Conditionally instantiating classes that use @Inject

Question

I'm trying to understand how to handle conditionally creating new instances of a class that uses @Inject. In the below example I have a factory that instantiates classes based on a parameter.

AnimalFactory does not have access to the injector the main class of my application has, so I can't use injector.getInstance(Cat.class)

class AnimalFactory {

    public IAnimal create(AnimalType type) {
        if (type.equals(AnimalType.CAT)) {
            return new Cat(); // Cat uses @Inject, so this won't work of course. But ???
        } else if (type.equals(AnimalType.DOG)) {
            return new Dog(); 
        }
    }
}

In the rest of my app, classes are injected into my constructors because I always need them. Guice creates an instance/singleton for each. But in this scenario, I do not want to create and inject instances for each animal because all but one are needed.

Answer 1

You can use a MapBinder as described here:

public class AnimalModule extends AbstractModule {
  public void configure() {
    MapBinder<AnimalType, IAnimal> animalBinder= MapBinder.newMapBinder(binder(), AnimalType.class, IAnimal.class);

    animalBinder.addBinding(AnimalType.DOG).to(Dog.class);
     ...
  }
}

And than use it in your factory:

class AnimalFactory {
   @Inject
   Map<AnimalType, IAnimal> animals;

   public IAnimal create(AnimalType type) {
       return animals.get(type);
   }
 }