最近一直在学习男神+大神任玉刚老师的Android开发艺术探索，我个人非常推荐这本书，写的真tm好，不好意思，有点激动。大家有空可以去看看。看完总觉得要写点什么吧，就将其中Bitmap加载和Cache的内容分享给还没有来得及看过的朋友们。

缓存策略在Android中有着广泛的。。。算了，先轻松一下，来张美女镇楼

老司机要发车了，来不及解释了，大家快上车啊。

言归正传（画风一转）

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，缓存策略变得更为重要。今天就带大家自己动手设计图片缓存工具。

提到缓存，不得不提目前常用的一种缓存算法-LRU（Least Recently Used），LRU是近期最少使用算法。采用这种算法思想的缓存有两种：LruCache和DiskLruCache，LruCache用于实现内存缓存，而DiskLruCache则充当了存储设备缓存，通过两者完美结合，就可以轻松实现我们的ImageLoader。

下面我们就正式开始了。。。

1.首先我们来看下两个核心的东西：LruCache和DiskLruCache。

1.1LruCache

LruCache的实现比较简单，大家可以参考它的源码。LruCache主要用来实现内存缓存，我们拿图片缓存来举例子，反手就是一段代码：

int maxMemory = (int) (Runtime.getRuntime().maxMemory() / 1024);
int cacheSize = maxMemory / 8;
mMemoryCache = new LruCache<String, Bitmap>(cacheSize) {
            @Override
            protected int sizeOf(String key, Bitmap value) {
                return value.getRowBytes() * value.getHeight() / 1024;
            }
        };

可以看出，我们只需要提供缓存的总容量大小并重写sizeOf方法即可。

除了LruCache的创建以外，还有缓存的获取和添加，这也很简单，从LruCache中获取一个缓存对象，如下所示。

mMemoryCache.get(key)

向LruCache中添加一个缓存对象，如下所示。

mMemoryCache.put(key,bitmap)

从Android3.1开始，LruCache就已经是Android源码的一部分了。

1.2DiskLruCache

DiskCahe用于实现存储设备缓存，已经得到了Android官方的推荐，但不属于SDK的一部分，我们要导入引用一下：

compile 'com.jakewharton:disklrucache:2.0.2'

1.2.1先来看下DiskLruCache的创建

DiskLruCache提供open方法用于创建自身，如下所示。

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

这里有四个参数，稍微解释下，有兴趣的朋友，下去可以深入了解下：

directory：磁盘缓存的存储路径。

appVersion：版本号，一般设为1即可。

valueCount：单个节点所对应的数据的个数，一般也设为1即可。

maxSize：缓存的总大小。

整个创建过程，反手又是一段代码：

private static final int DISK_CACHE_SIZE = 50*1024*1024;
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();
            }
        }

1.2.2DiskLruCache的缓存添加

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

这里的hashKeyFormUrl()方法主要是将url转成key，之所以这么做，是因为url中很可能有特殊字符，会影响url在Android中的直接使用。

有了文件输出流，接下来就是讲网络下载的图片时的文件流写入到文件系统上了：

private boolean downloadUrlToStream(String urlString, OutputStream outputStream) {
        HttpURLConnection urlConnection=null;
        BufferedOutputStream out = null;
        BufferedInputStream in = null;
        try {
            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) {
            e.printStackTrace();
        }finally {
            if(urlConnection!=null){
                urlConnection.disconnect();
            }
            MyUtils.close(out);
            MyUtils.close(in);
        }
        return false;
    }

还有不要忘记了，editor要提交写入操作。

editor.commit();

1.2.3DiskLruCache缓存的查找

private Bitmap loadBitmapFromDiskCache(String url, int reqWidth, int reqHeight)
    throws IOException{
        if (Looper.myLooper() == Looper.getMainLooper()) {
            throw new RuntimeException("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.decodeSampleFromFileDescriptor(fileDescriptor,reqWidth,reqHeight);
            if(bitmap !=null){
                addBitmapToMemoryCache(key,bitmap);
            }
        }
        return bitmap;
    }

代码的意思应该好理解，将url转成key，通过get方法得到一个Snapshot对象，通过这个对象即可得到缓存的输入流，有了输入流，还怕拿不到bitmap对象吗。

