Can I use a compiled library with a certain Spring Boot version in another project with a different Spring Boot version?

Question

We are developing a Spring Boot microservices with the typical multimodule scenario.

parent-pom
  module 1 (api)
  module 2 (server)
    module1-api <-- as dependency

We have a server module where we have our business logic, controllers, etc (Spring boot application) and we have an api module where we have our DTOs files and feign interfaces (not a spring boot application).

Nowadays all our applications have the same Spring Boot version, so we can inject api dependencies to use their methods and make REST calls.

service 1 (Spring boot 2.7)
   service2-api
   service3-api

service 2 (Spring boot 2.7)
service 3 (Spring boot 2.7)

The question is whether previously developed libraries using an older version of Spring would continue to function if a new service were to emerge with an updated version of Spring.

service 1 (Spring boot 2.7)
   service2-api
   service3-api

service 2 (Spring boot 2.7)
service 3 (Spring boot 2.7)

service 4 (Spring boot 3.2)
  service2-api (Spring boot 2.7) <--- Possible ???

Answer 1

The question is whether previously developed libraries using an older version of Spring would continue to function if a new service were to emerge with an updated version of Spring.

It depends. For example, if your application uses Spring classes, methods and so on that were present in the old version of Spring but have been removed in the new version, then when you try to run the code, you are liable to get classloading errors; e.g. ClassNotFoundError, NoSuchMethodError and so on.

If you had recompiled your application against the new version of Spring, then these issues (removed classes, methods, etc) would have shown up as compilation errors.

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Actually, when you migrate from SpringBoot 2.x to 3, you will almost certainly encounter breaking changes. This transition involves changing from javax.servlet.* to jakarta.servlet.* and so on.