Q: What are Spring Boot starters? How do they work under the hood?

Answer:

A starter is a curated dependency descriptor — a single Maven/Gradle artifact that pulls in a coherent set of libraries for a use case (web, JPA, security, etc.).

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-web</artifactId>
</dependency>

That single line brings:

  • spring-webmvc, spring-web
  • jackson-databind (JSON)
  • tomcat-embed-core (embedded server)
  • spring-boot-starter-json, -tomcat, -validation
  • compatible versions tested together via the BOM (spring-boot-dependencies).

Why Starters Exist

Pre-Boot Spring meant manual version juggling: which spring-webmvc works with which jackson with which validator-api? Starters solve this — pick a Boot version → all transitive versions known-good.

How Auto-Configuration Hooks In

Each starter brings classes annotated with @AutoConfiguration (Boot 2.7+) or @Configuration + listed in META-INF/spring/org.springframework.boot.autoconfigure.AutoConfiguration.imports. Boot scans these on startup.

@AutoConfiguration
@ConditionalOnClass(DataSource.class)
@ConditionalOnMissingBean(DataSource.class)
public class DataSourceAutoConfiguration { ... }

@ConditionalOnX annotations gate configuration:

  • @ConditionalOnClass — class on classpath?
  • @ConditionalOnMissingBean — user hasn't defined their own?
  • @ConditionalOnProperty — config flag set?

Common Starters

StarterBrings
spring-boot-starter-webMVC, Tomcat, Jackson
spring-boot-starter-webfluxWebFlux + Netty
spring-boot-starter-data-jpaHibernate, Spring Data JPA
spring-boot-starter-data-redisLettuce + Spring Data Redis
spring-boot-starter-securitySpring Security
spring-boot-starter-actuatorHealth/metrics endpoints
spring-boot-starter-testJUnit 5, AssertJ, Mockito, Spring Test
spring-boot-starter-validationJakarta Bean Validation

Custom Starter (Library Authors)

  1. Module: acme-spring-boot-starter (just dependency aggregator).
  2. Module: acme-spring-boot-autoconfigure (the actual @AutoConfiguration classes).
  3. Register classes in META-INF/spring/org.springframework.boot.autoconfigure.AutoConfiguration.imports.
  4. Provide @ConfigurationProperties for tunables.
@AutoConfiguration
@ConditionalOnClass(AcmeClient.class)
@EnableConfigurationProperties(AcmeProperties.class)
public class AcmeAutoConfiguration {
    @Bean
    @ConditionalOnMissingBean
    AcmeClient acmeClient(AcmeProperties p) {
        return AcmeClient.builder().url(p.url()).build();
    }
}

Override / Disable

  • Add your own @Bean of the same type → Boot's auto-config backs off (@ConditionalOnMissingBean).
  • Disable explicitly: 
    @SpringBootApplication(exclude = DataSourceAutoConfiguration.class)
  • Property: spring.autoconfigure.exclude=....

Key Takeaway

Starter = "pull dependencies" + "trigger auto-config". You stop writing infrastructure beans; convention does it. Override anywhere via @Bean or properties.