class MyApplication {
     private MessageService service;

    @Inject
    public void setService(MessageService svc){
        this.service=svc;
    }
}

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

public class ClientApplication {
     public static void main(String[] args) {
        Injector injector = Guice.createInjector(new AppInjector());
        MyApplication app = injector.getInstance(MyApplication.class);

        app.sendMessage(); // how does injection work here if there is no call of app.setService() ?

        // is this a correct way to change binding during a runtime? i guess no ;/
        app.setService(new AnotherService());
        app.sendMessage();
    }
}