Top 100 Spring Boot Interview Questions with Concise Answers

Basics & Core Concepts

1. What is Spring Boot?
   Spring Boot is an opinionated framework on top of the Spring Framework that simplifies building production-ready, stand-alone Spring applications via auto-configuration, starter dependencies, and embedded servers.

2. How is Spring Boot different from the Spring Framework?
   Spring provides core features like DI and AOP but requires manual configuration. Spring Boot adds auto-configuration, embedded servers, starters, and Actuator for rapid development with minimal setup.

3. What does the @SpringBootApplication annotation do?
   It is a meta-annotation combining @SpringBootConfiguration (or @Configuration), @EnableAutoConfiguration, and @ComponentScan. It bootstraps the application and enables auto-configuration and component scanning.

4. What is auto-configuration in Spring Boot?
   It automatically configures beans based on the classpath, properties, and conditions (e.g., @ConditionalOnClass, @ConditionalOnMissingBean) so you only override defaults when needed.

5. How does Spring Boot find auto-configuration classes?
   From META-INF/spring/org.springframework.boot.autoconfigure.AutoConfiguration.imports (or the older spring.factories in earlier versions). Classes are loaded conditionally.

6. What are Spring Boot starters?
   Curated dependency descriptors (POMs) that pull in a compatible set of libraries for a feature, e.g., spring-boot-starter-web for MVC + Tomcat + Jackson.

7. What is the spring-boot-starter-parent?
   A parent POM that manages dependency versions via the Spring Boot BOM and provides sensible defaults for plugins and properties.

8. How do you create a Spring Boot project?
   Primarily via Spring Initializr (start.spring.io), or manually with Maven/Gradle, or using the Spring Boot CLI for Groovy.

9. What is an embedded server in Spring Boot?
   A server like Tomcat (default), Jetty, or Undertow bundled inside the application so it runs as a standalone executable JAR without an external container.

10. What is the default embedded server and how do you change it?
    Tomcat. Exclude spring-boot-starter-tomcat and add spring-boot-starter-jetty (or Undertow, though Undertow support was removed in Boot 4).

Configuration & Profiles

11. What is application.properties / application.yml used for?
    Externalized configuration for server port, database, logging, custom properties, etc. YAML supports nested structures better.

12. What is the order of precedence for configuration properties?
    Command-line args > Java system properties > OS environment variables > profile-specific files > default application.properties/yml > Spring Boot defaults.

13. How do you override default properties?
    Via application.properties/yml, environment variables, command-line arguments, or @TestPropertySource in tests.

14. What are Spring profiles?
    Named sets of configuration and beans for different environments (dev, test, prod). Activate with spring.profiles.active.

15. How do you activate a profile?
    spring.profiles.active=prod in properties, --spring.profiles.active=prod on the command line, or SPRING_PROFILES_ACTIVE environment variable.

16. What is @Value annotation?
    Injects individual property values from the Environment into fields or parameters.

17. What is @ConfigurationProperties?
    Binds a group of related properties (with a prefix) to a POJO, supporting nested objects, lists, and validation. Preferred over multiple @Value for structured config.

18. What is externalized configuration?
    Configuration outside the code (files, env vars, args) so the same artifact can run in different environments without recompilation.

19. How do you disable auto-configuration for a specific class?
    Use @SpringBootApplication(exclude = SomeAutoConfiguration.class) or spring.autoconfigure.exclude property.

20. What is the purpose of spring.main.web-application-type=none?
    Disables the embedded web server for non-web (batch/CLI) applications.

Dependency Injection & Beans

21. Why is constructor injection preferred?
    Makes dependencies explicit and immutable (final), fails fast if missing, and is easier to unit-test without the Spring container. Single-constructor classes don't need @Autowired (since Spring 4.3).

22. What are the differences between @Component, @Service, @Repository, and @Controller?
    All are @Component stereotypes. @Service is for business logic, @Repository enables persistence exception translation, @Controller/@RestController for web handlers. Semantic + tooling differences.

23. When do you use @Bean vs @Component?
    @Component (and stereotypes) for classes you control that are scanned. @Bean methods in @Configuration classes for third-party objects or when custom construction logic is needed.

24. What are bean scopes in Spring?
    singleton (default, one per container), prototype (new instance each request), and web scopes: request, session, application, websocket.

25. What do @Primary and @Qualifier do?
    @Primary marks the preferred bean when multiple candidates exist. @Qualifier explicitly selects a named bean, overriding @Primary.

