IllegalStateException: Not supported on AsyncContext.startAsync(req, res)
@WebServlet(name="MyTest", urlPatterns={"/MyTest"}, asyncSupported=true)
public class MyTest extends HttpServlet {

    @Override
    public void doGet(HttpServletRequest req, HttpServletResponse res)
            throws ServletException, IOException {

        AsyncContext tmp = req.startAsync(req, res);
        ...

    }

}

req.setAttribute("org.apache.catalina.ASYNC_SUPPORTED", true);

((org.apache.catalina.connector.Request)req).setAsyncSupported(true);