What is Spring Boot, and how is it different from the Spring Framework?

Explain Spring Framework as the core ecosystem for building Java apps, including DI, AOP, MVC, data access, and more. Then position Spring Boot as an opinionated layer on top that makes Spring faster to start, with auto-configuration, starter dependencies, embedded servers, and production-ready features. A concrete example is building a REST API. With Spring Framework you often configure more pieces yourself, while with Spring Boot you can add something like spring-boot-starter-web, write a @RestController, and run it as a standalone app using an embedded Tomcat. Call out what Boot is not. It does not replace Spring, it packages and configures it, so you still need to understand core Spring concepts like beans, scopes, and how configuration is applied when things go wrong.

How does Spring Boot auto-configuration work?

Start by describing that auto-configuration uses classpath detection, property settings, and conditional annotations to configure beans for you. Mention @SpringBootApplication includes @EnableAutoConfiguration, and Boot loads auto-config classes listed under META-INF/spring/org.springframework.boot.autoconfigure.AutoConfiguration.imports. A practical way to explain it is with DataSource setup. If H2 is on the classpath, Boot can auto-create an in-memory DataSource, but if you add MySQL and set spring.datasource.url, it switches to a MySQL DataSource based on conditions like @ConditionalOnClass and @ConditionalOnProperty. Interviewers often want to see how you troubleshoot it. Mention enabling the condition evaluation report, for example by running with debug=true, and using actuator /actuator/conditions to see why a configuration did or did not apply.

What is dependency injection in Spring, and how do you define and wire beans?

Explain that dependency injection means Spring creates objects and provides their dependencies, which reduces tight coupling and makes testing easier. You can define beans with stereotypes like @Component, @Service, @Repository, or with @Bean methods in a @Configuration class, and Spring wires them by type. Give a short example: a PaymentService depends on a PaymentGateway interface. You create two implementations, then inject one into the service using constructor injection, optionally choosing with @Qualifier or @Primary. Mention common mistakes. Field injection makes testing and immutability harder, circular dependencies usually signal design issues, and missing component scanning often explains why a bean is not found.

What is the difference between @Component, @Service, @Repository, and @Controller?

Tell them these are all specializations of @Component, so they register beans, but each communicates intent and can enable extra behavior. @Controller and @RestController are for web endpoints, @Service is for business logic, and @Repository is for data access. A good example is exception translation. When you annotate a DAO with @Repository, Spring can translate persistence exceptions into Spring’s DataAccessException hierarchy, which gives you more consistent error handling across database technologies. Add a tip about clean layering. If you put database code in controllers or put HTTP concerns in services, your code gets harder to test and change, so show that you keep responsibilities separated.

Explain Spring Boot starters. Why do they matter?

Describe starters as curated dependency sets that bring in compatible libraries for a use case, like web, JPA, security, or test. They reduce dependency hunting and version conflicts because Spring Boot manages versions through dependency management. A concrete example is spring-boot-starter-web, which includes Spring MVC and an embedded server, plus JSON support through Jackson. For testing, spring-boot-starter-test pulls in JUnit, Mockito, AssertJ, and Spring Test so you can write slice tests and integration tests quickly. Share what to watch for. Adding too many starters can increase startup time and classpath complexity, and you should be comfortable excluding transitive dependencies when you have a conflict or want a different implementation.

How do you externalize configuration in Spring Boot, and what is the property precedence?

Explain that Spring Boot lets you configure apps through application.properties or application.yml, environment variables, command-line arguments, and profile-specific files. The idea is you keep code the same while changing behavior per environment, for example dev vs staging vs prod. Give an example using @ConfigurationProperties to bind a group of settings, such as retry settings or an S3 bucket configuration. You can then switch values using profiles like application-dev.yml and application-prod.yml, and activate with spring.profiles.active. Mention precedence at a high level without getting lost. Command-line args and environment variables typically override file-based configuration, and you should avoid hardcoding secrets in config files, instead use a secret manager or injected environment variables.

What is the difference between @RestController and @Controller?

Explain that @Controller is used for MVC views and usually returns a view name, while @RestController is a convenience annotation that combines @Controller and @ResponseBody. With @RestController, your methods typically return JSON or other serialized responses. A quick example is returning a User DTO from a GET endpoint. With @RestController, returning the object will be converted to JSON via HttpMessageConverters, often Jackson, and you can control the response with ResponseEntity. Mention pitfalls around serialization. Lazy-loaded JPA relationships can cause serialization errors or large payloads, and you should design API DTOs rather than returning entities directly in many production systems.

How do you handle exceptions in a Spring Boot REST API?

