Java Backend Developer Interview Questions: Spring Boot, Java, AWS, Docker, Kubernetes

💡 Java Backend Developer (3–6 Years) :: Interview Questions 🧠 Core Java & OOPs (Very Deep Questions) 1️⃣ What are SOLID principles? Can you explain them with examples? 2️⃣ Difference between abstraction and encapsulation — real-time examples. 3️⃣ Explain immutability — how to make a class immutable in Java. 4️⃣ How does HashMap work internally? What happens during resizing? 5️⃣ What’s the difference between ConcurrentHashMap and Collections.synchronizedMap()? 6️⃣ Explain thread lifecycle and difference between Thread.start() and run(). 7️⃣ What is the difference between ExecutorService and ForkJoinPool? 8️⃣ How do you implement custom sorting using Comparator and Comparable? 9️⃣ What is Java Memory Model (JMM) and volatile keyword? 🔟 How do you handle memory leaks in Java? 🌿 Spring Framework / Spring Boot 1️⃣1️⃣ What is the lifecycle of a Spring Bean? 1️⃣2️⃣ Difference between @Autowired and @Inject. 1️⃣3️⃣ How does @Transactional work internally? (Propagation + Isolation) 1️⃣4️⃣ What are Spring Boot Starters? 1️⃣5️⃣ How to create custom annotations in Spring Boot? 1️⃣6️⃣ What is the difference between ApplicationContext and BeanFactory? 1️⃣7️⃣ Explain how Spring handles dependency injection using reflection. 1️⃣8️⃣ What is the use of CommandLineRunner and ApplicationRunner? 1️⃣9️⃣ How do you externalize configuration in Spring Boot? 2️⃣0️⃣ What is Spring Boot Actuator and what endpoints are most used? 🧩 Microservices Architecture 2️⃣1️⃣ What is service discovery? How does Eureka/Consul handle it? 2️⃣2️⃣ How do microservices communicate with each other? (REST, Kafka, gRPC) 2️⃣3️⃣ What are common challenges in microservices and how do you solve them? 2️⃣4️⃣ How do you secure microservices (OAuth2/JWT)? 2️⃣5️⃣ What is distributed tracing? How do you implement it (Zipkin, Sleuth, OpenTelemetry)? 2️⃣6️⃣ Explain Circuit Breaker, Retry, and Rate Limiting patterns. 2️⃣7️⃣ How do you handle inter-service transactions? (Saga, 2PC) 2️⃣8️⃣ What is API Gateway and its role in microservices? 2️⃣9️⃣ Explain the role of Config Server in Spring Cloud. ⚙️ Coding / Problem Solving (EPAM Always Asks) 4️⃣1️⃣ Write a program to find the second highest number in an array. 4️⃣2️⃣ Reverse a linked list iteratively and recursively. 4️⃣3️⃣ Given a string, find the first non-repeated character. 4️⃣4️⃣ Implement an LRU Cache in Java. 4️⃣5️⃣ Find all pairs of integers whose sum equals a given target. 4️⃣6️⃣ Implement producer-consumer using wait() and notify(). 4️⃣7️⃣ Write a REST API to return top 5 highest salaries by department. 4️⃣8️⃣ Implement an Employee class and sort employees by salary, then name. 4️⃣9️⃣ Given a list of transactions, calculate total by user ID using streams. 5️⃣0️⃣ Implement rate limiting for API calls in Java.

