Dependency injection and custom annotation

Java EE provides a couple of extremely attractive options – DI (dependency injection feature) and custom annotations what allow qualify classes you develop. They bring remarkable flexibility in the job – but if you keep in mind the technique they use.

Couple of days ago I had to resolve some issue in the third-party code. I will simulate the case and keep the main idea of design to demonstrate what was the issue. Let’s create the basic interface and a custom annotation:

public interface SimpleInterface {
     String msg();
}

@Target({TYPE})
@Retention(RUNTIME)
public @interface QualifyerA {
    
} 

Also we create simple class and allow DI using @SessionScoped annotation.

@SessionScoped
@QualifyerA
public class SimpleClass 
implements SimpleInterface, Serializable {
    public String msg() { return "Got it"; }
}

Finally let’s create managed bean and the simple JSF to check results:

@ManagedBean
public class SimpleBean {
    SimpleInterface si = new SimpleClass();
    public String result() {
        if (si.getClass().isAnnotationPresent(QualifyerA.class))
                return si.msg();
        else
                return "Not annotated";
    }
    public String showClass() {
        return si.getClass().getName();
    }
}

A piece of JSF code:

    
        Hello, #{simpleBean.result()}
    

If we start the application on Web application server we get:

Hello, Got it

OK, everything is well so far. Let’s now use the injection instead of explicit instantiation:

public class SimpleBean {
    
    @Inject SimpleInterface si;
    
    public String result() {
        if (si.getClass().isAnnotationPresent(QualifyerA.class))
                return si.msg();
...

And we have failed:

Hello, Not annotated

Oops, we lost something, but what ? We can understand what happens if we look as the variable real class (
let’s change JSF code):

Hello, #{simpleBean.showClass()}

We get:

Hello, com.tray.SimpleClass$Proxy$_$$_WeldClientProxy

As you can see, Injection uses proxy class and, hence, we lost annotation information.
What we can do is to apply @Inherited to our custom annotation:

@Target({TYPE})
@Retention(RUNTIME)
@Inherited
public @interface QualifyerA {
   
}

After forcing the inheritance we gat what we need (don’t forget to restore original JSF code)

Hello, Got it