Web page for Java code to access its methods directly

I have a Java monolith application with Spring Core running on a Jetty server and I want to add some responsive web pages to it. The thing is that I'm not allowed to use web frameworks that run on another server or so (Spring MVC, RPC calls or any use of servers are not allowed as well). I need to use something that allows me to access the java code behind (to call methods directly and so) and to integrate a responsive html/css/js template with it. Any suggestions? Thanks