Spring Boot is a widely adopted framework in the Java ecosystem, simplifying the development of production-ready applications by providing defaults and eliminating boilerplate code. As a senior developer with over 10 years of experience, you are expected to have an in-depth understanding of the intricacies of Spring Boot. Here, we will discuss 35 interview questions to help you prepare for a Spring Boot interview.

1. What is Spring Boot, and how does it differ from the Spring Framework?

Answer: Spring Boot is an extension of the Spring Framework that simplifies the creation of production-ready applications. It provides defaults for various configurations and setups, eliminating boilerplate code. Spring Boot differs from the Spring Framework by offering:

Auto-configuration to automatically set up application context.
Opinionated defaults for application setup.
Embedded servers (e.g., Tomcat, Jetty) to run applications standalone.
Simplified dependency management.

2. Explain the key features of Spring Boot.

Answer: Key features of Spring Boot include:

Auto-configuration: Automatically configures Spring application based on the jar dependencies.
Starter dependencies: Simplifies Maven/Gradle configuration.
Embedded servers: Run applications with embedded Tomcat, Jetty, or Undertow.
Actuator: Provides production-ready features like monitoring and metrics.
Spring Initializr: Web-based tool to bootstrap a Spring Boot project.

3. How does Spring Boot auto-configuration work?

Answer: Spring Boot auto-configuration scans the classpath and configures beans based on the libraries present. It uses @Conditional annotations to apply configurations only when specific conditions are met. The META-INF/spring.factories file lists auto-configuration classes, which are then loaded by Spring Boot.

4. What is Spring Boot Starter? Name a few commonly used Starters.

Answer: Spring Boot Starters are dependency descriptors that aggregate common dependencies needed for a specific functionality. Examples of commonly used starters include:

spring-boot-starter-web: For web applications.
spring-boot-starter-data-jpa: For JPA and Hibernate.
spring-boot-starter-security: For security.
spring-boot-starter-test: For testing.

5. What is a Spring Boot Actuator?

Answer: Spring Boot Actuator provides production-ready features to help monitor and manage applications. It includes endpoints for health checks, metrics, info, environment properties, and more. Actuator endpoints can be accessed via HTTP or JMX.

6. Explain the role of application.properties and application.yml in Spring Boot.

Answer: application.properties and application.yml are configuration files used to define application properties in Spring Boot. These files allow configuring various aspects like server port, database connection, and custom application settings. The application.yml format supports hierarchical data structures and is more readable for complex configurations.

7. How do you configure a Spring Boot application to use a specific profile?

Answer: Profiles in Spring Boot can be configured using the spring.profiles.active property in application.properties or application.yml. For example:

spring.profiles.active=dev

Alternatively, profiles can be activated via the command line:

java -jar myapp.jar --spring.profiles.active=dev

8. What is the purpose of @SpringBootApplication annotation?

Answer: The @SpringBootApplication annotation is a convenience annotation that combines @Configuration, @EnableAutoConfiguration, and @ComponentScan annotations. It marks the main class of a Spring Boot application and triggers auto-configuration and component scanning.

9. How do you handle exceptions in Spring Boot?

Answer: Exceptions in Spring Boot can be handled using:

@ControllerAdvice and @ExceptionHandler annotations to define global exception-handling logic.
Custom error pages by defining error views in the src/main/resources/templates directory.
Spring Boot’s ErrorController to customize the error response.

10. What is the difference between @RestController and @Controller?

Answer: @RestController is a convenience annotation that combines @Controller and @ResponseBody. It is used for RESTful web services, returning data directly in the response body. @Controller is used for traditional MVC controllers where views are returned.

11. How do you implement security in a Spring Boot application?

Answer: Security in a Spring Boot application can be implemented using Spring Security. Key steps include:

Dependencies: Add spring-boot-starter-security to your project dependencies to include Spring Security.
Security Configuration: Create a configuration class with a SecurityFilterChain bean to define security rules using the Lambda DSL. Use authorizeHttpRequests to specify public and secured paths.
Authentication: Enable form-based login with formLogin() and HTTP Basic authentication with httpBasic().
UserDetailsService: Define a UserDetailsService bean to manage user authentication, typically backed by an in-memory user store for simplicity or a custom database authentication service for production.
Properties: Optionally, configure user credentials and roles in application.properties for quick setup.
Execution: Run your application to secure endpoints based on the specified configuration, ensuring robust and flexible security management.

