Google Guice Depedency Injection - Where exactly the object is created?

Question

I have the below code in which i dont understand where exactly the new instance of EmailService is created. I tried to check many other stackoverflow conversations, but still couldnt get it clearly.

public interface MessageService {
    void sendMessage(String msg, String recipient);
}

@Singleton
public class EmailService implements MessageService {

    @Override
    public void sendMessage(String msg, String recipient) {
        System.out.println("Sending Email to"+recipient+"Msg is:" + msg);
    }
}

public class MyApplication {
    private MessageService service;

    @Inject
    public MyApplication(MessageService service) {
      this.service = service;
    }

    public void sendMessage(String msg, String recipient) {
        this.service.sendMessage(msg, recipient);
    }
}

public class AppInjector extends AbstractModule {

    @Override
    protected void configure() {
      bind(MessageService.class).to(EmailService.class);
    }

}

public class ClientApplication {
    public static void main(String[] args) {
        Injector inj = Guice.createInjector(new AppInjector());
        MyApplication app = inj.getInstance(MyApplication.class);
        app.sendMessage("How are you?", "hello@hello.com");
    }
}

nowhere in this code, new instance of class EmailService is created something like (new EmailService()).

Answer 1

1. Via reflection Guice analyzes MyApplication's constructor and finds out that it depends on MessageService (public MyApplication(MessageService service)). Exactly this constructor is taken because it is marked with @Inject
2. Guice tries to find out binding for this interface. In AppInjector you specified that implementation for MessageService is EmailService (bind(MessageService.class).to(EmailService.class);)
3. EmailService is instantiated via Java Reflection API. It is done via Class.newInstance
4. After EmailService is created, it is passed as a parameter to MyApplication.class.newInstance() factory.

Notes:

• By default, if you have not specified any additional constructor, there is a default one without parameters, that's why EmailService has not dependencies.
• EmailService instance is a singleton because it is marked with @Singleton, so if there will be more dependencies on it, exactly the same instance will be injected
• If you would like to create binding to instance, you can use the following code: bind(MessageService.class).toInstance(new EmailService());
• Google libraries are always rich in terms of documentation. I advice you to read through this wiki: google/guice/wiki