When we define the annotations @Component, @Service, @Controller, @Repository with the classes, Spring automatically scan and identifies those classes and register the Bean definition with the ApplicationContext.

Difference between @Component, @Repository, @Controller & @Service annotations in Spring:

1. The main difference between @Component, @Repository, @Controller & @Service annotations is that these are used for different classification.

2. In our application we can have different layers like presentation, service, business, data access etc. To annotate a class for auto-detection by Spring, we should use the respective annotations as below.

   @Component - generic and can be used across the application. It is a basic auto component scan annotation, it indicates annotated class is a auto scan component.
   @Service - annotate classes at service layer level ( Indicates a Service component in the business layer).
   @Controller - annotated class indicates that it is a controller components, and mainly used at presentation layer.
   @Repository - a data access object (Indicates DAO component in the persistence layer which will act as database repository).

3. We can use @Component for any auto component scan, but for better readability and better standards we should use respective annotations. @Repository, @Controller & @Service annotations serve as specializations of @Component annotation.

Example of @Service and @Repository:

DAO layer

Define the DAO class as below:

EmployeeDAO.java

package com.example.employee.dao;
@Repository
public class EmployeeDAO
{
     @Override
     public String toString() {
     return "Hello , We are in EmployeeDAO";
   }
}

Service layer

Define Service class as below:

EmployeeService.java

package com.example.employee.services;
import org.springframework.beans.factory.annotation.Autowired;
import com.example.employee.dao.EmployeeDAO;
@Service
public class EmployeeService
{
@Autowired
EmployeeDAO employeeDAO;

@Override
public String toString() {
  return "EmployeeService [employeeDAO=" + employeeDAO + "]";
}
}

Now define the below code in app-context.xml

<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:context="http://www.springframework.org/schema/context"
xsi:schemaLocation="http://www.springframework.org/schema/beans
http://www.springframework.org/schema/beans/spring-beans-2.5.xsd
http://www.springframework.org/schema/context
http://www.springframework.org/schema/context/spring-context-2.5.xsd">

<context:component-scan base-package="com.example"/>
</beans>

Now define the below class to run the code:

App.java

package com.example.common;
import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;
import com.example.employee.services.EmployeeService;
public class App
{
public static void main( String[] args )
{
     ApplicationContext context = new ClassPathXmlApplicationContext(new String[] {"app-context.xml"});
     EmployeeService emp = (EmployeeService)context.getBean("employeeService");
     System.out.println(emp);
}
}

Output

 EmployeeService [employeeDAO=Hello , This is EmployeeDAO]

Hope this will help you :)