public class ErJiHuanCun extends ArrayAdapter<String> {
  /**
   * 記錄所有正在下載或等待下載的任務。
   */
  private Set<BitmapWorkerTask> taskCollection;

  /**
   * 圖片緩存技術的核心類，用于緩存所有下載好的圖片，在程序內(nèi)存達到設定值時會將最少最近使用的圖片移除掉。
   */
  private LruCache<String, Bitmap> mMemoryCache;

  /**
   * 圖片硬盤緩存核心類。
   */
  private DiskLruCache mDiskLruCache;

  /**
   * GridView的實例
   */
  private GridView mPhotoWall;

  /**
   * 記錄每個子項的高度。
   */
  private int mItemHeight = 0;

  public ErJiHuanCun(Context context, int textViewResourceId, String[] objects,
      GridView photoWall) {
    super(context, textViewResourceId, objects);
    mPhotoWall = photoWall;
    taskCollection = new HashSet<BitmapWorkerTask>();
    // 獲取應用程序最大可用內(nèi)存
    int maxMemory = (int) Runtime.getRuntime().maxMemory();
    int cacheSize = maxMemory / 8;
    // 設置圖片緩存大小為程序最大可用內(nèi)存的1/8
    mMemoryCache = new LruCache<String, Bitmap>(cacheSize) {
      @Override
      protected int sizeOf(String key, Bitmap bitmap) {
        return bitmap.getByteCount();
      }
    };
    try {
      // 獲取圖片緩存路徑
      File cacheDir = getDiskCacheDir(context, "thumb");
      if (!cacheDir.exists()) {
        cacheDir.mkdirs();
      }
      // 創(chuàng)建DiskLruCache實例，初始化緩存數(shù)據(jù)
      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;
    }
    // 給ImageView設置一個Tag，保證異步加載圖片時不會亂序
    imageView.setTag(url); 
    imageView.setImageResource(R.drawable.ic_launcher);
    loadBitmaps(imageView, url);
    return view;
  }

  /**
   * 將一張圖片存儲到LruCache中。
   * 
   * @param key
   *      LruCache的鍵，這里傳入圖片的URL地址。
   * @param bitmap
   *      LruCache的鍵，這里傳入從網(wǎng)絡上下載的Bitmap對象。
   */
  public void addBitmapToMemoryCache(String key, Bitmap bitmap) {
    if (getBitmapFromMemoryCache(key) == null) {
      mMemoryCache.put(key, bitmap);
    }
  }

  /**
   * 從LruCache中獲取一張圖片，如果不存在就返回null。
   * 
   * @param key
   *      LruCache的鍵，這里傳入圖片的URL地址。
   * @return 對應傳入鍵的Bitmap對象，或者null。
   */
  public Bitmap getBitmapFromMemoryCache(String key) {
    return mMemoryCache.get(key);
  }

  /**
   * 加載Bitmap對象。此方法會在LruCache中檢查所有屏幕中可見的ImageView的Bitmap對象，
   * 如果發(fā)現(xiàn)任何一個ImageView的Bitmap對象不在緩存中，就會開啟異步線程去下載圖片。
   */
  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();
    }
  }

  /**
   * 取消所有正在下載或等待下載的任務。
   */
  public void cancelAllTasks() {
    if (taskCollection != null) {
      for (BitmapWorkerTask task : taskCollection) {
        task.cancel(false);
      }
    }
  }

  /**
   * 根據(jù)傳入的uniqueName獲取硬盤緩存的路徑地址。
   */
  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);
  }

  /**
   * 獲取當前應用程序的版本號。
   */
  public int getAppVersion(Context context) {
    try {
      PackageInfo info = context.getPackageManager().getPackageInfo(context.getPackageName(),
          0);
      return info.versionCode;
    } catch (NameNotFoundException e) {
      e.printStackTrace();
    }
    return 1;
  }

  /**
   * 設置item子項的高度。
   */
  public void setItemHeight(int height) {
    if (height == mItemHeight) {
      return;
    }
    mItemHeight = height;
    notifyDataSetChanged();
  }

  /**
   * 使用MD5算法對傳入的key進行加密并返回。
   */
  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;
  }

  /**
   * 將緩存記錄同步到journal文件中。
   */
  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();
  }

  /**
   * 異步下載圖片的任務。
   * 
   *
   */
  class BitmapWorkerTask extends AsyncTask<String, Void, Bitmap> {

    /**
     * 圖片的URL地址
     */
    private String imageUrl;

    @Override
    protected Bitmap doInBackground(String... params) {
      imageUrl = params[0];
      FileDescriptor fileDescriptor = null;
      FileInputStream fileInputStream = null;
      Snapshot snapShot = null;
      try {
        // 生成圖片URL對應的key
        final String key = hashKeyForDisk(imageUrl);
        // 查找key對應的緩存
        snapShot = mDiskLruCache.get(key);
        if (snapShot == null) {
          // 如果沒有找到對應的緩存，則準備從網(wǎng)絡上請求數(shù)據(jù)，并寫入緩存
          DiskLruCache.Editor editor = mDiskLruCache.edit(key);
          if (editor != null) {
            OutputStream outputStream = editor.newOutputStream(0);
            if (downloadUrlToStream(imageUrl, outputStream)) {
              editor.commit();
            } else {
              editor.abort();
            }
          }
          // 緩存被寫入后，再次查找key對應的緩存
          snapShot = mDiskLruCache.get(key);
        }
        if (snapShot != null) {
          fileInputStream = (FileInputStream) snapShot.getInputStream(0);
          fileDescriptor = fileInputStream.getFD();
        }
        // 將緩存數(shù)據(jù)解析成Bitmap對象
        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) {
          // 將Bitmap對象添加到內(nèi)存緩存當中
            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);
      // 根據(jù)Tag找到相應的ImageView控件，將下載好的圖片顯示出來。
      ImageView imageView = (ImageView) mPhotoWall.findViewWithTag(imageUrl);
      if (imageView != null && bitmap != null) {
        imageView.setImageBitmap(bitmap);
      }
      taskCollection.remove(this);
    }

