Spring Boot is widely used for building scalable, production-ready microservices. In interviews, basic questions aren't enough. To truly assess expertise, interviewers dive deep into Spring Boot’s internals, design patterns, optimizations, and complex scenarios. Here’s a collection of advanced Spring Boot interview questions with one-liner answers.
1. Core Spring Boot Concepts
Q1: How does Spring Boot auto-configuration work internally?
Spring Boot uses @EnableAutoConfiguration, scans the classpath, and loads conditional beans via spring.factories.
Q2: How can you override an auto-configured bean?
Define the same bean explicitly in your @Configuration class with @Primary or @Bean.
Q3: What is the difference between @ComponentScan and @SpringBootApplication?
@SpringBootApplication includes @ComponentScan, @EnableAutoConfiguration, and @Configuration.
Q4: What design patterns does Spring Boot use internally?
Spring Boot heavily uses Factory, Proxy, Singleton, Template, and Dependency Injection patterns.
Q5: Explain the Spring Boot startup process in detail.
Spring Boot initializes context, loads properties, runs auto-configurations, registers beans, and starts embedded servers.
2. Spring Boot Internals
Q6: How does Spring Boot embed Tomcat and manage its lifecycle?
It creates an instance of TomcatServletWebServerFactory and starts it using WebServer.start().
Q7: How does Spring Boot manage application properties?
Properties are loaded from multiple sources (application.properties/yml, environment variables, system properties) and bound via @ConfigurationProperties.
Q8: How does Spring Boot handle dependency injection?
Uses a combination of constructor, setter, and field injection with proxies and bean post-processors.
Q9: What is the role of spring.factories in auto-configuration?
It registers configurations and components dynamically without explicit bean definitions.
Q10: How does Spring Boot handle circular dependencies?
By default, it throws an error, but it can be resolved using @Lazy or setter injection.
3. Spring Security & Authentication
Q11: How does Spring Security work in a Spring Boot application?
Spring Security registers filters, applies authentication & authorization, and integrates with OAuth2 & JWT.
Q12: Explain the difference between JWT and OAuth2 in Spring Boot security.
JWT is a token-based authentication method, whereas OAuth2 is an authorization framework.
Q13: How can you customize Spring Security authentication?
By implementing UserDetailsService and defining custom authentication providers.
Q14: What is the purpose of @PreAuthorize and @PostAuthorize?
They enable method-level security based on expressions.
Q15: How does Spring Boot handle CSRF protection by default?
CSRF protection is enabled by default but can be disabled via csrf().disable().
4. Spring Boot with Microservices
Q16: How does Spring Boot handle distributed transactions?
Spring Boot integrates with Saga, TCC patterns, and uses @Transactional with XA transactions.
Q17: What is Spring Cloud and how does it enhance Spring Boot microservices?
Spring Cloud provides service discovery, configuration management, circuit breakers, and API gateways.
Q18: How do you implement service-to-service authentication in Spring Boot microservices?
Using JWT, OAuth2, or API gateways like Spring Cloud Gateway.
Q19: What are circuit breakers in microservices, and how does Spring Boot implement them?
Circuit breakers prevent cascading failures, implemented using Resilience4j or Hystrix.
Q20: How does Spring Boot handle API rate limiting?
Using Redis, Guava RateLimiter, or Spring Cloud Gateway filters.
5. Performance Tuning and Debugging
Q21: How do you monitor Spring Boot applications in production?
Using Actuator, Prometheus, Grafana, and Micrometer.
Q22: What is the purpose of Spring Boot Actuator?
Provides production-ready features like metrics, health checks, and tracing.
Q23: How do you optimize memory usage in Spring Boot?
Use JVM tuning, bean scope optimizations, and lazy initialization.
Q24: How does Spring Boot handle request timeouts?
Configured via server.tomcat.connection-timeout or in WebFlux settings.
Q25: How do you debug slow Spring Boot applications?
Use profiling tools like JVisualVM, Flight Recorder, and distributed tracing.
6. Advanced Scenarios
Q26: How does Spring Boot handle event-driven architecture?
Uses ApplicationEventPublisher and asynchronous event listeners.
Q27: How do you implement multi-tenancy in Spring Boot?
Using database partitioning, schema-based separation, or context-based tenant resolution.
Q28: How does Spring Boot support reactive programming?
Through WebFlux, Project Reactor, and functional programming paradigms.
Q29: What are the differences between Spring MVC and WebFlux?
MVC is synchronous and blocking; WebFlux is asynchronous and non-blocking.
Q30: How do you implement custom starters in Spring Boot?
By defining auto-configurations and registering them in spring.factories.
Final Thoughts
Mastering Spring Boot requires deep understanding beyond just annotations and configurations. These advanced questions help evaluate real-world expertise in performance tuning, security, microservices, and design patterns. If you’re preparing for interviews, ensure hands-on experience with debugging, profiling, and optimizing Spring Boot applications.
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