1. What is the difference between Spring MVC and Spring WebFlux?

Answer:

• Spring MVC follows a synchronous (blocking) request-response model based on the Servlet API.

• Spring WebFlux is a reactive, non-blocking framework designed for handling high-concurrency applications using Reactor (Mono & Flux).

2. How does Spring Boot handle distributed transactions in microservices?

Answer: Spring Boot supports distributed transactions using:

• Two-phase commit (2PC) with XA transactions (e.g., Atomikos, Bitronix).

• Event-driven approaches like SAGA pattern using Kafka, RabbitMQ, or Apache Camel.

3. Explain the CAP theorem and how it applies to distributed databases.

Answer: The CAP theorem states that a distributed system can guarantee only two of the following three properties:

• Consistency (C): Every node sees the latest data.

• Availability (A): System remains operational despite failures.

• Partition Tolerance (P): System continues working despite network failures.
  Example: NoSQL databases like MongoDB favor Availability and Partition Tolerance (AP), whereas SQL databases lean towards Consistency and Partition Tolerance (CP).

4. How does Hibernate handle concurrency in multi-threaded environments?

Answer: Hibernate provides optimistic and pessimistic locking:

• Optimistic Locking: Uses a version number to prevent conflicts.

• Pessimistic Locking: Locks the database row until the transaction completes.

5. What is WebSocket, and how does it differ from REST APIs?

Answer:

• WebSockets provide a full-duplex communication channel between client and server (ideal for real-time applications like chat apps).

• REST APIs use stateless HTTP requests (best suited for CRUD operations).

6. How do you implement authentication and authorization in a Java Full Stack application?

Answer:

• Authentication: JWT, OAuth2, or LDAP with Spring Security.

• Authorization: Role-based access control (RBAC) or attribute-based access control (ABAC).

7. What is Circuit Breaker, and how does it improve microservices resilience?

Answer:
Circuit Breaker (implemented using Resilience4j or Hystrix) prevents cascading failures by:

• Opening the circuit if failures exceed a threshold.

• Closing the circuit after a cooldown period when the service recovers.

8. What are the differences between SQL and NoSQL databases?

FeatureSQL (MySQL, PostgreSQL)NoSQL (MongoDB, Cassandra)

SchemaFixed schemaSchema-less

ScalabilityVertical (scale-up)Horizontal (scale-out)

TransactionsACID-compliantBASE (eventual consistency)

Use CaseStructured dataLarge-scale, unstructured data

9. How does Kubernetes help in managing microservices?

Answer: Kubernetes provides:

• Container orchestration (auto-scaling, self-healing).

• Service discovery via Kubernetes DNS.

• Load balancing using Ingress Controllers.

• Config management via ConfigMaps and Secrets.

10. What is the difference between Synchronous and Asynchronous processing in Java?

Answer:

• Synchronous: The caller waits for the task to complete before proceeding (e.g., Java Thread join()).

• Asynchronous: The caller proceeds without waiting, using CompletableFuture, Executors, or Reactive programming (Project Reactor).

11. What is the N+1 select problem in JPA and how can you solve it?

Answer:
The N+1 problem occurs when one query retrieves N records, and then N additional queries are executed to fetch related entities.
Solution: Use JOIN FETCH, @EntityGraph, or batch fetching with appropriate fetch strategies (LAZY vs EAGER).

12. What is the purpose of the @Transactional annotation?

Answer:
@Transactional defines the scope of a single database transaction. It ensures that a sequence of operations either all succeed or all fail (rollback on error).

13. How does Spring Boot handle dependency injection internally?

Answer:
Spring Boot uses the Spring IoC container which scans components (@Component, @Service, etc.), creates bean definitions, and injects them where needed using @Autowired or constructor injection.

14. Explain how AOP (Aspect-Oriented Programming) is used in Spring.

Answer:
AOP allows separating cross-cutting concerns (e.g., logging, security).
In Spring, it's implemented using @Aspect and pointcuts to define "where" and "what" to execute.

15. How would you implement pagination in Spring Data JPA?

Answer:
Use the Pageable interface:

Page<User> findAll(Pageable pageable);

You can control page number, size, and sorting with it.

16. What is circuit breaking and how is it implemented in Java Microservices?

Answer:
Circuit breakers prevent repeated failures by cutting off calls to a failing service temporarily.
Implemented using Resilience4j, Hystrix, or Spring Cloud Circuit Breaker.

17. What are the advantages of reactive programming in Spring WebFlux?

Answer:

• Non-blocking I/O

• High scalability with fewer threads

• Better for streaming or event-driven apps

18. How does Hibernate manage caching?

Answer:
Hibernate supports:

• First-level cache: Default, session-scoped.

• Second-level cache: Shared across sessions, enabled via libraries like EhCache, Redis.

19. How do you secure REST APIs in a full stack Java app?

Answer:

• Use JWT for stateless authentication.

• Use Spring Security to define roles, access control, and token filters.

• Apply HTTPS and CORS policies.

20. What is the use of WebClient in Spring Boot?

Answer:
WebClient is a non-blocking, reactive alternative to RestTemplate. It is used to make asynchronous HTTP requests in Spring WebFlux apps.

21. What is Docker and how is it used in full stack Java development?

Answer:
Docker is a containerization platform that packages applications and dependencies together. In Java Full Stack, it allows you to containerize Spring Boot apps and run them with consistent environments.

22. What are the benefits of using GraphQL instead of REST?

Answer:

• Fetch only required data

• Single request for complex data

• Strongly typed schema
  Ideal for optimizing frontend-backend communication.

23. How do you handle concurrency in a Spring Boot application?

Answer:

• Use synchronized blocks for simple cases

• Use @Scheduled, ExecutorService, or @Async for background tasks

• Use Optimistic or Pessimistic Locking in JPA for DB-level concurrency

24. What are design patterns commonly used in Java Full Stack?

Answer:

• Singleton (Beans)

• Factory (Object creation)

• DAO (Data Access)

• MVC (Controller-View separation)

• Builder (Complex object creation)

• Proxy (AOP, security)

25. What is DevOps and how does it integrate with Java Full Stack development?

Answer:
DevOps is the practice of automating software delivery and infrastructure changes. In Java Full Stack, it includes:

• CI/CD pipelines (Jenkins, GitHub Actions)

• Containerization (Docker)

• Deployment (Kubernetes, AWS, Azure)