    /**
     * 建立HTTP請求，并獲取Bitmap對象。
     * 
     * @param imageUrl
     *      圖片的URL地址
     * @return 解析后的Bitmap對象
     */
    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;
    }

  }

}

/**
 * 照片墻主活動，使用GridView展示照片墻。
 * 
 * 
 */
public class MainActivity extends Activity {

  /**
   * 用于展示照片墻的GridView
   */
  private GridView mPhotoWall;

  /**
   * GridView的適配器
   */
  private ErJiHuanCun mAdapter;

  private int mImageThumbSize;
  private int mImageThumbSpacing;

  @Override
  protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    //mImageThumbSize = getResources().getDimensionPixelSize(
  //     R.dimen.image_thumbnail_size);
    //mImageThumbSpacing = getResources().getDimensionPixelSize(
  //     R.dimen.image_thumbnail_spacing);
    mPhotoWall = (GridView) findViewById(R.id.photo_wall);
    mAdapter = new ErJiHuanCun(this, 0, Images.imageThumbUrls,
        mPhotoWall);
    mPhotoWall.setAdapter(mAdapter);
/*   mPhotoWall.getViewTreeObserver().addOnGlobalLayoutListener(
        new ViewTreeObserver.OnGlobalLayoutListener() {

          @Override
          public void onGlobalLayout() {
            final int numColumns = (int) Math.floor(mPhotoWall
                .getWidth()
                / (mImageThumbSize + mImageThumbSpacing));
            if (numColumns > 0) {
              int columnWidth = (mPhotoWall.getWidth() / numColumns)
                  - mImageThumbSpacing;
              mAdapter.setItemHeight(columnWidth);
              mPhotoWall.getViewTreeObserver()
                  .removeGlobalOnLayoutListener(this);
            }
          }
        });*/
  }

  @Override
  protected void onPause() {
    super.onPause();
    //將緩存記錄同步到journal文件中
    mAdapter.flushCache();
  }

  @Override
  protected void onDestroy() {
    super.onDestroy();
    // // 退出程序時結束所有的下載任務
    mAdapter.cancelAllTasks();
  }

}

/**
 * 自定義正方形的ImageView
 * 
 */
public class MyImageView extends ImageView {


  public MyImageView(Context context, AttributeSet attrs, int defStyleAttr) {
    super(context, attrs, defStyleAttr);
    // TODO Auto-generated constructor stub
  }

  public MyImageView(Context context, AttributeSet attrs) {
    super(context, attrs);
    // TODO Auto-generated constructor stub
  }

  public MyImageView(Context context) {
    super(context);
    // TODO Auto-generated constructor stub
  }
  @Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
  // TODO Auto-generated method stub
    //將高度信息改成寬度即可
  super.onMeasure(widthMeasureSpec, widthMeasureSpec);
}
}

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
  xmlns:tools="http://schemas.android.com/tools"
  android:layout_width="match_parent"
  android:layout_height="match_parent"
  android:orientation="vertical"
  android:padding="5dp"
 >

<GridView
  android:id="@+id/photo_wall"
  android:layout_width="match_parent"
  android:layout_height="match_parent"
  android:gravity="center"
  android:horizontalSpacing="5dp"
  android:verticalSpacing="5dp"
  android:numColumns="3"
  android:stretchMode="columnWidth"
  />
</LinearLayout>

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
  android:layout_width="match_parent"
  android:layout_height="match_parent"
  android:orientation="vertical" >

<com.example.imageloader.MyImageView
  android:layout_width="match_parent"
  android:layout_height="0dp"
   android:id="@+id/photo"
  />
</LinearLayout>

public class ImageResizer {
 private static final String TAG="ImageResizer";

 public static Bitmap decodeSampledBitmapFromResource(Resources res,
     int resId,int reqWidth,int reqHeight){
   final 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 static Bitmap decodeSampleBithmapFromFileDescriptor(FileDescriptor fd,
     int reqWidth,int reqHeight){
   final 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);
 }
 public static int calculateInSampleSize(BitmapFactory.Options options,
     int reqWidth,int reqHeight){
   if(reqWidth==0||reqHeight==0)
     return 1;
   final int width=options.outWidth;
   final int height=options.outHeight;
   int inSampleSize=1;
   if(height>reqHeight||width>reqWidth  ){
     final int halfHeight=height/2;
     final int halfWidth=width/2;
     //盡最大限度的壓縮圖片，不能讓圖片的寬高比ImageView的寬高小，否則在將
     //圖片顯示到ImageView時，圖片會放大導致圖片失真
   while(halfHeight/inSampleSize>reqHeight&&halfWidth/inSampleSize>reqWidth){
      inSampleSize*=2;
    }
   }
   return inSampleSize;
 }
}