Android之图片缓存管理

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。

后两个步骤纯碎属于业务逻辑，暂且不表，这里来看一下手Q使用的图片缓存管理策略。
说到缓存管理，首先谈一下java中的强引用和弱引用

• 强引用：最普遍的引用，若一个对象具有强引用，那么GC绝不会回收它。如A a = new A()
• 弱引用： 弱引用又分为以下三类：
  • 软引用（SoftReference）: 这类引用只有当内存空间不足GC才会回收它
  • 弱引用（WeakReference）: 这类引用拥有更短的生命周期,GC扫描过程中一旦发现了此类引用，不管当前内存是否足够，立即回收
  • 虚引用（PhantomRefence）: 这类引用并不会决定对象的生命周期，如果一个对象仅持有虚引用，则任何时刻都可能被回收

下面来看看这样一个图片缓存类，为了更大限度使用缓存，它使用了强引用缓存（强引用）和弱引用缓存（弱引用）双重缓存，强引用缓存不会轻易被回收，用来保存常用数据，不常用的转入弱引用缓存。**

ImageCache.javapublic class ImageCache {    private static final String TAG = "ImageCache";    //CustomLruCache是一个继承了LruCache的继承类，它代表强引用缓存，它的缓存大小一般由业务方提供    private CustomLruCache<String, Drawable> mMemoryCache;// Default memory cache size    //这里设置的是弱引用缓存以及它所占据的空间大小    private static final int DEFAULT_MEM_CACHE_SIZE = 5; // 5MB    private final HashMap<String, WeakReference<Drawable>> mRefCache = new HashMap<String, WeakReference<Drawable>>();    public ImageCache(int memSize) {        memSize = Math.max(memSize, DEFAULT_MEM_CACHE_SIZE);        QLog.d(TAG, "Memory cache size = " + memSize + "MB");        mMemoryCache = new CustomLruCache<String, Drawable>(memSize * 1024 * 1024) {            //这里重写了LruCache的sizeOf方法，来计算每个图片资源所占用内存的字节数            @Override            protected int sizeOf(String key, Drawable drawable) {                if (drawable instanceof BitmapDrawable) {                    Bitmap bitmap = ((BitmapDrawable) drawable).getBitmap();                    if (bitmap != null) {                        //若是bitmap位图则直接计算它的大小                        return bitmap.getRowBytes() * bitmap.getHeight();                    }                    return 0;                } else if (drawable instanceof AnimationDrawable) {                    //若是逐帧动画，则首先获取它所有的帧数，再计算总共的大小                    AnimationDrawable anim = (AnimationDrawable) drawable;                    int count = anim.getNumberOfFrames();                    int memSize = 0;                    for (int i = 0; i < count; i++) {                        Drawable dr = anim.getFrame(i);                        if (dr instanceof BitmapDrawable) {                            Bitmap bitmap = ((BitmapDrawable) dr).getBitmap();                            if (bitmap != null) {                                memSize += bitmap.getRowBytes() * bitmap.getHeight();                            }                        }                    }                    return memSize;                }                return 0;            }        };    }    //从缓存中获取图片    public Drawable getImageFromMemCache(String key) {        Drawable memDrawable = null;        if (mMemoryCache != null) {            //首先从强引用缓存中获取图片，若找到的话，把元素移动到CustomLruCache的最后面，从而保证它在LRU算法中最后被删除？            //疑问，其实LinkedHashMap本身就存在LRU的算法机制，因此，get的时候，会自动移入到队列尾部            memDrawable = mMemoryCache.remove(key);            if (memDrawable != null) {                memDrawable = memDrawable.getConstantState().newDrawable();                mMemoryCache.put(key, memDrawable);                return memDrawable;            }        }        //强引用缓存中没有找到，开始在弱引用缓存中查找        WeakReference<Drawable> ref = mRefCache.get(key);        if (ref != null) {            //若找到的话，这里是否添加一步，将其从弱引用缓存移入强引用缓存中比较好            memDrawable = ref.get();            if (memDrawable == null) {                mRefCache.remove(key);            }        }        return memDrawable;    }    //添加图片到缓存，这里不理解为什么要向强引用缓存和弱引用缓存都要添加一份    public void addImageToCache(String data, Drawable drawable) {        // Add to memory cache        if (mMemoryCache != null && mMemoryCache.get(data) == null) {            mMemoryCache.put(data, drawable);            mRefCache.put(data, new WeakReference<Drawable>(drawable));        }    }    //从缓存中删除资源    public void removeImageFromCache(String data) {        if (mRefCache != null) {            mRefCache.remove(data);        }        if (mMemoryCache != null) {            mMemoryCache.remove(data);        }    }    public Drawable getImageFromDiskCache(String pathName) {        // TODO 暂不支持disk cache        return null;    }    public void clearCaches() {        // mDiskCache.clearCache();        mMemoryCache.evictAll();        mRefCache.clear();    }}   

