Top 21 Spring Boot Interview Questions and Answers

Spring Boot has become the de-facto standard for building modern, production-ready Java applications quickly and easily. Preparing for a Spring Boot interview requires a solid understanding of its core concepts and features. Here are 21 important interview questions with detailed answers to help you ace your interview:

1. What is Spring Boot?

Answer: Spring Boot is a framework that simplifies the process of building stand-alone, production-grade Spring-based applications with minimal configuration. It follows the “convention over configuration” principle, providing sensible defaults and reducing the boilerplate code required to set up a Spring application.

2. What are the key features of Spring Boot?

Answer: Key features include:

• Standalone: Applications can be run with minimal setup, often just a `java -jar` command.
• Embedded Servers: Supports embedded Tomcat, Jetty, and Undertow servers, eliminating the need for external web server deployment.
• Auto-configuration: Automatically configures Spring applications based on the dependencies present in the classpath.
• Starter Dependencies: Provides convenient dependency descriptors that group related dependencies together, simplifying project setup.
• Actuator: Offers built-in endpoints for monitoring and managing the application (health checks, metrics, etc.).
• Spring CLI: A command-line interface for quickly bootstrapping Spring Boot projects and running Groovy scripts.
• No XML Configuration: Favors Java-based configuration and annotations.
• DevTools: Provides development-time features like automatic application restarts and live reload.

3. What are Spring Boot Starters? Why are they important?

Answer: Spring Boot Starters are dependency descriptors that bundle together all the common dependencies needed for a specific type of application. For example, `spring-boot-starter-web` includes dependencies for building web applications (Spring MVC, Tomcat, Jackson, etc.).

Importance:

• Simplified Dependency Management: Reduces the need to manually add individual dependencies.
• Consistent Dependencies: Ensures that compatible versions of related libraries are used.
• Faster Project Setup: Speeds up the initial project configuration process.

4. What is Auto-configuration in Spring Boot? How does it work?

Answer: Auto-configuration is a feature in Spring Boot that automatically configures your Spring application based on the dependencies you’ve added. Spring Boot examines the classpath and, based on the presence of certain JARs, configures beans that are likely needed.

How it works: Spring Boot uses `@EnableAutoConfiguration` annotation (often implicitly included via `@SpringBootApplication`) along with `spring.factories` files present in the META-INF directory of various auto-configuration JARs. These files list configuration classes that Spring Boot attempts to apply based on conditions like the presence of specific classes or properties.

5. What is `@SpringBootApplication` annotation? What does it include?

Answer: `@SpringBootApplication` is a convenience annotation that combines three other Spring annotations:

• `@Configuration`: Marks the class as a source of bean definitions for the application context.
• `@EnableAutoConfiguration`: Enables Spring Boot’s auto-configuration mechanism.
• `@ComponentScan`: Tells Spring to look for other components, configurations, and services in the same package as the annotated class (and its sub-packages by default).

Using `@SpringBootApplication` simplifies the main application class setup.

6. How do you run a Spring Boot application? What are the different ways?

Answer: There are several ways to run a Spring Boot application:

• From an IDE: Most IDEs (IntelliJ IDEA, Eclipse, VS Code) provide a simple way to run the main application class (the one annotated with `@SpringBootApplication`).
• Using Maven/Gradle:
  • Maven: `mvn spring-boot:run`
  • Gradle: `gradle bootRun`
• As a JAR file: After building the application (e.g., using `mvn clean package` or `gradle bootJar`), you can run the generated executable JAR file using the command: `java -jar your-application.jar`.

7. What is Spring Boot Actuator? What are some of its important endpoints?

Answer: Spring Boot Actuator provides production-ready features to help you monitor and manage your application. It exposes a set of endpoints over HTTP or JMX.

Important Endpoints (examples):

• `/actuator/health`: Shows the health status of the application.
• `/actuator/info`: Displays general application information (can be customized).
• `/actuator/metrics`: Shows application metrics (memory usage, HTTP requests, etc.).
• `/actuator/beans`: Displays the list of Spring beans in the application context.
• `/actuator/env`: Shows the application’s environment properties.
• `/actuator/loggers`: Allows viewing and modifying the logging levels of the application.
• `/actuator/threaddump`: Performs a thread dump of the application.