🔥Interview Questions asked for Full-Stack Developer for 2+ YOE Spring Boot ❓What is Spring Boot, and how is it different from the traditional Spring Framework? ❓What is dependency injection, and how does Spring Boot implement it? ❓Explain the different types of Spring Bean scopes. ❓How do you create and consume REST APIs in Spring Boot? ❓What are @Component, @Service, and @Repository annotations used for? ❓How do you handle exceptions globally in Spring Boot? ❓What is the use of application.properties or application.yml? ❓How do you secure a Spring Boot REST API? ❓What is Spring Boot Actuator, and how is it useful? ✅Java ❓What are the main differences between an abstract class and an interface? ❓Explain the concept of multithreading in Java. ❓What is the difference between HashMap and ConcurrentHashMap? ❓How does garbage collection work in Java? ❓What is the difference between checked and unchecked exceptions? ❓What are functional interfaces and lambda expressions? ❓How do you handle immutability in Java classes? ❓Explain the concept of Stream API and its advantages. ❓What are the principles of OOP? ⚛️ React.js ❓What is the difference between functional and class components? ❓What are React Hooks, and why are they used? ❓Explain the concept of state and props in React. ❓What is the Virtual DOM, and how does it improve performance? ❓How do you handle side effects in React (useEffect)? ❓What is the purpose of useMemo and useCallback hooks? ❓How do you manage global state (Redux, Context API, Zustand)? ❓What is code splitting and lazy loading in React? ❓How do you handle API calls and errors in React? AWS ❓What are the main AWS services you’ve used? ❓How do you deploy a web application on AWS? ❓What is the difference between EC2 and Lambda? ❓How does AWS handle scalability and load balancing? ❓What is IAM, and how do you manage user permissions? ❓Explain the concept of VPC (Virtual Private Cloud). ❓How do you store and retrieve data using S3? ❓What is the difference between RDS and DynamoDB? ❓How do you monitor resources and set up alerts in AWS CloudWatch? Docker ❓What is Docker, and why is it used? ❓Explain the difference between an image and a container. ❓How do you create and run a Docker container? ❓What is a Dockerfile, and what are its main components? ❓How do you share or push a Docker image to a registry? ❓What is Docker Compose, and why is it useful? ❓How do you manage container networking in Docker? ❓How do you persist data in Docker containers (volumes)? ☸️Kubernetes ❓What is Kubernetes, and why is it used? ❓What are Pods, Deployments, and Services in Kubernetes? ❓How does Kubernetes handle scaling and load balancing? ❓What is a Namespace in Kubernetes? ❓How do you perform rolling updates in Kubernetes? ❓What is the role of etcd in Kubernetes? ❓How do you use ConfigMaps and Secrets? ❓How do you monitor and debug a failing Pod? ❓What is the difference between ReplicaSet and Deployment?

Most Asked Java, Spring Boot & Microservices Interview Questions (2025 Edition) 🚀 👉 The focus is not just on coding, but on core concepts + practical implementation. If you’re preparing for backend interviews, here’s a gold checklist of the most asked questions 👇 ✅ Core Java – Fundamentals & OOP 🔹 Abstract class vs Interface – when to use which? 🔹 final vs finally vs finalize() 🔹 String vs StringBuilder vs StringBuffer 🔹 equals() vs == 🔹 sleep() vs wait() 🔹 Serialization & marker interfaces 🚀 Java 8+ Features 🔸 Lambda & Functional Interfaces (@FunctionalInterface) 🔸 Stream API → map, filter, reduce, collect 🔸 map() vs flatMap() 🔸 Optional API (ifPresent, orElse) 🔸 CompletableFuture – async programming 🔸 Comparator vs Comparable ⚡ Multithreading & Concurrency 🔹 Thread vs Runnable 🔹 Synchronization & deadlock prevention 🔹 volatile vs Atomic variables 🔹 ExecutorService → execute() vs submit() 🔹 ThreadPoolExecutor – fixed vs cached pool 🔹 Producer-Consumer problem 🔹 ConcurrentHashMap vs HashMap 🌱 Spring Core & Spring Boot 🔹 @Component vs @Service vs @Repository vs @Controller 🔹 Auto-configuration – how it works 🔹 IoC & Dependency Injection (constructor/setter/field) 🔹 @Autowired & circular dependencies 🔹 Bean scopes (singleton, prototype, request, session) 🔹 Profiles (@Profile) 🔹 CommandLineRunner vs ApplicationRunner 🔒 Exception Handling & Security 🔹 @Controller vs @RestController 🔹 Global exception handling with @ControllerAdvice 🔹 Checked vs Unchecked exceptions 🔹 Spring Security – Authentication vs Authorization 🔹 JWT – expiration & refresh tokens 🔹 SecurityFilterChain (Spring Boot 3+) 🌐 Microservices & Cloud 🔹 Monolith vs Microservices 🔹 Service communication (REST, gRPC, Kafka) 🔹 API Gateway & Service Discovery (Eureka/Consul) 🔹 Circuit Breaker (Resilience4j, Hystrix) 🔹 Saga Pattern for distributed transactions 🔹 Centralized config (Spring Cloud Config) 🔹 Resilience → retry, fallback 🔹 Deployment → Docker & Kubernetes 🗄️ JPA, Databases & SQL 🔹 JPA Repositories & CRUD 🔹 @Transactional & propagation 🔹 Lazy vs Eager fetching 🔹 N+1 select problem 🔹 Hibernate caching (L1/L2) 🔹 Multiple DB configurations in Spring Boot 💡 Tip: Don’t just memorize answers — explain with real-world scenarios (production issues, scaling, debugging). 👉 Which topic do you struggle with the most — Java, Spring Boot, or Microservices? Drop your thoughts in the comments 👇 #Java #SpringBoot #Microservices #JavaDeveloper #InterviewPreparation #TechInterviews

#java 🔹 Day 31–60: Java Full Stack Interview Q&A (Part 3: Q30–Q43) 🔸 30. How do you structure a GitHub repo for a fullstack project? Ans: Use modular folders: - /backend/ → Spring Boot code - /frontend/ → React app - /docker/ → Dockerfiles - /k8s/ → Kubernetes manifests - /docs/ → Diagrams, README Include CI/CD workflows, version tags, and contribution guide. --- 🔸 31. What is the difference between API Gateway and Load Balancer? Ans: - API Gateway: Application-level routing, auth, rate limiting, logging. - Load Balancer: Network-level traffic distribution across instances. Gateway is smarter; Load Balancer is faster. --- 🔸 32. How do you implement centralized rate limiting in microservices? Ans: Use Spring Cloud Gateway filters + Redis counters. Limit requests per IP/token and return 429 Too Many Requests on breach. --- 🔸 33. What is the role of Spring Cloud Gateway? Ans: It acts as a single entry point for microservices, handling routing, filters, authentication, and resilience. --- 🔸 34. How do you monitor API latency and error rates? Ans: Expose /actuator/metrics, scrape with Prometheus, visualize in Grafana. Set alerts for latency spikes and error thresholds. --- 🔸 35. What is the difference between Pod, ReplicaSet, and Deployment in Kubernetes? Ans: - Pod: Basic unit (1+ containers) - ReplicaSet: Ensures desired pod count - Deployment: Manages ReplicaSet + rolling updates --- 🔸 36. How do you manage secrets in Kubernetes? Ans: Use Secret objects. Mount as env variables or volumes. Encrypt at rest and restrict RBAC access. --- 🔸 37. What is rolling update in Kubernetes? Ans: Gradual replacement of old pods with new ones. Ensures zero downtime and version control. --- 🔸 38. What is CAP theorem’s impact on system design? Ans: You must choose between Consistency, Availability, and Partition Tolerance. Most systems pick AP or CP depending on use case. --- 🔸 39. What is the difference between strong and eventual consistency? Ans: - Strong: Immediate sync across nodes - Eventual: Delayed sync, used in high-availability systems Strong = banking; Eventual = social media --- 🔸 40. How do you simulate network partition in local setup? Ans: Use Docker + iptables to block traffic between containers. Test fallback, retries, and consistency behavior. --- 🔸 41. What is optimistic UI and why is it useful? Ans: Frontend assumes success and updates UI instantly. Improves UX in async systems (e.g., React + Kafka). --- 🔸 42. How do you visualize logs and metrics together? Ans: Use ELK Stack for logs, Prometheus + Grafana for metrics. Integrate dashboards for unified observability. --- 🔸 43. What is the role of Micrometer in Spring Boot? Ans: It bridges Spring Boot metrics to monitoring tools like Prometheus, Datadog, etc. --- #JavaFullStack

