Spring boot auto configuration with dependency and without @ComponentScan

Question

Spring boot provides @ComponentScan to find packages to be scanned.

I am building a library which has @RestControllers inside with package com

Answer 1

Spring boot starter are special artifacts designed by Spring and used by Spring.
You can check that in the source code that contains mainly a
spring.provides file :

provides: spring-boot-actuator,micrometer-core

I don't know the exact way to process in the same way as Spring Boot Starter but as probably acceptable workaround, you could define in your jar a @Configuration class that specifies @ComponentScan("com.mylib").

 @Configuration
 @ComponentScan("com.mylib")
 public class MyLibConfig {
     //...
 }

In this way, clients of the jar need "only" to import the @Configuration class :

@Import(MyLibConfig.class)
@Configuration
public class ClientConfig{
  //...
}

Answer 2

You could create a Spring Boot Starter in the same style as the Spring Provided Starters. They are essentially a jar'd library with a a spring.factories file pointing to the @Configuration class to load with some other annotations on there to provide overriding/bean back off (@ConditionalOnMissingBean) and generally provide their own @ConfigurationProperties.

Stéphane Nicoll provided an excellent demo of how to build one.

https://github.com/snicoll-demos/hello-service-auto-configuration

It is also documented in the Spring Boot documentation. https://docs.spring.io/spring-boot/docs/current/reference/html/boot-features-developing-auto-configuration.html

The library approach would also work but I see no benefit in not making it a starter. Additionally for any library/starter I'd recommend dropping the @ComponentScan and just defining the beans in a @Configuration. This will work for sterotypes like @RestController etc. will function as normal if you create an @Bean out of it in a configuration.