spring by 59 comments

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

Download it – Spring-Auto-Scan-Component-Example.zip

59 comments on “Spring Auto scanning components”

  1. Can you not use Autowired to get the bean for CustomerService?

    Reply

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

    Reply

  5. 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.

    Reply

  6. 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.

    Reply

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

    Reply

  9. 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.

    Reply

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

    Reply

    1. Spring can inject dependencies to the fields marked with @Autowired only when the enclosing class itself resides as a bean in its container. SpringContext is simply unaware of instances not created by it and hence obviously cannot autowire the fields within those. In the above examples, spring is unaware of the ‘App’ class. So, to make @Autowired work for CustomerService field within App class, mark App class with @Component thereby informing spring to create a bean of the same and then obtain this bean object of App class through application context as done for CustomerService. This bean object will have CustomerService value set by Spring.

      Reply

  12. 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.

    Reply

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

      Reply

  16. 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 [email protected]. please sir.

    Reply

  17. 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.

    Reply

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

    Reply

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

      Reply

  20. “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());

    Reply

    1. 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()));

      Reply

  23. 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.

    Reply

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

    Best Regards.

    Reply

  25. 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

    Reply

  28. 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

    Reply

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

    Reply

  31. 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

    Reply

    1. 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.

      Reply

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

    Reply

  37. 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!

    Reply

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

    Reply

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

    Reply

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

      Reply

  45. 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 🙂

    Reply

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

    regards,
    vijay

    Reply

  47. 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 ?

    Reply

  49. Brilliant article! Easy and straightforward.
    Thank you!

    Reply

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

    Reply

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