Spring Boot – Architectural Interview Questions

 

🔷 Spring Boot – Architectural Interview Questions

🔹 1. Architecture Design & Modularization

Q1. How would you architect a Spring Boot-based microservices platform?

• Service registry (Eureka/Consul)

• Config server (Spring Cloud Config)

• Circuit breaker (Resilience4j/Hystrix)

• API Gateway (Spring Cloud Gateway)

• Observability (Micrometer + Prometheus + Grafana)

Q2. How do you design modular Spring Boot applications with shared components?

• Use Spring Boot starters for reusable modules.

• Apply DDD: separate bounded contexts.

• Package-by-feature vs. package-by-layer.

Q3. What are the pros/cons of using Spring Boot in microservices?

• ✅ Rapid development, opinionated setup

• ❌ Memory overhead, fat JAR size, complex upgrades

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🔹 2. Configuration & Environment Management

Q4. How do you handle configuration for multiple environments?

• Use application-{profile}.yml with @Profile.

• External config via Spring Cloud Config / HashiCorp Vault.

• Secrets management via KMS/Vault/SOPS.

Q5. How would you secure sensitive configurations in Spring Boot apps?

• Don’t hardcode secrets.

• Use encrypted properties (jasypt, Vault).

• Access secrets via mounted volume/environment variables.

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🔹 3. Performance & Scalability

Q6. How do you optimize startup time and memory usage in large Spring Boot apps?

• Use lazy initialization: spring.main.lazy-initialization=true.

• Avoid component scanning entire base package.

• Remove unused auto-configurations via spring.factories.

Q7. How do you scale a Spring Boot app horizontally?

• Stateless services.

• Externalize session/state (Redis, DB).

• Load balancing via gateway/ingress.

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🔹 4. Data Access & Persistence

Q8. How would you architect a system that supports multi-tenancy in Spring Boot?

• Schema-per-tenant with AbstractRoutingDataSource.

• Use Hibernate's MultiTenantConnectionProvider.

Q9. How do you avoid N+1 issues in Spring Data JPA?

• Fetch joins.

• @EntityGraph

• Custom DTO projections

Q10. How do you batch process millions of records in Spring Boot?

• Use Spring Batch with chunk processing.

• Parallel step execution, partitioning.

• JDBC cursor reader, paging reader.

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🔹 5. Security

Q11. How do you secure Spring Boot microservices?

• OAuth2/JWT tokens with spring-security-oauth2.

• API Gateway for auth propagation.

• Resource server configuration for validating JWTs.

Q12. How do you manage role-based authorization in REST APIs?

• Method-level security: @PreAuthorize, @Secured

• Hierarchical roles via custom RoleHierarchy.

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🔹 6. Observability & Resilience

Q13. How do you implement observability in Spring Boot apps?

• Actuator: expose /actuator/health, /metrics, /loggers.

• Use Micrometer + Prometheus for metrics.

• OpenTelemetry or Zipkin for tracing.

Q14. How do you design resilient REST APIs?

• Retry with RetryTemplate or Resilience4j.

• Circuit breaker patterns.

• Timeout fallback handlers.

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🔹 7. Plugin & Extensible Architecture

Q15. How can you implement a plugin-based architecture in Spring Boot?

• Dynamic class loading using PF4J or custom ClassLoader.

• SPI + Spring Boot conditional configuration.

• Isolated classpath for plugin jars.

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🔹 8. DevOps, CI/CD, Deployment

Q16. How do you containerize and deploy a Spring Boot app?

• Use layered JARs or buildpacks.

• Dockerize using multi-stage builds.

• Run as non-root, pass env config via k8s secrets/configmaps.

Q17. What strategies do you use for zero-downtime deployments?

• Blue-green deployments.

• Readiness/liveness probes.

• Graceful shutdown using ContextClosedEvent.

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🔹 9. Miscellaneous

Q18. How do you reduce memory footprint in Spring Boot apps?

• Exclude unused starters.

• Tune JVM: G1GC, memory limits.

• Avoid unnecessary caching inside app memory.

Q19. How do you manage dependencies and module boundaries in large Spring Boot projects?

• Split into multiple modules using Maven/Gradle.

• Use versioned interfaces for internal contracts.

Q20. How do you manage feature toggling in Spring Boot?

• Use tools like FF4J, Unleash, or Togglz.

