@Provides with @Named does not work for declared supertype variables

Question

I am currently exploring the possibilities of Guice and come across strange behavior - when I declare a variable as

@Inject @Named("dragon") Dragon dragon2; 

the injection works as expected, but when I want to declare dragon2 as an interface (it implements the Creature), i. e.

 @Inject @Named("dragon") Creature dragon2; 

I get an error

There is no implementation for warlock.rincewind.creatures.Creature annotated with @com.google.inject.name.Named(value=dragon) been linked.

Here is my provider method:

 @Named("dragon") @Provides public Dragon providesDragon() { Dragon d = new Dragon("Morkeleb"); return d; } 

I know there are many different ways to overcome this (the simplest is changing the type of return to the Creature), but I'm trying to get the reason for this restriction.

Answer 1

Consider, for example:

 interface Creature { } class Dragon implements Creature { } class Dog implements Creature { } public class Kennel { @Inject @Named("foo") Creature c; } public class KennelModule extends Abstract Module { @Override protected void configure() { bind(Dragon.class).annotatedWith(Names.named("foo")); bind(Dog.class).annotatedWith(Names.named("foo")); } } 

How does your injection know if you need to bind Dragon or Dog here?

Can not! Keys for injection are not covariant . Instead, you need to specify the subclass that you want to enter, and bind it to the interface or superclass that you are inserting into. In other words:

 public class KennelModule extends Abstract Module { @Override protected void configure() { bind(Creature.class).annotatedWith(Names.named("foo").to(Dragon.class); } } 

This will give you the behavior you want. Of course, you can also bind injections for Dragon , of course.