12. Explain the usage of @SpringBootTest annotation.

Answer: The @SpringBootTest annotation is used to create an application context for integration tests. It loads the full application context and can be used to test the entire Spring Boot application. It also supports configuring specific properties and profiles for tests.

13. How do you integrate Spring Boot with a database?

Answer: Integration with a database in Spring Boot can be achieved using Spring Data JPA. Steps include:

Adding spring-boot-starter-data-jpa dependency.
Configuring database properties in application.properties or application.yml.
Creating entity classes annotated with @Entity.
Creating repositories by extending JpaRepository.

14. What is the purpose of @Value annotation in Spring Boot?

Answer: The @Value annotation is used to inject values from property files into Spring Beans. It supports SpEL (Spring Expression Language) and can be used to inject single properties, lists, maps, and other complex types.

15. How do you configure logging in Spring Boot?

Answer: Logging in Spring Boot can be configured using application.properties or application.yml. By default, Spring Boot uses Logback for logging. You can customize logging levels, patterns, and appenders using properties such as:

logging.level.org.springframework=DEBUG
logging.file.name=app.log

16. What is Spring Boot DevTools, and how is it useful?

Answer: Spring Boot DevTools is a set of tools that improves the development experience. It includes features like automatic restart, live reload, and configuration for faster feedback during development. Adding spring-boot-devtools dependency activates these features.

17. Explain the use of @ConditionalOnProperty annotation.

Answer: The @ConditionalOnProperty annotation is used to conditionally enable or disable beans based on the presence or value of a property. It helps configure beans that should only be created when certain conditions are met.

18. How do you perform health checks in Spring Boot?

Answer: Health checks in Spring Boot are performed using the Actuator module. The /actuator/health endpoint provides health information about the application. Custom health indicators can be created by implementing the HealthIndicator interface.

19. What are the different ways to deploy a Spring Boot application?

Answer: Spring Boot applications can be deployed in various ways, including:

As a standalone application using the embedded server (e.g., java -jar myapp.jar).
Deploying the executable JAR or WAR to a traditional application server (e.g., Tomcat, JBoss).
Using cloud platforms like AWS, Azure, Google Cloud, or Pivotal Cloud Foundry.

20. How do you configure CORS in a Spring Boot application?

Answer: CORS (Cross-Origin Resource Sharing) can be configured in Spring Boot using:

Global configuration with a CorsConfigurationSource bean.
Specific configuration using the @CrossOrigin annotation on controller methods or classes.
Customizing the WebMvcConfigurer to set up CORS mappings.

21. Explain the use of @Scheduled annotation.

Answer: The @Scheduled annotation is used to schedule tasks in Spring Boot. It can be applied to methods to run them at fixed intervals, fixed delays, or using cron expressions. Enabling scheduling requires adding the @EnableScheduling annotation to a configuration class.

22. How do you handle asynchronous tasks in Spring Boot?

Answer: Asynchronous tasks in Spring Boot can be handled using:

@Async annotation to mark methods for asynchronous execution.
Enabling asynchronous processing with @EnableAsync annotation.
Configuring a TaskExecutor bean to manage async execution.

23. What is the purpose of @EnableAutoConfiguration?

Answer: The @EnableAutoConfiguration annotation enables Spring Boot’s auto-configuration mechanism. It attempts to automatically configure the Spring application based on the dependencies present in the classpath. It can be excluded or customized using the exclude and excludeName attributes.

24. How do you customize the banner in Spring Boot?

Answer: The banner displayed during Spring Boot startup can be customized by placing a banner.txt file in the src/main/resources directory. The banner can include ASCII art, text, or properties. The banner can also be disabled using spring.main.banner-mode=off.

