Guice: use the same instance independently and inside the binder

Question

Is it possible to create one singleton object, and being able to inject it independently as the one instance, or as a part of set or map?

For example, I have two instaces like this:

@Provides
@Singleton
@Named(value = "bike.listener")
fun bikeListener(
    mapper: ObjectMapper,
    amazonSNS: AmazonSNS,
    kafkaPublisher: KafkaPublisher
): PublishingListener {
    return PublishingListener(
        mapper = mapper,
        amazonSns = amazonSNS,
        kafkaPublisher = kafkaPublisher
    )
}

@Provides
@Singleton
@Named(value = "car.listener")
fun carListener(
    mapper: ObjectMapper,
    amazonSNS: AmazonSNS,
    kafkaPublisher: KafkaPublisher
): PublishingListener {
    return PublishingListener(
        mapper = mapper,
        amazonSns = amazonSNS,
        kafkaPublisher = kafkaPublisher,
    )
}

... and I use them in my classes by directly injecting them via @Named parameters. But, I also have a use case where I want to inject both of them in a set.

So something like:

@Inject
Set<PublishingListener> listeners; // carListener and bikeListener here

Is it possible to create MultiBinder which would use the already defined instances? How can I use the same instance in the set, and separately?

Answer 1

In your Guice Module, you can bind those instances to a set like this:

@Provides
fun publisherListenerSet(
    @Named("car.listener") val carListener: PublisherListener,
    @Named("bike.listener") val bikeListener: PublisherListener
): Set<PublisherListener> { 
    setOf(carListener, bikeListener)
}

Or you can use:

protected override fun configure() {
    val publishingListenerSetBinder = Multibinder.newSetBinder(binder(), PublisherListener::class.java)
    publishingListenerSetBinder.addBinding().annotatedWith(Names.named("car.listener"))!!.to(PublishingListener::class.java)
    publishingListenerSetBinder.addBinding().annotatedWith(Names.named("bike.listener"))!!.to(PublishingListener::class.java)
}

As you don’t actually call the provider method, the Injector can maintain the singleton-ness of the thing that the provider provides.