Drills

Build a JPA projection query with optional filters

Builds a bounded dynamic query that returns a read model, applies only supplied predicates, and uses deterministic ordering.

Build a validated Spring create endpoint with ProblemDetail errors

Implements request validation, a clean service boundary, a 201 response, and stable RFC 9457-style validation details.

JavaSpring Boot

Coalesce concurrent Java loads for the same key

Implements single-flight request coalescing so one load runs per key while unrelated keys remain concurrent.

Fix and regression-test a JPA N+1 query

Uses a bounded entity graph for a to-one association and proves the repair with Hibernate query statistics.

Implement an immutable USD money value object in Java

Builds a small immutable money type with explicit scale, rounding, arithmetic, ordering, and value equality.

Implement an optimistic version conflict in JPA

Carries the version observed by the client into a conditional update and returns an explicit conflict instead of silently overwriting newer data.

Implement a persisted idempotent POST with Spring and JPA

Uses a scoped idempotency key, request hash, unique constraint, and one transaction to create an order exactly once from the API perspective.

Implement stable keyset pagination with Spring Data JPA

Implements descending cursor pagination with a unique tie-breaker, bounded projection query, and next-cursor detection.

Run bounded concurrent HTTP fetches in Java

Fetches multiple URLs with bounded concurrency, per-request and batch deadlines, cancellation, ordered results, and executor cleanup.

Save an order and outbox event in one Spring transaction

Persists business state and durable publication intent atomically, leaving broker delivery to an idempotent retrying publisher.

Test Spring resource ownership with MockMvc

Tests anonymous, owner, non-owner, and missing-resource outcomes through the real Spring Security filter chain.

Verify PostgreSQL behavior with a Spring Boot Testcontainers test

Runs a Spring Boot integration test against PostgreSQL and proves a migration-backed database constraint at flush time.