#java 🔹 Day 31–60: Java Full Stack Interview Q&A (Q98–Q115) 🔸 98. How do you implement real-time updates in fullstack apps? Ans: Use WebSockets or Server-Sent Events (SSE). - Spring Boot: Use @EnableWebSocket and WebSocketHandler - React: Use native WebSocket API or libraries like socket.io-client Real-time updates are ideal for chat apps, dashboards, and notifications. --- 🔸 99. What is the difference between @Autowired and constructor injection? Ans: - @Autowired: Injects dependencies via field or setter - Constructor injection: Preferred for immutability, testability, and clear dependencies Spring Boot supports both, but constructor injection is recommended. --- 🔸 100. How do you showcase your fullstack journey to recruiters? Ans: - Create a GitHub portfolio with structured folders - Add screenshots, diagrams, and README summaries - Publish a LinkedIn post with roadmap highlights - Include live demo links and tech stack badges - Use hashtags like #JavaFullStack, #TechLegacy, #OpenToWork --- 🔸 101. What is the role of @Scheduled in Spring Boot? Ans: It runs background tasks at fixed intervals or cron expressions. Used for periodic jobs like cleanup, sync, or notifications. --- 🔸 102. How do you implement global exception handling in Spring Boot? Ans: Use @ControllerAdvice with @ExceptionHandler methods. Return custom error responses with appropriate HTTP status codes. --- 🔸 103. What is the difference between map() and flatMap() in Java Streams? Ans: - map(): Transforms each element - flatMap(): Flattens nested structures (e.g., List<List<T>> → List<T>) Used for stream chaining and data transformation. --- 🔸 104. How do you optimize SQL queries in Spring Boot apps? Ans: - Use indexes - Avoid N+1 queries (use @EntityGraph) - Use pagination (Pageable) - Profile queries with tools like Hibernate logs or DataGrip --- 🔸 105. What is the role of @PreAuthorize in Spring Security? Ans: It restricts access to methods based on roles or conditions. Example: @PreAuthorize("hasRole('ADMIN')") --- 🔸 106. How do you implement search + filter in React frontend? Ans: - Use controlled inputs - Debounce API calls - Apply filters client-side or via query params - Display results dynamically with pagination --- 🔸 107. What is the difference between @GetMapping and @RequestMapping? Ans: - @GetMapping: Shortcut for @RequestMapping(method = GET) - @RequestMapping: More flexible, supports multiple HTTP methods #JavaFullStack

🚀 Java Spring boot questions 1. Explain the difference between Comparable and Comparator. Which one is more flexible in real projects? 2. How does Garbage Collector decide which objects to clean? What is the difference between Minor GC & Major GC? 3. Why String is immutable in Java? What benefits does it give in multi-threaded environments? 4. How does ClassLoader work in Java? Can we have multiple classloaders in one JVM? 5. Difference between StringBuffer, StringBuilder, and normal String. Which one to use where? 🔹 Collections & Performance 6. How does CopyOnWriteArrayList work internally? When would you prefer it? 7. What is the difference between fail-fast and fail-safe iterators? 8. If you have millions of records to process, would you use Stream API or traditional loops? Why? 9. How does TreeMap maintain ordering? Can we provide custom sorting logic? 10. How do you find duplicates in a list using Java 8 features? 🔹 Spring & Spring Boot 11. What is the difference between @Transactional at class level vs method level? 12. Can you explain how Spring Boot auto-configuration works internally? 13. How do you configure multiple datasources in Spring Boot? 14. What’s the difference between @RequestParam, @PathVariable, and @RequestBody? 15. How do you monitor and optimize Spring Boot applications in production? 🔹 Microservices & System Design 16. How do you implement distributed logging in microservices? 17. What’s the difference between synchronous vs asynchronous communication in microservices? 18. How would you design a service to handle 1000+ requests per second? 19. How do you handle transactions that span across multiple microservices? 20. Can you explain the circuit breaker pattern? When have you used it? 🔹 Database & Real-time Scenarios 21. How do you implement pagination in SQL + Java? 22. If a query is taking 5 seconds, how would you debug & optimize it? 23. What’s the difference between optimistic and pessimistic locking? Which one is better in microservices? 24. How do you implement caching in Spring Boot + DB to reduce load? 25. Real-world scenario: If one service keeps failing in production, how will you identify whether it’s a code issue, infra issue, or DB issue? 👉 Over to you — If you recently gave an interview, which round felt most challenging for you? (Core Java / Spring Boot / Microservices / SQL)?