整个缓存策略是使用弱引用缓存和强引用缓存配合使用，并结合LRUCache，在尽可能地利用缓存的基础上，也大大提高了缓存命中率。我个人觉得这个类有改进的地方，比如，当LRUCache在移除元素的时候，默认是直接删除掉。这里更好的方式是重写LRUCache的entryRemoved方法，使得强引用缓存满的时候，会根据LRU算法将最近最久没有被使用的图片自动移入弱引用缓存，如下：

 mMemoryCache = new CustomLruCache<String, Drawable>(memSize * 1024 * 1024) {    //这里重写了LruCache的sizeOf方法，来计算每个图片资源所占用内存的字节数      @Override      protected int sizeOf(String key, Drawable drawable) {                .........      }      当强引用缓存满时，会自动调用这个方法，这时候，将移除的元素添加到弱引用缓存中      @Override      protected void entryRemoved(boolean evicted, String key, Drawable oldDrawable, Drawable newDrawable) {            if (oldDawable != null) {                mRefCache.put(data, new WeakReference<Drawable>(oldDawable));            }      }  };    

接下来看内存缓存类：ImageMemoryCachepublic class ImageMemoryCache {       private static final int SOFT_CACHE_SIZE = 15;  //软引用缓存容量    private static LruCache mLruCache;  //硬引用缓存    private static LinkedHashMap> 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(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(oldValue));            }        };        mSoftCache = new LinkedHashMap>(SOFT_CACHE_SIZE, 0.75f, true) {            private static final long serialVersionUID = 6040103833179403725L;            @Override            protected boolean removeEldestEntry(Entry> eldest) {                if (size() > SOFT_CACHE_SIZE){                        return true;                  }                  return false;             }        };    }                                                                                         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 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;    }                                                                                          public void addBitmapToCache(String url, Bitmap bitmap) {        if (bitmap != null) {            synchronized (mLruCache) {                mLruCache.put(url, bitmap);            }        }    }                                                                                      public void clearCache() {        mSoftCache.clear();    }}接下来看内存缓存类：ImageMemoryCachepublic 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());    }                                                                       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;    }                                                                       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");        }    }                                                                        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;    }                                                                       public void updateFileTime(String path) {        File file = new File(path);        long newModifiedTime = System.currentTimeMillis();        file.setLastModified(newModifiedTime);    }                                                                       private int freeSpaceOnSd() {        StatFs stat = new StatFs(Environment.getExternalStorageDirectory().getPath());        double sdFreeMB = ((double)stat.getAvailableBlocks() * (double) stat.getBlockSize()) / MB;        return (int) sdFreeMB;    }                                                                        private String convertUrlToFileName(String url) {        String[] strs = url.split("/");        return strs[strs.length - 1] + WHOLESALE_CONV;    }                                                                       private String getDirectory() {        String dir = getSDPath() + "/" + CACHDIR;        return dir;    }                                                                       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 "";        }    }                                                                    private class FileLastModifSort implements Comparator {        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;    }                                                              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：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;}