Interview

Spring & Spring Boot — Interview Questions (92+)

Conceptual coverage aligned with Spring Framework 6 / Spring Boot 3 (Jakarta EE namespace).

Detailed Questions

1. What is the Spring IoC container?

Short: Inversion of Control container that creates, wires, and manages beans.
Detailed: ApplicationContext loads bean definitions (annotations, Java config, XML), resolves dependencies via DI, manages lifecycle (@PostConstruct, @PreDestroy), and provides AOP, events, resource loading.
Example: @SpringBootApplication bootstraps SpringApplication.run().

2. Dependency Injection types?

Short: Constructor (preferred), setter, field injection.
Detailed: Constructor injection makes dependencies explicit, enables final fields, easy unit tests without Spring. Field @Autowired is convenient but hides deps and complicates testing. Spring 4.3+ auto-wires single-constructor beans.
Example: @Service public class OrderService(OrderRepository repo) {}

3. `@Component` vs `@Service` vs `@Repository` vs `@Controller`?

Short: Stereotypes — all @Component specializations for semantics and tooling.
Detailed: @Repository adds persistence exception translation. @Controller/@RestController for web. @Service marks business layer. Component scan picks all up equally otherwise.
Example: @RestController = @Controller + @ResponseBody.

4. Bean scopes?

Short: singleton (default), prototype, request, session, application (web).
Detailed: Singleton one per container; prototype new per injection/getBean. Request/session need web context. Misuse of prototype inside singleton — client gets one prototype instance unless ObjectProvider or @Lookup.
Example: @Scope(ConfigurableBeanFactory.SCOPE_PROTOTYPE).

5. `@Configuration` and `@Bean`?

Short: Java-based config; @Bean methods register objects in context.
Detailed: Full @Configuration classes CGLIB-enhanced so @Bean method calls return same singleton bean. @Bean lite mode without full config may create new instances per call.
Example: @Bean DataSource dataSource() { return HikariDataSource... }

6. `@Autowired` resolution?

Short: By type; @Qualifier or @Primary if multiple candidates.
Detailed: Optional dependency if zero-or-one. ObjectProvider<T> for lazy/optional multiple. Constructor injection fails fast at startup if missing bean (fail-fast wiring).
Example: @Qualifier("stripe") PaymentGateway gateway.

7. Spring Boot auto-configuration?

Short: Conditional beans based on classpath, properties, existing beans.
Detailed: META-INF/spring/org.springframework.boot.autoconfigure.AutoConfiguration.imports. @ConditionalOnClass, @ConditionalOnMissingBean. Starters bundle dependencies + auto-config.
Example: spring-boot-starter-web adds Tomcat, Jackson, MVC.

8. `application.properties` vs `application.yml`?

Short: Externalized config; YAML hierarchical; profiles application-dev.yml.
Detailed: @ConfigurationProperties binds prefix to POJO. @Value for single keys. Secrets via env vars / vault, not committed files.
Example: @ConfigurationProperties(prefix="app.mail") record MailProps(String host, int port) {}

9. Spring MVC request flow?

Short: DispatcherServlet → HandlerMapping → Controller → View/ResponseBody → ExceptionHandler.
Detailed: Filters (security) run first. HandlerAdapter invokes controller. HttpMessageConverter serializes JSON. @ControllerAdvice global exception mapping.
Example: @GetMapping("/users/{id}") User get(@PathVariable Long id).

10. `@Transactional` how it works?

Short: AOP proxy wraps bean; begins/commits/rolls back transaction per method.
Detailed: Self-invocation bypasses proxy (call this.save() — no tx). Rollback on unchecked by default; checked need rollbackFor. Propagation REQUIRED, REQUIRES_NEW, NESTED etc. Read-only optimization hint.
Example: @Transactional(readOnly=true) on query service.

11. Spring Data JPA repository magic?

Short: Interface extends JpaRepository; Spring generates implementation at runtime.
Detailed: Query methods parsed from method names (findByEmailAndActive). @Query for JPQL/native. Pagination via Pageable. Custom fragments with Impl suffix.
Example: Optional<User> findByEmail(String email);

12. Spring Security filter chain?

Short: Servlet filters authenticate and authorize before reaching controller.
Detailed: SecurityFilterChain bean configures HTTP rules, form/login, JWT resource server, CSRF. Method security @PreAuthorize uses AOP after authentication.
Example: authorizeHttpRequests(auth -> auth.requestMatchers("/public/**").permitAll()).