Java Backend Developer Interview Questions ➤ Core Java 1. Explain the scalability challenges with ConcurrentHashMap under high contention and how to optimize it. 2. How does the Java Memory Model (JMM) ensure visibility and ordering of operations in multi-core systems? 3. Compare synchronized blocks vs StampedLock vs VarHandle for concurrency control. 4. How do you debug and fix a memory leak in a production JVM? 5. Explain differences between CMS, G1GC, and ZGC – when would you choose each in production? 6. How would you design a high-throughput system using ForkJoinPool vs Virtual Threads (Project Loom)? 7. Explain false sharing in multi-threading and how to avoid it. 8. Difference between blocking queues and non-blocking (lock-free) data structures in Java. 9. How do you implement a custom classloader for hot-reloading? 10. How do you tune JVM for a low-latency trading-like system? ➤ Spring Boot 11. Explain how Spring Boot auto-configuration internally uses @ConditionalOnClass & @ConditionalOnMissingBean. 12. How would you debug circular dependency issues in a large Spring Boot project? 13. Explain how to secure a Spring Boot app using OAuth2 + JWT with key rotation. 14. How to implement distributed caching with Redis Cluster and avoid cache stampede? 15. What are Spring Cloud patterns for service-to-service authentication? 16. Explain Reactive programming in Spring WebFlux vs traditional MVC. 17. How do you enable backpressure handling in reactive pipelines? 18. programmatic vs declarative transaction management – when to use which? 19. How does Spring Boot manage bean lifecycle in Kubernetes (shutdown hooks, graceful termination)? 20. How do you handle feature toggles in Spring Boot at scale? ➤ JPA 21. Explain pitfalls of using EAGER fetching in large domain models and how to redesign them. 22. How does Hibernate’s dirty checking impact performance in batch operations? 23. How would you tune Hibernate 2nd-level cache in a distributed environment? 24. Explain how to handle schema evolution and migrations in microservices (Flyway/Liquibase). 25. How would you implement sharding & partitioning strategies in JPA? ➤ Microservices 26. How would you implement a global distributed transaction across 10+ microservices? 27. Compare Saga, CQRS, and Event Sourcing – when would you choose each at Walmart scale? 28. How do you design multi-region microservices for Walmart’s Black Friday traffic? 29. Explain API Gateway + Service Mesh (Istio/Linkerd) usage for enterprise-grade microservices. 30. How do you enforce idempotency in financial transactions APIs? 👋 Follow Payal Kumari for more freshers hiring, off-campus drives, and tech career opportunities! hashtag #Java hashtag #SpringBoot hashtag #Microservices

💻✨🚀 🔝 Top 20 Spring Boot Annotations Every Java Developer Must Know! 🚀✨💻 Spring Boot has transformed the way we build Java applications — making development faster, cleaner, and smarter. ⚙️ Its powerful annotations eliminate boilerplate code and make projects easier to manage. Let’s explore the most essential Spring Boot annotations every backend developer should know 👇 🌱 🔹 Core Annotations 1️⃣ @SpringBootApplication → Combines key annotations to launch your app. 2️⃣ @EnableAutoConfiguration → Auto-configures based on dependencies. 3️⃣ @ComponentScan → Scans packages for Spring-managed components. 4️⃣ @Configuration → Defines configuration classes. 5️⃣ @Bean → Creates and manages custom beans. 🧩 💡 Dependency Injection & Components 6️⃣ @Component → Marks a general Spring-managed component. 7️⃣ @Service → Defines a service (business logic layer). 8️⃣ @Repository → Handles data access + auto exception translation. 9️⃣ @Controller → Manages web MVC requests. 🔟 @RestController → Combines @Controller + @ResponseBody for REST APIs. 🧠 🌐 Request Handling Annotations 1️⃣1️⃣ @RequestMapping → Maps HTTP requests. 1️⃣2️⃣ @GetMapping, @PostMapping, @PutMapping, @DeleteMapping, @PatchMapping → For specific HTTP methods. 1️⃣3️⃣ @PathVariable → Binds URL path variables. 1️⃣4️⃣ @RequestParam → Reads query parameters. 1️⃣5️⃣ @RequestBody → Maps request body to a Java object. ⚙️ 🧾 Configuration & Validation 1️⃣6️⃣ @Value → Injects values from application.properties. 1️⃣7️⃣ @Valid / @Validated → Enables bean validation. 1️⃣8️⃣ @Autowired → Injects dependencies automatically. 💾 🔧 Advanced & Utility 1️⃣9️⃣ @Transactional → Manages database transactions. 2️⃣0️⃣ @ExceptionHandler → Handles exceptions gracefully in controllers. 🔥 In short: Spring Boot annotations make development simpler, scalable, and production-ready. They empower developers to focus on logic — not lengthy configuration. 💪 💼 I’m currently open to work in Java | Spring Boot | Backend Development — excited to contribute to impactful projects and grow with great teams! 🙌 #SpringBoot #Java #BackendDevelopment #Microservices #SoftwareEngineering #SpringFramework #Programming #OpenToWork #CareerGrowth #TechCommunity #Developers

