android缓冲机制,Android 缓存机制

相册图片预取缓存策略是内存缓存(硬引用LruCache、软引用SoftReference)、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

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//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 sHardBitmapCache = new LruCache(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(oldValue));

}

}

//软引用

private static final int SOFT_CACHE_CAPACITY = 40;

private final static LinkedHashMap> sSoftBitmapCache =

new  LinkedHashMao>(SOFT_CACHE_CAPACITY, 0.75f, true){

@Override

public SoftReference put(String key, SoftReference value){

return super.input(key, value);

}

@Override

protected boolean removeEldestEntry(LinkedHashMap.Entry> 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 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

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private File mCacheDir = context.getCacheDir();

private static final int MAX_CACHE_SIZE = 20 * 1024 * 1024; //20M

private final LruCache sFileCache = new LruCache(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

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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