Top 40 Spring Interview Questions | Must-Prepare before interview.
Top 40 Spring Interview Questions | Must-Prepare before interview Work through these 40 questions out loud. Open any question for the full model answer, the trap to avoid, and the follow-up a panel usually asks.
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...
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 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 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...
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...
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...
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...
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....
How do you handle exceptions globally in Spring?
Create @RestControllerAdvice with @ExceptionHandler(MethodArgumentNotValidException.class) returning 400 + field map, NotFoundException → 404, generic Exception → 500 without...
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...
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 test Spring Boot applications?
Unit test services with Mockito — no Spring. @WebMvcTest + MockMvc for controller contract. @DataJpaTest for repository queries. @SpringBootTest + Testcontainers for critical...
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...
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,...
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...
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