Spring Dependency Injection - An Introductory Tutorial

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.

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