Does GIN support anything like child injectors?

Question

I have one application that contains sub-applications. I would like to segregate the GIN injection so that each sub-application can have separate instances of the same core shared classes. I also want the injector to supply classes from some core modules to all sub-applications, so that singleton instances can be shared. e.g.

GIN Modules:
  Core - shared
  MetadataCache - one per sub-application
  UserProvider - one per sub-application

In Guice I can do this using createChildInjector, but I can't see an obvious equivalent in GIN.

Can I achieve something similar in GIN?

Answer 1

Here it is on SOF http://code.google.com/p/google-gin/wiki/PrivateModulesDesignDoc. Hope it helps you out.

Answer 2

I solved this thanks to the link given by @Abderrazakk, but as the link isn't very forthcoming with instructions I thought I'd add a sample solution here too:

Private GIN modules allow you to have a single level of hierarchical injection, where types registered inside a private module are only visible to other instances created within that module. Types registered inside any non-private modules are still available to all.

Example

Let's have some sample types to inject (and inject into):

public class Thing {
    private static int thingCount = 0;
    private int index;

    public Thing() {
        index = thingCount++;
    }

    public int getIndex() {
        return index;
    }
}

public class SharedThing extends Thing {
}

public class ThingOwner1 {
    private Thing thing;
    private SharedThing shared;

    @Inject
    public ThingOwner1(Thing thing, SharedThing shared) {
        this.thing = thing;
        this.shared = shared;
    }

    @Override
    public String toString() {
        return "" + this.thing.getIndex() + ":" + this.shared.getIndex();
    }
}

public class ThingOwner2 extends ThingOwner1 {
    @Inject
    public ThingOwner2(Thing thing, SharedThing shared) {
        super(thing, shared);
    }
}

Create two private modules like this (using ThingOwner2 for the second one):

public class MyPrivateModule1 extends PrivateGinModule {
  @Override
  protected void configure() {
    bind(Thing.class).in(Singleton.class);
    bind(ThingOwner1.class).in(Singleton.class);
  }
}

And a shared module:

public class MySharedModule extends AbstractGinModule {
    @Override
    protected void configure() {
        bind(SharedThing.class).in(Singleton.class);
    }
}

Now register the two modules in our injector:

@GinModules({MyPrivateModule1.class, MyPrivateModule2.class, MySharedModule.class})
public interface MyGinjector extends Ginjector {
    ThingOwner1 getOwner1();
    ThingOwner2 getOwner2();
}

Finally we can look and see that both ThingOwner1 and ThingOwner2 instances have the same SharedThing instance from the shared module, but different Thing instances from their private registrations:

System.out.println(injector.getOwner1().toString());
System.out.println(injector.getOwner2().toString());