Spring Boot Interview Questions 2026

How do you implement custom validation in Spring Boot? Explain the approach using @Constraint and ConstraintValidator, where to place the annotation, and how to return meaningful error messages through BindingResult.

Design a library management system. Include entities (Book, Member, Loan), relationships, and key operations: borrow, return, search, fine calculation. Discuss how to handle concurrent borrows of the last copy of a book.

What are design patterns? Explain Singleton, Factory, and Observer patterns with Java code examples. In which Spring Boot components are these patterns used internally?

What is the difference between checked and unchecked exceptions in Java? When should you use each? Explain the exception hierarchy and best practices for custom exceptions in a Spring Boot REST API.

Explain REST API best practices. What makes an API RESTful? Cover: proper use of HTTP verbs, status codes, versioning strategies (/v1/users vs Accept header), pagination (cursor vs offset), and HATEOAS.

How do you handle transactions in Spring Boot? Explain the @Transactional annotation, propagation levels (REQUIRED, REQUIRES_NEW, NESTED), isolation levels, and how Spring proxies intercept methods — including why self-invocation bypasses the proxy.

Design a REST API for an e-commerce cart system. Define the endpoints (add item, remove item, get cart, checkout), explain authentication with JWT, handle race conditions on inventory, and discuss idempotency for the checkout endpoint.

What are the SOLID principles? Explain each with a real-world Java example. Then describe a scenario where rigidly following SOLID leads to over-engineering, and how you balance pragmatism with good design.

Explain Spring Boot auto-configuration. How does @SpringBootApplication work under the hood? What is the difference between @Component, @Service, @Repository, and @Controller in Spring?

Design a library management system with online book reservation. Students can search, reserve, and borrow books. Librarians can manage inventory. Cover the database schema, REST APIs, and how you handle concurrent reservations of the last copy.

What is the circuit breaker pattern? Explain the three states (Closed, Open, Half-Open), when to trip and reset the breaker, and how Netflix's Hystrix library implements it. When would you NOT use a circuit breaker?

What does @SpringBootApplication actually do? Which three annotations does it combine, and what does each one do?

Design a REST controller in Spring Boot for a "create order" endpoint. What annotations and validation would you use to ensure a malformed request returns 400 instead of crashing?

Explain constructor injection vs field injection (@Autowired on a field) in Spring. Why do most teams now prefer constructor injection?

How do you implement centralized exception handling across all controllers in a Spring Boot application, so you don't repeat try/catch in every endpoint?

What does Spring Boot Actuator give you out of the box for a microservice running in production? Name at least 3 endpoints and what each is used for.