Start with the approach: use @ControllerAdvice or @RestControllerAdvice to centralize error handling and return consistent error responses. Pair that with custom exceptions and map them to HTTP status codes using @ExceptionHandler methods. Give an example: throw ResourceNotFoundException in a service, then in advice return a ResponseEntity with a structured body containing timestamp, status, message, and request path. This keeps controllers clean and makes client behavior predictable. Mention what interviewers often look for. You should not return stack traces to clients, you should log with enough context, and you should validate inputs with Bean Validation and return 400 errors with helpful field-level messages.

How do you test a Spring Boot application? What is the difference between unit tests and integration tests?

Explain that unit tests focus on a single class and use mocks for dependencies, while integration tests start more of the Spring context to verify wiring, configuration, and real behavior. In Spring Boot, you often combine JUnit with Mockito for unit tests, and use @SpringBootTest or slice annotations like @WebMvcTest and @DataJpaTest for targeted integration tests. A concrete example is testing a controller. For a fast test, use @WebMvcTest with MockMvc and mock the service, then verify status codes and JSON fields; for a deeper test, use @SpringBootTest with a random port and call the endpoint through HTTP. Share practical tips. Keep unit tests fast and numerous, keep integration tests focused and stable, and use Testcontainers when you need realistic dependencies like Postgres or Kafka without flakiness from shared test environments.

How do you secure a Spring Boot application?

Describe the typical approach using Spring Security, starting with authentication and then authorization. You configure security filter chains, define which endpoints require auth, and decide between session-based auth, HTTP Basic for internal services, or token-based auth like JWT for APIs. Give an example: protect /api/**, allow /health and /actuator/health, and enforce role-based access with annotations like @PreAuthorize("hasRole('ADMIN')"). If you use JWT, explain validating the token on each request and extracting claims into the SecurityContext. Mention common mistakes. Do not store passwords in plain text, use a strong password encoder, be careful with CORS and CSRF settings, and avoid exposing sensitive actuator endpoints without protection.

Tell me about a time you debugged a production issue in a Spring Boot service.

Situation: A Spring Boot API started returning intermittent 500 errors after a deployment during peak traffic. Task: You needed to identify the root cause quickly, reduce customer impact, and prevent recurrence. Action: You checked logs and tracing to correlate failures with a specific endpoint, then compared configuration between the previous and current release. You found a connection pool setting was changed, causing timeouts under load, so you rolled back the change, increased observability around database calls, and added an alert on pool saturation. Result: Error rates dropped back to baseline within 20 minutes, and the team later shipped a fix with safer defaults and a load test that reproduced the issue. You also documented the incident so future on-call engineers had a clear playbook.

Describe a time you improved performance or reliability in a Spring Boot application.

Situation: Your service handled user searches, but p95 latency grew over time and started missing SLOs. Task: You needed to improve latency without changing the external API contract. Action: You profiled the slowest paths, noticed repeated database calls for reference data, and added caching with clear TTLs plus cache metrics. You also optimized one query with an index and adjusted serialization by returning smaller DTOs instead of large nested objects. Result: p95 latency improved by about 35%, and the service stopped timing out during traffic spikes. The team kept the gains by adding a regression performance test in CI and a dashboard that tracked latency, error rate, and cache hit rate.

Tell me about a time you disagreed with a teammate on an architectural decision for a Spring Boot project.

Situation: Your team debated whether to split a Spring Boot application into multiple microservices right away. Task: You needed to express your concerns, align on goals, and still support the final decision. Action: You asked the team to clarify the current pain points, then proposed a short written comparison of options, including deployment complexity, data consistency, and testing overhead. You suggested starting with a modular monolith, adding clear package boundaries, and only extracting services when there was a proven scaling or ownership need. Result: The team agreed to modularize first and created a roadmap with measurable triggers for service extraction. You avoided premature complexity, and later the team extracted one module successfully because the boundaries were already clear.

spring boot Interview Questions: Complete Guide

Spring Boot interviews usually mix core Spring concepts with practical application design, debugging, and a bit of production thinking. You can expect questions on auto-configuration, dependency injection, REST APIs, data access, security, testing, and how you would run and monitor services in real environments. It can feel broad, but if you focus on explaining tradeoffs and showing how you have shipped features, you will come across as confident and credible.

Common Interview Questions

Behavioral Questions (STAR Method)

STAR Method: Structure your answers using Situation, Task, Action, and Result to tell compelling stories about your experience.