26. What is the difference between CommandLineRunner and ApplicationRunner?
    Both execute after the context is ready. CommandLineRunner receives raw String... args; ApplicationRunner receives structured ApplicationArguments. Use @Order for sequencing.

27. What is lazy initialization?
    Beans are created only when first requested (@Lazy or spring.main.lazy-initialization=true). Useful for faster startup but can hide errors until runtime.

28. What is the difference between @Autowired and @Resource?
    @Autowired is by type (Spring). @Resource is by name (JSR-250). Prefer constructor injection with @Autowired or implicit single constructor.

29. How does Spring handle circular dependencies?
    By default, it fails for constructor injection. Field/setter injection can sometimes resolve via proxies, but circular dependencies indicate design issues and should be avoided.

30. What is @ConditionalOnMissingBean?
    A condition that creates a bean only if no bean of the same type (or name) already exists, allowing user configuration to override auto-configuration.

REST & Web Layer

31. What is the difference between @Controller and @RestController?
    @RestController = @Controller + @ResponseBody. Methods return data (JSON/XML) directly instead of view names.

32. Explain @RequestParam, @PathVariable, and @RequestBody.
    @PathVariable extracts from URI path (/users/{id}). @RequestParam from query/form parameters. @RequestBody deserializes the request body into an object.

33. How do you handle exceptions globally?
    Use @RestControllerAdvice (or @ControllerAdvice) with @ExceptionHandler methods. Return ProblemDetail (RFC 9457) for standardized errors. Enable with spring.mvc.problemdetails.enabled=true.

34. How does request validation work?
    Add spring-boot-starter-validation. Annotate DTOs with constraints (@NotNull, @Size, etc.) and use @Valid on controller parameters. Failures throw MethodArgumentNotValidException.

35. What is the difference between @Valid and @Validated?
    @Valid is the standard Bean Validation trigger. @Validated (Spring) supports validation groups and method-level validation.

36. How do you enable CORS?
    Class/method @CrossOrigin, global WebMvcConfigurer.addCorsMappings, or CorsConfigurationSource in the security filter chain. Prefer specific origins over *.

37. What is content negotiation?
    Spring selects the response format (JSON, XML) based on the Accept header or URL extension, using HttpMessageConverters.

38. How do you return a custom HTTP status?
    Use ResponseEntity.status(HttpStatus.CREATED).body(...) or @ResponseStatus on methods/exceptions.

39. What is the difference between @GetMapping and @RequestMapping?
    @GetMapping is a composed annotation for HTTP GET only. @RequestMapping supports all methods and is more general.

40. How do you consume/produce specific media types?
    Use consumes and produces attributes on mapping annotations, or Content-Type/Accept headers.

Data Access & JPA

41. What does spring-boot-starter-data-jpa provide?
    Spring Data JPA, Hibernate, HikariCP connection pool, and auto-configuration of DataSource, EntityManagerFactory, and transaction manager.

42. What is the difference between CrudRepository and JpaRepository?
    CrudRepository provides basic CRUD. JpaRepository extends it with JPA-specific features like flush(), batch operations, and pagination support.

43. How do derived query methods work?
    Method names like findByEmailAndActiveTrue are parsed by Spring Data to generate queries automatically.

44. What is the N+1 query problem and how do you fix it?
    Loading a collection triggers one query per association due to lazy loading. Fix with JOIN FETCH, @EntityGraph, batch fetching, or DTO projections. Disable open-in-view if possible.

45. What does @Transactional do?
    Declares that a method runs inside a transaction. By default, rolls back on unchecked exceptions. Supports propagation, isolation, and timeout attributes.

46. What are common @Transactional pitfalls?
    Self-invocation bypasses the proxy (no transaction), only public methods are proxied, and default rollback is only for RuntimeExceptions. Prefer short, focused transactions.

47. What is transaction propagation?
    Defines how a transactional method joins an existing transaction. Common: REQUIRED (default, join or create), REQUIRES_NEW (suspend and create new), SUPPORTS, NOT_SUPPORTED, MANDATORY, NEVER.

48. How do you manage schema migrations?
    Use Flyway or Liquibase. Place versioned scripts in db/migration. Spring Boot auto-runs them on startup. Avoid ddl-auto=update in production.

49. What is spring.jpa.hibernate.ddl-auto?
    Controls Hibernate schema generation: none (default for non-embedded), validate, update, create, create-drop. Prefer migrations over update.

