How to create rest api which accepts post request using sparkjava (http://sparkjava.com/)

The service should support multiple concurrent users. This service will be used to create an excel by accepting json from user.

public static void main(String[] args) {
    post("/generateexcel", (req, res) -> {
        System.out.println(req.body());
        generateExcel(req.body());
        return "success";
    });
}

But the issue I am facing is, is the request is very big, the other users are getting blocked. Should I use another web framework?