java -jar starts the app.@Transactional, @Async and @Cacheable work through a proxy. A call from inside the same class skips the proxy, and the feature with it.SpringBootServletInitializer TipBoot doesn't replace Spring. It configures it, and you override what doesn't fit.
spring-boot-starter-webmvcspring-boot-starter-webdata-jpa, security, test or actuatorstart.spring.ioTipLeave the versions off. Boot's dependency management picks them.
@SpringBootConfiguration @EnableAutoConfiguration @ComponentScanTip@Spring combines all three.
TrapScanning starts at the main class's package. Put that class in a root package above your other classes.
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AskedExplain the @SpringBootApplication annotation.mvn spring-boot:run./gradlew bootRunmvn clean packagejava -jar app.jarjava -jarTipThe embedded server is Tomcat by default, or Jetty. Boot 4 dropped Undertow.
TrapThe server listens from step 5, so a request can arrive while the runners still work.
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AskedWhat happens when a Spring Boot application starts?ApplicationContext Creates singletons at startup, unlike BeanFactorySpringApplicationBuilder A child sees its parent's beans, not the reverseTrapCall new for a dependency and tests can't mock it.
@Autowiredrequired = false@Qualifier@Primary@Autowired@Inject@Inject is JSR-330, and @Named picks one@ResourceJSR-250, not on a constructorTipConstructors allow final fields and easy tests. Field injection hides what a class needs, so avoid it.
TrapEvery injection point is required by default, so startup fails when no bean matches.
@Component, or by layer @Service, @Repository, @Controller@BeanA method in a @Configuration class@ImportResourceTip@Repository also translates database exceptions into Data.
singletonprototyperequest, session, application, websocketTipA singleton is shared by every thread, so it should hold no changing state.
TrapA prototype injected into a singleton is injected once. Inject an ObjectProvider to get a fresh one.
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AskedExplain the different bean scopes in Spring.BeanFactoryPostProcessor BeanPostProcessor TrapPrototype beans get no shutdown callbacks. Spring doesn't track them.
BeanCurrentlyInCreationException @LazyA proxy fetches the bean on first callTipThe better fix removes the cycle. Move shared logic to a third bean, or publish an event.
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AskedHow does Spring handle circular dependencies?@Value("${app.name}") @ConfigurationProperties SERVER_PORT then sets server.portapplication-dev.properties @Profile("prod")--spring.profiles.active=prod SPRING_PROFILES_ACTIVE=prod TrapKeep secrets out of profile files. Supply them as environment variables, or from a secret store.
TrapAn external application. beats a packaged application-prod..
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AskedExplain Spring Boot's configuration precedence.logging.level.com.example=DEBUG logging.file.name=app.log TrapSpring extensions don't work in plain logback.xml. Use logback-spring..
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AskedExplain Spring Boot's logging configuration.@ConditionalOnClass @ConditionalOnMissingBean @ConditionalOnProperty spring.autoconfigure.exclude --debugIt prints the conditions reportTipYour configuration runs first, so @Conditional sees your bean and backs off.
@Conditional(MyCheck.class) MyCheck implements ConditionAutoConfiguration.imports spring.factories Removed in Boot 3Tip@Profile is built on @Conditional. Test your class with Application.
BufferingApplicationStartup Read Actuator's startup endpointspring.main.lazy-initialization=true TipHeavy @PostConstruct work blocks startup. Move it to first use or a background task.
TrapWith lazy initialization, a misconfigured bean fails on first use, not at startup.
DispatcherServlet HandlerMappingHandlerAdapterViewResolverHttpMessageConverter Trap@EnableWebMvc switches off Boot's MVC auto-configuration. Implement Web without it.
@ControllerIt returns a view name@RestController@Controller plus @ResponseBodyRouterFunction@GetMapping@PostMapping@PutMapping@PatchMapping@DeleteMappingTrap@RequestMapping with no method matches every HTTP method. Keep it for a class's path prefix, and use the shortcuts on methods.
@PathVariable Long id@RequestParam String q@RequestBody User userResponseEntityTrapParameters are required by default. A missing one, or one that won't convert, ends in 400 Bad Request.
