Top 20 Spring Boot REST API Interview Questions and Answers

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In this guide, we will discuss the top 20 frequently asked Spring Boot REST API interview questions for both beginners and experienced developers.

Let's begin with simple questions.

1. What is Spring Boot, and why is it used for REST API development?

Spring Boot is a framework that simplifies Java application development by removing XML configuration, providing default setups, and including an embedded server.

For REST API development, it is extremely popular because it reduces boilerplate and allows developers to create production-ready APIs with minimal effort.

Spring Boot automatically configures components like Jackson for JSON processing and Tomcat for serving HTTP requests. It also provides starter dependencies such as Spring Web, which gives immediate access to controllers, request mappings, exception handling, validation, and all REST-related features. Because of auto-configuration, a developer can focus on writing endpoints instead of worrying about server setup or deployment.

Spring Boot also integrates easily with data layers, security modules, logging frameworks, and actuator metrics, which are essential in real-world API applications. Overall, Spring Boot offers speed, simplicity, maintainability, and scalability, making it a natural choice for building RESTful services.

2. What is a REST Controller in Spring Boot?

A REST Controller is a special Spring component that receives HTTP requests and returns data, usually in JSON format. It is the foundation of building REST APIs in Spring Boot.

A REST Controller contains methods that map to specific HTTP operations, such as GET for retrieving data, POST for creating data, PUT for updating data, and DELETE for deleting data. Spring handles all the internal work like converting JSON into Java objects on incoming requests, and converting Java objects back into JSON in the response.

The role of the REST Controller is to act as a bridge between the client and the service layer, meaning it receives requests, passes them to the business logic, and sends structured responses. Using annotations makes REST development clearer and more readable. The controller does not handle business logic or database logic. Its job is simply request handling, response preparation, and communicating with the next service layer.

3. What is @RestController and how is it different from @Controller?

@RestController is a specialized annotation that tells Spring this class will expose REST endpoints and return data directly to the client. It is a combination of @Controller and @ResponseBody. Without @RestController, you would need to manually add @ResponseBody to every method to return JSON.

Using @RestController simplifies REST development because all methods return data by default rather than views. On the other hand, @Controller is used in web MVC applications where the method returns a view, such as a JSP or HTML page.

In typical enterprise REST development, @RestController is the standard choice because it avoids unnecessary view resolution and ensures JSON output by default.

In interviews, it is important to mention that @RestController improves readability, reduces the number of annotations, and works perfectly with mapping annotations like GetMapping or PostMapping to build APIs.

4. What does @RequestMapping do in Spring Boot?

@RequestMapping is used to map HTTP requests to specific handler methods or entire classes. When applied at the class level, it defines a base URI. When applied at the method level, it defines the exact path and request type.

It supports multiple HTTP methods, path variables, parameters, and content negotiation options. Although newer annotations like GetMapping and PostMapping exist, @RequestMapping remains the most flexible because it can configure advanced options like headers, consumes, and produces.

It improves clarity by telling Spring exactly which method should be invoked for a given request. For example, if you have your API grouped under a version prefix, using @RequestMapping at the class level ensures every method inherits the base path. This helps modularize the API and maintain clean organization. Interviewers want to hear that it gives fine-grained control over routing inside REST applications.

5. What are GetMapping, PostMapping, PutMapping, and DeleteMapping?

These annotations provide simplified mappings for common HTTP methods. GetMapping is used for read operations, PostMapping for creating resources, PutMapping for full updates, PatchMapping for partial updates, and DeleteMapping for deletions.

They improve readability because the HTTP method is part of the annotation itself rather than configured in RequestMapping. These method-level annotations follow REST architectural conventions, ensuring that your API aligns with industry standards. They also make the controller code cleaner and more expressive.

In interviews, it is important to mention that these annotations internally use RequestMapping with specific HTTP method values, which provides better clarity and simplicity, reducing confusion about which method handles which request type.

6. What is the difference between @PathVariable and @RequestParam?

@PathVariable extracts values from the URI path. For example, if the URL contains an identifier inside the path, this annotation reads it directly. It is best suited for data that uniquely identifies resources.

@RequestParam reads values from the query string, typically used for filtering, pagination, or optional parameters. For example, a search API may use query parameters to refine user input. The main difference is location.

Path variables are part of the URL structure, while request parameters are appended to the URL after a question mark.

In interview explanations, highlight that path variables model hierarchical resources, while request parameters provide additional optional details. Understanding these distinctions helps design clean and meaningful REST endpoints.

7. What is @RequestBody, and how does Spring convert JSON?

@RequestBody tells Spring to bind incoming JSON from the request body into a Java object. When a client sends a JSON payload, Spring uses an internal converter, usually Jackson, to map fields into the corresponding Java fields. This eliminates the need for manual parsing. The conversion is automatic as long as the names match.

This annotation is primarily used in POST and PUT operations that receive structured data.

Spring Boot auto-configures all necessary converters, making request handling seamless.

Interviewers expect you to mention that @RequestBody is essential for modern REST APIs and that Spring Boot handles the entire serialization and deserialization process transparently, reducing developer workload.

8. What is ResponseEntity, and why is it used?

ResponseEntity allows full control over the HTTP response. Instead of returning only data, you can also return status codes, custom headers, and structured output. For example, after creating a resource, you may want to return a “created” status code and a location header. ResponseEntity makes this possible. It also improves clarity and error handling by explicitly indicating the success or failure of each operation.

Using ResponseEntity is considered a best practice in enterprise APIs because it provides predictable and standardized responses. In interview answers, emphasize that it helps build robust APIs where clients receive meaningful status codes rather than relying on defaults.