Rapid-Fire (Q → A)

Core Spring modules? → Core, Context, AOP, Data, MVC, Security.
Bean lifecycle init? → @PostConstruct, InitializingBean, custom init.
Bean destroy? → @PreDestroy, DisposableBean.
ApplicationContext vs BeanFactory? → Context superset (events, i18n).
Component scan base? → @SpringBootApplication scan attribute.
@Import? → Pull config classes into context.
@Profile? → Conditional beans on profile.
active profile property? → spring.profiles.active.
Actuator endpoints? → health, metrics, env (secure them).
Starter parent? → Dependency version management.
spring.factories legacy? → AutoConfiguration.imports in Boot 3.
DevTools? → Restart, livereload dev only.
@RestControllerAdvice? → REST exception handler.
ResponseEntity? → Status + headers + body control.
@Valid? → Bean Validation trigger.
@RequestBody? → Deserialize JSON body.
@ResponseStatus? → HTTP status on method/exception.
Content negotiation? → Accept header / produces.
RestTemplate status? → RestClient / WebClient preferred.
WebClient reactive? → Non-blocking HTTP client.
@Async? → Separate thread pool execution.
@Scheduled? → Cron/fixed delay tasks.
@Cacheable? → Method result caching.
Cache abstraction? → Redis, Caffeine backends.
@EventListener? → ApplicationEvent handling.
ApplicationEventPublisher? → Publish domain events.
Transaction propagation REQUIRED? → Join or create.
REQUIRES_NEW? → Suspend and new transaction.
NESTED? → Savepoint nested (JDBC).
readOnly true benefit? → Flush optimization, routing replica.
LazyInitializationException? → Access lazy assoc outside session.
Open Session In View? → Controversial; keeps session for view.
EntityManager vs Session? → JPA vs Hibernate native.
PersistenceContext? → Managed entities scope.
Detached entity? → No longer tracked.
merge()? → Reattach detached state.
flush()? → Sync persistence context to DB.
clear()? → Detach all entities.
N+1 problem? → Join fetch or @EntityGraph.
@EntityGraph? → Specify fetch plan.
DTO projection? → Interface/class-based Spring Data.
Specification pattern? → Dynamic JPA queries.
Criteria API? → Type-safe programmatic queries.
Querydsl? → Alternative type-safe queries.
Flyway/Liquibase? → Schema migration with Boot.
HikariCP default pool? → Spring Boot 2+ default.
DataSource auto-config? → If JDBC on classpath.
JdbcTemplate? → Simplified JDBC without ORM.
RowMapper? → Map ResultSet row to object.
NamedParameterJdbcTemplate? → Named params :id.
@Mapper MyBatis? → SQL mapper alternative.
Spring Test @SpringBootTest? → Full context integration test.
@WebMvcTest slice? → MVC layer only mock.
@DataJpaTest slice? → JPA + in-memory DB.
@MockBean? → Replace bean in test context.
Testcontainers? → Real DB in Docker tests.
AOP proxy JDK vs CGLIB? → Interface JDK; class CGLIB.
@Aspect @Around? → Wrap method proceed().
Pointcut expression? → execution, @annotation, etc.
Self-invocation AOP fix? → Inject self or separate bean.
@Order on filters? → Filter chain ordering.
CORS config? → WebMvcConfigurer addCorsMappings.
CSRF REST? → Often disabled for stateless JWT APIs.
JWT resource server? → oauth2ResourceServer JWT.
OAuth2 client? → oauth2Client() for login.
Bean Validation @NotBlank? → String not empty trim.
@Validated on class? → Method-level validation groups.
Configuration metadata JSON? → IDE property hints.
ConditionalOnProperty? → Feature flags.
Spring Native/AOT? → GraalVM native images constraints.
Virtual threads Boot 3.2+? → spring.threads.virtual.enabled.
Observability Micrometer? → Metrics tracing logs.
@Timed custom metric? → Method timing.
Structured logging? → JSON log format.
Graceful shutdown? → server.shutdown=graceful.
Health groups? → liveness/readiness k8s.
Property placeholder? → ${app.name} in config.
@PropertySource? → Additional property files.
Environment abstraction? → Profiles + property sources.
FactoryBean? → Complex bean creation.

Question list