根据Uri获取文件绝对路径

protected String getAbsoluteImagePath(Uri uri)
   {
       // can post image
       String [] proj={MediaStore.Images.Media.DATA};
       Cursor cursor = managedQuery( uri,
                       proj,                 // Which columns to return
                       null,       // WHERE clause; which rows to return (all rows)
                       null,       // WHERE clause selection arguments (none)
                       null);                 // Order-by clause (ascending by name)  

       int column_index = cursor.getColumnIndexOrThrow(MediaStore.Images.Media.DATA);
       cursor.moveToFirst();  

       return cursor.getString(column_index);
   }