) { Bitmap bmp = BitmapFactory.decodeFile(path); if (bmp == null) { file.delete(); } else { updateFileTime(path); return bmp; } } return null; } /** 将图片存入文件缓存 **/ public void saveBitmap(Bitmap bm, String url) { if (bm == null) { return; } //判断sdcard上的空间 if (FREE_SD_SPACE_NEEDED_TO_CACHE > freeSpaceOnSd()) { //SD空间不足 return; } String filename = convertUrlToFileName(url); String dir = getDirectory(); File dirFile = new File(dir); if (!dirFile.exists()) dirFile.mkdirs(); File file = new File(dir +"/" + filename); try { file.createNewFile(); OutputStream outStream = new FileOutputStream(file); bm.compress(Bitmap.CompressFormat.JPEG, 100, outStream); outStream.flush(); outStream.close(); } catch (FileNotFoundException e) { Log.w("ImageFileCache", "FileNotFoundException"); } catch (IOException e) { Log.w("ImageFileCache", "IOException"); } } /** * 计算存储目录下的文件大小， * 当文件总大小大于规定的CACHE_SIZE或者sdcard剩余空间小于FREE_SD_SPACE_NEEDED_TO_CACHE的规定 * 那么删除40%最近没有被使用的文件 */ private boolean removeCache(String dirPath) { File dir = new File(dirPath); File[] files = dir.listFiles(); if (files == null) { return true; } if (!android.os.Environment.getExternalStorageState().equals( android.os.Environment.MEDIA_MOUNTED)) { return false; } int dirSize = 0; for (int i = 0; i < files.length; i++) { if (files[i].getName().contains(WHOLESALE_CONV)) { dirSize += files[i].length(); } } if (dirSize > CACHE_SIZE * MB || FREE_SD_SPACE_NEEDED_TO_CACHE > freeSpaceOnSd()) { int removeFactor = (int) ((0.4 * files.length) + 1); Arrays.sort(files, new FileLastModifSort()); for (int i = 0; i < removeFactor; i++) { if (files[i].getName().contains(WHOLESALE_CONV)) { files[i].delete(); } } } if (freeSpaceOnSd() <= CACHE_SIZE) { return false; } return true; } /** 修改文件的最后修改时间 **/ public void updateFileTime(String path) { File file = new File(path); long newModifiedTime = System.currentTimeMillis(); file.setLastModified(newModifiedTime); } /** 计算sdcard上的剩余空间 **/ private int freeSpaceOnSd() { StatFs stat = new StatFs(Environment.getExternalStorageDirectory().getPath()); double sdFreeMB = ((double)stat.getAvailableBlocks() * (double) stat.getBlockSize()) / MB; return (int) sdFreeMB; } /** 将url转成文件名 **/ private String convertUrlToFileName(String url) { String[] strs = url.split("/"); return strs[strs.length - 1] + WHOLESALE_CONV; } /** 获得缓存目录 **/ private String getDirectory() { String dir = getSDPath() + "/" + CACHDIR; return dir; } /** 取SD卡路径 **/ private String getSDPath() { File sdDir = null; boolean sdCardExist = Environment.getExternalStorageState().equals( android.os.Environment.MEDIA_MOUNTED); //判断sd卡是否存在 if (sdCardExist) { sdDir = Environment.getExternalStorageDirectory(); //获取根目录 } if (sdDir != null) { return sdDir.toString(); } else { return ""; } } /** * 根据文件的最后修改时间进行排序 */ private class FileLastModifSort implements Comparator<File> { public int compare(File arg0, File arg1) { if (arg0.lastModified() > arg1.lastModified()) { return 1; } else if (arg0.lastModified() == arg1.lastModified()) { return 0; } else { return -1; } } } }

从网络获取图片：

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public class ImageGetFromHttp { private static final String LOG_TAG = "ImageGetFromHttp"; public static Bitmap downloadBitmap(String url) { final HttpClient client = new DefaultHttpClient(); final HttpGet getRequest = new HttpGet(url); try { HttpResponse response = client.execute(getRequest); final int statusCode = response.getStatusLine().getStatusCode(); if (statusCode != HttpStatus.SC_OK) { Log.w(LOG_TAG, "Error " + statusCode + " while retrieving bitmap from " + url); return null; } final HttpEntity entity = response.getEntity(); if (entity != null) { InputStream inputStream = null; try { inputStream = entity.getContent(); FilterInputStream fit = new FlushedInputStream(inputStream); return BitmapFactory.decodeStream(fit); } finally { if (inputStream != null) { inputStream.close(); inputStream = null; } entity.consumeContent(); } } } catch (IOException e) { getRequest.abort(); Log.w(LOG_TAG, "I/O error while retrieving bitmap from " + url, e); } catch (IllegalStateException e) { getRequest.abort(); Log.w(LOG_TAG, "Incorrect URL: " + url); } catch (Exception e) { getRequest.abort(); Log.w(LOG_TAG, "Error while retrieving bitmap from " + url, e); } finally { client.getConnectionManager().shutdown(); } 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; } } }

最后，获取一张图片的流程就如下代码所示：

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/*** 获得一张图片,从三个地方获取,首先是内存缓存,然后是文件缓存,最后从网络获取 ***/ public Bitmap getBitmap(String url) { // 从内存缓存中获取图片 Bitmap result = memoryCache.getBitmapFromCache(url); if (result == null) { // 文件缓存中获取 result = fileCache.getImage(url); if (result == null) { // 从网络获取 result = ImageGetFromHttp.downloadBitmap(url); if (result != null) { fileCache.saveBitmap(result, url); memoryCache.addBitmapToCache(url, result); } } else { // 添加到内存缓存 memoryCache.addBitmapToCache(url, result); } } return result; }