I have a Struts 1 application and I am exploring different options for upgrading the system. One option is Spring MVC or Spring Boot. If I were to choose either framework to migrate the application to, can I do so transitionally (piece by piece) or would it be a complete wholesale change or complete rewrite. Don't know if I'm making myself understood.
Spring 3.0, had explicit support for integrating struts 1. This was two major versions of spring and 9 years ago. There is documentation here:
https://docs.spring.io/spring/docs/3.0.5.RELEASE/reference/web-integration.html#struts
Integration consisted in either using the ContextLoaderPlugin to allow spring to manage all your action classes, or to make your action classes aware of the spring context. A fair amount of struts specific code was available to do things like map struts action handlers to action beans in the spring context.
You could probably reproduce a lot of this in spring 5, but you'd have to do a lot of heavy lifting on your own. There are probably very few struts/spring 5 users out there to help you.
You could also go with spring 3.0 to make the integration easier, allowing you to do the migration in steps, but then you are in a different bad place. You have an old version of spring that fewer people use, and none of the goodness that 9 years of releases have wrought.
If you want to migrate your struts 1 app to spring, you are probably going to have to do it all at once. Redo your action classes as springmvc controllers and then rewrite your views (templates). spring mvc is pretty view agnostic and there are many good choices.
Good luck and have fun!