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Spring Boot simplifies Java application development by eliminating boilerplate code, providing auto-configuration, and enabling microservices architecture with ease. If you're diving into Spring Boot or preparing for job interviews, here’s a curated list of 20 essential topics every Java developer should know.

🌱 1. Introduction to Spring Boot

• What is Spring Boot?
• Difference: Spring Boot vs Spring Framework
• Key features (Auto-configuration, Embedded Servers, Starter Dependencies)
• Why Spring Boot is ideal for microservices

⚙️ 2. Setup and Configuration

• Using Spring Initializr
• Maven vs Gradle in Spring Boot projects
• Understanding application.properties vs application.yml
• Profile-based configuration (@Profile)

🧠 3. Auto-Configuration

• How Spring Boot automatically configures beans
• @EnableAutoConfiguration & @SpringBootApplication
• How to override or exclude auto-configurations

🌀 4. Application Lifecycle

• main() method and @SpringBootApplication
• CommandLineRunner vs ApplicationRunner
• Hooking into startup or shutdown events

🔁 5. DevTools

• Auto-reloading on file changes
• Hot swapping classes
• Enabling/Disabling debug logs during development

🧩 6. Dependency Injection (IoC)

• @Component, @Service, @Repository annotations
• @Autowired & @Qualifier
• Constructor vs setter injection

🌐 7. Building RESTful Web Services

• Creating REST endpoints using @RestController, @GetMapping, etc.
• Request handling: @PathVariable, @RequestParam, @RequestBody
• Global exception handling with @ControllerAdvice
• Content negotiation and versioning
• Pagination & sorting with Spring Data JPA

🗄️ 8. Data Access with Spring Data JPA

• Repositories: JpaRepository, CrudRepository
• Custom queries (JPQL, native SQL)
• Entity relationships: One-to-One, One-to-Many
• Working with H2, MySQL, PostgreSQL
• Connecting to NoSQL (MongoDB, Cassandra)

🔐 9. Security with Spring Boot

• Introduction to Spring Security
• Basic Auth, Form Login, JWT, OAuth2
• Securing REST APIs
• CORS and CSRF handling

✅ 10. Testing in Spring Boot

• Unit tests with JUnit and Mockito
• Integration tests using @SpringBootTest
• Testing REST APIs with MockMvc
• Repository testing with H2
• Using Testcontainers for database testing

🩺 11. Actuator for Monitoring

• Health and info endpoints (/actuator/health, /actuator/info)
• Customizing Actuator endpoints
• Adding metrics and custom health indicators

📃 12. Logging

• Built-in support for SLF4J, Logback
• External logging configuration (logback.xml, log4j2.xml)
• Changing log levels in application.properties

🧱 13. Microservices with Spring Boot

• Building microservices using Spring Boot
• Service discovery with Eureka
• Load balancing with Ribbon
• API Gateway with Spring Cloud Gateway
• Circuit Breakers with Resilience4j or Hystrix

📩 14. Messaging with Spring Boot

• JMS, RabbitMQ, Kafka integration
• Synchronous vs asynchronous messaging
• Event-driven architecture with Spring Cloud Stream

☁️ 15. Cloud Deployment

• Deploy Spring Boot to AWS, Azure, GCP
• Dockerize Spring Boot apps
• Use Kubernetes (K8s) for orchestration
• Centralized config using Spring Cloud Config Server

🔄 16. Reactive Programming with Spring WebFlux

• What is reactive programming?
• Using Mono and Flux
• Building non-blocking reactive APIs

🚀 17. Performance Tuning

• Enabling caching with EhCache, Redis
• Optimizing Hibernate queries (fetch joins, projections)
• Connection pool tuning (HikariCP)
• Metrics collection with Micrometer + Actuator

⏲️ 18. Scheduling and Async Tasks

• Run background jobs using @Scheduled
• Use @Async for non-blocking tasks
• Cron expressions in Spring

📁 19. File Uploads and Downloads

• Uploading files with MultipartFile
• Saving and serving static files
• Downloading files with correct content headers

🌍 20. Internationalization (i18n)

• Add support for multiple languages
• Load resource bundles
• Dynamically switch locales in UI/backend

📌 Bonus: Top Spring Boot Tools

ToolPurposeSpring InitializrProject generatorPostmanREST API testingDockerContainerizationTestcontainersContainer-based testingActuatorHealth & metricsLombokReduces boilerplate

🧪 Interview Tip

Prepare for common questions like:

• How does Spring Boot auto-configuration work?
• How do you secure a REST API?
• What is the difference between @Component and @Bean?
• How does Spring manage transactions?