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

The @Component annotations are:

@Controlller - a web controller ( Indicates a controller component in the presentation laye).

@Service - a business service facade ( Indicates a Service component in the business layer).

@Repository - a data access object (Indicates DAO component in the persistence layer).

@Autowired- to autowire the bean.

Example

In the below example we can show the Bean Autowiring using the @Autowired Annotation and it can auto detect the Department Bean and wire them to the Employee Bean. There is no need to use the ref=dept in the bean tag of Department. We can easily get the reference of Department and print the name of Department Name.

Employee.java

package com.babita;

import org.springframework.beans.factory.annotation.Autowired;

public class Employee {

    private int id;
    private String name;

    public Department getDepartment() {
        return department;
    }
    public void setDepartment(Department department) {
        this.department = department;
    }
    @Autowired
    private Department department;

    public int getId() {
        return id;
    }
    public void setId(int id) {
        this.id = id;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }


}

Department.java

package com.babita;

public class Department {

    private String name;

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }
}

Now define the below code in app-context.xml

app-context.xml

<?xml version="1.0" encoding="UTF-8"?>

<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-3.0.xsd
    http://www.springframework.org/schema/context
    http://www.springframework.org/schema/context/spring-context-3.0.xsd">

   <context:annotation-config/>

   <bean id="dep" class="com.babita.Department">
       <property name="name" value="java"></property>
   </bean>

   <bean id="emp" class="com.babita.Employee">
           <property name="name" value="babita"></property>
           <property name="id" value="001"></property>
   </bean>
</beans>

Now define the below class to run the code:

App.java

package com.app;

import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;

import com.babita.Department;
import com.babita.Employee;
import com.manish.Course;
import com.manish.Student;

public class App {

    /**
     * @param args
     */
    public static void main(String[] args) {

        ApplicationContext ctx=new ClassPathXmlApplicationContext(new String[]{"app-context.xml"});
        Employee emp=(Employee)ctx.getBean("emp");
        Department dep=emp.getDepartment();
        System.out.println("Department Name:="+dep.getName());
    }
}

Output:

Department Name:=java

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 :)