图片缓存

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

转http://www.eoeandroid.com/thread-212197-1-1.html