Spring Data & JPA interview questions

Entity lifecycle, lazy loading and N+1, transaction propagation, and generated-SQL debugging.

4 questions in Java & Spring.

Java and the Spring ecosystemA 22-minute read on how the topic fits together, before the questions.Java fundamentals75 short answers on one page, for revising rather than studying.

medium2

mediumConceptScenario

Nobody called save, but the UPDATE still went to the database. Explain how that happened.

Hibernate dirty checking tracks a managed JPA entity through the persistence context. At flush the provider compares the entity against its loaded snapshot and writes an UPDATE for whatever differs, so mutating a managed object inside a transaction is itself the instruction to persist. Use this spring data jpa answer to show the decision, trade-off, and evidence rather than a memorised definition.

5 minmid, senior, staff

mediumConceptScenario

This endpoint logs one query per row. Why are you seeing N+1 queries, and how do you fix them?

You get N+1 when one query loads N rows and something later dereferences a lazy association on each of them, firing a select apiece. Fix it per call site with join fetch, an entity graph, or batch fetching - not by changing mappings to EAGER.

6 minmid, senior, staff

hard2

hardScenarioConcept

A Spring Data repository method looks like one clean line, but the endpoint issues several hundred queries per request. How do you diagnose the N+1, and what would you actually change?

The repository call fetches parents in one query; the extra queries come later, when something touches a lazy association - usually the JSON serialiser, with open-session-in-view keeping the context alive so it succeeds silently. Diagnose by counting statements per request, then fix with a join fetch or entity graph, batch fetching, or a DTO projection.

4 minmid, senior, staff

hardConceptCodingScenario

You pass a thousand new entities to saveAll and the database sees a thousand separate inserts. How do you get them batched?

Spring Data JPA saveAll batching needs a Hibernate batch size, statements for the same table kept adjacent, and an identifier strategy that does not force a round trip per row. IDENTITY generation silently prevents insert batching, because Hibernate has to execute each insert to learn the key.

5 minmid, senior, staff