Questions to Ask the Interviewer

Show your interest by asking thoughtful questions

•How are your Spring Boot services deployed today, and what are the biggest reliability pain points you want this role to help with?
•What does your team’s approach to configuration and secrets look like across dev, staging, and production?
•How do you test services here, for example unit tests vs integration tests, and what is your target for build time and test coverage?
•Which observability tools do you use for logs, metrics, and tracing, and how do engineers typically debug issues in production?
•What are the main architectural tradeoffs you are dealing with right now, for example monolith vs microservices, and what drove those choices?

Interview Preparation Tips

Pick 2 projects you know well and practice explaining the architecture, database choices, and failure modes, because Spring Boot interviews often test depth more than breadth.

Write and run a small demo API before the interview, including validation, exception handling, and one integration test, so your answers stay concrete.

Practice troubleshooting talk tracks, such as how you would investigate a startup failure, a bean wiring error, or a slow endpoint, using logs and actuator.

Prepare a few tradeoff statements, for example when to use @SpringBootTest vs @WebMvcTest, or JWT vs sessions, and be ready to explain your reasoning.

Interview Preparation Checklist (1 Week → Day Of)

## Pre-interview (1 week out)

•Research the company (4–6 items): product lines, latest press, tech stack, annual revenue range, top competitors. Identify 2 recent projects or blog posts related to Spring Boot or microservices.
•Study the role (30–60 minutes): list required skills, map your experience to 3–5 bullets showing impact (metrics, e.g., reduced latency by 40%).
•Research interviewers: check LinkedIn for job history and shared connections; note 1–2 common topics to mention.
•Practice technical fundamentals: schedule 90 minutes/day for 5 days on Spring Boot topics (controllers, dependency injection, profiles, Actuator, testing).
•Behavioral prep: write 6 STAR stories tied to system design, debugging, teamwork, deadlines.
•Build or polish a demo: have 1 GitHub repo with README and instructions; include Dockerfile or docker-compose if relevant.

## Pre-interview (1 day out)

•Do a 60-minute mock interview: 30 minutes coding (Spring Boot endpoint, unit test), 30 minutes behavioral. Record or get feedback.
•Prepare logistics: 3 printed resumes, notebook, pen, charger, laptop with local app runnable. Confirm interview time and timezone.
•Plan route: leave buffer for traffic; aim to arrive 15 minutes early for onsite, 5–10 minutes early for virtual.
•Outfit: clean, comfortable, company-appropriate (smart-casual for most tech roles).
•Sleep: aim for 7–8 hours.

## Day of (hour before)

•Quick review: skim job description and your STAR bullets (10 minutes).
•Warm up: 20-minute light coding or whiteboard sketching; 5 minutes breathing/visualization.
•Final checklist: ID, resumes, laptop, charger, meeting link, water.

Actionable takeaway: follow this timeline, and log 10–12 interview-specific examples you can cite in answers.

8 Common Interview Mistakes and How to Fix Them

•Example: answering architecture questions without linking to the company’s microservices footprint.
•Problem: shows lack of interest and weak fit.
•Fix: mention one product or team fact in your first 2 answers.

•Example: tossing terms like "CQRS" without explaining trade-offs.
•Problem: confuses non-technical interviewers.
•Fix: state the term, give a one-sentence definition, and a real example.

•Example: vague "I improved performance" with no data.
•Problem: claims lack proof.
•Fix: include metric (e.g., cut response time from 800ms to 300ms) and your role.

•Example: spending 40 minutes on edge cases and not finishing core API.
•Problem: interviewer assesses completeness first.
•Fix: implement a working endpoint, then add tests and refinements.

•Example: arguing when an interviewer suggests a different design.
•Problem: signals poor collaboration.
•Fix: acknowledge, ask clarifying questions, present trade-offs.

•Example: avoiding eye contact, slouching.
•Problem: reduces perceived confidence.
•Fix: sit straight, nod, smile, use open hand gestures.

•Example: "Do you have any questions for me– followed by silence.
•Problem: misses chance to show curiosity.
•Fix: prepare 5 focused questions about team metrics, deployment cadence, or success criteria.

•Example: no thank-you note after final round.
•Problem: loses chance to reiterate fit.
•Fix: send a 2–3 sentence follow-up within 24 hours summarizing one high-impact point.

Actionable takeaway: pick three fixes to apply this week and track improvement in mock interviews.

Interview Success Stories: What Worked

Story 1 — Mid-level Backend Engineer

•Background: 4 years Java/Spring experience, limited distributed systems exposure.
•Preparation: spent 2 weeks building a small Spring Boot app with Kafka and Actuator; wrote 6 STAR stories and practiced 10 system-design sketches.
•Challenging moment: interviewer asked about data partitioning for a high-throughput topic. The candidate didn’t know a specific tool but used a clear framework: list constraints (throughput, ordering), propose partitions per key strategy, and back up with numbers (shard count = expected throughput / 1M messages per partition).
•Why it worked: structured thinking, concrete numbers, and a demo repo to reference.
•Lesson: prepare a lightweight demo that you can point to under pressure.