好了，介绍完两个核心的知识点，喝口雪碧压压惊。

接下来就是重头戏了，就是ImageLoader的实现。

2.ImageLoader的实现

要实现整个完整的ImageLoader功能，要怎么做呢？先捋下思想，应该具备以下几个功能：

-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

接下来，我们一步一步的实现。

2.1图片压缩功能的实现

将压缩图片功能抽出来单独形成一个类，就叫ImageResizer吧，看下实现：

public class ImageResizer {
    private static final String TAG = "ImageResizer";
    public Bitmap decodeSampleFromResource(Resources res, int resId, int reqWidth, int reqHeight) {
        BitmapFactory.Options options = new BitmapFactory.Options();
        options.inJustDecodeBounds = true;
        BitmapFactory.decodeResource(res, resId, options);
        options.inSampleSize = calculateInSampleSize(options, reqWidth, reqHeight);
        options.inJustDecodeBounds = false;
        return BitmapFactory.decodeResource(res,resId,options);
    }
    public Bitmap decodeSampleFromFileDescriptor(FileDescriptor fd, int reqWidth, int reqHeight) {
        BitmapFactory.Options options = new BitmapFactory.Options();
        options.inJustDecodeBounds = true;
        BitmapFactory.decodeFileDescriptor(fd, null, options);
        options.inSampleSize = calculateInSampleSize(options, reqWidth, reqHeight);
        options.inJustDecodeBounds = false;
        return BitmapFactory.decodeFileDescriptor(fd, null, options);
    }
    private int calculateInSampleSize(BitmapFactory.Options options, int reqWidth, int reqHeight) {
        if (reqWidth == 0 || reqHeight == 0) {
            return 1;
        }
        int width = options.outWidth;
        int height = options.outHeight;
        int inSampleSize = 1;
        if (width < reqWidth || height < reqHeight) {
            return inSampleSize;
        }
        inSampleSize *= 2;
        while (width /inSampleSize>=reqWidth&&height/inSampleSize>=reqHeight){
            inSampleSize *=2;
        }
        Log.e(TAG, "inSampleSize: "+inSampleSize);
       return inSampleSize;
    }
}

2.2内存缓存和磁盘缓存的实现

之前介绍过了，这里放下具体实现：

初始化：

public ImageLoader(Context context) {
        mImageResizer = new ImageResizer();
        mContext = context;
        int maxMemory = (int) (Runtime.getRuntime().maxMemory() / 1024);
        int cacheSize = maxMemory / 8;
        mMemoryCache = new LruCache<String, Bitmap>(cacheSize) {
            @Override
            protected int sizeOf(String key, Bitmap value) {
                return value.getRowBytes() * value.getHeight() / 1024;
            }
        };
        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();
            }
        }
    }

内存缓存添加和获取

private void addBitmapToMemoryCache(String key, Bitmap bitmap) {
        Log.e(TAG, "addBitmapToMemoryCache: ");
        if (getBitmapFromMemoryCache(key) == null) {
            mMemoryCache.put(key, bitmap);
        }
    }
private Bitmap getBitmapFromMemoryCache(String key) {
        return mMemoryCache.get(key);
    }

磁盘缓存添加和获取

private Bitmap loadBitmapFromHttp(String url, int reqWidth, int reqHeight)
            throws IOException {
        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)){
                editor.commit();
            }else {
                editor.abort();
            }
            mDiskLruCache.flush();
        }
        return loadBitmapFromDiskCache(url,reqWidth,reqHeight);
    }
    private Bitmap loadBitmapFromDiskCache(String url, int reqWidth, int reqHeight)
    throws IOException{
        if (Looper.myLooper() == Looper.getMainLooper()) {
            throw new RuntimeException("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.decodeSampleFromFileDescriptor(fileDescriptor,reqWidth,reqHeight);
            if(bitmap !=null){
                addBitmapToMemoryCache(key,bitmap);
            }
        }
        return bitmap;
    }

2.3同步加载和异步加载接口的设计

看下同步加载：

public Bitmap loadBitmap(String uri,int reqWidth,int reqHeight){
        Bitmap bitmap = loadBitmapFromMemCache(uri);
        if(bitmap!=null){
            Log.e(TAG, "loadBitmapFromMemCache,url "+uri);
            return bitmap;
        }
        try {
            bitmap = loadBitmapFromDiskCache(uri,reqWidth,reqHeight);
            if(bitmap!=null){
                Log.e(TAG, "loadBitmapFromDiskCache,url "+uri );
                return bitmap;
            }
            bitmap = loadBitmapFromHttp(uri,reqWidth,reqHeight);
            Log.e(TAG, "loadBitmapFromHttp,url "+uri);
        } catch (IOException e) {
            e.printStackTrace();
        }
        if(bitmap==null&&!mIsDiskLruCacheCreated){
            Log.e(TAG, "encounter error,DiskLruCache is not created.");
            bitmap = downLoadBitmapFromUrl(uri);
        }
        return bitmap;
    }

