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

About the Author

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mkyong
Founder of Mkyong.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

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Cor
Guest
Cor

Great tutorial, really clear and step by step.

I might add that if anyone is using JDK 1.8 and is getting the error “component-scan and its parser class are only available on JDK 1.5 and higher” after enabling the component-scan you might need to switch to JDK 1.7 as spring-context-2.5 and JDK 1.8 are apparently not compatible anymore.

Savita
Guest
Savita

Great explanation!! Your posts are really very useful and easy to understand. Thank you so much for your efforts.

arun singh
Guest
arun singh

you cleared the confusion

Karthikeyan
Guest
Karthikeyan

Fantastic Explanation!

sakthi
Guest
sakthi

Could you please create EAR project and explain spring mvc+hibernate using only with annotation please?

Create one ear project for spring mvc. jsp, controller, appinitializer alone in web module and call the dao from the ejb module or jar module.

Dunkin
Guest
Dunkin

Hi MKyong ,take a snickers ,you are not your self when you are hungry

Ayaskant Swain
Guest
Ayaskant Swain

Superb & easy to understand post. Thanks a ton Mr MKyoung for your contributions on your portal that helps thousands of Java/JEE developers across the globe to learn the technical stuff easily.

karthik
Guest
karthik

How is autowiring is different from component scanning.

Indra Eka Prasetya
Guest
Indra Eka Prasetya

Why do we still use CustomerService cust = (CustomerService)context.getBean(“customerService”); ? and not use @Autowired for CustomerService

jimbob
Guest
jimbob

I’m getting an error about the Java version needing to be 1.5 or greater (even though it is). I’m thinking the use of Spring 2.5 is the cause of this. I know the post is old, but maybe worth pointing that out.

wacik93
Guest
wacik93

You may use 1.8 JDK and that is why. Try to update your Spring to 4.0

The_1st_Hunter
Guest
The_1st_Hunter

Great article

lipe1233
Guest
lipe1233

help me a lot man! thanks!

lokanath Bagh
Guest
lokanath Bagh

Thanks a lot.. Good one..

Achilles Ram
Guest
Achilles Ram

Its a great post Sir, can i have one post for what is @Configurable, when should we use it? and its purpose? please mail me at nakirekantisaikumar@gmail.com. please sir.

Dinesh
Guest
Dinesh

HI Mkyong, I am getting below error org.springframework.beans.factory.BeanCreationException: Error creating bean with name ‘SAController’: but when I add bean id in xml it is working any solution will help.

paragshrivastava
Guest
paragshrivastava

This example miss the annotation-config in spring context file without which you cannot run @Autowire implemnetation classes.

v3gA
Guest
v3gA

component-scan implies the effects of the ‘annotation-config’ tag,

~$Nadeem
Guest
~$Nadeem

good article!!!

Preston
Guest
Preston

“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’.”

Is there a way to have spring automatically name the component with the value that is returned with CustomerService.class.getName() so you can then retrieve the component with

CustomerService cust = (CustomerService) context.getBean(CustomerService.class.getName());

malasang
Guest
malasang

no,that is not right.CustomerService.class.getName() get only get CustomerService not customerService.you can make the first character lower case like this.CustomerService cusomter =(CustomerService)context.getBean
(Introspector.decapitalize(CustomerService.class.getSimpleName()));

atul galande
Guest
atul galande

Which stereo type annotation used in web mvc module

Landscape
Guest
Landscape

NIce post, easy to understand.

trackback
Spring Auto Components Scanning with Constructor injection | Technology & Programming

[…] know how to use Auto Components Scanning and Consctuctor Injection individually. http://www.mkyong.com/spring/spring-auto-scanning-components/ […]

Nigel Thomas
Guest
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
Guest
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
Guest
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
Guest
sharan

Very nice article, my doubt got clear.

ash
Guest
ash

Nice article. clear and to the point.

Chandan
Guest
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

Shivang
Guest
Shivang

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

Anta
Guest
Anta

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

adrian
Guest
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

Veeranna
Guest
Veeranna

Cause you haven’t mentioned xsd’s perfectly. I’m using spring 3.x and I got the same error. Rectified that error after putting this code. So google it about xsd s or copy paste mine and change the spring version.

Veeranna
Guest
Veeranna
AJ
Guest
AJ

Add this attribute to the beans tag in your configuration xml. Change 2.5 to 3.0 if you’re using that version.

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