Spring Boot Interview Questions: Auto-Configuration, Security, Microservices, and More

🚀 Java & Spring Boot Interview Questions Every Developer Should Know Whether you're preparing for interviews or brushing up your fundamentals, here’s a curated list of important questions covering Core Java, Spring Boot, JPA, and Microservices 👇 🔹 𝐂𝐨𝐫𝐞 𝐉𝐚𝐯𝐚 & 𝐂𝐨𝐧𝐜𝐮𝐫𝐫𝐞𝐧𝐜𝐲 1. What is CompletableFuture and when do we use it? 2. Difference between Future and CompletableFuture 3. Internal working of HashMap 4. HashMap vs ConcurrentHashMap 5. How does the Java Memory Model (JMM) work? 6. What is the volatile keyword? 🔹 𝐒𝐩𝐫𝐢𝐧𝐠 𝐁𝐨𝐨𝐭 & 𝐁𝐚𝐜𝐤𝐞𝐧𝐝 𝐂𝐨𝐧𝐜𝐞𝐩𝐭𝐬 7. How does Spring Boot Auto-Configuration work? 8. How do we handle transactions in Spring Boot? 9. What are Propagation & Isolation levels in transaction management? 10. What is Dependency Injection? 🔹 𝐉𝐏𝐀 & 𝐇𝐢𝐛𝐞𝐫𝐧𝐚𝐭𝐞 11. Application is slow due to JPA – how do you improve performance? 12. What is Dirty Checking? 13. Difference between JPA and Hibernate 14. How to implement DTO Projection in JPA? (Write a query) 🔹 𝐌𝐢𝐜𝐫𝐨𝐬𝐞𝐫𝐯𝐢𝐜𝐞𝐬 𝐀𝐫𝐜𝐡𝐢𝐭𝐞𝐜𝐭𝐮𝐫𝐞 15. Two microservices sharing the same database – is it a good approach? 16. Two microservices updating the same row simultaneously – how do you handle it? 17. If one microservice goes down, how does it impact the system? How do you handle it? 18. How do you improve overall application performance? 💻 𝐏𝐫𝐨𝐠𝐫𝐚𝐦𝐦𝐢𝐧𝐠 𝐐𝐮𝐞𝐬𝐭𝐢𝐨𝐧𝐬 ✔️ Find all permutations of a string "abc" ✔️ Check if a string is balanced: "{[()]}" 𝐀𝐥𝐥 𝐓𝐡𝐞 𝐁𝐞𝐬𝐭✌ #Java #SpringBoot #Microservices #JPA #BackendDevelopment #InterviewPreparation #SoftwareEngineering

Java Backend Developer – 2nd Round Interview These 20 questions separate good developers from great ones. 1. How does the G1 Garbage Collector work? What are regions, and how does it decide what to collect? 2. What is a memory leak in Java? Walk through how you’d detect and fix one in production. 3. How does ReentrantLock differ from synchronized? When would you prefer one over the other? 4. Explain happens-before in Java Memory Model. Why does it matter in multithreaded code? 5. How does Spring’s @Transactional handle rollback internally? What are common pitfalls? 6. What is the difference between REQUIRED, REQUIRES_NEW and NESTED propagation in transactions? 7. How would you implement distributed locking across microservices? 8. How does Hibernate’s first-level vs second-level cache work? When does it hurt you? 9. Explain the N+1 problem in JPA. How do you detect and fix it? 10. How would you design an idempotent REST API? Why does it matter? 11. How does database connection pooling work? How do you tune HikariCP for high throughput? 12. What is eventual consistency? How would you handle it in a microservices architecture? 13. How do you implement optimistic vs pessimistic locking? When would you use each? 14. How would you design a rate limiter for a public API? 15. What is the Saga pattern? How does it compare to 2PC for distributed transactions? 16. How would you secure inter-service communication in a microservices setup? 17. How does Kafka ensure message ordering and exactly-once delivery? 18. How would you design a system that processes 1 million requests per day without downtime? 19. How do you do zero-downtime deployment for a Spring Boot service running in Kubernetes? 20. Your service’s p99 latency spiked from 80ms to 2s overnight. Walk me through your debugging process. Preparing for interviews? Start revising these today 𝗜’𝘃𝗲 𝗽𝗿𝗲𝗽𝗮𝗿𝗲𝗱 𝗶𝗻 𝗗𝗲𝗽𝘁𝗵 𝗝𝗮𝘃𝗮 𝗦𝗽𝗿𝗶𝗻𝗴𝗯𝗼𝗼𝘁 𝗯𝗮𝗰𝗸𝗲𝗻𝗱 𝗚𝘂𝗶𝗱𝗲, 𝟏𝟬𝟬𝟬+ 𝗽𝗲𝗼𝗽𝗹𝗲 𝗮𝗿𝗲 𝗮𝗹𝗿𝗲𝗮𝗱𝘆 𝘂𝘀𝗶𝗻𝗴 𝗶𝘁. 𝗚𝗲𝘁 𝘁𝗵𝗲 𝗴𝘂𝗶𝗱𝗲 𝗵𝗲𝗿𝗲: https://lnkd.in/dfhsJKMj keep learning, keep sharing ! #java #backend #javaresources

