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Spring Dependency Injection - An Introductory Tutorial

11.11.2008
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The concept of DI transcends Spring. Thus, you can accomplish DI without Spring as follows:

DI without Spring

 

package com.arcmind.springquickstart;

import java.math.BigDecimal;

public class AtmMain {
        
        public void main (String[] args) {
                AutomatedTellerMachine atm = new AutomatedTellerMachineImpl();
                ATMTransport transport = new SoapAtmTransport();
                /* Inject the transport. */           
                ((AutomatedTellerMachineImpl)atm).setTransport(transport);
                
                atm.withdraw(new BigDecimal("10.00"));
                
                atm.deposit(new BigDecimal("100.00"));
        }

}

Then injecting a different transport is a mere matter of calling a different setter method as follows:

Injecting a different dependency

                ATMTransport transport = new SimulationAtmTransport();
                ((AutomatedTellerMachineImpl)atm).setTransport(transport);

To use Spring to inject a dependency you could do the following:

Using Spring to manage dependencies

package com.arcmind.springquickstart;

import java.math.BigDecimal;

import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;

public class AtmMain {
        
        public static void main (String[] args) {
                ApplicationContext appContext = new ClassPathXmlApplicationContext("classpath:./spring/applicationContext.xml");
                AutomatedTellerMachine atm = (AutomatedTellerMachine) appContext.getBean("atm");
                
                atm.withdraw(new BigDecimal("10.00"));
                
                atm.deposit(new BigDecimal("100.00"));
        }

} 

/spring/applicationContext.xml file

<?xml version="1.0" encoding="UTF-8"?>
<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="atmTransport" class="com.arcmind.springquickstart.SoapAtmTransport" />


        <bean id="atm" class="com.arcmind.springquickstart.AutomatedTellerMachineImpl">
                <property name="transport" ref="atmTransport" />
        </bean>

</beans>

 

Figure 1 illustrates how Spring injects the dependency using property setter method injection. 


The application context is the central interface to the Spring DI container. In the application context, you declare two beans, atmTransport and atm, with a bean tag. Then you use the property tag to inject the atmTransport bean into the transport property. This effectively calls the setter method of the AutomatedTellerMachineImpl transport property (setTransport(...)).


The major capabilities that the application context provides include (taken from API docs):

  • Bean factory methods for accessing application components
  • The ability to load file resources in a generic fashion
  • The ability to resolve messages, supporting internationalization

The focus of this article is bean factory methods and DI.



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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