issue on sdcard in sdcard

i need to retrieve the images from sdcard,i used the following code.

String[] projection = {MediaStore.Images.Thumbnails._ID};
    cursor = managedQuery( MediaStore.Images.Thumbnails.EXTERNAL_CONTENT_URI,
            projection, 
            null,       
            null,
            MediaStore.Images.Thumbnails.IMAGE_ID);
    columnIndex = cursor.getColumnIndexOrThrow(MediaStore.Images.Thumbnails._ID);
    int s开发者_开发问答ize = cursor.getCount();

but the cursor always returns Zero.so i cann't get image from sdcard .whats the solution.

Do you add the permisson to your XML code?

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