Story 2 — Senior Full-Stack Engineer

•Background: 8 years, led small teams, had cloud deployment experience.
•Preparation: mapped role requirements to 7 achievements with metrics (e.g., reduced CI build time by 45%). Did three mock interviews with peers and one with a hiring manager friend.
•Challenging moment: panel asked to critique an existing API design shown on a whiteboard. The candidate calmly identified three issues (missing pagination, lack of idempotency, and unversioned endpoints), sketched improvements, and proposed migration steps with minimal downtime.
•Why it worked: clear trade-offs, migration plan, and leadership tone.
•Lesson: practice critiquing real APIs and always propose rollout steps.

Story 3 — Junior Dev with Limited Experience

•Background: 1.5 years, strong internship projects but no production ownership.
•Preparation: focused on fundamentals: Spring Boot controllers, testing with JUnit, and one end-to-end demo deployed to Heroku. Rehearsed answers for "Tell me about a time you failed."
•Challenging moment: asked to debug a failing test during the interview. The candidate talked through hypotheses, ran simple checks, found a missing MockBean, fixed it, and explained why the fix made the test reliable.
•Why it worked: calm troubleshooting skills and clear communication.
•Lesson: rehearse debugging aloud and keep a short checklist for common test failures.

Actionable takeaway: combine a small demo, numeric results, and practiced explanations to handle surprises.

Recommended Resources for Interview Prep

1. Book — "Spring in Action" (5th/6th ed.

•Why: deep, practical Spring Boot patterns and examples.
•Use when: building or refining a demo app; read 2–3 chapters before interviews.

•Why: hands-on projects and downloadable source code.
•Use when: you need guided, project-based practice over 10–20 hours.

•Why: standard algorithm problems used in many interviews.
•Use when: allocate 30–60 minutes daily for 4 weeks; focus on arrays, maps, and concurrency problems.

•Why: templates for designing services with concrete component diagrams.
•Use when: prepare 6 typical systems (e.g., messaging, metrics, auth) and practice timed sketches.

•Why: concise rules on versioning, pagination, and error handling.
•Use when: preparing to critique or propose API changes.

6. Mock interview service — Pramp or Interviewing.

•Why: real-time peer or expert practice with feedback.
•Use when: schedule 2–3 mock interviews within the final week.

•Why: current interview experiences and sample questions.
•Use when: research recent company-specific interview patterns.

•Why: common production deployment topics often discussed in senior interviews.
•Use when: prepare one pipeline example to describe during interviews.

Actionable takeaway: pick 3 resources and plan a 4-week study schedule with measurable goals.

spring boot Interview Questions: Complete Guide

Michael Rodriguez

Common Interview Questions

Behavioral Questions (STAR Method)

Questions to Ask the Interviewer

Interview Preparation Tips

Interview Preparation Checklist (1 Week → Day Of)

8 Common Interview Mistakes and How to Fix Them

Interview Success Stories: What Worked

Recommended Resources for Interview Prep

Common Interview Questions

Build your job search toolkit

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spring boot Interview Questions: Complete Guide

Michael Rodriguez

Common Interview Questions

Q1What is Spring Boot, and how is it different from the Spring Framework?

Q2How does Spring Boot auto-configuration work?

Q3What is dependency injection in Spring, and how do you define and wire beans?

Q4What is the difference between @Component, @Service, @Repository, and @Controller?

Q5Explain Spring Boot starters. Why do they matter?

Q6How do you externalize configuration in Spring Boot, and what is the property precedence?

Q7What is the difference between @RestController and @Controller?

Q8How do you handle exceptions in a Spring Boot REST API?

Q9How do you test a Spring Boot application? What is the difference between unit tests and integration tests?

Q10How do you secure a Spring Boot application?

Behavioral Questions (STAR Method)

B1Tell me about a time you debugged a production issue in a Spring Boot service.

B2Describe a time you improved performance or reliability in a Spring Boot application.

B3Tell me about a time you disagreed with a teammate on an architectural decision for a Spring Boot project.

Questions to Ask the Interviewer

Interview Preparation Tips

Interview Preparation Checklist (1 Week → Day Of)

8 Common Interview Mistakes and How to Fix Them

Interview Success Stories: What Worked

Recommended Resources for Interview Prep

Common Interview Questions

Build your job search toolkit

Related Articles

Cybersecurity Interview Questions in 2025

JavaScript Interview Questions in 2025

System Design Interview Questions in 2025

Node.js Interview Questions in 2025