@ComponentScan with multiple configuration class : Annotation Based Configuration

Question

As per Spring Doc-

Configures component scanning directives for use with @Configuration classes. Provides support parallel with Spring XML\'s

Answer 1

From the documentation:

Q1:

@Configuration is meta-annotated with @Component, therefore @Configuration classes are candidates for component scanning (typically using Spring XML's element) and therefore may also take advantage of @Autowired/@Inject like any regular @Component.

@Configuration classes may not only be bootstrapped using component scanning, but may also themselves configure component scanning using the @ComponentScan annotation:

@Configuration
@ComponentScan("com.acme.app.services")
public class AppConfig {
 // various @Bean definitions ...
}

Q2:

basePackageClasses

public abstract Class[] basePackageClasses

Type-safe alternative to basePackages() for specifying the packages to scan for annotated components. The package of each class specified will be scanned.

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Answer for your edited question

At some point you need to say to Spring where the beans your app needs are. You can use for example:

public static void main(String[] args) {  
    AnnotationConfigApplicationContext ctx = new AnnotationConfigApplicationContext();
    ctx.scan("com.acme");  
    ctx.refresh();
    MyService myService = ctx.getBean(MyService.class);
}

or using old xml style:

Using @ComponentScan at @Configuration level you are making sure all the dependencies for that config class are going to be available. Using @CompenentScan with basePackageClasses you are registering all the components available under the same package the class specified is.

If you have already let spring know what packages it needs to scan, in a different place or way, you do not need to use it. The piece of code at your Q2 is just an example of what spring is able to do.