: This is the heart of the system, containing business rules as Plain Old Java Objects (POJOs) . It uses Domain-Driven Design (DDD) building blocks like Entities, Value Objects, and Aggregates to model the problem without any dependency on external frameworks.
You are specifically looking for the version. Why 2021? Because that year marked a maturation of best practices. Earlier versions (2018-2019) often coupled the domain to libraries like Lombok or MapStruct . The 2021 editions focused on "Clean Dependencies" . : This is the heart of the system,