Interfacing with Java Application from Web App

I have created a simulation engine in Java. I want now to be able to expose it through a web App. How can I make it available to my users so that they can interact with the simulation engine in runtime? Basically the process should be similar to: 1) User access my application and configures his simulation parameters 2) Simulation starts 3) User is able to remotely modify/call some methods on specific objects during the simulation.

My idea was to expose a set of web services that will be used by users trough my Web App interface. However, I still have no idea how to connect this WSs to my "runtime". One of my ideas was to use directly DB to communicate ... but I guess there are better ways to do it (with probably better performance).

After some research I got to know about 2 approaches that I feel could be useful for my use case, but I don't know much about them. Those are: 1) Creating a thread and run it in the Servlet Context Listener Executor Service of a Tomcat Server 2) RMI

But still have no clear vision about implications of each one or even if there's any other better alternative