50. What is the difference between JPA and Hibernate?
    JPA is the specification (API). Hibernate is the most common implementation.

Security

51. What happens when you add spring-boot-starter-security?
    All endpoints require authentication by default. A default user is created with a random password logged at startup. Form login and HTTP Basic are enabled.

52. How do you configure security in modern Spring Boot (no WebSecurityConfigurerAdapter)?
    Define a SecurityFilterChain bean using the lambda DSL: http.authorizeHttpRequests(...).oauth2ResourceServer(...).csrf(...).

53. How do you secure a REST API with JWT?
    Use spring-boot-starter-oauth2-resource-server, configure issuer-uri or jwk-set-uri, and enable JWT validation in the security filter chain. Stateless, no sessions.

54. When should you disable CSRF?
    For stateless APIs using bearer tokens (JWT) in headers, as CSRF protects cookie-based sessions. Keep it enabled for browser form-based apps.

55. How does method-level security work?
    Enable with @EnableMethodSecurity. Use @PreAuthorize("hasRole('ADMIN')") or SpEL expressions. Applied via AOP proxies (self-invocation caveat applies).

56. What is the difference between authentication and authorization?
    Authentication verifies identity (who you are). Authorization checks permissions (what you can do).

57. How do you implement custom authentication?
    Implement AuthenticationProvider or use OAuth2/JWT resource server configuration, or custom filters in the security chain.

58. What is Spring Security's filter chain?
    A chain of filters that handle authentication, authorization, CSRF, CORS, etc. Order matters; customize via SecurityFilterChain.

Testing

59. What does @SpringBootTest do?
    Loads the full application context. Can start an embedded server. Useful for integration tests but slower than slice tests.

60. What is the difference between @WebMvcTest, @DataJpaTest, and @SpringBootTest?
    @WebMvcTest slices the web layer (controllers + MockMvc). @DataJpaTest slices the JPA layer (repositories + in-memory DB). @SpringBootTest loads everything.

61. How do you test REST controllers?
    Use @WebMvcTest + MockMvc to perform requests and assert status, content, and headers without starting a full server.

62. What is @MockBean (and its replacement in Boot 4)?
    Replaces a bean with a Mockito mock in the context. In Spring Boot 4, use @MockitoBean (or @MockitoSpyBean).

63. How do you test with profiles?
    Use @ActiveProfiles("test") or @SpringBootTest(properties = "spring.profiles.active=test").

Actuator, Monitoring & Production

64. What is Spring Boot Actuator?
    Provides production-ready endpoints for health, metrics, info, environment, loggers, etc. (/actuator/health, /actuator/metrics).

65. How do you expose Actuator endpoints?
    By default only health and info are exposed. Use management.endpoints.web.exposure.include=* (carefully) or specific ones. Secure them.

66. What is the health endpoint?
    Aggregates health indicators (DB, disk space, custom). Returns UP/DOWN. Supports liveness and readiness probes for Kubernetes.

67. How do you expose metrics?
    Actuator + Micrometer. Common registries: Prometheus (/actuator/prometheus), Datadog, etc. Use @Timed or custom meters.

68. What is graceful shutdown?
    Enabled by default in recent versions. Allows in-flight requests to complete before shutdown. Configure with server.shutdown=graceful and timeout.

69. How do you enable HTTP/2?
    Set server.http2.enabled=true (requires HTTPS in most cases).

Advanced Topics

70. What is @Async and how do you enable it?
    Runs a method in a separate thread. Enable with @EnableAsync. Configure a custom TaskExecutor for control. Return CompletableFuture or void.

71. How do you implement caching?
    Add spring-boot-starter-cache, @EnableCaching, and use @Cacheable, @CachePut, @CacheEvict. Supports Caffeine, Redis, Ehcache, etc.

72. How do you schedule tasks?
    @EnableScheduling + @Scheduled(fixedRate=..., fixedDelay=..., cron=...) on methods. Use a custom TaskScheduler for control.

73. What is the difference between fixedRate and fixedDelay?
    fixedRate: interval between method starts. fixedDelay: interval between method completion and next start.

74. How do you use WebClient vs RestTemplate vs RestClient?
    RestTemplate is legacy blocking. RestClient (Boot 3.2+) is the modern blocking fluent API. WebClient is reactive/non-blocking (WebFlux).

75. What is Spring WebFlux?
    Reactive web framework on Project Reactor. Non-blocking, suitable for high-concurrency, streaming, or backpressure scenarios. Uses Netty by default.