🚀 Spring Boot Interview Questions (Part 1) After sharing Core Java & Java 8 questions, here are Spring Boot questions that were frequently asked in my interviews 👇 🔹 Core Spring Concepts Explain Bean Lifecycle What is Bean Scope? What is the default scope? 🔹 Scope-Based Scenario If a class has singleton scope and it has a reference of prototype bean, how does it behave? @Scope("singleton") class A { B b; } @Scope("prototype") class B { } 👉 Follow-up: How can we ensure it behaves like prototype instead of singleton? 🔹 Important Comparisons Java Singleton vs Spring Singleton Scope Difference between @Primary and @Qualifier 🔹 Transaction Management Transactions, Propagation, and Isolation 👉 Scenario-based questions on propagation 🔹 Annotations & Config What annotations have you used? Explain them @RequestBody is used for JSON → Java object 👉 What if you want to support another data format? @ConfigurationProperties vs @Value @ConditionalOnProperty How to create beans based on environment or condition 🔹 Spring Security Spring Security basics JWT Authentication vs Authorization 🔹 Web Layer @PathVariable vs @RequestParam 🔹 Build Tool Maven Lifecycle 🔹 Spring Data JPA What is @Repository? interface StudentRepo extends JpaRepository<Student, Integer> { } 👉 Why Integer is used instead of int? 🔹 Configuration Files If same values are defined in both .properties and .yml, which one is loaded first? 🔹 Database How to use multiple databases in a single Spring Boot application? 🔹 APIs What are Spring Boot Starters? SOAP API vs REST API Advantages and disadvantages of REST API Difference between @RestController and @Controller 🔹 Dependency Injection What is Dependency Injection? Types of Dependency Injection Which one do you prefer and why? 🔹 Profiles & AOP Profiles in Spring Boot Spring AOP Filter vs Interceptor 🔹 Miscellaneous How to upload file and save in DB @Async vs CompletableFuture 👉 Follow for Part 2 (Advanced Spring Boot + Microservices Interview Questions) 🔁 Repost so others can benefit too #SpringBoot #Java #Backend #InterviewPreparation #Microservices

My recent interview experience related to Java, Spring Boot, REST APIs, Microservices, and Databases 👇 🔹 Core Java What is the difference between String, StringBuilder, and StringBuffer? How does HashMap work internally? Difference between ConcurrentHashMap and HashMap? What is the Java Memory Model (JMM)? Explain multithreading and synchronization What is the difference between ExecutorService and Thread? What is Optional class and why is it used? 🔹 Spring Boot What is Spring Boot and how is it different from Spring? What are starters in Spring Boot? Explain @SpringBootApplication What is dependency injection (DI)? Difference between @Component, @Service, @Repository, @Controller? What is Spring Boot Auto Configuration? How does application.properties/yml work? 🔹 REST APIs What is a RESTful API? Difference between PUT vs PATCH What are idempotent APIs? What is HTTP status code usage? How do you handle exception handling globally? What is @RestControllerAdvice? How to implement pagination and sorting? 🔹 Database (SQL + NoSQL) Difference between SQL vs NoSQL What is indexing and how does it improve performance? What is normalization vs denormalization? Explain ACID properties What is transaction management in Spring? Difference between JPA and Hibernate What is lazy vs eager loading? How to solve N+1 query problem? #Java #SpringBoot #Microservices #RESTAPI #BackendDevelopment #FullStackDeveloper #Hibernate #JPA #SQL #SystemDesign #SoftwareEngineering #TechInterview #InterviewPreparation #DevelopersOfLinkedIn #Coding #Programming #JavaDeveloper

