More than one question there:
There's nothing stopping you from using web.xml (it's still part of the Servlet spec). Most people prefer the Java initializers these days.
@EnableAutoConfiguration
works for any application context (it just uses features of Spring).But Spring Boot out of the box doesn't have a
ContextLoaderListener
that knows aboutSpringApplication
, so you miss some of the benefits of Boot if you do as you describe. There's one you might find useful here.Spring Boot Actuator relies on a few Servlet 3.0 features here and there so you need workarounds for a 2.5 environment (see this spring-boot-legacy prototype for details).
There's a sample app here that runs on GAE, currently deployed here: http://dsyerboot.appspot.com/.