I am creating a base API in JavaSE, which includes modules like MVP architecture, Service & Repository Layer, Event Model to fire events between presenter etc.

I am trying to implement all best practices of Software Design. Recently, I read about Dependency Injection & its advantages and decided to use Guice as DI framework.

After spending two days learning Guice, I am still confused with some design issues.

1. If I create an injector using Guice.createInjector(modules) with the core modules of my API,

   • How to make this injector instance available to my other API classes? Can an injector be injected and is it a good practice?
   • How to make this injector instance available to API user, so that they can use this injector to create instances?
   • Should I even make it available to API user, as it may be a bad practice?
   • Hence using it for AOP approach like this and custom injections

2. Is creating injectors a heavyweight operation, if so should there be only a few injectors per application?

3. Should injectors be created on startup stage of the application or could be created in between without affecting the application performance?

Edit

To understand usage of Provider instead of injecting injector here is an short MVP example:

interface ProductModel {

  // Some method declarations

}

interface ProductView {

  // Some method declarations

}

class ProductPresenter {

  // Using Guice for constructor injection
  @Inject
  ProductPresenter(ProductModel model, ProductView view) {
    this.model = model;
    this.view = view;
  }

}

class ProductViewImpl implements ProductView {

  // Method Implementations

}

class ProductModelImpl implements ProductModel {

  // Method Implementations

}

Now, as understood from below answers to use Provider<X> instead of injecting injector, here is the Provider class.

class ProductPresenterProvider implements Provider<ProductPresenter> {

  ProductPresenter get() {
    // But, How to provide the ProductPresenter without model and view instances!
  }

}