（转）Android技术积累:图片缓存管理

如果每次加载同一张图片都要从网络获取，那代价实在太大了。所以同一张图片只要从网络获取一次就够了，然后在本地缓存起来，之后加载同一张图片时就从缓存中加载就可以了。从内存缓存读取图片是最快的，但是因为内存容量有限，所以最好再加上文件缓存。文件缓存空间也不是无限大的，容量越大读取效率越低，因此可以设置一个限定大小比如10M，或者限定保存时间比如一天。

因此，加载图片的流程应该是：

1. 先从内存缓存中获取，取到则返回，取不到则进行下一步；

2. 从文件缓存中获取，取到则返回并更新到内存缓存，取不到则进行下一步；

3. 从网络下载图片，并更新到内存缓存和文件缓存。

接下来看内存缓存类：ImageMemoryCache

public class ImageMemoryCache {
    /**
     * 从内存读取数据速度是最快的，为了更大限度使用内存，这里使用了两层缓存。
     * 硬引用缓存不会轻易被回收，用来保存常用数据，不常用的转入软引用缓存。
     */
    private static final int SOFT_CACHE_SIZE = 15;  //软引用缓存容量
    private static LruCache<String, Bitmap> mLruCache;  //硬引用缓存
    private static LinkedHashMap<String, SoftReference<Bitmap>> mSoftCache;  //软引用缓存

    public ImageMemoryCache(Context context) {
        int memClass = ((ActivityManager)context.getSystemService(Context.ACTIVITY_SERVICE)).getMemoryClass();
        int cacheSize = 1024 * 1024 * memClass / 4;  //硬引用缓存容量，为系统可用内存的1/4
        mLruCache = new LruCache<String, Bitmap>(cacheSize) {
            @Override
            protected int sizeOf(String key, Bitmap value) {
                if (value != null)
                    return value.getRowBytes() * value.getHeight();
                else
                    return 0;
            }

            @Override
            protected void entryRemoved(boolean evicted, String key, Bitmap oldValue, Bitmap newValue) {
                if (oldValue != null)
                    // 硬引用缓存容量满的时候，会根据LRU算法把最近没有被使用的图片转入此软引用缓存
                    mSoftCache.put(key, new SoftReference<Bitmap>(oldValue));
            }
        };
        mSoftCache = new LinkedHashMap<String, SoftReference<Bitmap>>(SOFT_CACHE_SIZE, 0.75f, true) {
            private static final long serialVersionUID = 6040103833179403725L;
            @Override
            protected boolean removeEldestEntry(Entry<String, SoftReference<Bitmap>> eldest) {
                if (size() > SOFT_CACHE_SIZE){    
                    return true;  
                }  
                return false; 
            }
        };
    }

    /**
     * 从缓存中获取图片
     */
    public Bitmap getBitmapFromCache(String url) {
        Bitmap bitmap;
        //先从硬引用缓存中获取
        synchronized (mLruCache) {
            bitmap = mLruCache.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
                mLruCache.remove(url);
                mLruCache.put(url, bitmap);
                return bitmap;
            }
        }
        //如果硬引用缓存中找不到，到软引用缓存中找
        synchronized (mSoftCache) { 
            SoftReference<Bitmap> bitmapReference = mSoftCache.get(url);
            if (bitmapReference != null) {
                bitmap = bitmapReference.get();
                if (bitmap != null) {
                    //将图片移回硬缓存
                    mLruCache.put(url, bitmap);
                    mSoftCache.remove(url);
                    return bitmap;
                } else {
                    mSoftCache.remove(url);
                }
            }
        }
        return null;
    } 

    /**
     * 添加图片到缓存
     */
    public void addBitmapToCache(String url, Bitmap bitmap) {
        if (bitmap != null) {
            synchronized (mLruCache) {
                mLruCache.put(url, bitmap);
            }
        }
    }

    public void clearCache() {
        mSoftCache.clear();
    }
}

文件缓存类：ImageFileCache

public class ImageFileCache {
    private static final String CACHDIR = "ImgCach";
    private static final String WHOLESALE_CONV = ".cach";

    private static final int MB = 1024*1024;
    private static final int CACHE_SIZE = 10;
    private static final int FREE_SD_SPACE_NEEDED_TO_CACHE = 10;

    public ImageFileCache() {
        //清理文件缓存
        removeCache(getDirectory());
    }

    /** 从缓存中获取图片 **/
    public Bitmap getImage(final String url) {    
        final String path = getDirectory() + "/" + convertUrlToFileName(url);
        File file = new File(path);
        if (file.exists()) {
            Bitmap bmp = BitmapFactory.decodeFile(path);
            if (bmp == null) {
                file.delete();
            } else {
                updateFileTime(path);
                return bmp;
            }
        }
        return null;
    }

    /** 将图片存入文件缓存 **/
    public void saveBitmap(Bitmap bm, String url) {
        if (bm == null) {
            return;
        }
        //判断sdcard上的空间
        if (FREE_SD_SPACE_NEEDED_TO_CACHE > freeSpaceOnSd()) {
            //SD空间不足
            return;
        }
        String filename = convertUrlToFileName(url);
        String dir = getDirectory();
        File dirFile = new File(dir);
        if (!dirFile.exists())
            dirFile.mkdirs();
        File file = new File(dir +"/" + filename);
        try {
            file.createNewFile();
            OutputStream outStream = new FileOutputStream(file);
            bm.compress(Bitmap.CompressFormat.JPEG, 100, outStream);
            outStream.flush();
            outStream.close();
        } catch (FileNotFoundException e) {
            Log.w("ImageFileCache", "FileNotFoundException");
        } catch (IOException e) {
            Log.w("ImageFileCache", "IOException");
        }
    } 

