Guice: is it possible to inject modules?

Question

I have a Module that requires some Depedency. Is there a way Modules themselves can be injected? I realize this is a bit of a chicken and egg situation...

Answer 1

Using a provider or @Provides methods are great if you need a dependency to manually construct an object. However, what if you need something to help you decide how to configure the bindings themselves? It turns out you can use Guice to create (and configure) your module.

Here is a (contrived) example. First, the module we want to configure:

/**
 * Creates a binding for a Set which represents the food in a pantry.
 */
public class PantryModule extends AbstractModule {
  private final boolean addCheese;

  @Inject
  public ConditionalModule(@Named("addCheese") boolean addCheese) {
    this.addCheese = addCheese;
  }

  @Override
  protected void configure() {
    Multibinder pantryBinder = Multibinder
      .newSetBinder(binder(), String.class);

    pantryBinder.addBinding().toInstance("milk");

    if (addCheese) {
      pantryBinder.addBinding().toInstance("cheese");
    }

    pantryBinder.addBinding().toInstance("bread");
  }
}

The PantryModule expects a boolean value to be injected to decide whether or not it should include cheese in the pantry.

Next, we'll use Guice to configure the module:

// Here we use an anonymous class as the "configuring" module. In real life, you would 
// probably use a standalone module.
Injector injector = Guice.createInjector(new AbstractModule() {
  @Override
  protected void configure() {
    // No cheese please!
    bindConstant().annotatedWith(Names.named("addCheese")).to(false);
    bind(PantryModule.class);
  }
});

Module configuredConditionalModule = injector.getInstance(PantryModule.class);

Now that we have a configured module, we'll update our injector to use it...

//...continued from last snippet...
injector = injector.createChildInjector(configuredConditionalModule);

And finally we'll get the set of strings that represent our pantry:

//...continued from last snippet...
Set pantry = injector.getInstance(new Key>() {});

for (String food : pantry) {
  System.out.println(food);
}

If you put all the pieces together in a main method and run it, you'll get the following output:

milk
bread

If you change the binding to the "addCheese" boolean to true, you'll get:

milk
cheese
bread

This technique is cool, but probably only useful when you have control over the Injector instance and only when the module requires complex dependencies. Nonethless, I found a real need for this on a real project at work. If I did, then someone else might too.