@SpringBootApplication and Friends

The annotations that bootstrap your app and what each one actually enables.

Intermediate10 min read

Read these first

Properties, YAML, and Profiles

Why this matters

Knowing what each meta-annotation does lets you troubleshoot startup issues — like beans not being found or unwanted auto-config loading.
Boot-specific annotations (@ConditionalOn*, @ConfigurationProperties) are the vocabulary for custom starters and library integration.
Interview questions often ask what @SpringBootApplication actually contains.

spring-boot-starter-web

spring-boot-starterCore Boot + logging

spring-webmvcControllers and REST

tomcat-embed-coreEmbedded server

jackson-databindJSON serialisation

spring-boot-starter-web pulls in Tomcat, Jackson, Spring MVC and more with tested versions.

Decomposing @SpringBootApplication

Java

@SpringBootApplication
public class BookStoreApplication { ... }

Expands to three annotations:

Java

@SpringBootConfiguration   // alias for @Configuration
@EnableAutoConfiguration   // load auto-config classes from META-INF
@ComponentScan             // scan current package and below

Each serves a distinct purpose in the startup sequence.

Core bootstrap annotations

@SpringBootConfiguration — Marks the class as a source of @Bean definitions; typically only on the main class.
@EnableAutoConfiguration — Imports AutoConfigurationImportSelector which loads conditional config from META-INF/spring/.
@ComponentScan — Discovers @Component, @Service, @Repository, @Controller in the package tree.

Customising the scan root

Java

@SpringBootApplication(scanBasePackages = "com.example.bookstore")
public class BookStoreApplication { ... }

Exclude specific auto-configurations:

Java

@SpringBootApplication(exclude = {DataSourceAutoConfiguration.class})
public class BookStoreApplication { ... }

Useful when running BookStore without a database in a specific profile.

@Configuration and @Bean

Java

@Configuration
public class WebConfig {

    @Bean
    public FilterRegistrationBean<RequestLoggingFilter> loggingFilter() {
        FilterRegistrationBean<RequestLoggingFilter> bean = new FilterRegistrationBean<>();
        bean.setFilter(new RequestLoggingFilter());
        bean.addUrlPatterns("/api/*");
        return bean;
    }
}

@Configuration classes are processed at startup. @Bean methods register return values in the container. By default, @Bean methods are proxied so repeated calls return the same singleton.

Conditional annotations preview

Boot's auto-config relies on conditions you can use in your own config:

Java

@Configuration
@ConditionalOnProperty(name = "bookstore.cache.enabled", havingValue = "true")
public class CacheConfig {
    @Bean
    public CacheManager cacheManager() {
        return new ConcurrentMapCacheManager("books", "authors");
    }
}

The CacheManager bean exists only when the property is true. The auto-configuration module covers the full @ConditionalOn* family.

@ConfigurationProperties

Java

@ConfigurationProperties(prefix = "bookstore")
@Validated
public record BookStoreProperties(
    @NotNull Pricing pricing,
    @NotNull Catalog catalog
) {
    public record Pricing(String serviceUrl, int timeoutMs) {}
    public record Catalog(int pageSize) {}
}

Enable with @EnableConfigurationProperties(BookStoreProperties.class) or @ConfigurationPropertiesScan.

Web layer annotations

Java

@RestController
@RequestMapping("/api/books")
public class BookController {

    @GetMapping("/{id}")
    public BookResponse get(@PathVariable Long id) { ... }

    @PostMapping
    @ResponseStatus(HttpStatus.CREATED)
    public BookResponse create(@RequestBody @Valid CreateBookRequest request) { ... }
}

Annotation	Purpose
@GetMapping	Map HTTP GET
@PostMapping	Map HTTP POST
@PathVariable	Bind URI template variable
@RequestBody	Deserialize request body
@RequestParam	Bind query parameter
@ResponseStatus	Set HTTP status on response

@GetMapping
PurposeMap HTTP GET
@PostMapping
PurposeMap HTTP POST
@PathVariable
PurposeBind URI template variable
@RequestBody
PurposeDeserialize request body
@RequestParam
PurposeBind query parameter
@ResponseStatus
PurposeSet HTTP status on response

Transaction and caching annotations

Java

@Service
public class BookService {

    @Cacheable("books")
    public Book findById(Long id) { ... }

    @Transactional
    public Book updatePrice(Long id, BigDecimal price) { ... }
}

@Transactional starts a transaction boundary. @Cacheable stores return values in the configured cache — both are AOP proxies applied by Spring.

Testing annotations

Java

@WebMvcTest(BookController.class)
class BookControllerTest {
    @Autowired MockMvc mockMvc;
    @MockBean BookService bookService;
}

@WebMvcTest loads only the web layer. @MockBean replaces a container bean with a Mockito mock.

Quick recall

Everything you need if you only revisit this box.

@SpringBootApplication combines configuration, auto-config, and component scanning.
@Configuration + @Bean register objects the container cannot discover via scanning.
@ConditionalOnProperty and related annotations gate bean creation on environment state.
@ConfigurationProperties binds typed config from application.yml.
Web annotations (@GetMapping, @PathVariable) map HTTP to Java methods.
@Transactional and @Cacheable are declarative cross-cutting concerns applied via proxies.

Test yourself

Answer these before moving on — recall is what makes it stick.