I'll add: both Spring and Tomcat exemplify this quality that a lot of earlier (2000-era) Java frameworks have (others: Hibernate, actually literally anything that embraces XML...), of being "extensible mostly in name only". You can, in theory, implement modules that tie into various parts of their functionality -- and in practice I've found myself doing it occasionally -- but it's almost always regrettable.

Anything you figure out how to do in these will itself just become un-figure-outable for the next engineer who handles it.

(One of the most frustrating tasks in my engineering career involved trying to debug why Spring beans loaded by a Spring-Tomcat integration layer, in the initialization of a Tomcat container, weren't discovering XML files loaded at another stage of initialization. When I look back on that year I think I might have indirectly changed jobs because of it. Hah.)