• Conditional beans via @ConditionalOnProperty.

1. How would you architect a Spring Boot-based microservices platform?

Answer: A modern Spring Boot-based microservices architecture typically includes:

• Service Discovery: Using Eureka or Consul for dynamic discovery.

• API Gateway: Spring Cloud Gateway for routing, rate-limiting, and authentication.

• Config Management: Spring Cloud Config Server to externalize configuration.

• Resilience: Use Resilience4j for circuit breakers, retries, and timeouts.

• Security: OAuth2/JWT using Spring Security.

• Observability: Spring Boot Actuator + Micrometer + Prometheus + Grafana + Zipkin for monitoring and tracing.

• CI/CD: Dockerized services deployed via Kubernetes (Helm/ArgoCD).

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2. How do you manage environment-specific configurations?

Answer:

• Use application-{profile}.yml files with @Profile annotations.

• Externalize sensitive config to Spring Cloud Config or Vault.

• CI/CD pipelines inject environment variables during deployment.

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3. How do you secure a Spring Boot microservice?

Answer:

• Use Spring Security with OAuth2 resource server and JWT tokens.

• Propagate authentication via gateway.

• Secure endpoints with method-level annotations: @PreAuthorize, @RolesAllowed.

• Apply rate limiting and IP whitelisting.

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4. How do you enable resilience and fault tolerance?

Answer:

• Apply Resilience4j decorators like @Retry, @CircuitBreaker, @RateLimiter.

• Define fallback methods.

• Isolate remote calls using timeouts and bulkheads.

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5. How do you implement observability in Spring Boot?

Answer:

• Use Spring Boot Actuator for health and metrics endpoints.

• Integrate Micrometer with Prometheus.

• Use OpenTelemetry or Zipkin for distributed tracing.

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6. How do you design a plugin-based Spring Boot architecture?

Answer:

• Use PF4J or ServiceLoader with custom ClassLoaders.

• Load JARs at runtime and isolate plugin contexts.

• Register plugin beans dynamically using Spring BeanFactory or ApplicationContext.

📃 System Design Case Studies

Case Study 1: Event-Driven Order Management System

Components:

• Spring Boot microservices (Order, Inventory, Payment).

• Apache Kafka for asynchronous messaging.

• PostgreSQL for persistence.

• Spring Cloud Config, Eureka, Gateway.

• Zipkin + Micrometer for observability.

Architectural Decisions:

• Use Kafka to decouple services.

• Retry with backoff on transient failures.

• Implement Saga pattern for distributed transactions.

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Case Study 2: Data Ingestion and Processing Pipeline

Use Case: Periodically fetch and normalize data from multiple external APIs and persist in a unified format.

Solution:

• Spring Batch for ETL orchestration.

• Scheduled jobs using @Scheduled or Quartz.

• Apache Camel for routing and transformation.

• Plugins per source system (loaded via PF4J or SPI).

• Spring Data JPA + Hibernate for persistence.

• REST endpoints for triggering and monitoring jobs.

Key Features:

• Audit logging of job runs.

• Multitenancy and schema separation.

• Retry/failure handling in Batch jobs.

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Case Study 3: Scalable REST API Backend

Scenario: Build a scalable REST backend for a mobile app.

Solution:

• Stateless Spring Boot microservices with REST APIs.

• OAuth2 for security.

• Redis for session and rate-limit tracking.

• Kubernetes autoscaling based on Prometheus metrics.

Design Decisions:

• Use DTO mappers to isolate persistence.

• Caching layer for hot reads.

• Blue/Green deployment via ArgoCD.

Spring Boot Interview Questions with Answers (Senior/Lead Level)

General Architecture & Design

1. What are the core components of Spring Boot and how do they simplify Spring application development? Spring Boot provides autoconfiguration, opinionated defaults, and embedded servers (Tomcat/Jetty/Undertow) to reduce boilerplate and accelerate development.

2. How does Spring Boot auto-configuration work internally? Via spring.factories which registers conditional configuration classes using @ConditionalOn... annotations. These classes are auto-wired if conditions are met.

3. How would you structure a large Spring Boot application to ensure modularity and maintainability? Use layered architecture, split code by domain and module, follow clean architecture, extract reusable logic into libraries or plugins.

