This is the HAPI FHIR core library, a multi-module Java implementation of the HL7 FHIR standard. It provides FHIR object models for versions DSTU2, DSTU3, R4, R4B, R5, and an experimental R6, along with cross-version conversion, validation, terminology services, code generation, CLI tooling, and HTTP service endpoints. The architecture is a monolithic Maven project with modules for each FHIR version, a shared convertors module with pairwise converters, a validation engine with picocli-based CLI, and a services layer for rendering and terminology. Key entry points include VersionConvertorFactory for conversion, ValidationEngine for validation, and CLI commands for the validator. The codebase is actively maintained with CI/CD, security scanning, and regular releases to Maven Central. However, it suffers from significant duplication across version modules, some deprecated interfaces, and generated code that may drift from upstream specifications. The project demonstrates strong engineering practices with reusable security utilities (SSRF protection, zip-slip prevention), generic data structure utilities (MergedList, BaseList), and a well-structured conversion pattern using advisors and contexts.