Does Guice binding bind subclass as well?

Question

So I have a module which binds an Interface with an implementation class.

bind(ILocalStore.class).to(LocalStore.class);

Will this binding also inject the following constructor:

@Inject
public LocalStoreUser(LocalStore localstore) {
    this.localstore = localstore
}

Answer 1

The injection will work, but not because of the binding.

Guice treats lookups as if they were a simple Map<Key, Provider> lookup, where Key is composed of a (binding annotation, parameterized type) pair. Guice will not automatically walk up the type hierarchy for you, or otherwise modify the lookup key (checking for a non-annotated or raw type binding, for instance).

However, if LocalStore has a public no-argument constructor (or an @Inject-annotated constructor as you've listed), Guice can create the type directly, so the code above will work—it just has nothing to do with binding subclasses automatically.

---

Consider the following example:

interface A {}
interface B extends A {}
public class C implements B {}

• Without any bindings at all, you could inject C, because it has an implicit public no-arg constructor. The same would apply if C has an @Inject-annotated constructor. Requesting injection of A or B will fail.

• If you were to bind(A.class).to(C.class) you could inject A (explicit) or C (implicit) but not B. Guice will not bind the subinterface as well, so B has no implementation.

• Same goes for superclasses/superinterfaces: If you were to bind(B.class).to(C.class) you could inject B (explicit) or C (implicit) but not A, even though B extends A.

• If you were to bind(A.class).to(B.class) and bind(B.class).to(C.class), then you could inject A, B, or C without repeating yourself.