Android中的缓存策略—拉取网络图片，缓存本地

一、前期基础知识储备

缓存策略在Android中有着广泛的使用场景，尤其在图片加载这个场景下，缓存策略更是尤为重要。加载图片是消耗流量的，如果用户加载图片，每次都要从网络中拉取，不但耗费更好的流量，同时加载速度也不是每次都可以接受的。如何避免过多的流量消耗呢？那就是Android中的缓存了。

当程序第一次从网络中加载图片之后，就将其缓存在存储设备上，这样下次使用这张图片就不用再从网络上获取了，这样就为用户节省了流量。很多时候为了提高用户的体验，往往还会把图片在内存中再缓存一份，这样当用户应用打算从网络上请求一张图片时，程序会首先从内存中去获取，如果内存中没有那就从存储设备中去获取，如果存储设备中没有，那就从网络中下载这张图片。因为从内存中加载扸比从存储设备中加载图片要快，所以这样既提高了程序的效率又为用户节约了不必要的流量开销。

上述的三级缓存策略不仅仅适用于图片，也适用于其他文件类型。

缓存策略：包括缓存的添加、获取和删除。不管是内存缓存还是存储设备缓存，它们的缓存大小都是有限制的，因此在使用缓存时总要为缓存指定一个最大的容量。如果当容量满了，但是程序还需要添加缓存，这个时候就需要删除一些旧的缓存并添加新的缓存，如何定义缓存的新旧这就是一种策略。

目前常用的一种缓存算法是LRU（Least Recently Used），LRU是近期最少使用算法，它的核心思想就是当缓存满时，会优先淘汰那些近期最少使用的缓存对象。采用LRU算法的缓存有两种：LruCache和DiskLruCache，LruCache用于实现内存缓存，而DiskLruCache则充当了存储设备缓存，通过三者的完美结合，就可以很方便的实现一个具有很高实用价值的ImageLoader。

二、上代码，具体实现

1.LruCache

LruCache是Android3.1所提供的一个缓存类。为了兼容Android低版本，在使用LruCache时建议使用support-v4兼容包下的LruCache，而不是直接使用Android3.1提供的LruCache。

LruCache是一个泛型类，它内部采用了一个LinkedHashMap以强引用的方式实现存储外界的缓存对象，其提供了get和put方法来完成缓存的获取和添加操作。

强引用：直接的对象引用；
软引用：当一个对象只有软引用存在时，系统内存不足时此对象会被gc回收；
弱引用：当一个对象只用弱引用存在时，此对象会随时被gc回收。

LruCache的实现比较简单，以下是使用LruCache来实现内存缓存：

1）LruCache的典型初始化过程：

        int maxMemory = (int) (Runtime.getRuntime().maxMemory() / 1024);        int cacheSize = maxMemory / 8; // 总容量为当前进程的可用内存的1/8        mMemoryCache = new LruCache(cacheSize) {            @Override            protected int sizeOf(String key, Bitmap bitmap) {                //sizeOf方法用以计算缓存对象的大小 比如Bitmap对象的大小                return bitmap.getRowBytes() * bitmap.getHeight() / 1024;            }        };

上述代码中，只需要提供缓存的总容量大小并重写sizeOf方法即可。sizeOf方法的作用是计算缓存对象的大小，这里的大小单位和总容量的单位一致。对于上面的示例代码来说，总容量大小为当前进程可用内存的1/8，单位为KB。而sizeOf方法则完成了Bitmap对象的大小计算。

2）LruCache的添加、获取和删除：

从LruCache中获取一个缓存对象：mMemoryCache.get(key)

从LruCache中添加一个缓存对象：mMemoryCache.put(key, bitmap)

从LruCache中删除一个缓存对象：remove方法

从Android3.1开始，LruCache就是Android源码的一部分了。

2.DiskLruCache

DiskLruCache用于实现存储设备缓存，即磁盘缓存，它通过将缓存对象写入文件系统，从而实现缓存的效果。DiskLruCache得到了Android官方文档的推荐，但它不属于AndroidSDK的一部分，可以从网上找到它的源码，修改后添加到项目中。

1）DiskLruCache的创建：

DiskLruCache并不能通过构造方法来创建，它提供了open方法用于创建自身：

public static DiskLruCache open (File directory, int appVersion, int valueCount, long maxSize)

open方法有四个参数，其中第一个参数表示磁盘缓存在文件系统中的存储路径。缓存路径可以选择SD卡上的缓存目录，具体是指/sdcard/Android/data/package_name/cache目录，其中package_name表示当前应用的包名，当应用被卸载后，此目录会一并被删除。这里给出一个建议：如果应用卸载后就希望删除缓存文件，那么就选择SD卡上的缓存目录，如果希望保留缓存数据那就应该选择SD卡上的其他特定目录；

第二个参数表示应用的版本号，设为1即可；

第三个参数表示单个节点所对应的数据个数大小，一般设为1即可；

第四个参数表示缓存的总大小，当缓存超过这个设定值后，DiskLruCache就会自行清除一些缓存。

private static final long DISK_CACHE_SIZE = 1024 * 1024 * 50; //50MBFile diskCacheDir = getDiskCacheDir(mContext, "bitmap");        if (!diskCacheDir.exists()) {            diskCacheDir.mkdirs();        }        if (getUsableSpace(diskCacheDir) > DISK_CACHE_SIZE) {            try {                mDiskLruCache = DiskLruCache.open(diskCacheDir, 1, 1,                        DISK_CACHE_SIZE);                mIsDiskLruCacheCreated = true;            } catch (IOException e) {                e.printStackTrace();            }        }

2）DiskLruCache的缓存添加：

DiskLruCache的缓存添加操作是通过Editor对象完成的，Editor表示一个缓存对象的编辑对象。这里以图片缓存为例，首先需要获取图片url对应的key，然后根据key就可以edit()来获取Editor对象，如果这个对象正在被编辑，那么edit()就会返回null。之所以把url转换成key，是因为图片的url中很可能有特殊字符，这将影响url在Android中的直接使用，一般采用url的md5值作为key。

    //url转key DisLruCache 添加+读取    private String hashKeyFormUrl(String url) {        String cacheKey;        try {            final MessageDigest mDigest = MessageDigest.getInstance("MD5");            mDigest.update(url.getBytes());            cacheKey = bytesToHexString(mDigest.digest());        } catch (NoSuchAlgorithmException e) {            cacheKey = String.valueOf(url.hashCode());        }        return cacheKey;    }    private String bytesToHexString(byte[] bytes) {        StringBuilder sb = new StringBuilder();        for (int i = 0; i < bytes.length; i++) {            String hex = Integer.toHexString(0xFF & bytes[i]);            if (hex.length() == 1) {                sb.append('0');            }            sb.append(hex);        }        return sb.toString();    }