4. How do you manage configuration in a distributed Spring Boot environment (e.g., Kubernetes)? Use Spring Cloud Config or ConfigMaps and Secrets. Leverage profiles, and inject environment variables securely during container startup.

Advanced Spring Boot Features

5. Explain the role of @Conditional annotations in Spring Boot. They enable conditional bean registration. Examples include @ConditionalOnMissingBean, @ConditionalOnProperty to control configuration dynamically.

6. How does the Spring Boot ApplicationRunner differ from CommandLineRunner? Both are used to run logic after app startup. ApplicationRunner provides access to application arguments via ApplicationArguments, while CommandLineRunner gives raw String args.

7. What is a custom starter in Spring Boot, and how do you create one? It’s a reusable dependency that bundles autoconfiguration, starter dependencies, and default behavior. Create a separate module, provide META-INF/spring.factories, and use conditional beans.

Spring Security

8. How would you implement fine-grained access control using Spring Security? Use annotations like @PreAuthorize("hasRole('ADMIN')"), method-level security, and expression-based access control. Integrate with user roles/authorities in JWT claims.

9. What is the difference between OAuth2 and JWT? OAuth2 is a protocol for authorization; JWT is a token format often used to carry OAuth2 tokens. JWT is stateless and can embed claims.

10. How do you secure a REST API at both endpoint and method levels? Use HttpSecurity config to define endpoint access. Use annotations like @PreAuthorize, @Secured, or @RolesAllowed for method-level security.

Observability & Monitoring

11. How do you implement health checks, metrics, and tracing in Spring Boot? Use Spring Boot Actuator endpoints /health, /metrics. Integrate with Micrometer, Prometheus, Zipkin/Jaeger for metrics and distributed tracing.

12. Explain how you would trace a request across multiple Spring Boot services. Use OpenTelemetry or Sleuth + Zipkin. Inject correlation/tracing IDs in headers and propagate through all service calls.

Data & Persistence

13. What are the best practices for integrating Spring Data JPA in microservices? Use DTOs, lazy loading with caution, pagination, projections, entity graphs. Keep services schema-bound.

14. How do you handle N+1 query problems in JPA? Use @EntityGraph, batch fetching, join fetch in JPQL. Monitor queries using Hibernate statistics.

15. How do you implement pagination and sorting in Spring Boot REST APIs? Use Spring Data's Pageable and Sort parameters in repository methods and expose them via REST.

Exception Handling

16. How do you design a global exception handling mechanism in Spring Boot? Use @ControllerAdvice with @ExceptionHandler. Map exceptions to standardized error responses.

17. How do you propagate and standardize error responses across services? Define a consistent error DTO structure. Use Feign/Ribbon interceptors to map upstream error responses.

Scheduling & Batching

18. How does @Scheduled differ from Quartz? @Scheduled is simple for lightweight tasks. Quartz supports complex cron, distributed jobs, persistence, clustering, and retry policies.

19. How do you manage long-running jobs in Spring Batch with fault tolerance? Use chunk-based processing, checkpointing, RetryTemplate, listeners. Store metadata in DB for restart.

Messaging & Eventing

20. How would you implement asynchronous communication between Spring Boot services? Use Kafka or RabbitMQ. Spring Cloud Stream simplifies message handling. Events can be serialized POJOs or Avro.

21. What messaging systems have you used (e.g., Kafka, RabbitMQ) with Spring Boot, and why? Kafka for high-throughput event streaming; RabbitMQ for lightweight queueing. Spring integrates via @KafkaListener, @RabbitListener.

Testing Strategy

22. What are your strategies for writing unit, integration, and contract tests in a Spring Boot application? Use JUnit + Mockito for unit tests, @SpringBootTest for integration, WireMock or Testcontainers for contract tests.

23. How do you mock external service calls in your tests? Use WireMock or Mockito to simulate external endpoints. For integration, use Testcontainers with real service stubs.

CI/CD and Deployment

24. How do you design a Spring Boot application for containerized deployments (e.g., Docker + K8s)? Use Jib or Dockerfile to containerize. Externalize config. Leverage Kubernetes health/readiness probes and ConfigMaps/Secrets.

25. How do you implement blue/green or canary deployments in Spring Boot applications? Use Kubernetes deployment strategies with Argo Rollouts or Istio. Monitor real-time metrics before traffic shift.

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