Spring Boot Interview Questions

1. What is Spring Boot, and what are its key features?

Spring Boot is an open-source Java-based framework used to create stand-alone, production-grade Spring applications with minimal configuration. It simplifies the development process by providing default configurations and eliminating the need for boilerplate code. Key features of Spring Boot include auto-configuration, embedded servers (e.g., Tomcat), starter dependencies for easy dependency management, and production-ready features like metrics and health checks. Spring Boot also integrates seamlessly with the Spring ecosystem, including Spring Data, Spring Security, and Spring Cloud. Its convention-over-configuration approach makes it ideal for building microservices, REST APIs, and enterprise applications.

2. What is the difference between Spring and Spring Boot?

Spring is a comprehensive framework for building Java applications, offering modules like Spring MVC, Spring Data, and Spring Security. However, it requires extensive configuration, such as XML or Java-based setups. Spring Boot, on the other hand, is an extension of Spring that simplifies application development by providing auto-configuration and starter dependencies. It eliminates the need for manual configuration and allows developers to focus on writing business logic. While Spring is highly customizable, Spring Boot is designed for rapid development and deployment, making it ideal for modern, cloud-native applications.

3. What is auto-configuration in Spring Boot?

Auto-configuration in Spring Boot is a feature that automatically configures the application based on the dependencies present in the classpath. It reduces the need for manual configuration by providing sensible defaults. For example, if you include the spring-boot-starter-data-jpa dependency, Spring Boot automatically configures a DataSource and EntityManager. Auto-configuration is achieved through @Conditional annotations, which enable or disable configurations based on specific conditions. This feature simplifies development and ensures that applications are production-ready with minimal effort.

4. What are Spring Boot starter dependencies?

Spring Boot starter dependencies are a set of convenient dependency descriptors that simplify dependency management. They group commonly used dependencies for specific functionalities, such as web applications, data access, or security. For example, the spring-boot-starter-web dependency includes everything needed to build a web application, such as Spring MVC and embedded Tomcat. Starters reduce the risk of dependency conflicts and ensure compatibility between libraries. They are a key feature of Spring Boot, enabling developers to quickly set up and configure applications.

5. What is the purpose of the @SpringBootApplication annotation?

The @SpringBootApplication annotation is a combination of three annotations: @Configuration, @EnableAutoConfiguration, and @ComponentScan. It marks the main class of a Spring Boot application and enables auto-configuration and component scanning. For example:

@SpringBootApplication
public class MyApplication {
    public static void main(String[] args) {
        SpringApplication.run(MyApplication.class, args);
    }
}

This annotation simplifies the setup of Spring Boot applications by encapsulating common configurations. It is a best practice to use @SpringBootApplication for the main class.

6. What is the purpose of the application.properties file in Spring Boot?

The application.properties file in Spring Boot is used to configure application settings, such as server port, database connection details, and logging levels. It is located in the src/main/resources directory. For example:

server.port=8081
spring.datasource.url=jdbc:mysql://localhost:3306/mydb

Spring Boot automatically loads this file and applies the configurations. Alternatively, you can use application.yml for YAML-based configurations. The application.properties file is essential for customizing application behavior without modifying code.

7. What is the difference between @RestController and @Controller in Spring Boot?

The @Controller annotation in Spring Boot is used to define a controller class in a Spring MVC application. It typically returns a view name that resolves to an HTML page. For example:

@Controller
public class MyController {
    @GetMapping("/home")
    public String home() {
        return "home";
    }
}

The @RestController annotation, on the other hand, is a combination of @Controller and @ResponseBody. It is used in RESTful web services to return data directly (e.g., JSON or XML) instead of a view. For example:

@RestController
public class MyRestController {
    @GetMapping("/data")
    public String getData() {
        return "Hello, World!";
    }
}

@RestController is ideal for building APIs, while @Controller is used for traditional web applications.

8. What is the purpose of the @Autowired annotation in Spring Boot?

The @Autowired annotation in Spring Boot is used to inject dependencies automatically. It can be applied to fields, constructors, or setter methods. For example:

@Service
public class MyService {
    public String getMessage() {
        return "Hello, World!";
    }
}

@RestController
public class MyController {
    @Autowired
    private MyService myService;

    @GetMapping("/message")
    public String getMessage() {
        return myService.getMessage();
    }
}

@Autowired eliminates the need for manual dependency injection and ensures that Spring manages the lifecycle of beans. It is a key feature of Spring’s dependency injection mechanism.

