在Android 11中，应用的文件访问权限得到了加强，因此在访问外部存储器中的文件时，需要使用特定的方法来处理。以下是一个示例代码来读取外部存储上的文件：

if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.R
        && Environment.isExternalStorageManager()) {

    // Use the MediaStore API to read the file
    Uri uri = MediaStore.Downloads.EXTERNAL_CONTENT_URI.buildUpon().appendPath(filename).build();
    ContentResolver contentResolver = getContentResolver();
    String[] projection = {MediaStore.Downloads._ID, MediaStore.Downloads.DISPLAY_NAME, 
        MediaStore.Downloads.SIZE};
    try (Cursor cursor = contentResolver.query(uri, projection, null, null, null)) {
        int idColumn = cursor.getColumnIndexOrThrow(MediaStore.Downloads._ID);
        int nameColumn = cursor.getColumnIndexOrThrow(MediaStore.Downloads.DISPLAY_NAME);
        int sizeColumn = cursor.getColumnIndexOrThrow(MediaStore.Downloads.SIZE);
        if (cursor.moveToFirst()) {
            Log.d(TAG, "Found file " + cursor.getString(nameColumn) + " with size " + 
                cursor.getInt(sizeColumn));
            // Use the content resolver to open a stream to read the file
            try (InputStream is = contentResolver.openInputStream(uri)) {
                // Read the file as needed...
            } catch (IOException e) {
                Log.e(TAG, "Failed to read file", e);
            }
        } else {
            Log.e(TAG, "File not found");
        }
    } catch (Exception e) {
        Log.e(TAG, "Failed to retrieve file", e);
    }
} else {
    // Use legacy methods to read the file
    // For example, use the File API to open a FileInputStream
    File file = new File(Environment.getExternalStoragePublicDirectory(Environment.DIRECTORY_DOWNLOADS), filename);
    try (FileInputStream is = new FileInputStream(file)) {
        // Read the file as needed...
    } catch (IOException e) {
        Log.e(TAG, "Failed to read file", e);
    }
}

在使用MediaStore API时，需要请求用户授予应用访问外部存储器的权限，并且需要检查用户是否已经开启了外部存储器管理器的权限。如果用户未开启该权限，需要通过应用引导用户进行设置。