Spring Auto scanning components

Normally you declare all the beans or components in XML bean configuration file, so that Spring container can detect and register your beans or components. Actually, Spring is able to auto scan, detect and instantiate your beans from pre-defined project package, no more tedious beans declaration in in XML file.

Following is a simple Spring project, including a customer service and dao layer. Let’s explore the different between declare components manually and auto components scanning in Spring.

1. Declares Components Manually

See a normal way to declare a bean in Spring.

Normal bean.

package com.mkyong.customer.dao;
 
public class CustomerDAO 
{
	@Override
	public String toString() {
		return "Hello , This is CustomerDAO";
	}	
}

DAO layer.

package com.mkyong.customer.services;
 
import com.mkyong.customer.dao.CustomerDAO;
 
public class CustomerService 
{
	CustomerDAO customerDAO;
 
	public void setCustomerDAO(CustomerDAO customerDAO) {
		this.customerDAO = customerDAO;
	}
 
	@Override
	public String toString() {
		return "CustomerService [customerDAO=" + customerDAO + "]";
	}
 
}

Bean configuration file (Spring-Customer.xml), a normal bean configuration in Spring.

<beans xmlns="http://www.springframework.org/schema/beans"
	xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
	xsi:schemaLocation="http://www.springframework.org/schema/beans
	http://www.springframework.org/schema/beans/spring-beans-2.5.xsd">
 
	<bean id="customerService" class="com.mkyong.customer.services.CustomerService">
		<property name="customerDAO" ref="customerDAO" />
	</bean>
 
	<bean id="customerDAO" class="com.mkyong.customer.dao.CustomerDAO" />
 
</beans>

Run it

package com.mkyong.common;
 
import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;
 
import com.mkyong.customer.services.CustomerService;
 
public class App 
{
    public static void main( String[] args )
    {
    	ApplicationContext context = 
    	  new ClassPathXmlApplicationContext(new String[] {"Spring-Customer.xml"});
 
    	CustomerService cust = (CustomerService)context.getBean("customerService");
    	System.out.println(cust);
 
    }
}

output

CustomerService [customerDAO=Hello , This is CustomerDAO]

2. Auto Components Scanning

Now, enable Spring auto component scanning features.

Annotate with @Component to indicate this is class is an auto scan component.

package com.mkyong.customer.dao;
 
import org.springframework.stereotype.Component;
 
@Component
public class CustomerDAO 
{
	@Override
	public String toString() {
		return "Hello , This is CustomerDAO";
	}	
}

DAO layer, add @Component to indicate this is an auto scan component also.

package com.mkyong.customer.services;
 
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Component;
 
import com.mkyong.customer.dao.CustomerDAO;
 
@Component
public class CustomerService 
{
	@Autowired
	CustomerDAO customerDAO;
 
	@Override
	public String toString() {
		return "CustomerService [customerDAO=" + customerDAO + "]";
	}
}

Put this “context:component” in bean configuration file, it means, enable auto scanning feature in Spring. The base-package is indicate where are your components stored, Spring will scan this folder and find out the bean (annotated with @Component) and register it in Spring container.

<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.mkyong.customer" />
 
</beans>

Run it

package com.mkyong.common;
 
import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;
 
import com.mkyong.customer.services.CustomerService;
 
public class App 
{
    public static void main( String[] args )
    {
    	ApplicationContext context = 
    	   new ClassPathXmlApplicationContext(new String[] {"Spring-AutoScan.xml"});
 
    	CustomerService cust = (CustomerService)context.getBean("customerService");
    	System.out.println(cust);
 
    }
}

output

CustomerService [customerDAO=Hello , This is CustomerDAO]

This is how auto components scanning works in Spring.

Custom auto scan component name

By default, Spring will lower case the first character of the component – from ‘CustomerService’ to ‘customerService’. And you can retrieve this component with name ‘customerService’.

	CustomerService cust = (CustomerService)context.getBean("customerService");

To create a custom name for component, you can put custom name like this :

@Service("AAA")
public class CustomerService 
...

Now, you can retrieve it with this name ‘AAA’.

	CustomerService cust = (CustomerService)context.getBean("AAA");

Auto Components Scan Annotation Types

In Spring 2.5, there are 4 types of auto components scan annotation types

  • @Component – Indicates a auto scan component.
  • @Repository – Indicates DAO component in the persistence layer.
  • @Service – Indicates a Service component in the business layer.
  • @Controller – Indicates a controller component in the presentation layer.

So, which one to use? It’s really doesn’t matter. Let see the source code of @Repository,@Service or @Controller.

@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Component
public @interface Repository {
 
	String value() default "";
 
}

You will noticed that all @Repository,@Service or @Controller are annotated with @Component. So, can we use just @Component for all the components for auto scanning? Yes, you can, and Spring will auto scan all your components with @Component annotated.

It’s working fine, but not a good practice, for readability, you should always declare @Repository,@Service or @Controller for a specified layer to make your code more easier to read, as following :

DAO layer

package com.mkyong.customer.dao;
 
import org.springframework.stereotype.Repository;
 
@Repository
public class CustomerDAO 
{
	@Override
	public String toString() {
		return "Hello , This is CustomerDAO";
	}	
}

Service layer

package com.mkyong.customer.services;
 
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;
 
import com.mkyong.customer.dao.CustomerDAO;
 
@Service
public class CustomerService 
{
	@Autowired
	CustomerDAO customerDAO;
 
	@Override
	public String toString() {
		return "CustomerService [customerDAO=" + customerDAO + "]";
	}
 
}

Download Source Code

Tags :

About the Author

mkyong
Founder of Mkyong.com and HostingCompass.com, love Java and open source stuff. Follow him on Twitter, or befriend him on Facebook or Google Plus. If you like my tutorials, consider make a donation to these charities.