8. How do you customize Actuator endpoints? Change the base path? Enable/disable endpoints?

Answer: You can customize Actuator endpoints using properties in your `application.properties` or `application.yml` file:

• Change the base path: `management.endpoints.web.base-path=/manage`
• Enable/disable endpoints:
  • Enable all: `management.endpoints.web.exposure.include=*`
  • Disable all: `management.endpoints.web.exposure.exclude=*`
  • Enable specific: `management.endpoints.web.exposure.include=health,info,metrics`
  • Disable specific: `management.endpoints.web.exposure.exclude=env,beans`

9. How do you handle different environments (dev, prod, test) in Spring Boot?

Answer: Spring Boot provides several mechanisms for handling different environments:

• Profile-specific properties files: You can create separate property or YAML files for each environment (e.g., `application-dev.properties`, `application-prod.yml`). Spring Boot will load the properties from the default `application.properties/yml` and then overlay the properties from the active profile(s).
• `@Profile` annotation: You can use the `@Profile` annotation to conditionally enable specific Spring components or configurations based on the active profiles.
• Environment variables and command-line arguments: Spring Boot can also read configuration from environment variables and command-line arguments.

You can activate profiles using the `spring.profiles.active` property (e.g., in `application.properties` or as a command-line argument: `–spring.profiles.active=dev`).

10. What is Spring Data JPA? How does it simplify database interaction in Spring Boot?

Answer: Spring Data JPA is a Spring module that simplifies the development of JPA-based data access layers. It provides abstractions and utilities to reduce the amount of boilerplate code required for database interactions.

Simplification:

• Repositories: Spring Data JPA allows you to define repository interfaces that automatically get implemented by Spring. You often only need to declare methods based on naming conventions (e.g., `findByLastName`).
• Query Generation: Spring Data JPA can automatically generate database queries based on the method names in your repository interfaces.
• Auditing: Provides support for automatic auditing of entity creation and modification timestamps and users.
• Pagination and Sorting: Simplifies the implementation of pagination and sorting for query results.

11. How do you configure a database connection in Spring Boot?

Answer: You configure database connection properties in your `application.properties` or `application.yml` file. Common properties include:

• `spring.datasource.url`: The JDBC URL of the database.
• `spring.datasource.username`: The database username.
• `spring.datasource.password`: The database password.
• `spring.datasource.driver-class-name`: The JDBC driver class name.

Spring Boot auto-configures a `DataSource` bean based on these properties and the presence of a suitable JDBC driver in the classpath.

12. What is Spring Security? How can you implement authentication and authorization in Spring Boot?

Answer: Spring Security is a powerful and highly customizable framework for providing authentication and authorization to Spring applications.

Implementation in Spring Boot:

• Add `spring-boot-starter-security` dependency.
• Basic Authentication (default): By default, adding the starter will secure your application with HTTP Basic authentication and generate a default username and password on startup.
• Custom Authentication: You can customize authentication by creating a `@Configuration` class that extends `WebSecurityConfigurerAdapter` (or by using the `SecurityFilterChain` bean approach in newer Spring Security versions) and overriding methods like `configure(AuthenticationManagerBuilder auth)` to define user details services (e.g., in-memory, JDBC, LDAP).
• Authorization: You can define authorization rules by configuring method security using `@EnableMethodSecurity` and annotations like `@PreAuthorize`, `@PostAuthorize`, or by configuring request authorization in your `WebSecurityConfigurerAdapter` (or `SecurityFilterChain`) using `HttpSecurity` and methods like `authorizeHttpRequests()`.

13. How do you handle RESTful APIs in Spring Boot?

Answer: Spring Boot provides excellent support for building RESTful APIs using Spring MVC. Key annotations include:

• `@RestController`: A convenience annotation that combines `@Controller` and `@ResponseBody`, indicating that the methods in the class handle web requests and return data directly in the response body (e.g., JSON, XML).
• `@RequestMapping`: Maps HTTP requests to handler methods. Can be used at the class level (for a base path) and method level (for specific paths and HTTP methods).
• `@GetMapping`, `@PostMapping`, `@PutMapping`, `@DeleteMapping`: Specific shortcut annotations for mapping HTTP GET, POST, PUT, and DELETE requests.
• `@PathVariable`: Extracts values from the URI path.
• `@RequestParam`: Extracts values from the query parameters.
• `@RequestBody`: Binds the request body to a method parameter (often used for JSON or XML data).
• `@ResponseBody`: Indicates that the return value of a method should be directly written to the response body.
• `@ResponseStatus`: Sets the HTTP status code of the response.

14. What is dependency injection in Spring? How is it used in Spring Boot?

Answer: Dependency injection (DI) is a design pattern where an object receives its dependencies from external sources rather than creating them itself. Spring manages the creation and injection of these dependencies (beans) into other objects.

Usage in Spring Boot: Spring Boot leverages Spring’s DI extensively. You typically use annotations like `@Autowired` to inject dependencies into your classes. Spring’s container manages the lifecycle of these beans and ensures they are available when needed.