将图片的url转为key之后，就可以获取Editor对象了。对于这个key来说，如果当前不存在其他Editor对象，那么edit()就会返回一个新的Editor对象，通过它就可以得到一个文件输出流。

        String key = hashKeyFormUrl(url);        DiskLruCache.Editor editor = mDiskLruCache.edit(key);        if (editor != null) {            OutputStream outputStream = editor.newOutputStream(DISK_CACHE_INDEX);        }

当从网络中下载图片时，图片就可以通过这个文件输出流写入到文件系统上:

    public boolean downloadUrlToStream(String urlString,            OutputStream outputStream) {        HttpURLConnection urlConnection = null;        BufferedOutputStream out = null;        BufferedInputStream in = null;        try {            final URL url = new URL(urlString);            urlConnection = (HttpURLConnection) url.openConnection();            in = new BufferedInputStream(urlConnection.getInputStream(),                    IO_BUFFER_SIZE);            out = new BufferedOutputStream(outputStream, IO_BUFFER_SIZE);            int b;            while ((b = in.read()) != -1) {                out.write(b);            }            return true;        } catch (IOException e) {            Log.e(TAG, "downloadBitmap failed." + e);        } finally {            if (urlConnection != null) {                urlConnection.disconnect();            }            MyUtils.close(out);            MyUtils.close(in);        }        return false;    }

经过上面的步骤，其实并没有真正将图片写入文件系统，还必须通过Editor的commit()来提交写入操作，如果图片下载过程发生了异常，那么还可以通过Editor的abort()来回退整个操作。

 OutputStream outputStream = editor.newOutputStream(DISK_CACHE_INDEX);            if (downloadUrlToStream(url, outputStream)) {                // 图片下载正常                editor.commit();            } else {                // 图片下载异常                editor.abort();            }            mDiskLruCache.flush();

经过上面几个步骤，图片就已经被正确地写入到文件系统中，接下来图片的获取操作就不需要请求网络了。

3）DiskLruCache的缓存查找：

和缓存的添加过程类似，缓存的查找过程也需要将url转化为key，然后通过DiskLruCache的get方法得到一个Snapshot对象，接着再通过Snapshot对象即可得到缓存的文件输入流，有了文件输出流，自然就可以得到Bitmap对象了。为了避免图片加载过程中导致的OOM问题，一般不建议直接加载原始图片。可以通过文件流来得到它所对应的文件描述符，然后再通过BitmapFactory.decodeFileDescriptor方法来加载一整缩放后的图片。实现如下：

        Bitmap bitmap = null;        String key = hashKeyFormUrl(url);        DiskLruCache.Snapshot snapShot = mDiskLruCache.get(key);        if (snapShot != null) {            FileInputStream fileInputStream = (FileInputStream)snapShot.getInputStream(DISK_CACHE_INDEX);            FileDescriptor fileDescriptor = fileInputStream.getFD();            bitmap = mImageResizer.decodeSampledBitmapFromFileDescriptor(fileDescriptor,                    reqWidth, reqHeight);            if (bitmap != null) {                addBitmapToMemoryCache(key, bitmap);            }        }

除了上述方法之外，DiskLruCache还提供了remove和delete方法来用于磁盘缓存的删除操作。

三、ImageLoader的实现

一般来说，一个优秀的ImageLoader应该具备如下功能：

图片的同步加载
图片的异步加载
图片压缩
内存缓存
磁盘缓存
网络拉取

图片的同步加载是指能够以同步的方式向调用者提供所加载的图片；图片的异步加载是一个很有用的功能，很多时候调用者不想在单独的线程中以同步的方式来获取图片，这个时候ImageLoader内部就需要自己在线程中加载图片并将图片甚至所需的ImageView。图片压缩的作用更加毋庸置疑了，这是降低OOM概率的有效手段，ImageLoader必须合理地处理图片的压缩问题。

