android 用LruCache读取大图片并缓存(转)

图片预取缓存策略是内存缓存（硬引用LruCache、软引用SoftReference<Bitmap>）、外部文件缓存（context.getCachedDir()），缓存中取不到的情况下再向服务端请求下载图片。同时缓存三张图片（当前预览的这张，前一张以及后一张）。

1.内存缓存

•//需要导入外部jar文件 android-support-v4.jar 
•import android.support.v4.util.LruCache; 
•//开辟8M硬缓存空间 
•private final int hardCachedSize = 8*1024*1024; 
•//hard cache 
•private final LruCache<String, Bitmap> sHardBitmapCache = new LruCache<String, Bitmap>(hardCachedSize){ 
•@Override 
•public int sizeOf(String key, Bitmap value){ 
•return value.getRowBytes() * value.getHeight(); 
•} 
•@Override 
•protected void entryRemoved(boolean evicted, String key, Bitmap oldValue, Bitmap newValue){ 
•Log.v("tag", "hard cache is full , push to soft cache"); 
•//硬引用缓存区满，将一个最不经常使用的oldvalue推入到软引用缓存区 
•sSoftBitmapCahe.put(key, new SoftReference<Bitmap>(oldValue)); 
•} 
•} 
•//软引用 
•private static final int SOFT_CACHE_CAPACITY = 40; 
•private final static LinkedHashMap<String, SoftReference<Bitmap>> sSoftBitmapCache = 
•new LinkedHashMao<String, SoftReference<Bitmap>>(SOFT_CACHE_CAPACITY, 0.75f, true){ 
•@Override 
•public SoftReference<Bitmap> put(String key, SoftReference<Bitmap> value){ 
•return super.input(key, value); 
•} 
•@Override 
•protected boolean removeEldestEntry(LinkedHashMap.Entry<Stirng, SoftReference<Bitmap>> eldest){ 
•if(size() > SOFT_CACHE_CAPACITY){ 
•Log.v("tag", "Soft Reference limit , purge one"); 
•return true; 
•} 
•return false; 
•} 
•} 
•//缓存bitmap 
•public boolean putBitmap(String key, Bitmap bitmap){ 
•if(bitmap != null){ 
•synchronized(sHardBitmapCache){ 
•sHardBitmapCache.put(key, bitmap); 
•} 
•return true; 
•} 
•return false; 
•} 
•//从缓存中获取bitmap 
•public Bitmap getBitmap(String key){ 
•synchronized(sHardBitmapCache){ 
•final Bitmap bitmap = sHardBitmapCache.get(key); 
•if(bitmap != null) 
•return bitmap; 
•} 
•//硬引用缓存区间中读取失败，从软引用缓存区间读取 
•synchronized(sSoftBitmapCache){ 
•SoftReference<Bitmap> bitmapReference = sSoftBtimapCache.get(key); 
•if(bitmapReference != null){ 
•final Bitmap bitmap2 = bitmapReference.get(); 
•if(bitmap2 != null) 
•return bitmap2; 
•else{ 
•Log.v("tag", "soft reference 已经被回收"); 
•sSoftBitmapCache.remove(key); 
•} 
•} 
•} 
•return null; 
•} 

　　

2.外部文件缓存

•private File mCacheDir = context.getCacheDir(); 
•private static final int MAX_CACHE_SIZE = 20 * 1024 * 1024; //20M 
•private final LruCache<String, Long> sFileCache = new LruCache<String, Long>(MAX_CACHE_SIZE){ 
•@Override 
•public int sizeOf(String key, Long value){ 
•return value.intValue(); 
•} 
•@Override 
•protected void entryRemoved(boolean evicted, String key, Long oldValue, Long newValue){ 
•File file = getFile(key); 
•if(file != null) 
•file.delete(); 
•} 
•} 
•private File getFile(String fileName) throws FileNotFoundException { 
•File file = new File(mCacheDir, fileName); 
•if(!file.exists() || !file.isFile()) 
•throw new FileNotFoundException("文件不存在或有同名文件夹"); 
•return file; 
•} 
•//缓存bitmap到外部存储 
•public boolean putBitmap(String key, Bitmap bitmap){ 
•File file = getFile(key); 
•if(file != null){ 
•Log.v("tag", "文件已经存在"); 
•return true; 
•} 
•FileOutputStream fos = getOutputStream(key); 
•boolean saved = bitmap.compress(CompressFormat.JPEG, 100, fos); 
•fos.flush(); 
•fos.close(); 
•if(saved){ 
•synchronized(sFileCache){ 
•sFileCache.put(key, getFile(key).length()); 
•} 
•return true; 
•} 
•return false; 
•} 
•//根据key获取OutputStream 
•private FileOutputStream getOutputStream(String key){ 
•if(mCacheDir == null) 
•return null; 
•FileOutputStream fos = new FileOutputStream(mCacheDir.getAbsolutePath() + File.separator + key); 
•return fos; 
•} 
•//获取bitmap 
•private static BitmapFactory.Options sBitmapOptions; 
•static { 
•sBitmapOptions = new BitmapFactory.Options(); 
•sBitmapOptions.inPurgeable=true; //bitmap can be purged to disk 
•} 
•public Bitmap getBitmap(String key){ 
•File bitmapFile = getFile(key); 
•if(bitmapFile != null){ 
•Bitmap bitmap = BitmapFactory.decodeStream(new FileInputStream(bitmapFile), null, sBitmapOptions); 
•if(bitmap != null){ 
•//重新将其缓存至硬引用中 
•... 
•} 
•} 
•} 

3.从服务端下载图片
下载成功后调用1内存缓存的putBitmap()函数,缓存图片。
在外部文件缓存中也写入一份，调用2的putBitmap()函数.
4.预览图片的流程
1) 如果预览的图片在内存缓存区中，直接调用1的getBitmap()函数，获取bitmap数据(先在硬引用缓存区查找匹配，若硬引用区匹配失败，再去软引用区匹配)
2) 如果从内存缓存区读取失败，再从外部文件缓存中读取，调用2的getBitmap()函数
3) 如果从外部文件缓存中读取失败，则从服务端下载该图片，过程3.
5.生成key值

private static String generateKey(String fileId, int width, int height) { 
String ret = fileId + "_" + Integer.toString(width) + "x" + Integer.toString(height); 
return ret; 
} 

• String key = generateKey(...)即可生成唯一的key值

原文请看：http://androidkaifa.com/thread-52-1-1.html；