⚡ map vs flatMap in Java (Stream API) Definition: map() → Transforms each element 1:1 flatMap() → Transforms and flattens nested structures 🤔 Why use? 1. map() - When output is a single value per input - Simple transformations 2. flatMap() - When each element produces multiple values (collections/streams) - Avoid nested structures like List<List<T>> 💻 Example List<List<Integer>> list = Arrays.asList( Arrays.asList(1, 2), Arrays.asList(3, 4), Arrays.asList(5, 6) ); // map() → creates nested structure List<Stream<Integer>> mapResult = list.stream() .map(inner -> inner.stream()) .collect(Collectors.toList()); // flatMap() → flattens into single stream List<Integer> flatMapResult = list.stream() .flatMap(inner -> inner.stream()) .collect(Collectors.toList()); 🔄 Flow map() List<List> → Stream<List> → Stream<Stream> flatMap() List<List> → Stream<List> → Stream 🧠 Rule of Thumb 👉 If your transformation returns a single value → use map() 👉 If it returns a collection/stream → use flatMap() 👉 If you are preparing for Java backend interviews, connect & follow - I share short, practical backend concepts regularly. #Java #Backend #Streams #Java8 #CodingInterview #InterviewPrep #SoftwareEngineering

🚀 Java Backend Interview Series – Day 11 Spring is a must for backend roles 👇 🌱 Spring Core (Fundamentals): 1️⃣ What is IoC (Inversion of Control)? 2️⃣ What is Dependency Injection (DI)? Types? 3️⃣ Difference between IoC and DI? 4️⃣ What is Spring Bean? 5️⃣ Bean lifecycle in Spring? 6️⃣ What is @Component, @Service, @Repository? 7️⃣ What is @Autowired and how does it work? 8️⃣ @Primary vs @Qualifier? 9️⃣ Singleton bean in Spring vs Singleton in Java? 🔟 Are Spring beans thread-safe? 💡 Spring questions = real project understanding 📌 and Save this for revision 👇 Comment “NEXT” for such questions #Java #Spring #SpringBoot #BackendDevelopment #InterviewPrep #Developers #Coding

🧩 Optional in Java What is Optional? A wrapper to represent a value that may be absent, helps avoid null and makes intent clear. ✅ Why use it? 1. Prevents NullPointerException 2. Forces explicit handling of missing values 3. Cleaner and more readable APIs ⚖️ Correct vs Wrong Usage 1️⃣ Return Types ❌ User findUser(int id); // may return null ✅ Optional<User> findUser(int id); 2️⃣ Fields & Params ❌ Optional<String> name; // field void process(Optional<String>); // param 👉 Adds confusion + not intended use ✅ String name; void process(String data); 3️⃣ Accessing Value ❌ user.get(); // unsafe ✅ user.orElse(defaultUser); user.orElseThrow(); 4️⃣ Best Use (Chaining 💡) return Optional.ofNullable(user) .map(User::getAddress) .map(Address::getCity) .orElse("Unknown"); 🧠 Rule of Thumb 👉 Use Optional only for return types 👉 Avoid in fields, DTOs, method params 👉 Prefer map/flatMap over null checks 👉 If you are preparing for Java backend interviews, connect & follow - I share short, practical backend concepts regularly. #Java #SpringBoot #Backend #CleanCode #CodingTips #Developers #InterviewPrep #SoftwareEngineering

IF YOU THINK YOU’RE GOOD AT JAVA & SPRING BOOT, TRY ANSWERING THESE WITHOUT GOOGLE. These are real scenario-based questions interviewers use to test production-level thinking: Java (Runtime, JVM, Concurrency) 1) Your Java app slows down over time without errors. What could be happening internally? 2) CPU is low but response time is high. What might be blocking threads? 3) OutOfMemoryError occurs even with sufficient heap. Why? 4) Increasing heap size made performance worse. Explain. 5) Threads are free but requests are queued. Why? 6) GC pauses increased after a small change. What might have changed? 7) JVM doesn’t exit after main() completes. What’s holding it? 8) Parallel streams reduced performance instead of improving it. Why? 9) Memory usage keeps increasing slowly. What do you suspect? 10) Logging change caused slowdown. How? 11) ThreadLocal fixed one issue but caused another. Why? 12) ExecutorService tasks fail silently. What went wrong? 13) Retry logic overloaded the system. What mistake was made? 14) Deadlock happens only in production. Why? 15) Scaling instances made performance worse. Why? Spring Boot (Production Scenarios) 16) App works locally but fails in production. What do you check first? 17) APIs are fast locally but slow in prod. Why? 18) application.properties change didn’t reflect. Why? 19) CPU is low but requests timeout. What’s the bottleneck? 20) Multiple beans cause startup failure. How to fix? 21) @Transactional present but rollback doesn’t happen. Why? 22) DB connection pool exhausts under load. Why? 23) Scheduled jobs affect API latency. How to isolate? 24) App behaves differently in Docker vs local. Why? 25) New deployment but users see old behavior. What went wrong? 26) Logs missing in production but present locally. Where to check? 27) Async processing made performance worse. How? 28) Circuit breaker stays open even when service is healthy. Why? 29) Adding more resources didn’t improve performance. Why? 30) What Spring Boot decision you made could cause production issues? These questions are not about syntax. They test whether you can THINK when systems behave unexpectedly. I’ll share the detailed PDF with interested folks.