@NotBlank@Valid@ControllerAdvice @ExceptionHandler ProblemDetailTipA failed check throws Method, and the client gets 400 Bad Request.
TrapThe web starter doesn't include validation. Add spring-boot-starter-validation yourself.
MultipartFileResourceWith Content-Disposition: attachmentTrapString doesn't stop path traversal. Check that the normalized path stays in the upload folder.
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AskedHow do you implement file upload and download in Spring Boot?RestClientRestTemplateDeprecated in Spring 7.1WebClientMono<User>Flux<User>WebFlux, on Netty by defaultTrapOne blocking call, such as plain JDBC, stalls an event loop thread and delays every request on it.
@Entity@IdJpaRepositoryCrudRepositoryTrapSince Spring Data 3.0, Paging brings no CRUD methods.
findByLastNameexistsByEmail@Query("SELECT u FROM User u")nativeQuery = true runs plain SQL@ModifyingRun it in a transactionPageableSpring MVC reads page, size, sortPage<User>Slice<User>TipSpring checks property names when it creates the repository, so a typo fails at startup.
TrapSpring doesn't validate sort values. Check them against allowed property names.
@Transactional(readOnly = true)Hibernate skips dirty checkingREQUIREDREQUIRES_NEWTrapOnly unchecked exceptions roll back by default. A checked exception commits unless rollbackFor names it.
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AskedExplain Spring Boot's transaction management.spring-boot-starter-flywayflyway-coreV2__Add_phone.sql TipWith migrations, set ddl-auto to validate or none, so Hibernate never changes the schema.
TrapNever edit a script after it has run, because Flyway compares checksums. Add a new script.
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AskedHow do you implement database migration in Spring Boot?final methodsTrapA final method runs on the proxy itself and skips the advice.
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AskedWhat is the difference between JDK dynamic proxy and CGLIB proxy?@Aspect holds the advice, and a pointcut picks the methods@Before@After@AfterReturning@AfterThrowing@AroundTrapForget proceed() in @Around and the method silently doesn't run.
@Cacheable@CachePut@CacheEvictTipTurn it on with @EnableCaching.
TrapThe default Concurrent cache never expires entries.
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AskedHow do you configure and use caching in Spring Boot?@EnableAsync@AsyncCompletableFuture ApplicationEventPublisher @EventListener@TransactionalEventListener TrapEvents are synchronous by default. publishEvent blocks until every listener finishes.
SecurityContextHolder keeps the resultformLoginhttpBasicIt needs HTTPSTipThe starter alone locks every endpoint behind a generated password.
TrapA logged-in user without the role gets 403 Forbidden, not a login redirect.
permitAll()hasRole("ADMIN") BCryptPasswordEncoder UserDetailsService @PreAuthorizeTipTurn CSRF off only for a stateless API that doesn't use cookies.
TrapSpring checks requestMatchers in order, so put specific paths before anyRequest().
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AskedHow do you implement security in Spring Boot?FilterChainProxy It runs only the first SecurityFilterChain that matchesExceptionTranslationFilter oauth2ResourceServer A JwtDecoder checks signature and expiryOncePerRequestFilter Use STATELESS sessionsTipAdd your own filter with addFilterBefore.
TrapSpring Boot also registers a @Component filter with the servlet container. Turn that off with set on a Filter.
@InjectMocksTest one class@WebMvcTestTest a controller@DataJpaTestTest a repository@SpringBootTestLoad it all in MOCK modeRANDOM_PORTStart the real serverTipThe other modes are DEFINED_PORT and NONE.
TrapWith a real server, a @Transactional test can't roll back what the server wrote.
@Mock@MockitoBean@MockBeanRemoved in Boot 4MockMvc@AutoConfigureTestRestTemplate Trap@WebMvcTest skips @Service and ordinary @Component beans, so you mock each dependency.
/actuator/health /actuator/metrics /actuator/httpexchanges Boot 3 renamed it from httptracemanagement.endpoints.web.exposure.include HealthIndicatorTrapOnly health is exposed over HTTP by default. In production, set show-details to when-authorized.
@EnableConfigServer @CircuitBreakerIt returns a fallbackTrapWithout a circuit breaker, one failing service can make its callers fail too.
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