Fire event with subclass runtime type

Question

I want to fire an event in CDI whose type I can only determine on runtime. For instance, let's say there's some interface A with implementing classes AA and AB. I have two observers:

public void observeAA(@Observes AA aa) {
}

public void observeAA(@Observes AB ab) {
}

Then some event producer:

@Inject @Any
private Event<A> event;

public A getPayload();

public void fire() {
    this.event.fire(getPayload());
}

This doesn't work because A is neither a subtype of AA or AB (it's the other way around). I've noticed there's a select method that takes a subtype:

public <U extends T> Event<U> select(Class<U> subtype, Annotation... qualifiers);

However, it requires a correctly parameterized Class object, which (correct if I'm wrong), I can't build at runtime.

Is there any solution or will I have to use qualifiers (possibly an annotation with a Class<?> method)?

Artefacto · Accepted Answer · 2011-08-11T10:47:44.103

I ended up using a qualifier with a Class<?> member.

@Qualifier
@Target({TYPE, METHOD, PARAMETER, FIELD})
@Retention(RUNTIME)
public @interface EventType {
    Class<?> value();
}


public class Dispatcher {

    @Inject @Any
    private Event<A> event;

    public void fireEvent(A a) {
            this.event.select(
                    getTypeAnnotation(
                    a.getClass())).fire(a);
    }

    public static EventType getTypeAnnotation(
            final Class<?> type) {
        return (EventType) Proxy.newProxyInstance(
                Thread.currentThread().getContextClassLoader(),
                new Class<?>[]{EventType.class},
                new InvocationHandler() {

            @Override
            public Object invoke(Object proxy, Method method,
                    Object[] args) throws Throwable {
                if (method.equals(
                        EventType.class.getMethod("value"))) {
                    return type;
                } else if (method.equals(Annotation.class.getMethod(
                        "annotationType"))) {
                    return EventType.class;
                } else if (method.getName().equals("hashCode")) {
                    return 127 * "value".hashCode() ^ type.hashCode();
                } else if (method.getName().equals("equals")) {
                    return (args[0] instanceof EventType &&
                            ((EventType)args[0]).value()
                            .equals(type));
                }
                return null;
            }
        });
    }
}

public class X {
    public void observeA(
            @Observes @EventType(AA.class) A a) {
    ...

EDIT

This is a simpler way of instantiating the annotation:

public abstract static class ConfigTypeAnnotation
        extends AnnotationLiteral<ConfigType>
        implements ConfigType { }

public static ConfigType getConfigTypeAnnotation(final Class<?> type) {
    return new ConfigTypeAnnotation() {
        @Override
        public Class<?> value() {
            return type;
        }
    };
}

score 0 · Answer 2 · answered Aug 16 '14 at 09:13

0

I had a similar requirement and ended up injecting BeanManager to fire the event.

answered Aug 16 '14 at 09:13

Sam Lawrance

1

score 0 · Answer 3 · answered Aug 04 '11 at 14:02

0

Why don't you use

public void observeA(@Observes A a) {
}

in which you decide what to do according to "a" implementation class ?

public void observeA(@Observes A a) {
    if (a instanceof AA)
    {
     ...
    }
    else
    ...
}

answered Aug 04 '11 at 14:02

Antoine Sabot-Durand

4,725
15
33

Because I can't centralize that logic. The observer needs to change private state, etc. I *could* do `if (!(a instanceof AA)) return;`, but I'd have to put that in every observer. – Artefacto Aug 04 '11 at 14:55

Fire event with subclass runtime type

3 Answers3