This repository represents a very simple proof of concept of creating a monolithic spring-boot project with separation of internal components via Kotlin APIs (hiding their internals from others). This is done via utilising the internal Kotlin visibility modifier, which limits the visibility to the scope of the current module (which provides more isolation than Java's package protection - think of intentionally creating the same package as in your dependency to access it's package protected classes). The goal of this sample project is just to verify this works well with Spring and other libraries (it is also a migration of spring-boot-monolith to Gradle modules and Kotlin).
The APIs are very loosely coupled and it would be quite easy to change e.g. the persistence for each "service" or transform this to use microservices.
The project uses spring-data as a persistence layer for all the "services". It also takes advantage of ObjectMapper to reduce the boilerplate needed.
Following figure demonstrates the isolation by trying to use ItemEntity (internal to item module) in the order module:
