Guice: One “Provider” for multiple implementations

Question

I have an interface that has 20 or so annotated implementations. I can inject the correct one if I know which I need at compile time, but I now need to dynamically inject on

Answer 1

If all you want is to get an instance programmatically, you can inject an Injector. It's rarely a good idea--injecting a Provider is a much better idea where you can, especially for the sake of testing--but to get a binding reflectively it's the only way to go.

class YourClass {
  final YourDep yourDep;  // this is the dep to get at runtime

  @Inject YourClass(Injector injector) {
    YourAnnotation annotation = deriveYourAnnotation();
    // getProvider would work here too.
    yourDep = injector.getInstance(Key.get(YourDep.class, annotation));
  }
}

If you're trying write a Provider that takes a parameter, the best way to express this is to write a small Factory.

class YourDepFactory {
  @Inject @A Provider aProvider;
  @Inject @B Provider bProvider;
  // and so forth

  Provider getProvider(YourParameter parameter) {
    if (parameter.correspondsToA()) {
      return aProvider;
    } else if (parameter.correspondsToB()) {
      return bProvider;
    }
  }

  YourDep get(YourParameter parameter) {
    return getProvider(parameter);
  }
}