    /**
     * 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
     */
    private boolean removeCache(String dirPath) {
        File dir = new File(dirPath);
        File[] files = dir.listFiles();
        if (files == null) {
            return true;
        }
        if (!android.os.Environment.getExternalStorageState().equals(
                android.os.Environment.MEDIA_MOUNTED)) {
            return false;
        }

        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, new FileLastModifSort());
            for (int i = 0; i < removeFactor; i++) {
                if (files[i].getName().contains(WHOLESALE_CONV)) {
                    files[i].delete();
                }
            }
        }

        if (freeSpaceOnSd() <= CACHE_SIZE) {
            return false;
        }

        return true;
    }

    /** 修改文件的最后修改时间 **/
    public void updateFileTime(String path) {
        File file = new File(path);
        long newModifiedTime = System.currentTimeMillis();
        file.setLastModified(newModifiedTime);
    }

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

    /** 将url转成文件名 **/
    private String convertUrlToFileName(String url) {
        String[] strs = url.split("/");
        return strs[strs.length - 1] + WHOLESALE_CONV;
    }

    /** 获得缓存目录 **/
    private String getDirectory() {
        String dir = getSDPath() + "/" + CACHDIR;
        return dir;
    }

    /** 取SD卡路径 **/
    private String getSDPath() {
        File sdDir = null;
        boolean sdCardExist = Environment.getExternalStorageState().equals(
                android.os.Environment.MEDIA_MOUNTED);  //判断sd卡是否存在
        if (sdCardExist) {
            sdDir = Environment.getExternalStorageDirectory();  //获取根目录
        }
        if (sdDir != null) {
            return sdDir.toString();
        } else {
            return "";
        }
    }

    /**
     * 根据文件的最后修改时间进行排序
     */
    private class FileLastModifSort 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;
            }
        }
    }

}

从网络获取图片：

public class ImageGetFromHttp {
    private static final String LOG_TAG = "ImageGetFromHttp";

    public static Bitmap downloadBitmap(String url) {
        final HttpClient client = new DefaultHttpClient();
        final HttpGet getRequest = new HttpGet(url);

        try {
            HttpResponse response = client.execute(getRequest);
            final int statusCode = response.getStatusLine().getStatusCode();
            if (statusCode != HttpStatus.SC_OK) {
                Log.w(LOG_TAG, "Error " + statusCode + " while retrieving bitmap from " + url);
                return null;
            }

            final HttpEntity entity = response.getEntity();
            if (entity != null) {
                InputStream inputStream = null;
                try {
                    inputStream = entity.getContent();
                    FilterInputStream fit = new FlushedInputStream(inputStream);
                    return BitmapFactory.decodeStream(fit);
                } finally {
                    if (inputStream != null) {
                        inputStream.close();
                        inputStream = null;
                    }
                    entity.consumeContent();
                }
            }
        } catch (IOException e) {
            getRequest.abort();
            Log.w(LOG_TAG, "I/O error while retrieving bitmap from " + url, e);
        } catch (IllegalStateException e) {
            getRequest.abort();
            Log.w(LOG_TAG, "Incorrect URL: " + url);
        } catch (Exception e) {
            getRequest.abort();
            Log.w(LOG_TAG, "Error while retrieving bitmap from " + url, e);
        } finally {
            client.getConnectionManager().shutdown();
        }
        return null;
    }

    /*
     * An InputStream that skips the exact number of bytes provided, unless it reaches EOF.
     */
    static class FlushedInputStream extends FilterInputStream {
        public FlushedInputStream(InputStream inputStream) {
            super(inputStream);
        }

        @Override
        public long skip(long n) throws IOException {
            long totalBytesSkipped = 0L;
            while (totalBytesSkipped < n) {
                long bytesSkipped = in.skip(n - totalBytesSkipped);
                if (bytesSkipped == 0L) {
                    int b = read();
                    if (b < 0) {
                        break;  // we reached EOF
                    } else {
                        bytesSkipped = 1; // we read one byte
                    }
                }
                totalBytesSkipped += bytesSkipped;
            }
            return totalBytesSkipped;
        }
    }
}

最后，获取一张图片的流程就如下代码所示：

/*** 获得一张图片,从三个地方获取,首先是内存缓存,然后是文件缓存,最后从网络获取 ***/
public Bitmap getBitmap(String url) {
    // 从内存缓存中获取图片
    Bitmap result = memoryCache.getBitmapFromCache(url);
    if (result == null) {
        // 文件缓存中获取
        result = fileCache.getImage(url);
        if (result == null) {
            // 从网络获取
            result = ImageGetFromHttp.downloadBitmap(url);
            if (result != null) {
                fileCache.saveBitmap(result, url);
                memoryCache.addBitmapToCache(url, result);
            }
        } else {
            // 添加到内存缓存
            memoryCache.addBitmapToCache(url, result);
        }
    }
    return result;
}