@WebServlet(urlPatterns = "/my/servlet",
        asyncSupported = true)
public class MyServlet extends HttpServlet {

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) {
        AsyncContext asyncContext = req.getAsyncContext();

        asyncContext.start(()->{
            try {
                resp.getWriter().println("helloWorld");

            } catch (IOException e) {
                e.printStackTrace();
            }
            asyncContext.complete();
        });
    }
}

There was an unexpected error (type=Internal Server Error, status=500).
It is illegal to call this method if the current request is not in asynchronous mode (i.e. isAsyncStarted() returns false)