Interview Preparation/Question : Senior Java Backend Developer – Spring Boot & Microservices 🔹 Java 1. What are virtual threads in Java 21 and how do they improve concurrency? 2. Explain the concept of sealed classes and their practical use. 3. What is the difference between record and a traditional Java POJO? 4. How do Optional.of(), Optional.ofNullable(), and Optional.empty() differ? 5. What are the advantages of using the Stream API in Java? 6. How do you handle checked vs unchecked exceptions effectively? 7. What are switch expressions and pattern matching in Java 17+? 8. How does Java’s garbage collector (ZGC or G1) improve memory efficiency? 9. What are soft, weak, and phantom references, and when are they used? 10. How does CompletableFuture enable asynchronous programming in Java? 🔹 Spring + Spring Boot 11. How does Spring Boot simplify dependency management and configuration? 12. What is the difference between @Configuration, @Bean, and @Component annotations? 13. How do you configure exception handling using @RestControllerAdvice? 14. How does Spring Boot handle externalized configuration using YAML and properties files? 15. How do you secure REST APIs using JWT and Spring Security? 16. What is the purpose of Spring Profiles and how do you activate them? 17. How does Spring Boot Actuator help in production monitoring? 18. How do you integrate Kafka with Spring Boot applications? 19. How do you manage database migrations with Flyway or Liquibase? 20. How do you implement async event-driven communication using @Async or messaging queues? 🔹 Microservices 21. How do you design resilient microservices with Circuit Breaker and Retry mechanisms? 22. What is the difference between API Gateway and Service Mesh? 23. How do you handle data consistency in distributed microservices? 24. How do you implement centralized configuration in a microservices architecture? 🔹 Coding Questions 25. Write a Java program to find the first non-repeating character in a string. 26. Implement a method to reverse a linked list iteratively. 27. Write a function to check if a binary tree is height-balanced. 🔹 Others 33. What is the difference between Docker images and containers? 34. How do you configure centralized logging using ELK or EFK stack? 35. How does Kafka guarantee at-least-once message delivery? 36. What are liveness and readiness probes in Kubernetes, and why are they important? 37. How does AWS IAM ensure resource-level access control? 38. How do you implement monitoring and alerting using Prometheus and Grafana? 🏷️ Hashtags: hashtag #JavaInterview #SpringBoot #Microservices #Java21 #Kafka #Docker

#java 🔹 Day 31–60: Java Full Stack Interview Q&A (Q70–Q83) 🔸 70. How do you prepare for fullstack interviews after completing a roadmap? Ans: - Revise key concepts from each phase - Practice coding (DSA + SQL + system design) - Build and polish a showcase project - Share learnings on GitHub and LinkedIn - Join mock interviews and tech communities --- 🔸 71. What is the difference between @RestController and @Controller in Spring Boot? Ans: - @RestController: Combines @Controller + @ResponseBody, returns JSON directly - @Controller: Used for MVC, returns views (e.g., HTML templates) --- 🔸 72. How do you implement pagination and sorting in Spring Boot? Ans: Use Pageable and Sort in repository methods. Example: `java Page<User> findAll(Pageable pageable); ` --- 🔸 73. What is the role of @Transactional in Spring Boot? Ans: It manages transaction boundaries. Ensures atomicity — either all DB operations succeed or all rollback. --- 🔸 74. How do you handle cross-origin requests in Spring Boot + React? Ans: Use @CrossOrigin annotation or configure CORS globally. React frontend must match allowed origins in backend. --- 🔸 75. What is the difference between SQL and NoSQL databases? Ans: - SQL: Structured, relational, ACID compliant (e.g., MySQL, Postgres) - NoSQL: Flexible schema, scalable, eventual consistency (e.g., MongoDB, Cassandra) --- 🔸 76. How do you secure Docker containers in production? Ans: - Use minimal base images - Scan for vulnerabilities - Avoid root user - Use secrets via env vars or volumes - Enable network isolation --- 🔸 77. What is the role of @Value and @ConfigurationProperties in Spring Boot? Ans: - @Value: Injects single property - @ConfigurationProperties: Binds entire config block to POJO --- 🔸 78. How do you implement file upload in Spring Boot + React? Ans: - React: Use FormData and Axios - Spring Boot: Use MultipartFile in controller - Store file in DB or filesystem --- 🔸 79. What is the difference between PUT and PATCH in REST APIs? Ans: - PUT: Full update (replaces entire resource) - PATCH: Partial update (modifies specific fields) --- 🔸 80. How do you handle environment-specific configs in Spring Boot? Ans: Use application-{profile}.yml (e.g., application-dev.yml) Activate profile via spring.profiles.active --- 🔸 81. What is the role of @Scheduled in Spring Boot? Ans: Runs tasks at fixed intervals or cron expressions. Used for background jobs like cleanup, sync, etc. --- 🔸 82. How do you implement global exception handling in Spring Boot? Ans: Use @ControllerAdvice + @ExceptionHandler Return custom error responses with status codes --- 🔸 83. What is the difference between map() and flatMap() in Java Streams? Ans: - map(): Transforms each element - flatMap(): Flattens nested structures (e.g., List<List<T>> → List<T>) --- #JavaFullStack