private void saveBmpToSd(Bitmap bm, Stringurl) { 
    if (bm == null) { 
      Log.w(TAG, " trying to savenull bitmap"); 
      return; 
    } 
     //判断sdcard上的空间 
    if (FREE_SD_SPACE_NEEDED_TO_CACHE >freeSpaceOnSd()) { 
      Log.w(TAG, "Low free space onsd, do not cache"); 
      return; 
    } 
    String filename =convertUrlToFileName(url); 
    String dir = getDirectory(filename); 
    File file = new File(dir +"/" + filename); 
    try { 
      file.createNewFile(); 
      OutputStream outStream = newFileOutputStream(file); 
      bm.compress(Bitmap.CompressFormat.JPEG, 100, outStream); 
      outStream.flush(); 
      outStream.close(); 
      Log.i(TAG, "Image saved tosd"); 
    } catch (FileNotFoundException e) { 
      Log.w(TAG,"FileNotFoundException"); 
    } catch (IOException e) { 
      Log.w(TAG,"IOException"); 
    } 
  } 

/** 
* 计算sdcard上的剩余空间 
* @return 
*/ 
private int freeSpaceOnSd() { 
  StatFs stat = newStatFs(Environment.getExternalStorageDirectory() .getPath()); 
  double sdFreeMB = ((double)stat.getAvailableBlocks() * (double) stat.getBlockSize()) / MB; 
  return (int) sdFreeMB; 
} 

/** 
* 修改文件的最后修改时间 
* @param dir 
* @param fileName 
*/ 
private void updateFileTime(String dir,String fileName) { 
  File file = new File(dir,fileName);     
  long newModifiedTime =System.currentTimeMillis(); 
  file.setLastModified(newModifiedTime); 
} 

/** 
*计算存储目录下的文件大小，当文件总大小大于规定的CACHE_SIZE或者sdcard剩余空间小于FREE_SD_SPACE_NEEDED_TO_CACHE的规定 
* 那么删除40%最近没有被使用的文件 
* @param dirPath 
* @param filename 
*/ 
private void removeCache(String dirPath) { 
  File dir = new File(dirPath); 
  File[] files = dir.listFiles(); 
  if (files == null) { 
    return; 
  } 
  int dirSize = 0; 
  for (int i = 0; i < files.length;i++) { 
    if(files[i].getName().contains(WHOLESALE_CONV)) { 
      dirSize += files[i].length(); 
    } 
  } 
  if (dirSize > CACHE_SIZE * MB ||FREE_SD_SPACE_NEEDED_TO_CACHE > freeSpaceOnSd()) { 
    int removeFactor = (int) ((0.4 *files.length) + 1); 
 
    Arrays.sort(files, newFileLastModifSort()); 
 
    Log.i(TAG, "Clear some expiredcache files "); 
 
    for (int i = 0; i <removeFactor; i++) { 
 
      if(files[i].getName().contains(WHOLESALE_CONV)) { 
 
        files[i].delete();        
 
      } 
 
    } 
 
  } 
 
} 
/** 
* 删除过期文件 
* @param dirPath 
* @param filename 
*/ 
private void removeExpiredCache(StringdirPath, String filename) { 
 
  File file = new File(dirPath,filename); 
 
  if (System.currentTimeMillis() -file.lastModified() > mTimeDiff) { 
 
    Log.i(TAG, "Clear some expiredcache files "); 
 
    file.delete(); 
 
  } 
 
} 

/** 
* TODO 根据文件的最后修改时间进行排序 * 
*/ 
classFileLastModifSort implements Comparator<File>{ 
  public int compare(File arg0, File arg1) { 
    if (arg0.lastModified() >arg1.lastModified()) { 
      return 1; 
    } else if (arg0.lastModified() ==arg1.lastModified()) { 
      return 0; 
    } else { 
      return -1; 
    } 
  } 
} 

private final HashMap<String, Bitmap>mHardBitmapCache = new LinkedHashMap<String, Bitmap>(HARD_CACHE_CAPACITY/ 2, 0.75f, true) { 
    @Override 
    protected booleanremoveEldestEntry(LinkedHashMap.Entry<String, Bitmap> eldest) { 
      if (size() >HARD_CACHE_CAPACITY) { 
        //当map的size大于30时，把最近不常用的key放到mSoftBitmapCache中，从而保证mHardBitmapCache的效率 
        mSoftBitmapCache.put(eldest.getKey(), newSoftReference<Bitmap>(eldest.getValue())); 
        return true; 
      } else 
        return false; 
    } 
  }; 

