Auto-configuration and Starters

Starters are dependency bundles (e.g., the web starter brings Spring MVC, Jackson and Tomcat). Auto-configuration classes create beans conditionally (@ConditionalOnClass, @ConditionalOnMissingBean, @ConditionalOnProperty) — for example a DataSource when a JDBC driver is on the classpath and a URL is configured. Defining your own bean of the same type usually makes Boot back off. Run with --debug to see the condition evaluation report.

// Override Boot's default ObjectMapper customization without replacing everything
@Configuration
class JacksonConfig {
    @Bean
    Jackson2ObjectMapperBuilderCustomizer customizer() {      // Boot 3.x API; Boot 4 uses Jackson 3 builder customizers
        return builder -> builder.failOnUnknownProperties(true);
    }
}

// Writing your own conditional auto-config (in a shared library)
@AutoConfiguration
@ConditionalOnClass(AuditClient.class)
@EnableConfigurationProperties(AuditProperties.class)
public class AuditAutoConfiguration {
    @Bean
    @ConditionalOnMissingBean
    AuditClient auditClient(AuditProperties props) {
        return new AuditClient(props.endpoint());
    }
}

Register library auto-configurations in META-INF/spring/org.springframework.boot.autoconfigure.AutoConfiguration.imports.

Common pitfalls

• Replacing a whole auto-configured bean when a customizer would do.
• Excluding auto-configurations blindly to 'fix' startup errors.
• Not checking the condition report when a bean is unexpectedly missing.

Key points

• Starters bundle dependencies
• Conditional beans back off when you define your own
• --debug shows the condition report

Further reading: Spring Boot: Auto-configuration