public function getByUri(Request $request)
    {
        if (!$request->headers->has('X-Islandora-URI')) {
            return new Response('Require the X-Islandora-URI header', 400);
        }
        $uri = $request->headers->get('X-Islandora-URI');
        if (is_array($uri)) {
            // Can only return one Location header.
            $uri = reset($uri);
        }
        $uri = $this->urlMapper->findUrls($uri);
        $headers = [];
        if ($uri) {
            $headers['Location'] = $uri;
        }
        return new Response(null, ($uri ? 200 : 404), $headers);
    }