Top 10 Java Interview Questions on System Design, Architecture & Best Practices – 2025
When you reach the mid-level or final round interviews, questions move beyond syntax. Interviewers start focusing on how you design systems, manage services, scale components, and apply clean architecture principles. Here are 10 real-world questions with practical answers that will help you ace those rounds.
1. How does your project architecture look?
Answer: We followed a layered architecture: Controller → Service → Repository. At the top, we use REST controllers to expose endpoints, services for business logic, and repositories for DB interaction using Spring Data JPA. We also use DTOs for external contracts and ModelMapper for conversions.
2. Difference between Monolithic and Microservices Architecture?
Monolithic: Entire app is built and deployed as one unit. Easy to develop initially but hard to scale and maintain.
Microservices: App is broken into small independent services. Easier to scale, deploy, test but complex in terms of communication and data management.
3. What is HLD and LLD in Java projects?
HLD (High Level Design): Focuses on system overview – modules, flow, communication, databases, tech stack.
LLD (Low Level Design): Focuses on class-level design, method signatures, interactions, design patterns.
4. How do services communicate in your application?
Answer: REST-based communication using FeignClient. We use load balancer for service discovery, and Feign handles fallback logic using Hystrix/Resilience4j for circuit breaking.
5. What design patterns have you used?
Answer:
- Singleton – For service beans
- Factory – For creating notification handlers
- Builder – For DTO construction
- Strategy – For payment method selection logic
6. How do you secure APIs?
Answer: Spring Security with JWT. Each request contains token, validated in filter. For RBAC, we use @PreAuthorize annotations and maintain roles in DB.
7. How do you manage logs in a distributed system?
Answer: We use SLF4J + Logback. Logs are enriched with traceId and userId. For centralized logging, we use ELK Stack (Elasticsearch, Logstash, Kibana).
8. How do you structure DB in a microservice?
Answer: Each microservice owns its own DB schema (DB per service). We use Flyway for version control. Common data is synced via APIs or event queues.
9. How would you design a Notification Service?
Answer: Notification module is decoupled. Events (e.g., OrderPlaced) are sent to Kafka queue. Notification service listens, maps template, and triggers Email/SMS/Push. Easy to scale independently.
10. How do you handle API versioning?
Answer: Versioning is done via URI like /api/v1/. We maintain backward compatibility by versioning controller classes and using DTO versioning when needed.
Final Tips:
- Give architecture diagrams or draw them if asked
- Mention tools used (Kafka, Redis, ELK, Jenkins)
- Give real examples, not just theory
📅 Last Updated: 20 May 2025
📎 #JavaSystemDesign #BackendArchitecture #SpringBootDesign #MicroservicesInterview
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