Top 50 JPA Interview Questions and Answers (2026 Edition)
Top 50 JPA Interview Questions and Answers (2026 Edition) Work through these 50 questions out loud. Open any question for the full model answer, the trap to avoid, and the follow-up a panel usually asks.
A database connection pool is getting exhausted. How do you identify and solve it?
Enable `spring.datasource.hikari.leak-detection-threshold`. Watch `hikaricp.connections.active` and threads blocked waiting. Fix leaks with try-with-resources; avoid long...
What happens if @Transactional fails midway? How do you verify rollback?
Default: runtime exception marks rollback-only; transaction commits nothing on exit. ```java @Transactional(rollbackFor = Exception.class) public void placeOrder(Order o) {...
How do you manage schema changes without breaking production?
Add nullable column first; deploy code reading/writing both; backfill; then remove old in later release. ```sql -- V12__add_phone.sql ALTER TABLE users ADD COLUMN phone...
Database is healthy but app shows connection timeout. Why?
DB healthy from admin console but app waits for free connection. Check Hikari pending threads. ```yaml spring.datasource.hikari.maximum-pool-size: 10...
How does Spring Boot auto-configuration work?
@SpringBootApplication enables @EnableAutoConfiguration, which loads candidate configs from starters. Each auto-config uses @ConditionalOn* to register beans only when...
Describe the Spring Bean lifecycle.
Container instantiates the bean, injects dependencies, runs BeanPostProcessors, then @PostConstruct or InitializingBean. Singleton lives for context lifetime; prototype creates...
What is dependency injection and why does Spring use it?
DI supplies collaborators from outside the class — typically via constructor. Problem solved: tight coupling to concrete classes, hard unit tests, scattered object...
@RestController vs @Controller?
@Controller is for traditional MVC — return "user/list" view name. @RestController adds @ResponseBody so return types become JSON/XML body — standard for REST APIs. Use...
Explain JWT authentication flow in Spring Security.
POST /login with credentials → auth provider validates → issue JWT with exp/sub/roles. Subsequent requests include Bearer token. JwtAuthenticationFilter runs before...
How do you handle exceptions globally in Spring?
Create @RestControllerAdvice with @ExceptionHandler(MethodArgumentNotValidException.class) returning 400 + field map, NotFoundException → 404, generic Exception → 500 without...
What is CSRF and how does Spring Security handle it?
CSRF tricks logged-in user's browser to POST to your site. Spring stores CsrfToken and validates on mutating requests. For SPA with session cookies, expose token via...
Bean scopes in Spring — singleton vs prototype?
Default singleton shares one instance — stateless services. Prototype for per-operation objects. Request/session scopes tie to web lifecycle with proxies. Prototype inside...
How do you version REST APIs?
We use /api/v1/users for clarity. Non-breaking: add optional JSON fields. Breaking: launch v2 path, maintain v1 for 6 months with Deprecation header. Spring maps...
How does spring.config.import work for externalized configuration?
In K8s: spring.config.import=optional:configtree:/etc/secrets/,optional:configtree:/etc/config/ mounts volumes as properties. Vault:...
How do Spring Boot Actuator health probes work with Kubernetes?
Enable probes, configure K8s readinessProbe on /actuator/health/readiness, liveness on /actuator/health/liveness. Custom HealthIndicator for critical deps on readiness group...
What are GraalVM native image considerations for Spring Boot?
Use spring-boot-maven-plugin native profile + GraalVM 21. Run integration tests on native binary. Register reflection for Jackson/custom serializers. Trade longer CI builds for...
How does Bean Validation work in Spring Boot 3?
DTO fields: @NotBlank, @Email, @Size. Controller: createUser(@Valid @RequestBody UserDto dto). Global @RestControllerAdvice maps MethodArgumentNotValidException to 400 + field...
Explain @ConfigurationProperties vs @Value.
@ConfigurationProperties(prefix = "app.payment") record PaymentProps(int timeout, String apiUrl) {} — validated, testable, grouped. Use @Value only for single feature flags....
How do you enable virtual threads in Spring Boot 3.2+?
application.yml: spring.threads.virtual.enabled=true on Java 21. Verify integration tests pass — especially security, transactions, MDC logging. Replace custom fixed thread...
What is ProblemDetail (RFC 7807) in Spring 6 error handling?
@ExceptionHandler(NotFoundException.class) ProblemDetail handle(NotFoundException ex) { return ProblemDetail.forStatusAndDetail(NOT_FOUND, ex.getMessage()); } — consistent 404...
When should you use spring.main.lazy-initialization?
Use in test slices or large batch apps where most beans unused per run. Production APIs: fix slow @PostConstruct and DB migrations instead. Combine with...
How do you expose custom metrics with Micrometer in Spring Boot?
Service method: orderCounter.increment(); timer.record(() -> process()). Add Tags.of("tenant", tenantId) for multi-tenant. Expose...
springdoc-openapi vs legacy springfox — what should new projects use?
Add springdoc-openapi-starter-webmvc-ui dependency. Annotate with @Operation, @Schema on DTOs. Disable in prod via springdoc.api-docs.enabled=false or secure endpoint. Generate...
How do Spring profiles work for dev, staging, and production?
Base application.yml shared defaults. application-dev.yml local Docker Compose. application-prod.yml from K8s ConfigMap + secrets. Activate via SPRING_PROFILES_ACTIVE=prod....
What is Spring Boot Docker Compose dev services support?
Add compose.yaml with postgres and redis services. Include spring-boot-docker-compose on runtimeOnly. Boot starts Postgres, sets JDBC URL — zero local install. CI uses...
