android读取大图片并缓存

最近开发电视版的云存储应用，要求”我的相册“模块有全屏预览图片的功能，全屏分辨率是1920*1080超清。
UI组件方面采用Gallery+ImageSwitcher组合，这里略过，详情参见google
Android API。相册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