再看下异步加载

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);
                  Message message = mMainHandler.obtainMessage(MESSAGE_POST_RESULT,result);
                  message.sendToTarget();
//                  sendToTarget();
              }
          }
      };
      THREAD_POOL_EXECUTOR.execute(loadBitmapTask);
    }

仔细的同学发现了，这里面好像用到了线程池和Handler，关于线程池的内容这里就不扯了，大家自己去度娘。

这里直接把撸好的代码放上来：

private Handler mMainHandler = new Handler(Looper.getMainLooper()){
        @Override
        public void handleMessage(Message msg) {
           LoaderResult result = (LoaderResult) msg.obj;
            ImageView imageView = result.imageView;
            imageView.setImageBitmap(result.bitmap);
            String uri = (String) imageView.getTag(TAG_KEY_URI);
            if(uri.equals(result.uri)){
                imageView.setImageBitmap(result.bitmap);
            }else {
                Log.d(TAG, "set image bitmap,but url has changed,ignored!");
            }
        }
    };
private static final ThreadFactory sThreadFactory = new ThreadFactory() {
        private final AtomicInteger mCount = new AtomicInteger(1);
        @Override
        public Thread newThread(@NonNull Runnable r) {
            return new Thread(r,"ImageLoader#"+mCount.getAndIncrement());
        }
    };
public static final Executor THREAD_POOL_EXECUTOR = new ThreadPoolExecutor(
            CORE_POOL_SIZE,MAXIMUM_POOL_SIZE,
            KEEP_ALIVE, TimeUnit.SECONDS
            ,new LinkedBlockingDeque<Runnable>(),sThreadFactory);