内存缓存和磁盘缓存时ImageLoader的核心。

以下为完整实现：

1）ImageLoader

/** * 缓存：当应用第一次从网络加载图片之后，就将其缓存到内存和存储设备中，下次使用时就不必再从网络中获取； * 三级缓存：再次请求时，先访问内存，没有就再去访问存储设备，如果两者都没有再从网络中进行下载； * 缓存三要素：添加、读取、删除 * 缓存算法：LRU，近期使用最少算法，采用LRU算法的缓存有两种：LruCache和DisLruCache * LruCache：Android3.1提供的缓存类，v4包下可以兼容到早期版本。线程安全，提供了get和put方法来完成获取和添加操作 * DisLruCache：没有加入源码，本项目中可以copy一份出来。将缓存对象写入文件系统，从而实现缓存的效果。 *  创建：open()①SD卡上的缓存目录(getDiskCacheDir(context,"name")getCacheDir/getExternalCacheDir)，②1；③1；④缓存的总容量。 *  添加：Editor缓存对象的编辑对象，url转为key，根据key通过edit()来获取Editor对象，通过后者可以获取到一个文件输出流(OutputStream)，然后写入到文件系统中。commit提交；abort回退。 *        OutputStream outputStream = editor.newOutputStream(DISK_CACHE_INDEX); *  查找：添加类似：url转为key，通过get方法获取Snapshot对象，通过后者获取一个文件输入流，有了文件输入流(FileInputStream)，自然可以得到Bitmap查找对象了。 *        FileInputStream fileInputStream = (FileInputStream)snapShot.getInputStream(DISK_CACHE_INDEX); *  为了避免加载图片过程中导致的OOM问题，一般不建议直接加载原始图片，磁盘缓存时使用BitmapFactory.decodeFileDescriptor(）方法来加载一张缩放后的图片。 * *  加载的对象会优先缓存在磁盘缓存中，添加缓存成功后再往内存中缓存一份。读取的顺序是反过来的。 */public class ImageLoader {    private static final String TAG = "ImageLoader";    public static final int MESSAGE_POST_RESULT = 1;    private static final int CPU_COUNT = Runtime.getRuntime().availableProcessors();    private static final int CORE_POOL_SIZE = CPU_COUNT + 1;    private static final int MAXIMUM_POOL_SIZE = CPU_COUNT * 2 + 1;    private static final long KEEP_ALIVE = 10L;    private static final int TAG_KEY_URI = R.id.imageloader_uri;    private static final long DISK_CACHE_SIZE = 1024 * 1024 * 50; //50MB    private static final int IO_BUFFER_SIZE = 8 * 1024;    private static final int DISK_CACHE_INDEX = 0;    private boolean mIsDiskLruCacheCreated = false;    private static final ThreadFactory sThreadFactory = new ThreadFactory() {        private final AtomicInteger mCount = new AtomicInteger(1);        public Thread newThread(Runnable r) {            return new Thread(r, "ImageLoader#" + mCount.getAndIncrement());        }    };    //创建线程池 用于执行网络请求这一耗时操作    public static final Executor THREAD_POOL_EXECUTOR = new ThreadPoolExecutor(            CORE_POOL_SIZE, MAXIMUM_POOL_SIZE,            KEEP_ALIVE, TimeUnit.SECONDS,            new LinkedBlockingQueue(), sThreadFactory);    //创建Handler 用于发送消息 切回主线程执行UI操作    private Handler mMainHandler = new Handler(Looper.getMainLooper()) {        @Override        public void handleMessage(Message msg) {            LoaderResult result = (LoaderResult) msg.obj;            ImageView imageView = result.imageView;            String uri = (String) imageView.getTag(TAG_KEY_URI);            if (uri.equals(result.uri)) {                imageView.setImageBitmap(result.bitmap);            } else {                Log.w(TAG, "set image bitmap,but url has changed, ignored!");            }        };    };    private Context mContext;    private ImageResizer mImageResizer = new ImageResizer();    private LruCache mMemoryCache; // 内存缓存    private DiskLruCache mDiskLruCache; // 磁盘缓存    private ImageLoader(Context context) {        mContext = context.getApplicationContext();        // 内存缓存 LruCache初始化        int maxMemory = (int) (Runtime.getRuntime().maxMemory() / 1024);        int cacheSize = maxMemory / 8; // 总容量为当前进程的可用内存的1/8        mMemoryCache = new LruCache(cacheSize) {            @Override            protected int sizeOf(String key, Bitmap bitmap) {                //sizeOf方法用以计算缓存对象的大小 比如Bitmap对象的大小                return bitmap.getRowBytes() * bitmap.getHeight() / 1024;            }        };        // 磁盘缓存 DisLruCache初始化        File diskCacheDir = getDiskCacheDir(mContext, "bitmap");        if (!diskCacheDir.exists()) {            diskCacheDir.mkdirs();        }        if (getUsableSpace(diskCacheDir) > DISK_CACHE_SIZE) {            try {                mDiskLruCache = DiskLruCache.open(diskCacheDir, 1, 1,                        DISK_CACHE_SIZE);                mIsDiskLruCacheCreated = true;            } catch (IOException e) {                e.printStackTrace();            }        }    }    /**     * build a new instance of ImageLoader     * @param context     * @return a new instance of ImageLoader     */    public static ImageLoader build(Context context) {        return new ImageLoader(context);    }    // 添加进LruCache 添加缓存对象    private void addBitmapToMemoryCache(String key, Bitmap bitmap) {        if (getBitmapFromMemCache(key) == null) {            mMemoryCache.put(key, bitmap);        }    }    // LruCache读取出 获取缓存对象    private Bitmap getBitmapFromMemCache(String key) {        return mMemoryCache.get(key);    }    /**     * 异步加载图片     * load bitmap from memory cache or disk cache or network async, then bind imageView and bitmap.     * NOTE THAT: should run in UI Thread     * @param uri http url     * @param imageView bitmap's bind object     */    public void bindBitmap(final String uri, final ImageView imageView) {        bindBitmap(uri, imageView, 0, 0);    }    public void bindBitmap(final String uri, final ImageView imageView,            final int reqWidth, final int reqHeight) {        imageView.setTag(TAG_KEY_URI, uri);        Bitmap bitmap = loadBitmapFromMemCache(uri);        if (bitmap != null) {            imageView.setImageBitmap(bitmap);            return;        }        Runnable loadBitmapTask = new Runnable() {            @Override            public void run() {                Bitmap bitmap = loadBitmap(uri, reqWidth, reqHeight);                if (bitmap != null) {                    LoaderResult result = new LoaderResult(imageView, uri, bitmap);                    mMainHandler.obtainMessage(MESSAGE_POST_RESULT, result).sendToTarget();                }            }        };        THREAD_POOL_EXECUTOR.execute(loadBitmapTask);    }    /**     * 同步加载图片     * load bitmap from memory cache or disk cache or network.     * @param uri http url     * @param reqWidth the width ImageView desired     * @param reqHeight the height ImageView desired     * @return bitmap, maybe null.     */    public Bitmap loadBitmap(String uri, int reqWidth, int reqHeight) {        Bitmap bitmap = loadBitmapFromMemCache(uri); // 内存缓存中读取        if (bitmap != null) {            Log.d(TAG, "loadBitmapFromMemCache,url:" + uri);            return bitmap;        }        try {            bitmap = loadBitmapFromDiskCache(uri, reqWidth, reqHeight); // 磁盘缓存的查找 即读取            if (bitmap != null) {                Log.d(TAG, "loadBitmapFromDisk,url:" + uri);                return bitmap;            }            bitmap = loadBitmapFromHttp(uri, reqWidth, reqHeight); // 磁盘缓存的添加 当磁盘缓存查找失败时调用该方法            Log.d(TAG, "loadBitmapFromHttp,url:" + uri);        } catch (IOException e) {            e.printStackTrace();        }        if (bitmap == null && !mIsDiskLruCacheCreated) {            Log.w(TAG, "encounter error, DiskLruCache is not created.");            bitmap = downloadBitmapFromUrl(uri); // 前面两次读取缓存Bitmap失败 重现调用网络拉取        }        return bitmap;    }    // LruCache加载图片 调用上面的方法    private Bitmap loadBitmapFromMemCache(String url) {        final String key = hashKeyFormUrl(url);        return getBitmapFromMemCache(key);    }    //DisLruCache 添加 Editor -耗时操作    //添加的方法最终会在最后调用一次磁盘查找的方法，返回一个Bitmap，所以这是一个有返回值的方法    private Bitmap loadBitmapFromHttp(String url, int reqWidth, int reqHeight)            throws IOException {        // 写入磁盘的操作是耗时操作 不能在主线程中执行 利用主线程Looper进行判断是否为主线程        if (Looper.myLooper() == Looper.getMainLooper()) {            throw new RuntimeException("can not visit network from UI Thread.");        }        if (mDiskLruCache == null) {            return null;        }                String key = hashKeyFormUrl(url);        DiskLruCache.Editor editor = mDiskLruCache.edit(key);        if (editor != null) {            OutputStream outputStream = editor.newOutputStream(DISK_CACHE_INDEX);            if (downloadUrlToStream(url, outputStream)) {                // 图片下载正常                editor.commit();            } else {                // 图片下载异常                editor.abort();            }            mDiskLruCache.flush();        }        return loadBitmapFromDiskCache(url, reqWidth, reqHeight);    }    //DisLruCache 缓存查找 Snapshot -耗时操作    //磁盘缓存的查找方法，得到查找对象后，会调用内存缓存的添加方法，将对象添加进内存中    private Bitmap loadBitmapFromDiskCache(String url, int reqWidth,            int reqHeight) throws IOException {        // 读取磁盘的操作是耗时操作 不能在主线程中执行 利用主线程Looper进行判断是否为主线程        if (Looper.myLooper() == Looper.getMainLooper()) {            Log.w(TAG, "load bitmap from UI Thread, it's not recommended!");        }        if (mDiskLruCache == null) {            return null;        }        Bitmap bitmap = null;        String key = hashKeyFormUrl(url);        DiskLruCache.Snapshot snapShot = mDiskLruCache.get(key);        if (snapShot != null) {            FileInputStream fileInputStream = (FileInputStream)snapShot.getInputStream(DISK_CACHE_INDEX);            FileDescriptor fileDescriptor = fileInputStream.getFD();            bitmap = mImageResizer.decodeSampledBitmapFromFileDescriptor(fileDescriptor,                    reqWidth, reqHeight);            if (bitmap != null) {                addBitmapToMemoryCache(key, bitmap);            }        }        return bitmap;    }    //外界调用该方法时传入文件输出流对象，和Url，就可以直接将下载好的图片写入到文件系统上    //DisLruCache判断下载是否异常 写入是否成功 最后决定是com_mit() 还是ab_ort()    //通过Editor对象获取文件输出流之后，当从网络中下载图片之后，再利用这个输出流将图片写入到文件系统上    // OutputStream输出流 write()写入方法    // 注意这里已经执行了从网络中拉取图片的方法，所以调用磁盘添加方法时，实际已经进行了网络图片的拉取    public boolean downloadUrlToStream(String urlString,            OutputStream outputStream) {        HttpURLConnection urlConnection = null;        BufferedOutputStream out = null;        BufferedInputStream in = null;        try {            final URL url = new URL(urlString);            urlConnection = (HttpURLConnection) url.openConnection();            in = new BufferedInputStream(urlConnection.getInputStream(),                    IO_BUFFER_SIZE);            out = new BufferedOutputStream(outputStream, IO_BUFFER_SIZE);            int b;            while ((b = in.read()) != -1) {                out.write(b);            }            return true;        } catch (IOException e) {            Log.e(TAG, "downloadBitmap failed." + e);        } finally {            if (urlConnection != null) {                urlConnection.disconnect();            }            MyUtils.close(out);            MyUtils.close(in);        }        return false;    }    // HttpURL_Connection 网络拉取图片    private Bitmap downloadBitmapFromUrl(String urlString) {        Bitmap bitmap = null;        HttpURLConnection urlConnection = null;        BufferedInputStream in = null;        try {            final URL url = new URL(urlString);            urlConnection = (HttpURLConnection) url.openConnection();            in = new BufferedInputStream(urlConnection.getInputStream(),                    IO_BUFFER_SIZE);            bitmap = BitmapFactory.decodeStream(in);        } catch (final IOException e) {            Log.e(TAG, "Error in downloadBitmap: " + e);        } finally {            if (urlConnection != null) {                urlConnection.disconnect();            }            MyUtils.close(in);        }        return bitmap;    }    //url转key DisLruCache 添加+读取    private String hashKeyFormUrl(String url) {        String cacheKey;        try {            final MessageDigest mDigest = MessageDigest.getInstance("MD5");            mDigest.update(url.getBytes());            cacheKey = bytesToHexString(mDigest.digest());        } catch (NoSuchAlgorithmException e) {            cacheKey = String.valueOf(url.hashCode());        }        return cacheKey;    }    private String bytesToHexString(byte[] bytes) {        StringBuilder sb = new StringBuilder();        for (int i = 0; i < bytes.length; i++) {            String hex = Integer.toHexString(0xFF & bytes[i]);            if (hex.length() == 1) {                sb.append('0');            }            sb.append(hex);        }        return sb.toString();    }    public File getDiskCacheDir(Context context, String uniqueName) {        boolean externalStorageAvailable = Environment                .getExternalStorageState().equals(Environment.MEDIA_MOUNTED);        final String cachePath;        if (externalStorageAvailable) {            cachePath = context.getExternalCacheDir().getPath();            //  -/storage/emulated/0/Android/data/com.camera.fhuji.camera/cache 应用数据目录下的外部存储目录，优先使用        } else {            cachePath = context.getCacheDir().getPath();            //  -/data/user/0/com.camera.fhuji.camera/cache 应用数据目录下的内部存储目录，尽量少用        }        //  -/storage/emulated/0/Android/data/com.camera.fhuji.camera/cache/bitmap        return new File(cachePath + File.separator + uniqueName);    }    @TargetApi(VERSION_CODES.GINGERBREAD)    private long getUsableSpace(File path) {        if (Build.VERSION.SDK_INT >= VERSION_CODES.GINGERBREAD) {            return path.getUsableSpace();        }        final StatFs stats = new StatFs(path.getPath());        return (long) stats.getBlockSize() * (long) stats.getAvailableBlocks();    }    private static class LoaderResult {        public ImageView imageView;        public String uri;        public Bitmap bitmap;        public LoaderResult(ImageView imageView, String uri, Bitmap bitmap) {            this.imageView = imageView;            this.uri = uri;            this.bitmap = bitmap;        }    }}

