Spring Auto scanning components

Posted 2020-10-10 田园沃土

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 + "]";
	}

}

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