Spring Bean Lifecycle and Scopes

Implement or sketch code for Spring Bean Lifecycle and Scopes. Explain the logic, complexity, and pros/cons of this approach.

Answers use simple, clear English.

Audio N/A

Quick interview answer

Logic: Container creates beans: instantiate → inject → @PostConstruct → use → @PreDestroy. Scopes: singleton (default), prototype, request, session.

Detailed answer

Logic: Container creates beans: instantiate → inject → @PostConstruct → use → @PreDestroy. Scopes: singleton (default), prototype, request, session. Complexity notes included in code section when present. Pros: Consistent lifecycle hooks; easy testing with @SpringBootTest. Cons: Prototype injected into singleton stays single accidental instance unless scoped proxy. Core: Container creates beans: instantiate → inject → @PostConstruct → use → @PreDestroy. Scopes: singleton (default), prototype, request, session. Real-time example: @Service OrderService injected into @RestController; @PostConstruct loads cache on startup. Pros: Consistent lifecycle hooks; easy testing with @SpringBootTest. Cons: Prototype injected into singleton stays single accidental instance unless scoped proxy. Common mistakes: Calling @Transactional methods via this (self-invocation bypasses proxy). Best practices: Constructor injection for required deps; lazy init only when startup cost high. Audience level: Junior.

Full explanation

Container creates beans: instantiate → inject → @PostConstruct → use → @PreDestroy. Scopes: singleton (default), prototype, request, session.

Real example & use case

@Service OrderService injected into @RestController; @PostConstruct loads cache on startup.

Pros & cons

Pros: Consistent lifecycle hooks; easy testing with @SpringBootTest. Cons: Prototype injected into singleton stays single accidental instance unless scoped proxy.

Code example

@Service
public class CacheWarmup {
    private final UserRepo repo;

    public CacheWarmup(UserRepo repo) { this.repo = repo; }

    @PostConstruct
    void warm() {
        repo.findTop10Active();
    }
}

Practice code · java (view only · no execution)

@Service
public class CacheWarmup {
    private final UserRepo repo;

    public CacheWarmup(UserRepo repo) { this.repo = repo; }

    @PostConstruct
    void warm() {
        repo.findTop10Active();
    }
}

Common mistakes

Calling @Transactional methods via this (self-invocation bypasses proxy).

Best practices

Constructor injection for required deps; lazy init only when startup cost high.

Follow-up questions

How would you test Spring Bean Lifecycle and Scopes?
What metrics prove Spring Bean Lifecycle and Scopes is healthy in prod?
How does Spring Bean Lifecycle and Scopes change at 10× traffic?

#spring-bean-lifecycle#junior