Spring Boot: ComponentScan vs Declared Bean in an Autoconfigured Jar

Question

Suppose I have a jar with a Spring Component called MyComponent. This jar is a Spring Boot "autoconfigured" jar, meaning that it has a Configuration class (annotated with @Configuration), and additionally, a META-INF/spring.factories file on the classpath. This jar is not an executable jar by itself; it is a library that is meant for inclusion in a Spring Boot application.

These files look as follows:

MyComponent.java, in package com.mine.components:

@Component
public class MyComponent {

    private static final Logger logger = LoggerFactory.getLogger(MyComponent.class);

    @PostConstruct
    public void init() {
        logger.info("MyComponent inited");
    }
}

MyConfiguration.java, in package com.mine.config:

@Configuration
@ComponentScan(basePackages = "com.mine.components")
public class MyConfiguration {
}

spring.factories, in META-INF under src/main/resources:

org.springframework.boot.autoconfigure.EnableAutoConfiguration=com.mine.config.MyConfiguration

If I include this jar in a Spring Boot project with the above three files, the MyComponent component is NOT detected (the log message never prints).

But if I instead remove the @ComponentScan and declare MyComponent using the @Bean annotation as follows, it is detected:

@Bean
public MyComponent myComponent() {
    return new MyComponent();
}

Why?

Answer 1

Difference beetween ComponentScan and declared Bean inside @Configuration class:

@ComponentScan: You enable auto-scanning (default using current folder path), optionally you can specify an basePackage where spring will found yours beans.

@ComponentScan(basePackages = "com.mine.components") You're saying to Spring that in this package("com.mine.components"), you'll define yours beans typically using annotations (@Component, @Entity, @Controller, @Service, @Repository or more).

@Bean: This way you define your beans manually inside @Configuration class, but Spring has to discover your configuration class, usually using @ComponentScan, @SpringBootApplication.

META-INF/spring.factories: you define an custom autoconfiguration