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Spring Dependency Injection - An Introductory Tutorial

11.11.2008
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Avoiding XML hell with component-scan tag and @Service, @Component, @Repository annotations

Imagine an application with hundreds of managed objects and the size of the XML configuration file(s). You can manage objects with Spring without putting them in the applicationContext files by marking them with @Service, @Component, or @Repository, and telling Spring where to find the objects. Spring will next scan the classpath looking for these beans and then automatically manage their dependencies.
To perform this feat, you must configure a context:component-scan tag passing the packages you would like Spring to scan as follows:

Using component-scan tag in applicationContext.xml

<?xml version="1.0" encoding="UTF-8"?>
<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.arcmind.springquickstart"/>
        
</beans>

 

Then you mark your beans with the @Service, @Component, or @Repository as follows:

AutomatedTellerMachine class using @Service

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.beans.factory.annotation.Qualifier;
import org.springframework.stereotype.Service;

@Service ("atm")
public class AutomatedTellerMachineImpl implements AutomatedTellerMachine{
        
        @Autowired (required=true)
        @Qualifier ("default")
        private ATMTransport transport;

 

Three transports using @Component

import org.springframework.beans.factory.annotation.Qualifier;
import org.springframework.stereotype.Component;

@Component("standardTransport")
@Qualifier("default")
public class StandardAtmTransport implements ATMTransport {

        public void communicateWithBank(byte[] datapacket) {
                ...
        }

}

@Component("soapTransport")
public class SoapAtmTransport implements ATMTransport {

        public void communicateWithBank(byte[] datapacket) {
                ...
        }

}

@Component("simulationTransport")
public class SimulationAtmTransport implements ATMTransport {

        public void communicateWithBank(byte[] datapacket) {
                ...
        }

}

 

Notice that there is a @Qualifier annotation used in the StandardAtmTransport to denote it as the default transport for this application. For new projects, it makes sense to use annotations for objects that don't often change their dependencies. Avoiding XML and using annotation is the new trend in DI; some say it is a best practice. Figure 5 illustrates injection using this technique.



About the author

Rick Hightower is CTO of Mammatus and is an expert on Java and Cloud Computing. Rick is invovled in Java CDI advocacy and Java EE. CDI Implementations - Resin Candi - Seam Weld - Apache OpenWebBeans

Published at DZone with permission of its author, Rick Hightower.

(Note: Opinions expressed in this article and its replies are the opinions of their respective authors and not those of DZone, Inc.)

Comments

Murphree Mukada replied on Thu, 2008/11/27 - 7:48am

Brilliant! Concise and clear. Thanks a million Rick.

Rick Hightower replied on Sun, 2008/11/30 - 6:33pm in response to: George Jiang

The Spring AOP article has been written and is waiting to be copy edited.

Viraf Karai replied on Sat, 2009/02/21 - 3:14pm

Well written, but it might still be a tad difficult for newcomers to Spring to grasp fully.

I'm not in the anti-XML camp, so I don't feel very comfortable with this new trend towards using annotations willy-nilly. In fact I feel that it diminishes the ability to view the big picture. I do like the new 'context' and 'p' namespaces. Some of the annotations like @Required and @Transactional have been well-thought out by the SpringSource folks and I commend them for that.

Just one final note and it's not meant to be a sales pitch - IntelliJ IDEA 8.x has brilliant Spring support and provides really good code completion as well as linking Java interfaces with Spring bean definitions. I've been using it for about 8 months now and am continually amazed at it's capabilities.

Looking forward to your future articles - especially AOP - a great passion of mine.

Rick Reumann replied on Tue, 2009/09/01 - 2:54pm

I'm really curious though, how often do people have that many different implementations at application startup time? I've been using Spring and Guice for a while now and often when I've completed my app I look back and wonder "ok, what did I really gain here having my concrete implemenations defined in an xml or single Java file?"

 I don't care much about using Mock objects in Testing either, but if I did, I could definitely see the advantage of DI there. 

 Of course I like containers doing some DI for me, like the EJB3 DI stuff etc, but in your typical CRUD app I simply find that I've added one more layer of abstraction that isn't needed. In the early days I was also always a purist and coded everything to an Interface - but in real life, how often, for example, are you swapping out Service class or DAO implemnations? 

 I feel like a heretic and wondering if I'm alone :)

 

Pradeep Kumar replied on Wed, 2012/09/19 - 12:59pm

Very nice, no words thanks much..!!

But please add spring MVC 3.0 also.

Krishna Pokala replied on Tue, 2012/10/02 - 9:08am

Superb intro to DI. Thanks Rick

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