Question 1

What is Inversion of Control (IoC) in Spring?

Accepted Answer

IoC is a principle where the Spring framework manages object creation and lifecycle, rather than the developer. This reverses the traditional control flow where application code directly instantiates its dependencies. Spring uses an IoC container to manage beans and their dependencies declaratively.

Question 2

What is auto-configuration in Spring Boot?

Accepted Answer

Auto-configuration automatically configures Spring beans based on dependencies present in the classpath. For example, if spring-boot-starter-data-jpa is present, Spring Boot automatically configures a DataSource, EntityManagerFactory, and TransactionManager. This significantly reduces the manual configuration required.

Question 3

What is a Spring Boot Starter?

Accepted Answer

A starter is a dependency descriptor that automatically bundles all necessary libraries for a specific use case, with compatible versions tested together. This eliminates the need to manually search for each dependency and automatically manages transitive dependencies. For example, spring-boot-starter-web includes Spring MVC, embedded Tomcat, Jackson for JSON, and all their compatible dependencies.

Question 4

What is the main difference between application.properties and application.yml in Spring Boot?

Accepted Answer

Both files are used to configure Spring Boot, but use different formats. YAML format (.yml) allows hierarchical structure with indentation, making it more readable for complex configurations. Properties format (.properties) uses flat key=value syntax. Spring Boot automatically loads either one, with priority to .properties if both exist.

Question 5

What is @ControllerAdvice in Spring MVC?

Accepted Answer

@ControllerAdvice allows centralizing exception handling for all application controllers. This annotation creates a global component that intercepts exceptions thrown by any @Controller or @RestController. This avoids duplicating error handling logic in each controller and promotes error response consistency.

Question 6

What is the role of the @Valid annotation in Spring?

Accepted Answer

@Valid triggers Bean Validation constraints (JSR 303/380) on the annotated object. This annotation comes from the jakarta.validation package and is recognized by Spring to automatically validate DTOs in controllers. Use @Valid with BindingResult to retrieve errors without throwing an exception.

Question 7

What is JPA (Java Persistence API)?

Accepted Answer

JPA is a standard Java specification that defines common interfaces for relational object persistence. It allows mapping Java classes to relational database tables. Hibernate, EclipseLink and OpenJPA are implementations of this specification. Using JPA allows changing ORM implementation without rewriting business code.

Question 8

What is the primary role of the @Transactional annotation in Spring?

Accepted Answer

@Transactional indicates that a method should execute within a transactional context, ensuring atomicity of database operations. Spring automatically creates an AOP proxy around the class to handle transaction start, commit and rollback. This declarative approach greatly simplifies transaction management compared to manual programming with TransactionTemplate or PlatformTransactionManager.

Question 9

What is the main role of the UserDetailsService interface in Spring Security?

Accepted Answer

UserDetailsService is responsible for loading user information from the data source (database, LDAP, etc.) during authentication. Its loadUserByUsername() method returns a UserDetails object containing the username, password, roles and authorities. Spring Security then uses this information to validate credentials and build the SecurityContext.

Question 10

What is the structure of a JWT (JSON Web Token)?

Accepted Answer

A JWT consists of three parts separated by dots: Header (algorithm and type), Payload (claims/data), and Signature (integrity verification). Each part is Base64URL encoded. This structure allows secure and verifiable information transmission between two parties.

Spring Boot interview questions

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