9. What is the purpose of the @Component annotation in Spring Boot?

The @Component annotation in Spring Boot is a generic stereotype for any Spring-managed component. It marks a class as a Spring bean, making it eligible for auto-detection during component scanning. For example:

@Component
public class MyComponent {
    public String getMessage() {
        return "Hello, World!";
    }
}

Specialized annotations like @Service, @Repository, and @Controller are derived from @Component and provide additional semantics. @Component is used for general-purpose beans that do not fit into specific categories.

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

The @Value annotation in Spring Boot is used to inject values from properties files, environment variables, or system properties into fields or method parameters. For example:

@Value("${app.name}")
private String appName;

If the application.properties file contains app.name=MyApp, the appName field will be set to "MyApp". The @Value annotation is useful for externalizing configuration and making applications more flexible.

11. What is the purpose of the @Configuration annotation in Spring Boot?

The @Configuration annotation in Spring Boot indicates that a class declares one or more @Bean methods and can be processed by the Spring container to generate bean definitions. For example:

@Configuration
public class MyConfig {
    @Bean
    public MyService myService() {
        return new MyService();
    }
}

@Configuration classes are a key part of Spring’s Java-based configuration, allowing developers to define beans programmatically instead of using XML.

12. What is the purpose of the @Bean annotation in Spring Boot?

The @Bean annotation in Spring Boot is used to define a bean in a @Configuration class. It marks a method as a bean producer, and the return value of the method is registered as a bean in the Spring context. For example:

@Configuration
public class MyConfig {
    @Bean
    public MyService myService() {
        return new MyService();
    }
}

@Bean is useful for defining third-party library beans or custom beans that cannot be annotated with @Component.

13. What is the purpose of the @Profile annotation in Spring Boot?

The @Profile annotation in Spring Boot is used to conditionally enable or disable beans based on the active profile. For example:

@Profile("dev")
@Service
public class DevService {
    public String getMessage() {
        return "Development Mode";
    }
}

If the dev profile is active, the DevService bean will be created. Profiles are useful for managing environment-specific configurations, such as development, testing, and production.

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

The @Conditional annotation in Spring Boot is used to conditionally register beans based on specific conditions. It is the foundation of Spring Boot’s auto-configuration. For example:

@Conditional(MyCondition.class)
@Bean
public MyService myService() {
    return new MyService();
}

Custom conditions can be implemented by creating a class that implements the Condition interface. @Conditional is a powerful feature for creating flexible and dynamic configurations.

15. What is the purpose of the SpringApplication class in Spring Boot?

The SpringApplication class in Spring Boot is used to bootstrap and launch a Spring application. It provides methods like run() to start the application and customize its behavior. For example:

public class MyApplication {
    public static void main(String[] args) {
        SpringApplication.run(MyApplication.class, args);
    }
}

SpringApplication also supports features like custom banners, application events, and command-line arguments. It is the entry point for all Spring Boot applications.

Certainly! Here are the next set of Spring Boot interview questions and detailed answers:

16. What is the purpose of the CommandLineRunner interface in Spring Boot?

The CommandLineRunner interface in Spring Boot is used to execute code after the application context is loaded but before the application starts running. It is often used for initialization tasks, such as loading data or performing setup operations. For example:

@Component
public class MyRunner implements CommandLineRunner {
    @Override
    public void run(String... args) throws Exception {
        System.out.println("Application started with args: " + Arrays.toString(args));
    }
}

The run() method is invoked with the command-line arguments passed to the application. CommandLineRunner is useful for tasks that need to be executed once during application startup.

17. What is the purpose of the ApplicationRunner interface in Spring Boot?

The ApplicationRunner interface in Spring Boot is similar to CommandLineRunner but provides more advanced access to application arguments. It allows you to work with ApplicationArguments, which provides methods to retrieve argument values. For example:

@Component
public class MyRunner implements ApplicationRunner {
    @Override
    public void run(ApplicationArguments args) throws Exception {
        System.out.println("Non-option args: " + args.getNonOptionArgs());
        System.out.println("Option args: " + args.getOptionNames());
    }
}

ApplicationRunner is useful when you need to handle command-line arguments in a structured way, such as parsing options and flags.