2）图片压缩ImageResizer

public class ImageResizer {    private static final String TAG = "ImageResizer";    public ImageResizer() {    }    public Bitmap decodeSampledBitmapFromResource(Resources res,            int resId, int reqWidth, int reqHeight) {        // First decode with inJustDecodeBounds=true to check dimensions        final BitmapFactory.Options options = new BitmapFactory.Options();        options.inJustDecodeBounds = true;        BitmapFactory.decodeResource(res, resId, options);        // Calculate inSampleSize        options.inSampleSize = calculateInSampleSize(options, reqWidth,                reqHeight);        // Decode bitmap with inSampleSize set        options.inJustDecodeBounds = false;        return BitmapFactory.decodeResource(res, resId, options);    }    public Bitmap decodeSampledBitmapFromFileDescriptor(FileDescriptor fd, int reqWidth, int reqHeight) {        // First decode with inJustDecodeBounds=true to check dimensions        final BitmapFactory.Options options = new BitmapFactory.Options();        options.inJustDecodeBounds = true;        BitmapFactory.decodeFileDescriptor(fd, null, options);        // Calculate inSampleSize        options.inSampleSize = calculateInSampleSize(options, reqWidth,                reqHeight);        // Decode bitmap with inSampleSize set        options.inJustDecodeBounds = false;        return BitmapFactory.decodeFileDescriptor(fd, null, options);    }    public int calculateInSampleSize(BitmapFactory.Options options,            int reqWidth, int reqHeight) {        if (reqWidth == 0 || reqHeight == 0) {            return 1;        }        // Raw height and width of image        final int height = options.outHeight;        final int width = options.outWidth;        Log.d(TAG, "origin, w= " + width + " h=" + height);        int inSampleSize = 1;        if (height > reqHeight || width > reqWidth) {            final int halfHeight = height / 2;            final int halfWidth = width / 2;            // Calculate the largest inSampleSize value that is a power of 2 and            // keeps both            // height and width larger than the requested height and width.            while ((halfHeight / inSampleSize) >= reqHeight                    && (halfWidth / inSampleSize) >= reqWidth) {                inSampleSize *= 2;            }        }        Log.d(TAG, "sampleSize:" + inSampleSize);        return inSampleSize;    }}

