How to write your own auto-configuration class.

Writing your own auto-configuration means a @AutoConfiguration class, guarded by @Conditional annotations, listed in AutoConfiguration.imports. This is exactly how you'd build a reusable library or company starter that "just works" when added to a project.

Step 1 — the configuration class

Use @AutoConfiguration (Boot 3's dedicated annotation, a @Configuration specialization that also carries ordering metadata). Guard it so it backs off correctly:

@AutoConfiguration
@ConditionalOnClass(AcmeClient.class)              // only if the library is present
@EnableConfigurationProperties(AcmeProperties.class)
public class AcmeAutoConfiguration {

    @Bean
    @ConditionalOnMissingBean                       // user can override
    public AcmeClient acmeClient(AcmeProperties props) {
        return AcmeClient.builder()
                .endpoint(props.getEndpoint())
                .timeout(props.getTimeout())
                .build();
    }
}

@ConfigurationProperties(prefix = "acme")
public class AcmeProperties {
    private String endpoint = "https://api.acme.com";
    private Duration timeout = Duration.ofSeconds(5);
    // getters/setters
}

Step 2 — register it in the imports file

Boot won't find the class unless it's listed:

# src/main/resources/META-INF/spring/
#   org.springframework.boot.autoconfigure.AutoConfiguration.imports
com.acme.autoconfigure.AcmeAutoConfiguration

(Add spring-boot-autoconfigure-processor and the file is generated for you.)

Step 3 — control ordering

@AutoConfiguration takes before/after, and you can use @AutoConfigureBefore, @AutoConfigureAfter, @AutoConfigureOrder for finer control. Order matters when your config depends on another's beans existing first — e.g. you need the DataSource before configuring your own component:

@AutoConfiguration(after = DataSourceAutoConfiguration.class)
@ConditionalOnBean(DataSource.class)
public class AcmeJdbcAutoConfiguration { ... }

The design rules that make it well-behaved

• @ConditionalOnClass at the top so the class is inert when the library is absent (never throws NoClassDefFoundError).
• @ConditionalOnMissingBean on every @Bean so a user's own definition wins.
• @ConfigurationProperties for configuration, not scattered @Value.
• Don't @ComponentScan from an auto-config class — declare beans explicitly so you never accidentally pull in the user's packages.
• Ship it as the autoconfigure module of a starter so consumers get the library + config in one dependency.