Interview Questions
Spring Boot

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

This is a very common interview question because it verifies whether the candidate understands the role of Spring Boot within the Spring ecosystem.

Spring is a comprehensive framework providing many features such as dependency injection, bean management, Spring MVC, Spring Data, and Spring Security.

Spring Boot simplifies using Spring by providing auto-configuration, ready-to-use dependencies via starters, an embedded server, and a faster setup to build an application.

2. What is dependency injection in Spring?

Dependency injection is a core principle of Spring. Instead of a class creating its own dependencies, Spring supplies them automatically.

For example, a service might need a repository. Rather than running a new directly, we let Spring inject the repository into the service.

This makes the code more testable, decoupled, and easier to maintain. In an interview, explain that Spring manages objects called beans and wires them together via its container.

3. What is Inversion of Control (IoC)?

Inversion of Control means the developer no longer directly controls the creation and lifecycle of certain objects. The Spring container takes care of it.

Without Spring, a class might instantiate its dependencies itself with new. With Spring, dependencies are declared and then supplied by the framework.

IoC is the overarching principle, and dependency injection is a concrete way to implement it.

4. What is the difference between @Component, @Service, and @Repository?

These three annotations are used to declare Spring beans, but they carry different semantic meanings.

@Component is the generic annotation. It simply indicates that a class should be detected and managed by Spring.

@Service is used for the business layer. It makes the role of the class explicit within the application's architecture.

@Repository is used for the data access layer. It also allows Spring to translate certain persistence-related exceptions.

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

@Controller is used in Spring MVC to handle web requests and typically return a view, such as an HTML page.

@RestController is used to build REST APIs. It combines @Controller and @ResponseBody, which means methods return data directly in the HTTP response body, often as JSON.

In an interview, you should clearly highlight that @RestController is the natural choice for exposing a REST API with Spring Boot.

6. What is @SpringBootApplication used for?

@SpringBootApplication is the primary annotation of a Spring Boot application. It bundles several essential behaviors together.

It notably combines @Configuration, @EnableAutoConfiguration and @ComponentScan.

This means it allows you to declare a Spring configuration, enable Spring Boot auto-configuration, and scan for components in the current package and its sub-packages.

7. What is auto-configuration in Spring Boot?

Auto-configuration is a mechanism that allows Spring Boot to automatically configure certain beans based on the dependencies present in the project classpath.

For example, if Spring Boot detects a web dependency, it can automatically configure the necessary components to run a web application with an embedded server.

The goal is to reduce manual configuration and allow developers to get started much faster.

8. What is the difference between application.properties and application.yml?

application.properties and application.yml are both used to configure a Spring Boot application.

The application.properties file uses a simple key-value syntax, while application.yml uses a hierarchical structure that is more readable for nested configurations.

The choice often comes down to team preference. Both formats allow you to configure the server, database, logging, profiles, or custom application properties.

9. What is the purpose of @Transactional in Spring?

@Transactional allows you to manage a transaction around a method or a class. It is a very common topic in Java backend interviews.

If the method completes successfully, the transaction can be committed. If an exception occurs, Spring can perform a rollback depending on the exception type and the configuration used.

In an interview, you should also know that @Transactional relies on Spring dynamic proxies. This explains common pitfalls, especially when an annotated method is called from within the same class.

10. What is the difference between @RequestParam, @PathVariable, and @RequestBody?

These annotations are used to extract different parts of an HTTP request in a Spring controller.

@PathVariable extracts a value from the URL path, such as an ID in /users/12.

@RequestParam extracts a query parameter, such as ?page=1 or ?sort=name.

@RequestBody extracts the request body, typically a JSON object sent in a POST or PUT request.

What are the most frequent Spring Boot interview questions?

The most frequent Spring Boot interview questions cover the difference between Spring and Spring Boot, dependency injection, IoC, annotations, REST controllers, auto-configuration, Spring Data JPA, transactions, and security.

How to prepare for a junior Spring Boot interview?

To prepare for a junior Spring Boot interview, you need to understand Spring fundamentals, know how to build a REST API, explain core annotations, understand dependency injection, and know the basics of JPA and transactions.

Do you need to master Java before Spring Boot?

Yes. Spring Boot is built on Java. You need to know the core language, interfaces, exceptions, collections, annotations, and object-oriented programming principles to properly understand Spring Boot.

Is Spring Boot enough for a Java backend interview?

Spring Boot is essential for a Java backend role, but it is not always enough. You also need to know Java, SQL, REST, JPA, Hibernate, transactions, testing, and architectural best practices.