BhrikutiSoft

@Configuration indicates that the class can be used by the Spring IoC container as a source of bean definitions. The @Bean annotation tells Spring that a method annotated with @Bean will return an object that should be registered as a bean in the Spring application context.

@Configuration is the heart of the Java-based configuration mechanism that was introduced in Spring 3. It provides an alternative to XML-based configuration.

Here is an example which will clear the concept of @Configuration.

Import com.mukesh;

Import org.springframework.context.annotation.*;

@Configuration

public class SpringConfigExample{

@Bean

public MyBean mybean(){

return new MyBean();

}

The above code is same as context file configuration of a bean class mybean. The method mybean() return a object which consider as bean class . Because of it annotated as @Bean .

<beans>

</beans>

Java Example :--

package com.mukesh;

import org.springframework.context.annotation.*;

@Configuration

public class SpringConfigExample {

@Bean

public MyBean mybean(){

return new MyBean();

}

Here is the content of MyBean.java file

package com.mukesh;

public class MyBean {

private String message;

public void setMessage(String message){

this.message = message;

}

public void getMessage(){

System.out.println("Your Message : " + message);

}

Following is the content of the MainApp.java file

package com.mukesh;

import org.springframework.context.ApplicationContext;

import org.springframework.context.annotation.*;

public class MainApp {

public static void main(String[] args) {

ApplicationContext ctx =

new AnnotationConfigApplicationContext(.class);

MyBean myBean = ctx.getBean(MyBean.class);

myBean.setMessage("Hello World!");

myBean.getMessage();

}

Here how you can use properties file using @Configuration

@Configuration

@PropertySource("classpath:root/test.props")

public class SampleConfig {

@Value("${test.prop}")

private String attr;

@Bean

public SampleService sampleService() {

return new SampleService(attr);

}

@Bean

public static PropertySourcesPlaceholderConfigurer

placeHolderConfigurer() {

return new PropertySourcesPlaceholderConfigurer();

}

Here we can use @Configuration annotation to import java class and xml file .

import org.springframework.context.annotation.Configuration;

import org.springframework.context.annotation.ImportResource;

@Configuration

@ImportResource("classpath:/config/spring.xml")

public class AppConfig {

}

AppConfig.java

import org.springframework.context.annotation.Configuration;

import org.springframework.context.annotation.ImportResource;

import org.springframework.context.annotation.Import;

@Configuration

@Import({ AppConfigWeb.class })

@ImportResource("classpath:/config/spring.xml")

public class AppConfig {

}

Here we know about @Component .

Indicates that an annotated class is a "component". Such classes are considered as candidates for auto-detection when using annotation-based configuration and classpath scanning.

We can directly use @Component for each and every bean, but for better understanding and maintainability of a large application, we use @Controller, @Service, @Repository.

Purpose of each annotation:

1) @Controller -> Classes annotated with this, are intended to receive a request from the client side. The first request comes to the Dispatcher Servlet, from where it passes the request to the particular controller using the value of @RequestMapping annotation.

2) @Service -> Classes annotated with this, are intended to manipulate data, that we receive from the client or fetch from the database. All the manipulation with data should be done in this layer.

3) @Repository -> Classes annotated with this, are intended to connect with database. It can also be considered as DAO(Data Access Object) layer. This layer should be restricted to CRUD (create, retrieve, update, delete) operations only. If any manipulation is required, data should be sent be send back to @Service layer.

If we interchange their place(use @Repository in place of @Controller), our application will work fine.

The main purpose of using three different @annotations is to provide better Modularity to the Enterprise application.