9. How does Spring Boot handle exceptions in REST APIs?

Spring Boot provides a structured approach to exception handling, ensuring that errors return clean, meaningful JSON responses rather than default server pages.

The framework supports annotations like ExceptionHandler for handling exceptions at the controller level, and ControllerAdvice for global exception handling. You can customize messages, error codes, and additional metadata for consistency. This is crucial because clients rely on stable and predictable error formats.

Proper exception handling also improves debugging and user experience. Mention in interviews that Spring Boot encourages centralized exception management, separating error logic from business logic, and ensuring the entire API behaves uniformly when unexpected conditions occur.

10. What is auto-configuration in Spring Boot?

Auto-configuration automatically sets up components based on the project's dependencies. If the Spring Web dependency is available, Spring Boot automatically configures Tomcat, dispatchers, JSON mapping, and other components.

Developers do not need to write configuration files or manually set up beans. This feature dramatically speeds up development by eliminating repetitive setup tasks.

Auto-configuration also allows overriding defaults when needed, giving flexibility without sacrificing simplicity. In interviews, emphasize that this is one of Spring Boot’s biggest advantages because it minimizes configuration effort while remaining fully extensible.

11. What is Dependency Injection in Spring?

Dependency injection is a technique where Spring creates objects and injects them wherever needed. Instead of manually constructing objects inside your classes, you rely on Spring to manage their lifecycle.

This promotes loose coupling, improves testability, and separates creation logic from usage. Spring injects dependencies using constructors, setters, or field injection.

Most modern recommendations prefer constructor injection for better clarity and immutability. Dependency injection is core to Spring’s design and is fundamental when building scalable REST APIs.

Interviewers expect you to highlight that DI makes applications modular and easier to maintain.

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

All three are Spring-managed beans, but each represents a specific application layer.

Component is a generic annotation for any Spring bean.

Service marks business-logic classes and indicates that the class performs service operations.

Repository marks data access classes and activates exception translation, converting low-level database exceptions into higher-level Spring exceptions.

While all three behave similarly at runtime, using the correct annotation improves readability, structure, and maintainability.

Interviewers look for your understanding that these annotations clarify architectural intent and enable layered design practices.

13. What is Spring Data JPA, and why is it useful for REST APIs?

Spring Data JPA simplifies database access by eliminating boilerplate queries. It provides built-in CRUD operations and supports derived query methods based on method naming conventions. This makes it easy to create APIs that interact with databases without writing complex SQL.

The framework automatically handles transactions, mapping, and repository implementation.

For REST APIs, this significantly reduces development time and ensures a clean, consistent data access layer. Mention in interviews that Spring Data JPA allows rapid development while still supporting custom queries when needed.

14. What is an embedded server?

An embedded server is a server, such as Tomcat or Jetty, that ships with the Spring Boot application. When you run your application as a standalone JAR, the embedded server starts automatically. This eliminates the need to deploy applications manually to external servers.

It simplifies deployment pipelines and supports container-based architectures.

Developers can focus on coding instead of server configuration. This approach is ideal for microservices, as each service can run independently on its own server instance.

15. How is validation handled in Spring Boot REST APIs?

Validation ensures incoming data follows defined rules before the application processes it.

Spring Boot integrates with the Java Validation API to support annotations such as @NotNull, @Size, @Email, and @Min. These annotations are applied to model fields, and Spring automatically validates them when data arrives via RequestBody. If validation fails, Spring generates structured error messages.

In interviews, highlight that validation prevents incorrect or malicious data from entering the system and improves API reliability.

16. What is content negotiation in Spring Boot?

Content negotiation determines the format in which data is returned, such as JSON or XML. Clients specify their preferred format through the Accept header. Spring analyzes this header and produces the response accordingly. This makes APIs flexible and compatible with different systems.

Content negotiation is essential in enterprise APIs because different clients may require different formats. Spring Boot handles this automatically unless customizations are needed.

17. What is Spring Boot Actuator?

Spring Boot Actuator provides production-ready endpoints for monitoring and managing applications. It exposes information about health, metrics, logs, environment, and application status. These endpoints help DevOps teams monitor API performance, track resource usage, and detect issues in real time.

Actuator is essential for microservices because each service must expose health information for load balancers and monitoring systems. In interviews, emphasize that Actuator improves observability, which is critical in distributed systems.

18. What is CORS, and how does Spring handle it?

CORS stands for Cross-Origin Resource Sharing. Browsers restrict cross-domain requests for security reasons. When building REST APIs used by front-end applications like React or Angular, CORS must be configured to allow cross-domain requests.

Spring allows enabling CORS globally or per controller. Proper configuration ensures that legitimate clients can access the API without exposing security vulnerabilities.

In interviews, mention that CORS is essential when your API serves web or mobile clients hosted on different domains.

19. What is the difference between monolithic and microservices architecture?

A monolithic architecture bundles all features into one large application. While simple to start, it becomes difficult to scale and maintain as the application grows.

A microservices architecture divides the application into independent services, each responsible for a specific functionality. These services communicate through REST APIs. Microservices improve scalability, fault isolation, and deployment flexibility. However, they introduce complexity in communication and monitoring.

In interviews, highlight that Spring Boot is widely used to build microservices because it provides lightweight, self-contained services.

20. How do you secure REST APIs in Spring Boot?

Securing REST APIs typically involves using Spring Security, JWT tokens, OAuth2, or API keys.

The most common approach today is JWT, where the client sends a token with every request. The server validates the token and grants access.

Spring Boot integrates tightly with these security models. You can enforce authorization rules, protect endpoints, and customize authentication logic easily.

In interviews, emphasize that security is mandatory in real-world APIs, and Spring provides a complete security framework that supports stateless authentication needed for distributed systems.

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