Spring Core & Boot

🌱 Spring Core & Spring Boot (target: Boot 4.x / Framework 7)

The secret to Spring mastery

Spring is not magic. It’s reflection + proxies + conventions. Once you understand IoC, bean post-processors, AOP proxies, and auto-configuration, every annotation becomes readable.

Core → Advanced

Spring Framework core

• IoC & DI: constructor injection, @Component scanning vs @Bean config
• Bean scopes, lifecycle, BeanPostProcessor, BeanFactoryPostProcessor
• ApplicationContext, events (ApplicationEventPublisher, @TransactionalEventListener)
• AOP: JDK dynamic proxy vs CGLIB, self-invocation pitfall (why @Transactional sometimes doesn’t work)
• SpEL, Environment, property sources

Spring Boot

• Auto-configuration: @Conditional*, how to read the conditions report (--debug)
• Externalized config, @ConfigurationProperties (typed, validated), profiles
• Starters; writing your own starter (great interview talking point)
• Spring MVC: DispatcherServlet flow, controllers, validation (@Valid), @ControllerAdvice, ProblemDetail
• RestClient (sync), WebClient, HTTP interface clients (@HttpExchange)
• Virtual threads (spring.threads.virtual.enabled=true)
• WebFlux/Reactor: understand it and know when it’s worth it (rarely, now that virtual threads exist)
• Actuator: health, metrics (Micrometer), info, graceful shutdown
• Observability: Micrometer + OpenTelemetry tracing
• Spring Modulith: enforce module boundaries in a modular monolith ⭐
• Testing: @SpringBootTest, slices (@WebMvcTest, @DataJpaTest), Testcontainers with @ServiceConnection
• Docker Compose support, Buildpacks / layered jars, GraalVM native images, CDS
• Scheduling, async (@Async), caching abstraction (@Cacheable + Redis)

🧪 Labs (🟢 warm-up → 🟡 core → 🔴 hard → ⚫ boss)

• 🟢 Print the conditions report; override an auto-configured bean
• 🟡 A BeanPostProcessor logging creation time and proxy type for every bean
• 🔴 Mini DI container from scratch (W5 lab): scanning, constructor injection, cycle detection, a JDK-proxy @Timed
• 🔴 A custom starter: orbit-idempotency-spring-boot-starter with auto-config + conditions + tests
• ⚫ Step through refresh() and DispatcherServlet.doDispatch(); write the sequence from memory

🧠 Cognitive tasks

• Predict: does @Transactional apply in 5 tricky snippets? (self-call, private, final, @Async, interface proxy) Then verify
• Code archaeology: how does @ConditionalOnMissingBean let your bean win?

🛰️ Orbit integration

• orbit-api, orbit-knowledge, and orbit-tools are Spring Boot services
• Spring Modulith boundaries in orbit-api

Go deeper

⚙️ Spring Internals · 🧩 GoF Patterns (Java & Go) · Enterprise & DDD Patterns

Resources

• spring.io guides + reference docs ⭐ · Spring Academy (free courses) ⭐
• Spring Start Here (Spilcă) · Spring in Action 6e (Walls)
• YouTube: Dan Vega, Josh Long (Spring Tips), Laurențiu Spilcă
• Baeldung (reference)

Interview questions interview-q

• How does auto-configuration work?
• Why doesn’t @Transactional work on a private/self-invoked method?
• Bean scopes? Circular dependencies: why they’re bad, and how Boot handles them now?
• Filter vs Interceptor vs AOP?