Comments

  • Pingback: alkaline water()

  • Pingback: water ionizer()

  • Pingback: tv online, online tv()

  • Pingback: watch tv show episodes()

  • Pingback: stream movies()

  • Pingback: stream movies()

  • Pingback: INCREASELEPTIN.COM()

  • Pingback: Blue Coaster33()

  • Pingback: Spring Auto Components Scanning with Constructor injection | Technology & Programming()

  • Nigel Thomas

    Hi,
    I have a doubt regarding @Component

    In one of the classes I have seen like
    @Component(“CORE.TestClass”)
    @Scope(“singleton”)
    public class TestClass implements ITestHelper{

    After running the application it will throw an error like:

    Caused By: org.springframework.beans.factory.BeanCreationException: Error creating bean with name ‘CORE.CoherenceCacheHelper’ defined in URL [zip:D:/system12.1.2.0.40.66.68/DefaultDomain/servers/DefaultServer/tmp/_WL_user/ADF-TEST/21n7qx/war/WEB-INF/lib/TestClass.class]: Instantiation of bean failed; nested exception is org.springframework.beans.BeanInstantiationException: Could not instantiate bean class [TestClass]: Constructor threw exception; nested exception is java.lang.ExceptionInInitializerError
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.instantiateBean(AbstractAutowireCapableBeanFactory.java:883)

    Why is this error occurring ?

    Nigel.

  • joe

    Hi Mkyong,
    This is the most well explained article i’ve read in a while. Many thanks for sharing your expertise!

    Best Regards.

  • Pankaj Kumar Mishra

    Hello Sir,
    Could please help me by providing example/s or link of example/s which usage

    @Configuration
    @Import
    @ImportResource
    Annotation at a time–> as these are also supported by Auto Component Scan. I want to understand their relation.

    with following Annotaion-
    @Component, @Service, @Repository, @Controller, @Endpoint
    , @Bean, @Lazy, @Scope, @Order, @Primary, @Profile, @DependsOn

  • sharan

    Very nice article, my doubt got clear.

  • http://[email protected] ash

    Nice article. clear and to the point.

  • Chandan

    Hi Mkyong,

    I have a doubt. Usually a spring mvc application will have dao, service, controller and form (component) which will all be treated as beans. So in the bean xml how do i declare these

  • http://[email protected] Shivang

    The tutorial is just mind blowing. Very thanks to you MKYong

  • Anta

    Nice post … explained the concept in such an easy way.

  • adrian

    hello,

    i saw this example. Very clear.
    The problem I have is that I got the error in the file xml on the line with <context:component-scan…. The error says that it cannot find a definition for "<context…" because it cannot find the xsd definition file.
    I use eclipse and spring 2.5 and I got this error for all other projects I have.

    Thank you,
    Adrian

  • Mahender

    your superb…….

  • Kumaran

    crystal clear…

  • http://alikemaltasci.blogspot.com Ali Kemal

    Thanks :)

  • devi

    great Article

  • Peter Jerald

    Thanks bro, your articles saved me in various situations, Please keep doing the same.

  • Alex

    Thank you for your posts. Your English is not the best and really frustrating but your excellent tutorials make up for it. Keep up the great work!

  • Saravanan

    Awesome… I have used this concept in my project, but haven’t understand clearly. After reading this article got clear concept… Thanks

  • bruno

    Thank you for this post, clear and objective.

  • Sharath

    Thanks a lot mkYong.. the above post gives good understanding.

  • ahmet

    why do we use @Override ?

    • js518

      If not , toString() will print com.xx.xx.xx@112 other than info you want to .

  • Pritish

    Excellent post !

  • Nitesh

    simple and easy..
    thanx

  • Levan

    Thanks…

  • Borgy Manotoy

    I have a related question regarding autowiring :)

    I created a new controller, MyController.java… I wanted to autowire some service interfaces (AccountService, CompetitionService).

    In my application, there are already other existing controllers (i.e InvitationController.java, CompetitionController.java, etc) which does not have any autowire issues.

    all of them belongs to the same package (i.e. com.test.app), I also added the line below in the XML file.

    now, I really do not know why I am having difficulties in autowiring the service interfaces in the newly created controller.

    I tried almost copying the code of the most simple existing controller but still I have no luck of making the autowiring works.. :-(

    any idea?

    hope somebody can help me… I’m just a noob in spring :)

  • http://www.kogantivijay.com vijay

    Can you explain why you don’t need to Include in the configuration file as explained in your other Tuturial ?

    regards,
    vijay

  • Alebald

    Could you please tell me, how to use Spring Auto Scanning
    for multiple path. say i have my Action Objects at “com.personal.server.handler” and
    Dao at “com.personal.server.dao.handler”. Sould i use two explicit tag in my configuration.xml ?

  • alex

    thx ! this article easy to understand, really great

  • Pingback: Jersey + Spring integration example()

  • Dhafir

    Brilliant article! Easy and straightforward.
    Thank you!

  • anil

    What about perfomance issues with Auto Scanning Component compared to Manual Components Declaration ?

  • Pingback: Spring MVC hello world annotation example()

  • Pingback: Spring Tutorials | J2EE Web Development Tutorials()

  • Noman

    hmmmm nice great post….

  • Pingback: Spring – Filtering Components in Auto Scanning | Spring()