Spring Boot

Are Spring singleton beans thread-safe?

Separates container instance reuse from concurrency guarantees and designs singleton services around stateless or safely coordinated state.

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.

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.

How can @Transactional tests create false confidence?

Finds failures hidden by test-owned transactions, automatic rollback, open persistence contexts, missing flushes, and separate server threads.

Java & Spring Backend

Java and Spring Boot interview preparation grounded in real backend work.

A backend-focused Java and Spring preparation path covering language fundamentals, the JVM, collections, concurrency, Spring Boot, web APIs, persistence, security, testing, reliability, and production diagnosis.