Difference between register() and @ComponentScan

There is no difference. It are different ways of registering a bean in the Spring application context. Like you can use XML or Java based configuration. In the end it is all metadata used to construct the ApplicationContext. The one with the registerBean is considered the (sort of) functional style of registering beans. This is mainly being utilized to be able to write the Kotlin bean registration DSL (AFAIK).

Although they work differently, i.e. registering components directly as opposed to scan the classpath for beans that match the selection criteria (default is to detect @Component). The end result is the same.

For all of them a BeanDefinition is being created by Spring. The exact type of BeanDefinition will differ based on the type of configuration. For XML and Properties based it will, generally, be a RootBeanDefinition, when detected with component scanning a ScannedBeanDefinition etc. For the registerBean it will be a ClassDerivedBeanDefinition.

BeanDefinitions are the recipes on how to create beans.

All the different ways as shown below will have the same end result. An ApplicationContext with a FooService.

public class FooService { ... }

XML

<bean id="fooService" class="FooService"/>

XML with Component Scan

@Service
public class FooService { ... }

<context:component-scan />

Java Config

@Bean
public FooService fooService() {
  return new FooService();
}

Java Config with Component Scan

@Service
public class FooService { ... }

@ComponentScan
public FooConfig {}

Functional Style DSL

context.registerbean("fooService", FooService.class, () -> new FooService());

Properties Based

fooService.(class)=FooService

Then use the PropertiesBeanDefinitionReader to load it (going to be deprecated or removed as of Spring 5.3).

Basically, in simple terms :

• @ComponentScan search for all the classes annotated with @Component, @Service, @Repository & @Controller and creates and register their beans in IOC Container from a package you define.

• Whereas when you call register(Class<?>... componentClasses) method from AnnotationConfigApplicationContext class, you need to define which classes (annotated with @Component, @Service, @Repository & @Controller) you want to create & register beans in IOC container.

So, you can say that @ComponentScan is package-based bean registration whereas register(Class<?>... componentClasses) method is class based bean registration.