Cracking the Spring Core Interview: Tips and Must-Know Questions
Cracking the Spring Core Interview: Tips and Must-Know Questions Work through these 12 questions out loud. Open any question for the full model answer, the trap to avoid, and the follow-up a panel usually asks.
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...
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....
BeanFactoryPostProcessor vs BeanPostProcessor — explain both.
BFPP: replace ${placeholder} in definitions via PropertySourcesPlaceholderConfigurer. BPP: @Autowired injection, @PostConstruct detection, AOP proxy wrapping. Custom BPP can...
Describe the Spring Bean lifecycle.
Container instantiates the bean, injects dependencies, runs BeanPostProcessors, then @PostConstruct or InitializingBean. Singleton lives for context lifetime; prototype creates...
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...
@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...
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...
Multiple beans of the same type exist and Spring throws an error. How do you fix it?
Spring needs disambiguation when two `@Service` implement the same interface. ```java @Primary @Service public class StripePaymentGateway implements PaymentGateway {} @Service...
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...
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...
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