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；