HAPI FHIR

HAPI provides pre-built Java classes for every FHIR resource across all released versions (DSTU2, STU3, R4, R5). The library uses a code generation engine that transforms the core FHIR StructureDefinitions into native Java objects, ensuring type safety and eliminating manual boilerplate. Each generated class includes getters, setters, and fluent builders for every defined element, including complex datatypes like HumanName, CodeableConcept, and Identifier.

The HAPI FHIR Validator is an embedded instance of the official HL7 validation toolchain, capable of checking resource conformance against the base specification and any custom ImplementationGuide or StructureDefinition. It validates:

• Cardinality and required elements
• ValueSet binding strength (required vs. extensible)
• FHIRPath invariant constraints
• Terminology membership against CodeSystems like SNOMED CT and LOINC This allows developers to catch conformance errors at build time or runtime before data enters a clinical repository.

HAPI includes a production-grade FHIR server that can be embedded in any Java application or deployed as a standalone service. It natively implements the full FHIR RESTful API, including CRUD operations, search with chained parameters, paging, history, and transactional Bundles. The server supports multitenancy and can be configured with custom interceptors for authentication, audit logging, and data enrichment, making it suitable for enterprise clinical data repositories.

HAPI ships with a complete FHIRPath engine that allows developers to execute path-based expressions against FHIR resources. This is critical for evaluating complex invariants defined in profiles and for extracting specific data elements from deeply nested structures. The engine supports all standard FHIRPath functions, including aggregation, filtering, and mathematical operations, enabling dynamic data extraction without hardcoded traversal logic.

Every FHIR resource must contain a human-readable narrative section. HAPI includes a Thymeleaf-based narrative generator that automatically produces HTML representations of clinical data from the structured elements. This ensures that even systems without sophisticated rendering capabilities can display a basic, standards-compliant summary of a patient's allergies, medications, or lab results directly from the resource payload.

HAPI integrates with the SUSHI compiler, the reference implementation for FHIR Shorthand (FSH). This allows teams to author profiles, extensions, and implementation guides using the concise FSH domain-specific language and compile them directly into HAPI-compatible StructureDefinitions. This streamlines the process of defining custom constraints and value sets for specific healthcare use cases like mCODE or US Core.