Android二级缓存加载图片实现照片墙功能

实现二级缓存加载图片的功能，在使用DiskLruCache时，需先在工程中添加名为libcore.io的包，并将DiskLruCache.Java文件放进去。DiskLruCache直接百度下载即可。

在GridView的适配器中，为ImageView添加图片时，先从内存缓存中加载，内存中无缓存的话则在磁盘缓存中加载，磁盘缓存也没有的话开启线程下载，然后将下载的图片缓存到磁盘，内存中。下载的图片最好先进行压缩，文章最后给出了压缩代码，但本例中并未实现压缩。

public class ErJiHuanCun extends ArrayAdapter<String> {    private Set<BitmapWorkerTask> taskCollection;    private LruCache<String, Bitmap> mMemoryCache;    private DiskLruCache mDiskLruCache;    private GridView mPhotoWall;    private int mItemHeight = 0;  public ErJiHuanCun(Context context, int textViewResourceId, String[] objects,      GridView photoWall) {    super(context, textViewResourceId, objects);    mPhotoWall = photoWall;    taskCollection = new HashSet<BitmapWorkerTask>();    // 获取应用程序最大可用内存    int maxMemory = (int) Runtime.getRuntime().maxMemory();    int cacheSize = maxMemory / 8;    // 设置图片缓存大小为程序最大可用内存的1/8    mMemoryCache = new LruCache<String, Bitmap>(cacheSize) {      @Override      protected int sizeOf(String key, Bitmap bitmap) {        return bitmap.getByteCount();      }    };    try {      // 获取图片缓存路径      File cacheDir = getDiskCacheDir(context, "thumb");      if (!cacheDir.exists()) {        cacheDir.mkdirs();      }      // 创建DiskLruCache实例，初始化缓存数据      mDiskLruCache = DiskLruCache          .open(cacheDir, getAppVersion(context), 1, 10 * 1024 * 1024);    } catch (IOException e) {      e.printStackTrace();    }  }  @Override  public View getView(int position, View convertView, ViewGroup parent) {    final String url = getItem(position);    View view;    if (convertView == null) {      view = LayoutInflater.from(getContext()).inflate(R.layout.photo_layout, null);    } else {      view = convertView;    }    final ImageView imageView = (ImageView) view.findViewById(R.id.photo);    if (imageView.getLayoutParams().height != mItemHeight) {      imageView.getLayoutParams().height = mItemHeight;    }    // 给ImageView设置一个Tag，保证异步加载图片时不会乱序    imageView.setTag(url);     imageView.setImageResource(R.drawable.ic_launcher);    loadBitmaps(imageView, url);    return view;  }    public void addBitmapToMemoryCache(String key, Bitmap bitmap) {    if (getBitmapFromMemoryCache(key) == null) {      mMemoryCache.put(key, bitmap);    }  }    public Bitmap getBitmapFromMemoryCache(String key) {    return mMemoryCache.get(key);  }    public void loadBitmaps(ImageView imageView, String imageUrl) {    try {      Bitmap bitmap = getBitmapFromMemoryCache(imageUrl);      if (bitmap == null) {        BitmapWorkerTask task = new BitmapWorkerTask();        taskCollection.add(task);        task.execute(imageUrl);      } else {        if (imageView != null && bitmap != null) {          imageView.setImageBitmap(bitmap);        }      }    } catch (Exception e) {      e.printStackTrace();    }  }    public void cancelAllTasks() {    if (taskCollection != null) {      for (BitmapWorkerTask task : taskCollection) {        task.cancel(false);      }    }  }    public File getDiskCacheDir(Context context, String uniqueName) {    String cachePath;    if (Environment.MEDIA_MOUNTED.equals(Environment.getExternalStorageState())        || !Environment.isExternalStorageRemovable()) {      cachePath = context.getExternalCacheDir().getPath();    } else {      cachePath = context.getCacheDir().getPath();    }    return new File(cachePath + File.separator + uniqueName);  }    public int getAppVersion(Context context) {    try {      PackageInfo info = context.getPackageManager().getPackageInfo(context.getPackageName(),          0);      return info.versionCode;    } catch (NameNotFoundException e) {      e.printStackTrace();    }    return 1;  }    public void setItemHeight(int