哇哈哈哈哈哈哈哈哈。。。。。

终于要接近尾声了，提取的不好，大家多见谅（有兴趣的同学一定要去看看任玉刚老师的书）。

下面还是给下ImageLoader的完整代码。

public class ImageLoader {
    private static final String TAG = "ImageLoader";
    private Context mContext;
    private LruCache<String, Bitmap> mMemoryCache;
    private DiskLruCache mDiskLruCache;
    private Bitmap bitmap;
    private static final int DISK_CACHE_SIZE = 50*1024*1024;
    private boolean mIsDiskLruCacheCreated;
    private static final int DISK_CACHE_INDEX  = 0;
    private ImageResizer mImageResizer;
    private String cachePath;
    private static final int IO_BUFFER_SIZE = 8*1024;
    private static final int CPU_COUNT = Runtime.getRuntime().availableProcessors();
    private static final int TAG_KEY_URI = R.id.imageloader_uri;
    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 Handler mMainHandler = new Handler(Looper.getMainLooper()){
        @Override
        public void handleMessage(Message msg) {
           LoaderResult result = (LoaderResult) msg.obj;
            ImageView imageView = result.imageView;
            imageView.setImageBitmap(result.bitmap);
            String uri = (String) imageView.getTag(TAG_KEY_URI);
            if(uri.equals(result.uri)){
                imageView.setImageBitmap(result.bitmap);
            }else {
                Log.d(TAG, "set image bitmap,but url has changed,ignored!");
            }
        }
    };
    private static final ThreadFactory sThreadFactory = new ThreadFactory() {
        private final AtomicInteger mCount = new AtomicInteger(1);
        @Override
        public Thread newThread(@NonNull Runnable r) {
            return new Thread(r,"ImageLoader#"+mCount.getAndIncrement());
        }
    };
    public static final Executor THREAD_POOL_EXECUTOR = new ThreadPoolExecutor(
            CORE_POOL_SIZE,MAXIMUM_POOL_SIZE,
            KEEP_ALIVE, TimeUnit.SECONDS
            ,new LinkedBlockingDeque<Runnable>(),sThreadFactory);
    private static final int MESSAGE_POST_RESULT = 1;
    public ImageLoader(Context context) {
        mImageResizer = new ImageResizer();
        mContext = context;
        int maxMemory = (int) (Runtime.getRuntime().maxMemory() / 1024);
        int cacheSize = maxMemory / 8;
        mMemoryCache = new LruCache<String, Bitmap>(cacheSize) {
            @Override
            protected int sizeOf(String key, Bitmap value) {
                return value.getRowBytes() * value.getHeight() / 1024;
            }
        };
        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();
            }
        }
    }
    @TargetApi(Build.VERSION_CODES.GINGERBREAD)
    private long getUsableSpace(File path) {
         if(Build.VERSION.SDK_INT> Build.VERSION_CODES.GINGERBREAD){
             return path.getUsableSpace();
         }
        StatFs statFs = new StatFs(path.getPath());
         return statFs.getBlockSize()*statFs.getAvailableBlocks();
    }
    private File getDiskCacheDir(Context mContext, String uniqueName) {
     boolean externalStorageAvailable = Environment
             .getExternalStorageState().equals(Environment.MEDIA_MOUNTED);
       if(externalStorageAvailable){
            cachePath = mContext.getExternalCacheDir().getPath();
       }else {
           cachePath = mContext.getCacheDir().getPath();
       }
       return new File(cachePath+File.separator+uniqueName);
    }
    /**
     * 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);
        if(bitmap!=null){
            Log.e(TAG, "loadBitmapFromMemCache,url "+uri);
            return bitmap;
        }
        try {
            bitmap = loadBitmapFromDiskCache(uri,reqWidth,reqHeight);
            if(bitmap!=null){
                Log.e(TAG, "loadBitmapFromDiskCache,url "+uri );
                return bitmap;
            }
            bitmap = loadBitmapFromHttp(uri,reqWidth,reqHeight);
            Log.e(TAG, "loadBitmapFromHttp,url "+uri);
        } catch (IOException e) {
            e.printStackTrace();
        }
        if(bitmap==null&&!mIsDiskLruCacheCreated){
            Log.e(TAG, "encounter error,DiskLruCache is not created.");
            bitmap = downLoadBitmapFromUrl(uri);
        }
        return bitmap;
    }
    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);
                  Message message = mMainHandler.obtainMessage(MESSAGE_POST_RESULT,result);
                  message.sendToTarget();
//                  sendToTarget();
              }
          }
      };
      THREAD_POOL_EXECUTOR.execute(loadBitmapTask);
    }
    private Bitmap downLoadBitmapFromUrl(String uri) {
        Bitmap bitmap = null;
        HttpURLConnection urlConnection = null;
        BufferedInputStream in = null;
        try {
            URL url = new URL(uri);
            urlConnection = (HttpURLConnection) url.openConnection();
            in = new BufferedInputStream(urlConnection.getInputStream(),IO_BUFFER_SIZE);
            bitmap = BitmapFactory.decodeStream(in);
        } catch (IOException e) {
            Log.e(TAG, "Error in downloadBitmap:"+e);
        }finally {
            if(urlConnection!=null){
                urlConnection.disconnect();
            }
            MyUtils.close(in);
        }
        return bitmap;
    }
    private Bitmap loadBitmapFromMemCache(String url) {
        String key = hashKeyFormUrl(url);
        Bitmap bitmap = getBitmapFromMemoryCache(key);
        return bitmap;
    }
    private void addBitmapToMemoryCache(String key, Bitmap bitmap) {
        Log.e(TAG, "addBitmapToMemoryCache: ");
        if (getBitmapFromMemoryCache(key) == null) {
            mMemoryCache.put(key, bitmap);
        }
    }
    private Bitmap getBitmapFromMemoryCache(String key) {
        return mMemoryCache.get(key);
    }
    private Bitmap loadBitmapFromHttp(String url, int reqWidth, int reqHeight)
            throws IOException {
        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)){
                editor.commit();
            }else {
                editor.abort();
            }
            mDiskLruCache.flush();
        }
        return loadBitmapFromDiskCache(url,reqWidth,reqHeight);
    }
    private Bitmap loadBitmapFromDiskCache(String url, int reqWidth, int reqHeight)
    throws IOException{
        if (Looper.myLooper() == Looper.getMainLooper()) {
            throw new RuntimeException("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.decodeSampleFromFileDescriptor(fileDescriptor,reqWidth,reqHeight);
            if(bitmap !=null){
                addBitmapToMemoryCache(key,bitmap);
            }
        }
        return bitmap;
    }
    private boolean downloadUrlToStream(String urlString, OutputStream outputStream) {
        HttpURLConnection urlConnection=null;
        BufferedOutputStream out = null;
        BufferedInputStream in = null;
        try {
            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) {
            e.printStackTrace();
        }finally {
            if(urlConnection!=null){
                urlConnection.disconnect();
            }
            MyUtils.close(out);
            MyUtils.close(in);
        }
        return false;
    }
    private String hashKeyFormUrl(String url) {
        String cacheKey;
        MessageDigest mDigest;
        try {
            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();
    }
}

自己模仿着写了个项目分享到github上了。
Github地址