Question 1

What is dependency injection and why does Spring use it?

Accepted Answer

Dependency injection is a pattern where an object's dependencies are provided ("injected") from the outside rather than created internally. Spring's IoC container manages object creation and wiring, which makes classes easier to test (you can inject mocks) and decouples components from their concrete dependencies.

Question 2

What's the difference between @Component, @Service, @Repository, and @Controller?

Accepted Answer

All four are stereotypes of @Component and get auto-detected/registered as Spring beans. They're semantically different: @Service marks business logic, @Repository marks data-access classes (and adds exception translation for persistence exceptions), @Controller marks web layer classes. Functionally, @Component is the generic base.

Question 3

What is Spring Boot auto-configuration?

Accepted Answer

Auto-configuration automatically configures Spring beans based on the dependencies on your classpath and properties in application.properties/yml — e.g. adding spring-boot-starter-web auto-configures an embedded Tomcat server and Spring MVC, with sensible defaults you can override.

Question 4

What's the difference between @RequestParam, @PathVariable, and @RequestBody?

Accepted Answer

@RequestParam reads query string parameters (?name=value). @PathVariable reads segments from the URL path (/users/{id}). @RequestBody deserializes the HTTP request body (typically JSON) into a Java object.

Question 5

What is a Spring Bean's default scope, and what other scopes exist?

Accepted Answer

The default scope is singleton — one shared instance per Spring container. Other scopes include prototype (a new instance every time it's requested), and web-specific scopes like request and session.

Question 6

How does Spring Boot handle application configuration?

Accepted Answer

Via application.properties or application.yml files, environment variables, command-line args, and profile-specific files (application-{profile}.yml). Spring Boot follows a defined precedence order, and values can be injected with @Value or bound to typed classes with @ConfigurationProperties.

Question 7

What is the purpose of @Transactional?

Accepted Answer

It wraps a method in a database transaction — if the method completes normally the transaction commits; if it throws an unchecked exception (by default), it rolls back. It's commonly applied at the service layer, not the repository layer.

Question 8

What's the difference between JpaRepository and CrudRepository?

Accepted Answer

CrudRepository provides basic CRUD operations. JpaRepository extends it and adds JPA-specific features like batch operations, flushing, and pagination/sorting support (it also extends PagingAndSortingRepository).

Question 9

How do you handle exceptions globally in a Spring Boot REST API?

Accepted Answer

With @ControllerAdvice combined with @ExceptionHandler methods — this centralizes error handling so you don't repeat try/catch logic in every controller, and lets you return consistent error response shapes (e.g. a standard error DTO with status/message).

Question 10

What is Spring Security's filter chain?

Accepted Answer

A chain of servlet filters that intercepts every HTTP request to apply authentication and authorization logic before it reaches your controller — e.g. checking a JWT, redirecting unauthenticated users, or enforcing role-based access rules.

Spring Boot Interview Questions

Spring Boot Interview Questions