Springeasy

What do `@Configuration` and `@Bean` do in Spring?

Answer

`@Configuration` marks a class that defines Spring beans. `@Bean` marks a method whose return value should be managed by Spring as a bean (created, injected, lifecycle handled).

@Configuration
class AppConfig {
  @Bean
  Clock clock() {
    return Clock.systemUTC();
  }
}

Advanced answer

Deep dive

Expanding on the short answer — what usually matters in practice:

Context (tags): configuration, bean, spring-core
Lifecycle: what happens at runtime (render/build, request/response, background jobs).
Caching: where cache lives, cache keys, how to invalidate without chaos.
Security: authn/authz, secrets, attack surface (SSRF/CSRF).
Explain the "why", not just the "what" (intuition + consequences).
Trade-offs: what you gain/lose (time, memory, complexity, risk).
Edge cases: empty inputs, large inputs, invalid inputs, concurrency.

Examples

Here’s an additional example (building on the short answer):

@Configuration
class AppConfig {
  @Bean
  Clock clock() {
    return Clock.systemUTC();
  }
}

Common pitfalls

Too generic: no concrete trade-offs or examples.
Mixing average-case and worst-case (e.g., complexity).
Ignoring constraints: memory, concurrency, network/disk costs.

Interview follow-ups

When would you choose an alternative and why?
What production issues show up and how do you diagnose them?
How would you test edge cases?