@WebServlet(urlPatterns = {"/photo"})
public class PhotoServlet extends HttpServlet {

    @PersistenceContext
    EntityManager entityManager;

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        try {
            int id = Integer.parseInt(req.getParameter("id"));
            Photo photo = entityManager.find(Photo.class, id);

            if (photo == null) {
                resp.getWriter().append("Image not found");
                resp.setStatus(404);
                return;
            }

            resp.setContentType("image/jpeg");
            resp.getOutputStream().write(photo.getData());

        } catch (NumberFormatException e) {
            resp.getWriter().append("Invalid request");
            resp.setStatus(400);
        }
    }
}