Then probably you don’t need MVC controllers. Take a look at the Organizing Business Logic chapter from the manual. Maybe middleware services or some other approach described in this chapter is what you’re looking for.
Also, it may be useful to walk through the Bicycle Workhop step-by-step guide. It also contains examples of how to organize a business logic.
I still don’t understand your task, but anyway, from any place of your code you can use any Java library. Send HTTP requests with Apache Http Components (https://hc.apache.org/) or any other lib you like.