25. How do you test RESTful web services in Spring Boot?

Answer: Testing RESTful web services in Spring Boot can be done using:

@WebMvcTest for testing controllers.
MockMvc to perform HTTP requests and assert responses.
@RestClientTest for testing REST clients.
TestRestTemplate for integration testing of REST endpoints.

Here are additional interview questions focusing on Spring Boot microservices for experienced developers.

26. What is microservices architecture, and how does it differ from monolithic architecture?

Answer: Microservices architecture is an architectural style that structures an application as a collection of loosely coupled, independently deployable services. Each service represents a specific business functionality and communicates with other services over standard protocols such as HTTP/REST. This contrasts with monolithic architecture, where the entire application is built as a single unit, making it harder to scale and maintain.

27. Explain the role of Spring Cloud in microservices.

Answer: Spring Cloud provides tools for developers to quickly build some of the common patterns in distributed systems, such as configuration management, service discovery, circuit breakers, intelligent routing, distributed sessions, and more. It leverages existing Spring projects like Spring Boot, Spring Data, and Spring Security to provide a robust ecosystem for developing microservices.

28. What is service discovery, and how is it implemented in Spring Boot microservices?

Answer: Service discovery is a mechanism for automatically detecting and managing the network locations of service instances. In Spring Boot microservices, service discovery can be implemented using Spring Cloud Netflix Eureka, which provides a service registry where services can register themselves and discover other services.

29. Explain the purpose of a Config Server in Spring Cloud.

Answer: A Config Server in Spring Cloud centralizes the management of external properties for applications across all environments. It allows configuration to be stored in a Git repository, providing versioned and consistent configuration management. Spring Cloud Config Server exposes these configurations as REST endpoints, enabling applications to retrieve their configuration at startup or runtime.

30. How do you secure Spring Boot microservices?

Answer: Securing Spring Boot microservices involves several practices:

Authentication and Authorization: Using OAuth2 and JWT for secure token-based authentication.
Service-to-Service Communication: Implementing mutual TLS or OAuth2 for secure communication between services.
API Gateway: Using Spring Cloud Gateway or Zuul to handle security centrally.
Configuring Security Policies: Defining security policies using Spring Security.

31. What is the role of an API Gateway in microservices architecture?

Answer: An API Gateway acts as a single entry point for all client requests, routing them to the appropriate microservice. It provides cross-cutting concerns such as authentication, rate limiting, load balancing, caching, and logging. Spring Cloud Gateway is a popular choice for implementing API gateways in Spring Boot microservices.

32. How do you handle distributed transactions in Spring Boot microservices?

Answer: Handling distributed transactions in Spring Boot microservices can be challenging. Approaches include:

Saga Pattern: Breaking down transactions into a series of smaller, compensatable transactions.
Event-Driven Architecture: Using events to maintain eventual consistency across services.
Two-Phase Commit: Though less common in microservices, it can be used where strict ACID transactions are necessary.

33. What is the difference between synchronous and asynchronous communication in microservices?

Answer: Synchronous communication in microservices involves direct interaction between services where the caller waits for the response (e.g., HTTP/REST). Asynchronous communication involves services communicating via messaging systems (e.g., RabbitMQ, Kafka) where the caller does not wait for the response, allowing for more decoupling and better scalability.

34. How do you implement distributed logging in Spring Boot microservices?

Answer: Distributed logging in Spring Boot microservices can be implemented using:

Log Aggregation Tools: Tools like ELK (Elasticsearch, Logstash, Kibana) stack or EFK (Elasticsearch, Fluentd, Kibana) stack to collect, process, and visualize logs.
Correlation IDs: Including a unique identifier in logs to trace a request across multiple services.

35. What are the best practices for database management in microservices?

Answer: Best practices for database management in microservices include:

Database per Service: Each microservice should have its own database to ensure loose coupling.
Data Replication: Use data replication to share data between services if necessary.
Event-Driven Architecture: Use events to propagate changes between services.
CQRS: Command Query Responsibility Segregation to separate read and write operations.

Also, check out:

15+ Spring Boot Microservices Interview Questions and Answers.

Spring Boot Interview Questions for 5 Years Experience

Spring Boot Interview Questions for 10 Years of Experience