18. What is the purpose of the @EnableAutoConfiguration annotation in Spring Boot?

The @EnableAutoConfiguration annotation in Spring Boot enables auto-configuration, which automatically configures the application based on the dependencies present in the classpath. For example, if you include the spring-boot-starter-data-jpa dependency, Spring Boot will automatically configure a DataSource and EntityManager. This annotation is typically used in combination with @Configuration and @ComponentScan in the @SpringBootApplication annotation. Auto-configuration simplifies development by reducing the need for manual configuration.

19. What is the purpose of the @ComponentScan annotation in Spring Boot?

The @ComponentScan annotation in Spring Boot is used to scan for Spring components (e.g., @Component, @Service, @Repository, @Controller) in specified packages. It is typically included in the @SpringBootApplication annotation. For example:

@SpringBootApplication
@ComponentScan(basePackages = "com.example")
public class MyApplication {
    public static void main(String[] args) {
        SpringApplication.run(MyApplication.class, args);
    }
}

By default, @ComponentScan scans the package of the class annotated with @SpringBootApplication and its sub-packages. It ensures that all Spring-managed components are detected and registered in the application context.

20. What is the purpose of the @Repository annotation in Spring Boot?

The @Repository annotation in Spring Boot is used to mark a class as a repository, which is a component responsible for data access and manipulation. It is a specialization of the @Component annotation and is typically used with Spring Data JPA. For example:

@Repository
public interface UserRepository extends JpaRepository<User, Long> {
}

The @Repository annotation enables exception translation, converting data access exceptions into Spring’s DataAccessException hierarchy. It is a best practice to use @Repository for data access layers.

21. What is the purpose of the @Service annotation in Spring Boot?

The @Service annotation in Spring Boot is used to mark a class as a service component, which contains business logic. It is a specialization of the @Component annotation and is typically used in the service layer of an application. For example:

@Service
public class UserService {
    public String getMessage() {
        return "Hello, World!";
    }
}

The @Service annotation makes the class eligible for auto-detection during component scanning and ensures that it is managed by the Spring container. It is a best practice to use @Service for business logic.

22. What is the purpose of the @Transactional annotation in Spring Boot?

The @Transactional annotation in Spring Boot is used to define transactional boundaries for methods or classes. It ensures that a method is executed within a transaction, and if an exception occurs, the transaction is rolled back. For example:

@Service
public class UserService {
    @Autowired
    private UserRepository userRepository;

    @Transactional
    public void saveUser(User user) {
        userRepository.save(user);
    }
}

The @Transactional annotation can be customized with attributes like propagation, isolation, and rollbackFor. It is essential for managing database transactions in Spring Boot applications.

23. What is the purpose of the @EnableCaching annotation in Spring Boot?

The @EnableCaching annotation in Spring Boot enables caching in the application. It is typically used in combination with caching providers like EhCache, Hazelcast, or Redis. For example:

@SpringBootApplication
@EnableCaching
public class MyApplication {
    public static void main(String[] args) {
        SpringApplication.run(MyApplication.class, args);
    }
}

Once caching is enabled, you can use the @Cacheable, @CachePut, and @CacheEvict annotations to manage cached data. Caching improves application performance by reducing redundant computations and database queries.

24. What is the purpose of the @Cacheable annotation in Spring Boot?

The @Cacheable annotation in Spring Boot is used to cache the result of a method. When the method is called with the same arguments, the cached result is returned instead of executing the method again. For example:

@Service
public class UserService {
    @Cacheable("users")
    public User getUserById(Long id) {
        // Fetch user from database
        return userRepository.findById(id).orElse(null);
    }
}

The @Cacheable annotation can be customized with attributes like value, key, and condition. It is a powerful feature for optimizing performance in data-intensive applications.

25. What is the purpose of the @Scheduled annotation in Spring Boot?

The @Scheduled annotation in Spring Boot is used to schedule tasks to run at fixed intervals or specific times. It is typically used in combination with the @EnableScheduling annotation. For example:

@Component
public class MyScheduler {
    @Scheduled(fixedRate = 5000)
    public void runTask() {
        System.out.println("Task executed at: " + new Date());
    }
}

The @Scheduled annotation supports attributes like fixedRate, fixedDelay, and cron for defining scheduling rules. It is useful for automating repetitive tasks, such as sending notifications or cleaning up data.