android 网络图片双缓存

Android每次加载图片很浪费时间。所以设计了一个图片缓存技术来解决每次android手机加载图片的问题

 

内存的读取速度是最快的，然后是文件的读取速度，最后是网络资源的读取

 

既然内存的读取时间最快，我们好好利用内存资源。将内存再分两层缓存

强引用缓存不会轻易被回收，来保存常用数据，不常用的资源放入软引用缓存中。

对于硬引用和软引用的介绍：

⑴强引用（StrongReference）
    强引用是使用最普遍的引用。如果一个对象具有强引用，那垃圾回收器绝不会回收它。当内存空间不足，Java虚拟机宁愿抛出OutOfMemoryError错误，使程序异常终止，也不会靠随意回收具有强引用的对象来解决内存不足的问题。

⑵软引用（SoftReference）

如果一个对象只具有软引用，则内存空间足够，垃圾回收器就不会回收它；如果内存空间不足了，就会回收这些对象的内存。只要垃圾回收器没有回收它，该对象就可以被程序使用。软引用可用来实现内存敏感的高速缓存

 1，对于强引用和软引用的使用，我们首先去强引用缓存中去找图片资源，当没有发现时，就去软引用缓存中。当强引用的超额时，将最后使用的资源放入软引用缓存中，使用到软引用的资源时 ，则将资源重新放回强引用缓存池中。

 2，内存缓存池中找不到，就去文件中查找，

 

3，再找不到就只好去网络上下载了，下载到以后，分别将资源放入文件缓存和内存缓存中

 

Android对于InputStream流有个小bug在慢速网络的情况下可能产生中断，可以考虑重写FilterInputStream处理skip方法来解决这个bug。 BitmapFactory类的decodeStream方法在网络超时或较慢的时候无法获取完整的数据，这里我 们通过继承FilterInputStream类的skip方法来强制实现flush流中的数据，主要原理就是检查是否到文件末端，告诉http类是否继续。

 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;

        }

    }

 

 主界面读取图片

public class MainActivity extends Activity {

    private ImageMemoryCache memoryCache;
    private ImageFileCache fileCache;
    private ImageView imageView;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        memoryCache=new ImageMemoryCache(this);
        fileCache=new ImageFileCache();
        imageView=(ImageView) findViewById(R.id.img);
        Bitmap b=getBitmap("http://f.hiphotos.baidu.com/album/w%3D2048/sign=7aa167f79f2f07085f052d00dd1cb999/472309f7905298228f794c7bd6ca7bcb0b46d4c4.jpg");
        imageView.setImageBitmap(b);



    }

    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;
    }

}

public class MainActivity extends Activity {

	private ImageMemoryCache memoryCache;
	private ImageFileCache fileCache;
	private ImageView imageView;
	@Override
	protected void onCreate(Bundle savedInstanceState) {
		super.onCreate(savedInstanceState);
		setContentView(R.layout.main);
		memoryCache=new ImageMemoryCache(this);
		fileCache=new ImageFileCache();
		imageView=(ImageView) findViewById(R.id.img);
		Bitmap b=getBitmap("http://f.hiphotos.baidu.com/album/w%3D2048/sign=7aa167f79f2f07085f052d00dd1cb999/472309f7905298228f794c7bd6ca7bcb0b46d4c4.jpg");
		imageView.setImageBitmap(b);
	
		
		
	}

	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;
	}

}

 内存中读取

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 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;
            }
        };
    }

    /**
     * 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 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();
    }
}

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 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; 
            }
        };
    }
                                                                                  
    /**
     * 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 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();
    }
}

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 {
        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;
            }
        }
    }

}

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 {
        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;
            }
        }
    }
                                                            
}

网络下载图片

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;
        }
    }
}

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;
        }
    }
}

 权限

<uses-permission android:name="android.permission.INTERNET" />
   <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />