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Spring Dependency Injection - An Introductory Tutorial

11.11.2008
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Using constructor instead of setter

Another option when using Spring is to use constructor arguments instead of setter methods to inject dependencies. This keeps things more pure from an object-oriented design standpoint as an object has to be created with all of its collaborators (a.k.a. dependencies) it needs to fulfill its role.

Using constructors, injection is much like using setter methods as follows:

Application context for constructor injection
        

 <bean id="standardTransport" class="com.arcmind.springquickstart.StandardAtmTransport"/>
        
        <bean id="atm" class="com.arcmind.springquickstart.AutomatedTellerMachineImpl">
                <constructor-arg ref="standardTransport" />
        </bean>

 

Notice the use of the constructor-arg tag. This implies that the constructor takes transport as a single argument.

Adding a constructor to AutomatedTellerMachineImpl

public class AutomatedTellerMachineImpl implements AutomatedTellerMachine{
        
        private ATMTransport transport;
        
        public AutomatedTellerMachineImpl (ATMTransport transport) {
                this.transport = transport;
        }

The above example should keep the object purists in your group happy. However, the setter injection style makes test-driven development a bit easier. In practice, the setter method approach is used more often.
Figure 2 illustrates how the constructor injection occurs. 

 

If you have many constructors in the same class with a variable number of arguments, Spring will try to pick the best fit. However, you can give Spring some hints as follows:

Application context for constructor injection with a hint for Spring

        <bean id="atm" class="com.arcmind.springquickstart.AutomatedTellerMachineImpl">
                <constructor-arg index="0" ref="standardTransport" />
        </bean>

Under some circumstances, you can even specify the type attribute that you want Spring to use to resolve the constructor argument in cases when there are more than one possible constructor match.  Most times, you don't have to specify index or type. Your mileage may vary.



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.

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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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