/** 
 *当mHardBitmapCache的key大于30的时候，会根据LRU算法把最近没有被使用的key放入到这个缓存中。 
 *Bitmap使用了SoftReference，当内存空间不足时，此cache中的bitmap会被垃圾回收掉 
 */ 
private final staticConcurrentHashMap<String, SoftReference<Bitmap>> mSoftBitmapCache =new ConcurrentHashMap<String,SoftReference<Bitmap>>(HARD_CACHE_CAPACITY / 2); 

从缓存中获取数据：
/** 
   * 从缓存中获取图片 
   */ 
  private Bitmap getBitmapFromCache(Stringurl) { 
    // 先从mHardBitmapCache缓存中获取 
    synchronized (mHardBitmapCache) { 
      final Bitmap bitmap =mHardBitmapCache.get(url); 
      if (bitmap != null) { 
        //如果找到的话，把元素移到linkedhashmap的最前面，从而保证在LRU算法中是最后被删除 
        mHardBitmapCache.remove(url); 
        mHardBitmapCache.put(url,bitmap); 
        return bitmap; 
      } 
    } 
    //如果mHardBitmapCache中找不到，到mSoftBitmapCache中找 
    SoftReference<Bitmap>bitmapReference = mSoftBitmapCache.get(url); 
    if (bitmapReference != null) { 
      final Bitmap bitmap =bitmapReference.get(); 
      if (bitmap != null) { 
        return bitmap; 
      } else { 
        mSoftBitmapCache.remove(url); 
      } 
    } 
    return null; 
  } 

/** 
 * 异步下载图片 
 */ 
class ImageDownloaderTask extendsAsyncTask<String, Void, Bitmap> { 
   private static final int IO_BUFFER_SIZE= 4 * 1024; 
   private String url; 
   private finalWeakReference<ImageView> imageViewReference; 
   public ImageDownloaderTask(ImageViewimageView) { 
     imageViewReference = newWeakReference<ImageView>(imageView); 
   } 
 
  @Override 
   protected BitmapdoInBackground(String... params) { 
     final AndroidHttpClient client =AndroidHttpClient.newInstance("Android"); 
     url = params[0]; 
     final HttpGet getRequest = newHttpGet(url); 
     try { 
       HttpResponse response =client.execute(getRequest); 
       final int statusCode =response.getStatusLine().getStatusCode(); 
       if (statusCode !=HttpStatus.SC_OK) { 
         Log.w(TAG, "从" +url + "中下载图片时出错!,错误码:" + statusCode); 
         return null; 
       } 
       final HttpEntity entity =response.getEntity(); 
       if (entity != null) { 
         InputStream inputStream =null; 
         OutputStream outputStream =null; 
         try { 
           inputStream =entity.getContent(); 
           finalByteArrayOutputStream dataStream = new ByteArrayOutputStream(); 
           outputStream = newBufferedOutputStream(dataStream, IO_BUFFER_SIZE); 
           copy(inputStream,outputStream); 
           outputStream.flush(); 
           final byte[] data =dataStream.toByteArray(); 
           final Bitmap bitmap =BitmapFactory.decodeByteArray(data, 0, data.length); 
           return bitmap; 
         } finally { 
           if (inputStream !=null) { 
            inputStream.close(); 
           } 
           if (outputStream !=null) { 
            outputStream.close(); 
           } 
          entity.consumeContent(); 
         } 
       } 
     } catch (IOException e) { 
       getRequest.abort(); 
       Log.w(TAG, "I/O errorwhile retrieving bitmap from " + url, e); 
     } catch (IllegalStateException e) { 
       getRequest.abort(); 
       Log.w(TAG, "Incorrect URL:" + url); 
     } catch (Exception e) { 
       getRequest.abort(); 
       Log.w(TAG, "Error whileretrieving bitmap from " + url, e); 
     } finally { 
       if (client != null) { 
         client.close(); 
       } 
     } 
     return null; 
   }