@Service
public class MyService {

    @Autowired
    private MyRepository myRepository;

    // ...
}

15. What are Spring Beans? How are they managed in Spring Boot?

Answer: Spring Beans are objects that are managed by the Spring IoC (Inversion of Control) container. The container is responsible for instantiating, configuring, and managing the lifecycle of these beans.

Management in Spring Boot: Spring Boot builds upon Spring’s bean management. Beans are typically defined using:

• `@Component`: A generic stereotype annotation for any Spring-managed component.
• `@Service`: A specialization of `@Component` for service layer components.
• `@Repository`: A specialization of `@Component` for data access layer components.
• `@Controller`: A specialization of `@Component` for MVC controller components.
• `@RestController`: A specialization of `@Controller` and `@ResponseBody`.
• `@Configuration` and `@Bean`: Methods annotated with `@Bean` within a class annotated with `@Configuration` define beans.

Spring Boot’s auto-configuration also creates and manages many beans automatically based on the project’s dependencies.

16. How do you handle logging in Spring Boot? What are the common logging frameworks used?

Answer: Spring Boot provides default logging configuration using Logback. You can customize logging levels, appenders, and formats in `application.properties` or `application.yml`.

Common Logging Frameworks:

• Logback (default): Spring Boot’s default logging implementation.
• Log4j2: Another popular Java logging framework that can be used instead of Logback by including its starter dependency (`spring-boot-starter-log4j2`) and excluding the Logback starter.
• java.util.logging: The standard Java logging API, which Spring Boot also supports.

17. What is Spring Boot DevTools? How is it useful during development?

Answer: Spring Boot DevTools is a set of development-time tools that aim to improve the development experience.

Usefulness:

• Automatic Application Restarts: Automatically restarts the application whenever changes are detected on the classpath (excluding static resources).
• Live Reload: Automatically refreshes the browser when static resources (HTML, CSS, JavaScript) are modified.
• Property Defaults: Provides sensible defaults for development-time properties (e.g., caching).
• Remote Debugging Support: Enables easier remote debugging.
• Spring Boot Actuator Enhancements: Provides additional information in Actuator endpoints during development.

To use DevTools, you add the `spring-boot-devtools` dependency to your project.

18. How do you test Spring Boot applications? What are some common testing annotations?

Answer: Spring Boot provides comprehensive support for testing applications at various levels.

Common Testing Annotations:

• `@SpringBootTest`: Used to load the full Spring application context for integration tests.
• `@WebMvcTest`: Used for testing Spring MVC controllers without loading the full application context. It auto-configures Spring MVC infrastructure and limits the beans loaded.
• `@DataJpaTest`: Used for testing JPA repositories. It configures an in-memory database by default and focuses only on JPA-related components.
• `@JdbcTest`: Used for testing JDBC-based code. Configures an in-memory database and a `JdbcTemplate`.
• `@RestClientTest`: Used for testing REST clients using `RestTemplate` or `WebClient`. It provides a way to mock external REST calls.
• `@MockBean`: Creates and injects a Mockito mock for a Spring-managed bean.
• `@Autowired`: Used to inject the components being tested.
• `@Test`: Marks a method as a test case (from JUnit).

19. What is Spring Batch? How can you integrate it with Spring Boot?

Answer: Spring Batch is a comprehensive framework for building robust batch processing applications. It provides features for logging, transaction management, job processing statistics, job restartability, skip and retry, and resource management.

Integration with Spring Boot: Spring Boot provides excellent integration with Spring Batch through the `spring-boot-starter-batch` dependency. Adding this starter automatically configures the necessary infrastructure for running batch jobs. You can then define your batch jobs using Spring Batch’s core concepts like `Job`, `Step`, `Tasklet`, and `ItemReader`/`ItemWriter`/`ItemProcessor` with standard Spring annotations.

20. What is Spring Cloud? How does it relate to Spring Boot?

Answer: Spring Cloud is a framework built on top of Spring Boot that provides tools for building distributed systems (microservices) in the cloud. It offers solutions for common distributed system patterns like configuration management, service discovery, circuit breaking, routing, and more.

Relationship with Spring Boot: Spring Boot provides the foundational building blocks for individual microservices. Spring Cloud then adds the necessary components and abstractions to orchestrate and manage these microservices in a cloud environment.

21. How do you handle static resources (CSS, JavaScript, images) in Spring Boot?

Answer: Spring Boot automatically serves static resources located in the following default locations in the classpath:

• `/static`
• `/public`
• `/resources`
• `/META-INF/resources`

You can place your static assets in any of these directories, and Spring Boot will serve them under the application’s root path (e.g., http://localhost:8080/css/style.css). You can also customize the locations for static