Forcing Dependency Injection

[DartmanX2]

I am trying to inject a dependency into a class. The class is named TechnicianViewHelper and looks something like this:

public class TechnicianViewHelper {

    private TechnicianService technicianService;

    @Required
    public void setTechnicianService(TechnicianService technicianService) {
        this.technicianService = technicianService;
    }

    private int technicianId;

    public void setTechnicianId(int technicianId) {
        this.technicianId = technicianId;
    }

    public List<Technician> getUnapprovedTechnicians() {
        List<Technician> unapprovedTechnicianList = technicianService.getUnapprovedTechnicians();
        return unapprovedTechnicianList;
        //return technicianService.getUnapprovedTechnicians();
    }
}

Since this is a webapp, the related bean definitions in applicationContext.xml look something like this:

    <bean id="technicianService" class="mil.af.amc.projecttracker.service.TechnicianServiceImpl">
        <property name="technicianDao">
            <ref bean="technicianDao" />
        </property>
        <property name="personDao">
            <ref bean="personDao" />
        </property>
        <property name="gradeDao">
            <ref bean="gradeDao" />
        </property>
        <property name="baseDao">
            <ref bean="baseDao" />
        </property>
        <property name="organizationDao">
            <ref bean="organizationDao" />
        </property>
        <property name="officeSymbolDao">
            <ref bean="officeSymbolDao" />
        </property>
        <property name="permissionDao">
            <ref bean="permissionDao" />
        </property>
        <property name="projectDao">
            <ref bean="projectDao" />
        </property>
    </bean>

<bean id="technicianViewHelper" class="mil.af.amc.projecttracker.web.viewhelpers.TechnicianViewHelper" depends-on="technicianService">
        <property name="technicianService">
            <ref bean="technicianService" />
        </property>
    </bean>

I'm reasonably sure that technicianService is working correctly: my LoginAction defined in action-servlet.xml pulls the same service and works fine.

I've also tried, in desperation, switching the technicianViewHelper to be autowired by name. But when I attempt to debug the TechnicianViewHelper class, the debug shows the technician service to be null, and a NullPointerException is thrown.

Am I just making a stupid mistake here?

Jason

[Colin Yates]

You are retrieving this bean from Spring right, and not just "new TechnicianViewHelper()"?

BTW; if the dependency is required I would consider using constructor injection.

I would also avoid autowiring like the plague...really

[DartmanX2]

No, I'm definitely using Spring.

I've changed the bean definition for technicianViewHelper to the following:

    <bean id="technicianViewHelper" class="mil.af.amc.projecttracker.web.viewhelpers.TeamViewHelper">
        <constructor-arg type="mil.af.amc.projecttracker.service.TechnicianService" index="1">
            <ref bean="technicianService" />
        </constructor-arg>
    </bean>

and put the following constructor into TechnicianViewHelper:

    public TechnicianViewHelper(TechnicianService technicianService) {
        this.technicianService = technicianService;
    }

However, the application will now not instantiate, giving the following error:


2007-08-07 10:51:46.677 ERROR J2EE HTTP-00004 Internal error raised tyring to instantiate web-application: projecttracker defined in web site OC4J 10g (10.1.3) Default Web Site. Error creating bean with name technicianViewHelper' defined in ServletContext resource [/WEB-INF/classes/applicationContext.xml]: 1 constructor arguments specified but no matching constructor found in bean 'technicianViewHelper' (hint: specify index and/or type arguments for simple parameters to avoid type ambiguities)

Again, I'm probably making silly mistakes here.

(One more thing: TechnicianService is an interface, and the implementation is TechnicianServiceImpl, if that helps.

[Colin Yates]

Your bean definition is for TeamViewHelper, but your java code is for TechnicianViewHelper

[DartmanX2]

Okay, that scream you just heard was likely me... I must've hit the autocomplete in IDEA too quickly. Thanks for the assistance... I'm on to an entirely new exception now...

(And I usually try to avoid autowiring because the explanation "it's magic" doesn't help the maintainers much...)

[DartmanX2]

Alright, back to square one... I can't use a <constructor-arg> because the TechnicanViewHelper is used in a JSP as a bean:

    <jsp:useBean id="technicianViewHelper"
                 scope="page"
                 class="mil.af.amc.projecttracker.web.viewhelpers.TechnicianViewHelper" />

Unfortunately, jsp:useBean requires a no-argument constructor. If it weren't for that, the constructor-arg method seems to work.

Here is the current bean definition for the TechnicianViewHelper:

    <bean id="technicianViewHelper" class="mil.af.amc.projecttracker.web.viewhelpers.TechnicianViewHelper">
        <property name="technicianService">
            <ref bean="technicianService" />
        </property>
    </bean>

Any more ideas?

[iwein]

I think you're on to the core of the problem. jsp:useBean requires a no argument constructor, because it will use this constructor to instantiate a bean...

Check out http://forum.springframework.org/showthread.php?t=36326.

[Colin Yates]

Alright, back to square one... I can't use a <constructor-arg> because the TechnicanViewHelper is used in a JSP as a bean:

I generally put any beans that the view needs into the Model that the view renders.

In other words rather than using jsp:useBean I would modify my controller to put it into the model, the view just picks it up out of the model.