Java REST API Design Principles

Apply REST API design principles for Spring Boot applications.

What is covered in this Skill?

• HTTP methods (GET, POST, PUT, PATCH, DELETE) — semantic consistency
• Resource URI design
• HTTP status codes
• Request/response DTOs with lean contracts
• API versioning (URI, header, or media type — applied consistently)
• Bean Validation at the boundary (@Valid/@Validated on controller inputs, 400 on failure)
• Pagination, sorting, and filtering (Page/Pageable with caps)
• ISO-8601 instants with offset (OffsetDateTime, Instant) in JSON contracts
• Content negotiation (JSON default; vendor media types when meaningful)
• Idempotency-Key support for POST creates; 409 Conflict for collisions
• ETag concurrency with If-Match/If-None-Match; 412 Precondition Failed / 304 Not Modified
• HTTP caching discipline (Cache-Control, ETag, Last-Modified)
• Deprecation and sunset headers (Deprecation, Sunset, Link rel="successor-version)"
• Error handling
• API security (TLS, authentication, authorization, input validation)
• API contract: OpenAPI file as source of truth for API-first (OpenAPI Generator)
• Controller advice and problem details (RFC 7807)

Scope: Apply recommendations based on the reference rules and good/bad code examples.

Constraints

Before applying any REST API changes, ensure the project compiles. If compilation fails, stop immediately. After applying improvements, run full verification.

• MANDATORY: Run ./mvnw compile or mvn compile before applying any change
• SAFETY: If compilation fails, stop immediately
• VERIFY: Run ./mvnw clean verify or mvn clean verify after applying improvements
• BEFORE APPLYING: Read the reference for detailed rules and good/bad patterns
• EDGE CASE: If request scope is ambiguous, stop and ask a clarifying question before applying changes
• EDGE CASE: If required inputs, files, or tooling are missing, report what is missing and ask whether to proceed with setup guidance

When to use this skill

• Review Java code for Spring Boot REST API
• Apply best practices for Spring Boot REST API in Java code
• Design Spring Boot REST controllers and DTOs
• Add Problem Details error responses in Spring Boot REST
• Improve pagination validation or idempotency in Spring APIs

Workflow

1. Read reference and assess project context

Read references/302-frameworks-spring-boot-rest.md and inspect the current project setup before proposing changes.

2. Gather scope and decide target improvements

Identify requested outcomes, constraints, and the minimum safe set of changes to apply.

3. Apply framework-aligned changes

Implement or refactor configuration/code following the reference patterns and project conventions.

4. Run verification and report results

Execute appropriate build/tests and summarize what changed, what was verified, and any follow-up actions.

Reference

For detailed guidance, examples, and constraints, see references/302-frameworks-spring-boot-rest.md.