Java Backend Developer 1st Round Interview These 20 questions are commonly asked in 2026 1. How does HashMap work internally? Explain buckets, hashing, and resizing. 2. What is the difference between HashMap and ConcurrentHashMap? 3. How does JVM memory architecture work? (Heap, Stack, Metaspace) 4. What are the common causes of OutOfMemoryError in production, and how would you troubleshoot it? 5. How can you make a class thread-safe? Provide a practical example. 6. Explain the complete lifecycle of a Spring Bean. 7. How does Dependency Injection function internally in Spring? 8. What happens internally when you use @Transactional? 9. What is the difference between @Component, @Service, and @Repository? 10. How do you implement global exception handling in Spring Boot? 11. What is lazy vs eager loading in JPA, and when can it lead to performance issues? 12. How do you manage concurrent updates to the same database row? 13. Explain ACID properties with a real-world banking transaction example. 14. How do you optimize a slow-performing SQL query? 15. How would you design pagination and sorting in a REST API? 16. How does JWT-based authentication work step by step? 17. If your API becomes slow under load, how would you identify the bottleneck? 18. REST vs Messaging (Kafka) when would you choose each approach? 19. How would you Dockerize a Spring Boot application? 20. If two microservices fail during communication, how do you implement fault tolerance? 𝗞𝗲𝗲𝗽𝗶𝗻𝗴 𝘁𝗵𝗶𝘀 𝗶𝗻 𝗺𝗶𝗻𝗱, 𝗜 𝘄𝗲𝗻𝘁 𝗱𝗲𝗲𝗽 𝗮𝗻𝗱 𝗱𝗼𝗰𝘂𝗺𝗲𝗻𝘁𝗲𝗱 𝗲𝘃𝗲𝗿𝘆𝘁𝗵𝗶𝗻𝗴 𝗶𝗻𝘁𝗼 𝗮 𝗝𝗮𝘃𝗮 𝗕𝗮𝗰𝗸𝗲𝗻𝗱 𝗗𝗲𝘃𝗲𝗹𝗼𝗽𝗲𝗿 𝗚𝘂𝗶𝗱𝗲. 𝗚𝗲𝘁 𝘁𝗵𝗲 𝗚𝘂𝗶𝗱𝗲 𝗵𝗲𝗿𝗲: https://lnkd.in/duh8Y4nV Use Code 𝗦𝗗𝗘𝟭𝟬 Stay Hungry, Stay Foolish!!

java interview questions 🔹 Core Java & OOP - Explain OOP principles. - Difference between Abstract Class and Interface. - Difference between final, finally, and finalize. - How do you create an immutable object? - String vs StringBuilder vs StringBuffer. - Types of exceptions in Java. - How do you handle NullPointerException? 🔹 Java 8 & Best Practices - How to handle null checks using Optional in Java 8? - How to handle null checks using annotations (e.g., @NotNull, @NotBlank)? 🔹 Multithreading & Concurrency - If two threads are using the same resource, how will you handle it? 🔹 Spring Boot & Backend Development - How to validate incoming JSON requests? - What is Spring Cloud? How is it useful? - How do you handle exceptions in a Spring Boot application? - Difference between @Component, @Service, and @Repository. - How do microservices communicate with each other? - What are the different HTTP methods used in REST APIs? - What is API versioning? 🔹 Design & Architecture - Explain SOLID principles. - Singleton vs Prototype scope. - What is Dependency Injection? - Explain the Repository Pattern. 🔹 Database & Persistence - JPA vs Hibernate. - How do you establish a database connection in Spring Boot? - What are stored procedures? - How do you optimize database queries? - Lazy vs Eager loading. 🔹 Testing & Tools - Difference between @Mock and @MockBean. - Which version control tool are you using? - How do you handle merge conflicts while pushing code to a developer branch? - Have you used Jenkins? Why is it used? 🔹 AI in Development - Are you using AI tools for coding? - What are their pros, cons, and risks? - How do you validate AI-generated code? #Java #SpringBoot #Microservices #BackendDeveloper #InterviewPreparation #TechInterview #JavaDeveloper #SoftwareEngineering