height) {    if (height == mItemHeight) {      return;    }    mItemHeight = height;    notifyDataSetChanged();  }    public String hashKeyForDisk(String key) {    String cacheKey;    try {      final MessageDigest mDigest = MessageDigest.getInstance("MD5");      mDigest.update(key.getBytes());      cacheKey = bytesToHexString(mDigest.digest());    } catch (NoSuchAlGorithmException e) {      cacheKey = String.valueOf(key.hashCode());    }    return cacheKey;  }    public void flushCache() {    if (mDiskLruCache != null) {      try {        mDiskLruCache.flush();      } catch (IOException e) {        e.printStackTrace();      }    }  }  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();  }    class BitmapWorkerTask extends AsyncTask<String, Void, Bitmap> {        private String imageUrl;    @Override    protected Bitmap doInBackground(String... params) {      imageUrl = params[0];      FileDescriptor fileDescriptor = null;      FileInputStream fileInputStream = null;      Snapshot snapShot = null;      try {        // 生成图片URL对应的key        final String key = hashKeyForDisk(imageUrl);        // 查找key对应的缓存        snapShot = mDiskLruCache.get(key);        if (snapShot == null) {          // 如果没有找到对应的缓存，则准备从网络上请求数据，并写入缓存          DiskLruCache.Editor editor = mDiskLruCache.edit(key);          if (editor != null) {            OutputStream outputStream = editor.newOutputStream(0);            if (downloadUrlToStream(imageUrl, outputStream)) {              editor.commit();            } else {              editor.abort();            }          }          // 缓存被写入后，再次查找key对应的缓存          snapShot = mDiskLruCache.get(key);        }        if (snapShot != null) {          fileInputStream = (FileInputStream) snapShot.getInputStream(0);          fileDescriptor = fileInputStream.getFD();        }        // 将缓存数据解析成Bitmap对象        Bitmap bitmap = null;        if (fileDescriptor != null) {        // bitmap = BitmapFactory.decodeFileDescriptor(fileDescriptor);         WindowManager wm= (WindowManager)getContext().getSystemService(Context.WINDOW_SERVICE);         int width=wm.getDefaultDisplay().getWidth();         bitmap= ImageResizer.decodeSampleBithmapFromFileDescriptor(fileDescriptor,width/3,width/3);        }        if (bitmap != null) {          // 将Bitmap对象添加到内存缓存当中            addBitmapToMemoryCache(params[0], bitmap);        }        return bitmap;      } catch (IOException e) {        e.printStackTrace();      } finally {        if (fileDescriptor == null && fileInputStream != null) {          try {            fileInputStream.close();          } catch (IOException e) {          }        }      }      return null;    }    @Override    protected void onPostExecute(Bitmap bitmap) {      super.onPostExecute(bitmap);      // 根据Tag找到相应的ImageView控件，将下载好的图片显示出来。      ImageView imageView = (ImageView) mPhotoWall.findViewWithTag(imageUrl);      if (imageView != null && bitmap != null) {        imageView.setImageBitmap(bitmap);      }      taskCollection.remove(this);    }        private 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(), 8 * 1024);        out = new BufferedOutputStream(outputStream, 8 * 1024);        int b;        while ((b = in.read()) != -1) {          out.write(b);        }        return true;      } catch (final IOException e) {        e.printStackTrace();      } finally {        if (urlConnection != null) {          urlConnection.disconnect();        }        try {          if (out != null) {            out.close();          }          if (in != null) {            in.close();          }        } catch (final IOException e) {          e.printStackTrace();        }      }      return false;    }  }}