Guice: inject different implementation depending on who is getting it?

Question

I have two third-party classes, both of which take an implementation of an Authorizer interface. I need to inject each with a different implementation.

If I do an @Provides, how can I implement it so that it provides the implementation required at run time? The provider has no idea who is asking for the injection.

In theory I could use @Named, but I can't modify the code being injected. I want to do something like:

bind(Authorizer.class).to(ImplA.class).for(SomeClass.class)
bind(Authorizer.class).to(ImplB.class).for(SomeOtherClass.class)

Obviously, the "for" code doesn't exist, but is there some equivalent way to do this?

Answer 1

You can achieve this using Private Modules, which let you install (mutually-inaccessible) conflicting bindings to be used in constructing a limited set of non-conflicting exposed bindings. This is often seen as a solution to the robot legs problem, in which you'd want to (for instance) expose a @Left Leg and a @Right Leg where the Leg object is exactly the same but you've bound different Foot implementations (LeftFoot and RightFoot) further down in the hierarchy.

At this point, you're not specifying "who is getting it", but you're exposing a slightly different Injector graph for one consumer versus the other.

install(new PrivateModule() {
  bind(Authorizer.class).to(ImplA.class);
  expose(SomeClass.class);
});
install(new PrivateModule() {
  bind(Authorizer.class).to(ImplB.class);
  expose(SomeOtherClass.class);
});