76. How do you configure multiple data sources?
    Define multiple DataSource beans with @Primary and @Qualifier, plus separate EntityManagerFactory and TransactionManager configurations.

77. What is @ConditionalOnProperty?
    Creates a bean only if a property matches a value (or is present). Useful for feature flags.

78. How does DevTools work?
    Provides automatic restart on classpath changes, LiveReload, and remote debugging support. Exclude from production.

79. What is the difference between JAR and WAR packaging?
    JAR with embedded server is preferred (standalone). WAR is for deployment to an external servlet container.

80. How do you package a Spring Boot app as a Docker image?
    Use the Spring Boot Maven/Gradle plugin's build-image goal (Cloud Native Buildpacks) or a Dockerfile with multi-stage builds.

Microservices & Cloud

81. What is Spring Cloud?
    A set of tools for building distributed systems on Spring Boot: service discovery, config server, circuit breakers, API gateway, etc.

82. What is service discovery?
    Dynamic registration and lookup of service instances (e.g., Eureka, Consul, or Kubernetes DNS) so clients don't hardcode addresses.

83. What is a circuit breaker?
    A resilience pattern (Resilience4j) that fails fast and provides fallbacks when a downstream service is unhealthy, preventing cascade failures.

84. What is Spring Cloud Config?
    Centralized external configuration server that serves properties from Git or other backends, with support for encryption and profiles.

85. What is an API Gateway in Spring Cloud?
    Spring Cloud Gateway routes requests, applies filters (auth, rate limiting, path rewriting), and acts as the single entry point.

86. How do you implement distributed tracing?
    Micrometer Tracing + OpenTelemetry (or Zipkin/Brave). Propagates trace IDs across services via headers.

Recent Changes & Best Practices (Boot 3/4)

87. What are the key changes in Spring Boot 3?
    Jakarta EE namespace (jakarta.*), Java 17 minimum, native image support, ProblemDetail errors, virtual threads support (Java 21), Micrometer Observation.

88. What are major changes in Spring Boot 4?
    Spring Framework 7, Jackson 3 (tools.jackson), modular starters (e.g., spring-boot-starter-webmvc), Undertow removed, JUnit 4 removed, @MockBean replaced by @MockitoBean, built-in retry, API versioning support.

89. Why was Undertow removed in Boot 4?
    Lack of support for Servlet 6.1 (Jakarta EE 11). Use Tomcat 11 or Jetty 12.

90. How do you handle Jackson 3 migration?
    Package change from com.fasterxml.jackson to tools.jackson. Update imports; optionally set spring.jackson.use-jackson2-defaults=true for compatibility.

91. What is the recommended migration path to Boot 4?
    Upgrade to Boot 3.5 first, eliminate all deprecation warnings, then migrate to 4.x. Use the properties migrator dependency temporarily.

92. What is native image support?
    GraalVM native compilation for faster startup and lower memory. Requires careful configuration of reflection, resources, and dynamic proxies.

93. How do you enable virtual threads?
    spring.threads.virtual.enabled=true (Java 21+). Suitable for I/O-bound workloads; be cautious with synchronized blocks and pinning.

94. What is API versioning support in Boot 4?
    Native support via @GetMapping(version = "2.0") or headers/paths, with auto-configuration for MVC and WebFlux.

95. Why prefer RestClient over RestTemplate?
    RestTemplate is in maintenance mode. RestClient offers a modern, fluent, builder-style API with better defaults and is the recommended blocking client.

Miscellaneous & Practical

96. How do you log in Spring Boot?
    SLF4J + Logback (default). Configure levels in properties (logging.level.com.example=DEBUG). Use structured logging for production.

97. What is the difference between @ControllerAdvice and @RestControllerAdvice?
    @RestControllerAdvice is @ControllerAdvice + @ResponseBody, so handlers return data directly (ideal for REST).

98. How do you customize the error response?
    Implement ErrorController, use @ControllerAdvice, or rely on ProblemDetail. Override BasicErrorController if needed.

99. What is the purpose of @EnableAutoConfiguration?
    Triggers auto-configuration. Usually not used directly because it is included in @SpringBootApplication.

100. What are best practices for Spring Boot applications?
     Prefer constructor injection, keep transactions short, use profiles, externalize config, secure Actuator, use Flyway/Liquibase, write slice tests, monitor with Actuator + metrics, and follow the principle of convention over configuration while overriding only when necessary.