java - gson.toJson(obj) ローカル クラス

Question

gson.toJson(obj)これを行うと が戻るのはなぜnullですか?

public class LoginServlet extends HttpServlet {
    @Override
    public void doGet(HttpServletRequest req, HttpServletResponse resp) throws IOException {
        UserService userService = UserServiceFactory.getUserService();
        User user = userService.getCurrentUser();
        Gson gson = new Gson();

        if (user != null) {
            resp.setContentType("application/json");
            resp.getWriter().println(gson.toJson(user));
        } else {
            class Url {
                private String url;
                Url(String url) {
                    this.url=url;
                }
            }
            Url obj = new Url(userService.createLoginURL(req.getRequestURI()));
            resp.setContentType("application/json");
            resp.getWriter().println(gson.toJson(obj));
        }
    }
}

Urlクラスの外でクラスを定義すると、LoginServletそれは機能し、url オブジェクトの json 文字列を返しますか?

class Url {
    private String url;
    Url(String url) {
        this.url=url;
    }
}

public class LoginServlet extends HttpServlet {
    @Override
    public void doGet(HttpServletRequest req, HttpServletResponse resp) throws IOException {
        UserService userService = UserServiceFactory.getUserService();
        User user = userService.getCurrentUser();
        Gson gson = new Gson();

        if (user != null) {
            resp.setContentType("application/json");
            resp.getWriter().println(gson.toJson(user));
        } else {
            Url obj = new Url(userService.createLoginURL(req.getRequestURI()));
            resp.setContentType("application/json");
            resp.getWriter().println(gson.toJson(obj));
        }
    }
}