We’ll here focus on some minor differences among them.

First the Similarity

First point worth highlighting again is that with respect to scan-auto-detection and dependency injection for BeanDefinition all these annotations (viz., @Component, @Service, @Repository, @Controller) are the same. We can use one in place of another and can still get our way around.

Differences between @Component, @Repository, @Controller and @Service

@Component

This is a general-purpose stereotype annotation indicating that the class is a spring component.

What’s special about @Component
<context:component-scan> only scans @Component and does not look for @Controller, @Service and @Repository in general. They are scanned because they themselves are annotated with @Component.

Just take a look at @Controller, @Service and @Repository annotation definitions:

@Component
public @interface Service {
    ….
}

@Component
public @interface Repository {
    ….
}

@Component
public @interface Controller {
    …
}

Thus, it’s not wrong to say that @Controller, @Service and @Repository are special types of @Component annotation. <context:component-scan> picks them up and registers their following classes as beans, just as if they were annotated with @Component.

Special type annotations are also scanned, because they themselves are annotated with @Component annotation, which means they are also @Components. If we define our own custom annotation and annotate it with @Component, it will also get scanned with <context:component-scan>

@Repository

This is to indicate that the class defines a data repository.

What’s special about @Repository?

In addition to pointing out, that this is an Annotation based Configuration, @Repository’s job is to catch platform specific exceptions and re-throw them as one of Spring’s unified unchecked exception. For this, we’re provided with PersistenceExceptionTranslationPostProcessor, that we are required to add in our Spring’s application context like this:

<bean class="org.springframework.dao.annotation.PersistenceExceptionTranslationPostProcessor"/>

This bean post processor adds an advisor to any bean that’s annotated with @Repository so that any platform-specific exceptions are caught and then re-thrown as one of Spring’s unchecked data access exceptions.

@Controller

The @Controller annotation indicates that a particular class serves the role of a controller. The @Controller annotation acts as a stereotype for the annotated class, indicating its role.

What’s special about @Controller?

We cannot switch this annotation with any other like @Service or @Repository, even though they look same. The dispatcher scans the classes annotated with @Controller and detects methods annotated with @RequestMapping annotations within them. We can use @RequestMapping on/in only those methods whose classes are annotated with @Controller and it will NOT work with @Component, @Service, @Repository etc…

_{Note: If a class is already registered as a bean through any alternate method, like through @Bean or through @Component, @Service etc… annotations, then @RequestMapping can be picked if the class is also annotated with @RequestMapping annotation. But that’s a different scenario.}

@Service

@Service beans hold the business logic and call methods in the repository layer.

What’s special about @Service?

Apart from the fact that it’s used to indicate, that it’s holding the business logic, there’s nothing else noticeable in this annotation; but who knows, Spring may add some additional exceptional in future.

What else?

Similar to above, in the future Spring may add special functionalities for @Service, @Controller and @Repository based on their layering conventions. Hence, it’s always a good idea to respect the convention and use it in line with layers.