3）Activity中使用ImageLoader

 mImageLoader.bindBitmap(uri, imageView, mImageWidth, mImageWidth);

4）附录：DiskLruCache源码（自主添加进项目）

/* * Copyright (C) 2011 The Android Open Source Project * * Licensed under the Apache License, Version 2.0 (the "License"); * you may not use this file except in compliance with the License. * You may obtain a copy of the License at * *      http://www.apache.org/licenses/LICENSE-2.0 * * Unless required by applicable law or agreed to in writing, software * distributed under the License is distributed on an "AS IS" BASIS, * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. * See the License for the specific language governing permissions and * limitations under the License. */package com.camera.fhuji.camera.art.loader;import java.io.BufferedInputStream;import java.io.BufferedWriter;import java.io.Closeable;import java.io.EOFException;import java.io.File;import java.io.FileInputStream;import java.io.FileNotFoundException;import java.io.FileOutputStream;import java.io.FileWriter;import java.io.FilterOutputStream;import java.io.IOException;import java.io.InputStream;import java.io.InputStreamReader;import java.io.OutputStream;import java.io.OutputStreamWriter;import java.io.Reader;import java.io.StringWriter;import java.io.Writer;import java.lang.reflect.Array;import java.nio.charset.Charset;import java.util.ArrayList;import java.util.Arrays;import java.util.Iterator;import java.util.LinkedHashMap;import java.util.Map;import java.util.concurrent.Callable;import java.util.concurrent.ExecutorService;import java.util.concurrent.LinkedBlockingQueue;import java.util.concurrent.ThreadPoolExecutor;import java.util.concurrent.TimeUnit;/** ****************************************************************************** * Taken from the JB source code, can be found in: * libcore/luni/src/main/java/libcore/io/DiskLruCache.java * or direct link: * https://android.googlesource.com/platform/libcore/+/android-4.1.1_r1/luni/src/main/java/libcore/io/DiskLruCache.java ****************************************************************************** * * A cache that uses a bounded amount of space on a filesystem. Each cache * entry has a string key and a fixed number of values. Values are byte * sequences, accessible as streams or files. Each value must be between {@code * 0} and {@code Integer.MAX_VALUE} bytes in length. * * The cache stores its data in a directory on the filesystem. This * directory must be exclusive to the cache; the cache may delete or overwrite * files from its directory. It is an error for multiple processes to use the * same cache directory at the same time. * * 
This cache limits the number of bytes that it will store on the * filesystem. When the number of stored bytes exceeds the limit, the cache will * remove entries in the background until the limit is satisfied. The limit is * not strict: the cache may temporarily exceed it while waiting for files to be * deleted. The limit does not include filesystem overhead or the cache * journal so space-sensitive applications should set a conservative limit. * * 
Clients call {@link #edit} to create or update the values of an entry. An * entry may have only one editor at one time; if a value is not available to be * edited then {@link #edit} will return null. * 
 *     When an entry is being created it is necessary to *         supply a full set of values; the empty value should be used as a *         placeholder if necessary. *     
When an entry is being edited, it is not necessary *         to supply data for every value; values default to their previous *         value. * 
 * Every {@link #edit} call must be matched by a call to {@link Editor#commit} * or {@link Editor#abort}. Committing is atomic: a read observes the full set * of values as they were before or after the commit, but never a mix of values. * * Clients call {@link #get} to read a snapshot of an entry. The read will * observe the value at the time that {@link #get} was called. Updates and * removals after the call do not impact ongoing reads. * * This class is tolerant of some I/O errors. If files are missing from the * filesystem, the corresponding entries will be dropped from the cache. If * an error occurs while writing a cache value, the edit will fail silently. * Callers should handle other problems by catching {@code IOException} and * responding appropriately. */public final class DiskLruCache implements Closeable {    static final String JOURNAL_FILE = "journal";    static final String JOURNAL_FILE_TMP = "journal.tmp";    static final String MAGIC = "libcore.io.DiskLruCache";    static final String VERSION_1 = "1";    static final long ANY_SEQUENCE_NUMBER = -1;    private static final String CLEAN = "CLEAN";    private static final String DIRTY = "DIRTY";    private static final String REMOVE = "REMOVE";    private static final String READ = "READ";    private static final Charset UTF_8 = Charset.forName("UTF-8");    private static final int IO_BUFFER_SIZE = 8 * 1024;    /*     * This cache uses a journal file named "journal". A typical journal file     * looks like this:     *     libcore.io.DiskLruCache     *     1     *     100     *     2     *     *     CLEAN 3400330d1dfc7f3f7f4b8d4d803dfcf6 832 21054     *     DIRTY 335c4c6028171cfddfbaae1a9c313c52     *     CLEAN 335c4c6028171cfddfbaae1a9c313c52 3934 2342     *     REMOVE 335c4c6028171cfddfbaae1a9c313c52     *     DIRTY 1ab96a171faeeee38496d8b330771a7a     *     CLEAN 1ab96a171faeeee38496d8b330771a7a 1600 234     *     READ 335c4c6028171cfddfbaae1a9c313c52     *     READ 3400330d1dfc7f3f7f4b8d4d803dfcf6     *     * The first five lines of the journal form its header. They are the     * constant string "libcore.io.DiskLruCache", the disk cache's version,     * the application's version, the value count, and a blank line.     *     * Each of the subsequent lines in the file is a record of the state of a     * cache entry. Each line contains space-separated values: a state, a key,     * and optional state-specific values.     *   o DIRTY lines track that an entry is actively being created or updated.     *     Every successful DIRTY action should be followed by a CLEAN or REMOVE     *     action. DIRTY lines without a matching CLEAN or REMOVE indicate that     *     temporary files may need to be deleted.     *   o CLEAN lines track a cache entry that has been successfully published     *     and may be read. A publish line is followed by the lengths of each of     *     its values.     *   o READ lines track accesses for LRU.     *   o REMOVE lines track entries that have been deleted.     *     * The journal file is appended to as cache operations occur. The journal may     * occasionally be compacted by dropping redundant lines. A temporary file named     * "journal.tmp" will be used during compaction; that file should be deleted if     * it exists when the cache is opened.     */    private final File directory;    private final File journalFile;    private final File journalFileTmp;    private final int appVersion;    private final long maxSize;    private final int valueCount;    private long size = 0;    private Writer journalWriter;    private final LinkedHashMap lruEntries            = new LinkedHashMap(0, 0.75f, true);    private int redundantOpCount;    /**     * To differentiate between old and current snapshots, each entry is given     * a sequence number each time an edit is committed. A snapshot is stale if     * its sequence number is not equal to its entry's sequence number.     */    private long nextSequenceNumber = 0;    /* From java.util.Arrays */    @SuppressWarnings("unchecked")    private static  T[] copyOfRange(T[] original, int start, int end) {        final int originalLength = original.length; // For exception priority compatibility.        if (start > end) {            throw new IllegalArgumentException();        }        if (start < 0 || start > originalLength) {            throw new ArrayIndexOutOfBoundsException();        }        final int resultLength = end - start;        final int copyLength = Math.min(resultLength, originalLength - start);        final T[] result = (T[]) Array                .newInstance(original.getClass().getComponentType(), resultLength);        System.arraycopy(original, start, result, 0, copyLength);        return result;    }    /**     * Returns the remainder of 'reader' as a string, closing it when done.     */    public static String readFully(Reader reader) throws IOException {        try {            StringWriter writer = new StringWriter();            char[] buffer = new char[1024];            int count;            while ((count = reader.read(buffer)) != -1) {                writer.write(buffer, 0, count);            }            return writer.toString();        } finally {            reader.close();        }    }    /**     * Returns the ASCII characters up to but not including the next "\r\n", or     * "\n".     *     * @throws EOFException if the stream is exhausted before the next newline     *     character.     */    public static String readAsciiLine(InputStream in) throws IOException {        // TODO: support UTF-8 here instead        StringBuilder result = new StringBuilder(80);        while (true) {            int c = in.read();            if (c == -1) {                throw new EOFException();            } else if (c == '\n') {                break;            }            result.append((char) c);        }        int length = result.length();        if (length > 0 && result.charAt(length - 1) == '\r') {            result.setLength(length - 1);        }        return result.toString();    }    /**     * Closes 'closeable', ignoring any checked exceptions. Does nothing if 'closeable' is null.     */    public static void closeQuietly(Closeable closeable) {        if (closeable != null) {            try {                closeable.close();            } catch (RuntimeException rethrown) {                throw rethrown;            } catch (Exception ignored) {            }        }    }    /**     * Recursively delete everything in {@code dir}.     */    // TODO: this should specify paths as Strings rather than as Files    public static void deleteContents(File dir) throws IOException {        File[] files = dir.listFiles();        if (files == null) {            throw new IllegalArgumentException("not a directory: " + dir);        }        for (File file : files) {            if (file.isDirectory()) {                deleteContents(file);            }            if (!file.delete()) {                throw new IOException("failed to delete file: " + file);            }        }    }    /** This cache uses a single background thread to evict entries. */    private final ExecutorService executorService = new ThreadPoolExecutor(0, 1,            60L, TimeUnit.SECONDS, new LinkedBlockingQueue());    private final Callable cleanupCallable = new Callable() {        @Override public Void call() throws Exception {            synchronized (DiskLruCache.this) {                if (journalWriter == null) {                    return null; // closed                }                trimToSize();                if (journalRebuildRequired()) {                    rebuildJournal();                    redundantOpCount = 0;                }            }            return null;        }    };    private DiskLruCache(File directory, int appVersion, int valueCount, long maxSize) {        this.directory = directory;        this.appVersion = appVersion;        this.journalFile = new File(directory, JOURNAL_FILE);        this.journalFileTmp = new File(directory, JOURNAL_FILE_TMP);        this.valueCount = valueCount;        this.maxSize = maxSize;    }    /**     * Opens the cache in {@code directory}, creating a cache if none exists     * there.     *     * @param directory a writable directory     * @param appVersion     * @param valueCount the number of values per cache entry. Must be positive.     * @param maxSize the maximum number of bytes this cache should use to store     * @throws IOException if reading or writing the cache directory fails     */    public static DiskLruCache open(File directory, int appVersion, int valueCount, long maxSize)            throws IOException {        if (maxSize <= 0) {            throw new IllegalArgumentException("maxSize <= 0");        }        if (valueCount <= 0) {            throw new IllegalArgumentException("valueCount <= 0");        }        // prefer to pick up where we left off        DiskLruCache cache = new DiskLruCache(directory, appVersion, valueCount, maxSize);        if (cache.journalFile.exists()) {            try {                cache.readJournal();                cache.processJournal();                cache.journalWriter = new BufferedWriter(new FileWriter(cache.journalFile, true),                        IO_BUFFER_SIZE);                return cache;            } catch (IOException journalIsCorrupt) {//                System.logW("DiskLruCache " + directory + " is corrupt: "//                        + journalIsCorrupt.getMessage() + ", removing");                cache.delete();            }        }        // create a new empty cache        directory.mkdirs();        cache = new DiskLruCache(directory, appVersion, valueCount, maxSize);        cache.rebuildJournal();        return cache;    }    private void readJournal() throws IOException {        InputStream in = new BufferedInputStream(new FileInputStream(journalFile), IO_BUFFER_SIZE);        try {            String magic = readAsciiLine(in);            String version = readAsciiLine(in);            String appVersionString = readAsciiLine(in);            String valueCountString = readAsciiLine(in);            String blank = readAsciiLine(in);            if (!MAGIC.equals(magic)                    || !VERSION_1.equals(version)                    || !Integer.toString(appVersion).equals(appVersionString)                    || !Integer.toString(valueCount).equals(valueCountString)                    || !"".equals(blank)) {                throw new IOException("unexpected journal header: ["                        + magic + ", " + version + ", " + valueCountString + ", " + blank + "]");            }            while (true) {                try {                    readJournalLine(readAsciiLine(in));                } catch (EOFException endOfJournal) {                    break;                }            }        } finally {            closeQuietly(in);        }    }    private void readJournalLine(String line) throws IOException {        String[] parts = line.split(" ");        if (parts.length < 2) {            throw new IOException("unexpected journal line: " + line);        }        String key = parts[1];        if (parts[0].equals(REMOVE) && parts.length == 2) {            lruEntries.remove(key);            return;        }        Entry entry = lruEntries.get(key);        if (entry == null) {            entry = new Entry(key);            lruEntries.put(key, entry);        }        if (parts[0].equals(CLEAN) && parts.length == 2 + valueCount) {            entry.readable = true;            entry.currentEditor = null;            entry.setLengths(copyOfRange(parts, 2, parts.length));        } else if (parts[0].equals(DIRTY) && parts.length == 2) {            entry.currentEditor = new Editor(entry);        } else if (parts[0].equals(READ) && parts.length == 2) {            // this work was already done by calling lruEntries.get()        } else {            throw new IOException("unexpected journal line: " + line);        }    }    /**     * Computes the initial size and collects garbage as a part of opening the     * cache. Dirty entries are assumed to be inconsistent and will be deleted.     */    private void processJournal() throws IOException {        deleteIfExists(journalFileTmp);        for (Iterator i = lruEntries.values().iterator(); i.hasNext(); ) {            Entry entry = i.next();            if (entry.currentEditor == null) {                for (int t = 0; t < valueCount; t++) {                    size += entry.lengths[t];                }            } else {                entry.currentEditor = null;                for (int t = 0; t < valueCount; t++) {                    deleteIfExists(entry.getCleanFile(t));                    deleteIfExists(entry.getDirtyFile(t));                }                i.remove();            }        }    }    /**     * Creates a new journal that omits redundant information. This replaces the     * current journal if it exists.     */    private synchronized void rebuildJournal() throws IOException {        if (journalWriter != null) {            journalWriter.close();        }        Writer writer = new BufferedWriter(new FileWriter(journalFileTmp), IO_BUFFER_SIZE);        writer.write(MAGIC);        writer.write("\n");        writer.write(VERSION_1);        writer.write("\n");        writer.write(Integer.toString(appVersion));        writer.write("\n");        writer.write(Integer.toString(valueCount));        writer.write("\n");        writer.write("\n");        for (Entry entry : lruEntries.values()) {            if (entry.currentEditor != null) {                writer.write(DIRTY + ' ' + entry.key + '\n');            } else {                writer.write(CLEAN + ' ' + entry.key + entry.getLengths() + '\n');            }        }        writer.close();        journalFileTmp.renameTo(journalFile);        journalWriter = new BufferedWriter(new FileWriter(journalFile, true), IO_BUFFER_SIZE);    }    private static void deleteIfExists(File file) throws IOException {//        try {//            Libcore.os.remove(file.getPath());//        } catch (ErrnoException errnoException) {//            if (errnoException.errno != OsConstants.ENOENT) {//                throw errnoException.rethrowAsIOException();//            }//        }        if (file.exists() && !file.delete()) {            throw new IOException();        }    }    /**     * Returns a snapshot of the entry named {@code key}, or null if it doesn't     * exist is not currently readable. If a value is returned, it is moved to     * the head of the LRU queue.     */    public synchronized Snapshot get(String key) throws IOException {        checkNotClosed();        validateKey(key);        Entry entry = lruEntries.get(key);        if (entry == null) {            return null;        }        if (!entry.readable) {            return null;        }        /*         * Open all streams eagerly to guarantee that we see a single published         * snapshot. If we opened streams lazily then the streams could come         * from different edits.         */        InputStream[] ins = new InputStream[valueCount];        try {            for (int i = 0; i < valueCount; i++) {                ins[i] = new FileInputStream(entry.getCleanFile(i));            }        } catch (FileNotFoundException e) {            // a file must have been deleted manually!            return null;        }        redundantOpCount++;        journalWriter.append(READ + ' ' + key + '\n');        if (journalRebuildRequired()) {            executorService.submit(cleanupCallable);        }        return new Snapshot(key, entry.sequenceNumber, ins);    }    /**     * Returns an editor for the entry named {@code key}, or null if another     * edit is in progress.     */    public Editor edit(String key) throws IOException {        return edit(key, ANY_SEQUENCE_NUMBER);    }    private synchronized Editor edit(String key, long expectedSequenceNumber) throws IOException {        checkNotClosed();        validateKey(key);        Entry entry = lruEntries.get(key);        if (expectedSequenceNumber != ANY_SEQUENCE_NUMBER                && (entry == null || entry.sequenceNumber != expectedSequenceNumber)) {            return null; // snapshot is stale        }        if (entry == null) {            entry = new Entry(key);            lruEntries.put(key, entry);        } else if (entry.currentEditor != null) {            return null; // another edit is in progress        }        Editor editor = new Editor(entry);        entry.currentEditor = editor;        // flush the journal before creating files to prevent file leaks        journalWriter.write(DIRTY + ' ' + key + '\n');        journalWriter.flush();        return editor;    }    /**     * Returns the directory where this cache stores its data.     */    public File getDirectory() {        return directory;    }    /**     * Returns the maximum number of bytes that this cache should use to store     * its data.     */    public long maxSize() {        return maxSize;    }    /**     * Returns the number of bytes currently being used to store the values in     * this cache. This may be greater than the max size if a background     * deletion is pending.     */    public synchronized long size() {        return size;    }    private synchronized void completeEdit(Editor editor, boolean success) throws IOException {        Entry entry = editor.entry;        if (entry.currentEditor != editor) {            throw new IllegalStateException();        }        // if this edit is creating the entry for the first time, every index must have a value        if (success && !entry.readable) {            for (int i = 0; i < valueCount; i++) {                if (!entry.getDirtyFile(i).exists()) {                    editor.abort();                    throw new IllegalStateException("edit didn't create file " + i);                }            }        }        for (int i = 0; i < valueCount; i++) {            File dirty = entry.getDirtyFile(i);            if (success) {                if (dirty.exists()) {                    File clean = entry.getCleanFile(i);                    dirty.renameTo(clean);                    long oldLength = entry.lengths[i];                    long newLength = clean.length();                    entry.lengths[i] = newLength;                    size = size - oldLength + newLength;                }            } else {                deleteIfExists(dirty);            }        }        redundantOpCount++;        entry.currentEditor = null;        if (entry.readable | success) {            entry.readable = true;            journalWriter.write(CLEAN + ' ' + entry.key + entry.getLengths() + '\n');            if (success) {                entry.sequenceNumber = nextSequenceNumber++;            }        } else {            lruEntries.remove(entry.key);            journalWriter.write(REMOVE + ' ' + entry.key + '\n');        }        if (size > maxSize || journalRebuildRequired()) {            executorService.submit(cleanupCallable);        }    }    /**     * We only rebuild the journal when it will halve the size of the journal     * and eliminate at least 2000 ops.     */    private boolean journalRebuildRequired() {        final int REDUNDANT_OP_COMPACT_THRESHOLD = 2000;        return redundantOpCount >= REDUNDANT_OP_COMPACT_THRESHOLD                && redundantOpCount >= lruEntries.size();    }    /**     * Drops the entry for {@code key} if it exists and can be removed. Entries     * actively being edited cannot be removed.     *     * @return true if an entry was removed.     */    public synchronized boolean remove(String key) throws IOException {        checkNotClosed();        validateKey(key);        Entry entry = lruEntries.get(key);        if (entry == null || entry.currentEditor != null) {            return false;        }        for (int i = 0; i < valueCount; i++) {            File file = entry.getCleanFile(i);            if (!file.delete()) {                throw new IOException("failed to delete " + file);            }            size -= entry.lengths[i];            entry.lengths[i] = 0;        }        redundantOpCount++;        journalWriter.append(REMOVE + ' ' + key + '\n');        lruEntries.remove(key);        if (journalRebuildRequired()) {            executorService.submit(cleanupCallable);        }        return true;    }    /**     * Returns true if this cache has been closed.     */    public boolean isClosed() {        return journalWriter == null;    }    private void checkNotClosed() {        if (journalWriter == null) {            throw new IllegalStateException("cache is closed");        }    }    /**     * Force buffered operations to the filesystem.     */    public synchronized void flush() throws IOException {        checkNotClosed();        trimToSize();        journalWriter.flush();    }    /**     * Closes this cache. Stored values will remain on the filesystem.     */    public synchronized void close() throws IOException {        if (journalWriter == null) {            return; // already closed        }        for (Entry entry : new ArrayList(lruEntries.values())) {            if (entry.currentEditor != null) {                entry.currentEditor.abort();            }        }        trimToSize();        journalWriter.close();        journalWriter = null;    }    private void trimToSize() throws IOException {        while (size > maxSize) {//            Map.Entry toEvict = lruEntries.eldest();            final Map.Entry toEvict = lruEntries.entrySet().iterator().next();            remove(toEvict.getKey());        }    }    /**     * Closes the cache and deletes all of its stored values. This will delete     * all files in the cache directory including files that weren't created by     * the cache.     */    public void delete() throws IOException {        close();        deleteContents(directory);    }    private void validateKey(String key) {        if (key.contains(" ") || key.contains("\n") || key.contains("\r")) {            throw new IllegalArgumentException(                    "keys must not contain spaces or newlines: \"" + key + "\"");        }    }    private static String inputStreamToString(InputStream in) throws IOException {        return readFully(new InputStreamReader(in, UTF_8));    }    /**     * A snapshot of the values for an entry.     */    public final class Snapshot implements Closeable {        private final String key;        private final long sequenceNumber;        private final InputStream[] ins;        private Snapshot(String key, long sequenceNumber, InputStream[] ins) {            this.key = key;            this.sequenceNumber = sequenceNumber;            this.ins = ins;        }        /**         * Returns an editor for this snapshot's entry, or null if either the         * entry has changed since this snapshot was created or if another edit         * is in progress.         */        public Editor edit() throws IOException {            return DiskLruCache.this.edit(key, sequenceNumber);        }        /**         * Returns the unbuffered stream with the value for {@code index}.         */        public InputStream getInputStream(int index) {            return ins[index];        }        /**         * Returns the string value for {@code index}.         */        public String getString(int index) throws IOException {            return inputStreamToString(getInputStream(index));        }        @Override public void close() {            for (InputStream in : ins) {                closeQuietly(in);            }        }    }    /**     * Edits the values for an entry.     */    public final class Editor {        private final Entry entry;        private boolean hasErrors;        private Editor(Entry entry) {            this.entry = entry;        }        /**         * Returns an unbuffered input stream to read the last committed value,         * or null if no value has been committed.         */        public InputStream newInputStream(int index) throws IOException {            synchronized (DiskLruCache.this) {                if (entry.currentEditor != this) {                    throw new IllegalStateException();                }                if (!entry.readable) {                    return null;                }                return new FileInputStream(entry.getCleanFile(index));            }        }        /**         * Returns the last committed value as a string, or null if no value         * has been committed.         */        public String getString(int index) throws IOException {            InputStream in = newInputStream(index);            return in != null ? inputStreamToString(in) : null;        }        /**         * Returns a new unbuffered output stream to write the value at         * {@code index}. If the underlying output stream encounters errors         * when writing to the filesystem, this edit will be aborted when         * {@link #commit} is called. The returned output stream does not throw         * IOExceptions.         */        public OutputStream newOutputStream(int index) throws IOException {            synchronized (DiskLruCache.this) {                if (entry.currentEditor != this) {                    throw new IllegalStateException();                }                return new FaultHidingOutputStream(new FileOutputStream(entry.getDirtyFile(index)));            }        }        /**         * Sets the value at {@code index} to {@code value}.         */        public void set(int index, String value) throws IOException {            Writer writer = null;            try {                writer = new OutputStreamWriter(newOutputStream(index), UTF_8);                writer.write(value);            } finally {                closeQuietly(writer);            }        }        /**         * Commits this edit so it is visible to readers.  This releases the         * edit lock so another edit may be started on the same key.         */        public void commit() throws IOException {            if (hasErrors) {                completeEdit(this, false);                remove(entry.key); // the previous entry is stale            } else {                completeEdit(this, true);            }        }        /**         * Aborts this edit. This releases the edit lock so another edit may be         * started on the same key.         */        public void abort() throws IOException {            completeEdit(this, false);        }        private class FaultHidingOutputStream extends FilterOutputStream {            private FaultHidingOutputStream(OutputStream out) {                super(out);            }            @Override public void write(int oneByte) {                try {                    out.write(oneByte);                } catch (IOException e) {                    hasErrors = true;                }            }            @Override public void write(byte[] buffer, int offset, int length) {                try {                    out.write(buffer, offset, length);                } catch (IOException e) {                    hasErrors = true;                }            }            @Override public void close() {                try {                    out.close();                } catch (IOException e) {                    hasErrors = true;                }            }            @Override public void flush() {                try {                    out.flush();                } catch (IOException e) {                    hasErrors = true;                }            }        }    }    private final class Entry {        private final String key;        /** Lengths of this entry's files. */        private final long[] lengths;        /** True if this entry has ever been published */        private boolean readable;        /** The ongoing edit or null if this entry is not being edited. */        private Editor currentEditor;        /** The sequence number of the most recently committed edit to this entry. */        private long sequenceNumber;        private Entry(String key) {            this.key = key;            this.lengths = new long[valueCount];        }        public String getLengths() throws IOException {            StringBuilder result = new StringBuilder();            for (long size : lengths) {                result.append(' ').append(size);            }            return result.toString();        }        /**         * Set lengths using decimal numbers like "10123".         */        private void setLengths(String[] strings) throws IOException {            if (strings.length != valueCount) {                throw invalidLengths(strings);            }            try {                for (int i = 0; i < strings.length; i++) {                    lengths[i] = Long.parseLong(strings[i]);                }            } catch (NumberFormatException e) {                throw invalidLengths(strings);            }        }        private IOException invalidLengths(String[] strings) throws IOException {            throw new IOException("unexpected journal line: " + Arrays.toString(strings));        }        public File getCleanFile(int i) {            return new File(directory, key + "." + i);        }        public File getDirtyFile(int i) {            return new File(directory, key + "." + i + ".tmp");        }    }}

一、前期基础知识储备

二、上代码，具体实现

2.DiskLruCache

三、ImageLoader的实现

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