How does Micrometer Tracing replace Spring Cloud Sleuth?
Dependencies: micrometer-tracing-bridge-otel, opentelemetry-exporter-otlp. Traces propagate Authorization-safe headers across RestClient/WebClient calls. Correlate logs with...
What is the difference between @Configuration and @Component?
AppConfig @Configuration with @Bean DataSource — calling dataSource() from another @Bean method returns same instance. Mistake: @Component Config { @Bean Service s() { return...
BeanFactoryPostProcessor vs BeanPostProcessor — explain both.
BFPP: replace ${placeholder} in definitions via PropertySourcesPlaceholderConfigurer. BPP: @Autowired injection, @PostConstruct detection, AOP proxy wrapping. Custom BPP can...
@Primary vs @Qualifier — when to use each?
Two DataSource beans: @Primary mainDataSource for JPA, @Qualifier("analytics") for reporting repo. Prefer @Qualifier when both are equally valid — @Primary for sensible...
ApplicationContext vs BeanFactory — what extra does ApplicationContext provide?
BeanFactory sufficient for minimal DI tests. ApplicationContext adds @EventListener infrastructure, @MessageSource for i18n, Environment integration. Spring Boot creates...
How does Spring Environment resolve property precedence?
Debug with --debug or Environment.getPropertySources(). K8s secret as env var overrides application-prod.yml — intentional for secrets. Document precedence for team. Use...
What does @Lazy mean on injection and on @Configuration?
ServiceA needs ServiceB and vice versa — @Lazy on one injection breaks cycle via proxy. Heavy ReportGenerator @Lazy on admin-only path. Prefer refactoring to remove cycles....
How do Spring application events work with @EventListener?
OrderService publishes OrderCreatedEvent after save. EmailListener @EventListener sends confirmation. @TransactionalEventListener(AFTER_COMMIT) prevents email on rolled-back...
How configure Spring Security OAuth2 Resource Server for JWT?
Configure issuer-uri pointing to IdP. SecurityFilterChain: authorizeHttpRequests(auth ->...
What is the N+1 query problem and how do you fix it?
Loading 100 orders then accessing each order's items triggers 101 queries. Fix with JOIN FETCH in JPQL, @EntityGraph on repository method, or a DTO query selecting only...
How does @Transactional work?
Spring creates a proxy around @Transactional beans. Calling createOrder() from another bean starts a transaction on the thread-bound EntityManager; commit on success, rollback...
What are Spring Boot starters?
spring-boot-starter-web brings spring-webmvc, Tomcat, Jackson with tested versions and enables WebMvcAutoConfiguration. You declare one starter instead of 10 coordinates....
Difference between @Component, @Service, @Repository?
All register as singleton beans via component scan. Choose by layer for readability: @Repository on DAOs (plus SQLException translation), @Service on business logic, @Component...
What is the difference between JPA and Hibernate?
JPA is the API contract; Hibernate is the engine under Spring Boot by default. You write @Entity per JPA spec; Hibernate handles SQL generation. Spring Data JpaRepository adds...
How do you test Spring Boot applications?
Unit test services with Mockito — no Spring. @WebMvcTest + MockMvc for controller contract. @DataJpaTest for repository queries. @SpringBootTest + Testcontainers for critical...
What changed migrating Spring Boot 2 to Spring Boot 3 (Jakarta EE)?
Upgrade to Java 17+, replace imports javax.persistence → jakarta.persistence, update Spring Security config to SecurityFilterChain lambda DSL, run OpenRewrite migration recipe,...
Lazy vs eager fetching in JPA — trade-offs?
Default associations LAZY. Service method needing orders+items uses JOIN FETCH in repository query. Disable spring.jpa.open-in-view=false — forces explicit fetch in service...
When use @Query native SQL vs JPQL in Spring Data JPA?
JPQL for standard entity fetches and portable logic. Native for PostgreSQL-specific jsonb queries, recursive CTEs, or performance-tuned index hints after EXPLAIN. Interface...
How do JPA Specifications (Criteria API) help dynamic queries?
UserSearchSpec.withStatus(status).and(withNameLike(name)) — repository.findAll(spec, pageable). Criteria API verbose — wrap in static factory methods. Alternative: Querydsl or...
What is JPA auditing with @CreatedDate and @LastModifiedDate?
@EnableJpaAuditing + AuditorAware<String> from SecurityContextHolder. Entity: @CreatedDate Instant createdAt; @LastModifiedDate Instant updatedAt. Works with auditing on save()...
How do Flyway/Liquibase integrate with Spring Boot JPA?
Prod: spring.jpa.hibernate.ddl-auto=validate, Flyway V1__init.sql through Vn. CI: @Testcontainers PostgreSQL + Flyway migrate + integration tests. Never rename column without...
Best practices for Spring Data pagination with Pageable?
Controller: @PageableDefault(size=20, sort="createdAt", direction=DESC) Pageable p. Cap size at 100. Admin reports: keyset pagination on id > lastId for deep pages — OFFSET...
What is Hibernate second-level cache and when enable it?
Enable for ProductCategory, Currency — READ_ONLY, rarely changed. spring.jpa.properties.hibernate.cache.use_second_level_cache=true with Caffeine region factory. Avoid on...
How implement multi-tenancy in Spring Security context?
JWT includes tenant_id — converter sets TenantContextHolder. Base entity @Column tenantId validated on save. Repository methods always include tenant predicate — or Hibernate...
What do Java Records provide and their limitations?
DTO: record UserResponse(String id, String name) {} — JSON mapping works. Service layer immutable value objects. Not for entities needing setters/lazy proxies yet. Combine with...
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