The Custom Instantiator Official Vaadin documentation is now available, check it out. I’m keeping this text for historic reasons mostly.

The official recommended way now is to introduce your own custom InstantiatorFactory which creates Instantiators:

• If you’re using Spring, simply create a class implementing the InstantiatorFactory interface and annotate it with @Component.
• When not using Spring, the standard ServiceLoader machinery is used: you create a file named com.vaadin.flow.di.InstantiatorFactory whose content is the fully qualified name of your class implementing the InstantiatorFactory. The file goes into the /src/main/resources/META-INF/services folder.

The factory is simple:

public class MyInstantiatorFactory implements InstantiatorFactory {
  @Override
  public Instantiator createInstantitor(VaadinService service) {
    return new MyInstantiator(service);
  }
}

How to implement an Instantiator

All Vaadins 14 - 23.2.0 does not implement instantiator chaining: there’s no mechanism for “your instantiator returns null and thus the DefaultInstantiator is used”. Therefore, you must extend the DefaultInstantiator to properly implement all the default behavior yourself. Example:

public class MyInstantiator extends DefaultInstantiator {
    public MyInstantiator(VaadinService service) {
        super(service);
    }

    // this one creates all @Routes
    @Override
    public <T extends HasElement> T createRouteTarget(Class<T> routeTargetType, NavigationEvent event) {
        if (routeTargetType == MainView.class) {
            // some special construction perhaps
            final MainView view = new MainView("Param 1", "Param 2");
            return ((T) view);
        }
        return super.createRouteTarget(routeTargetType, event);
    }
    
    // perhaps override other methods? For example SpringInstantiator overrides
    // also createComponent() (simply creates a bean), getI18NProvider() (creates a bean or falls back to super)
    // getServiceInitListeners() (looks up all beans implementing VaadinServiceInitListener.class and concats that with super)
    // and getOrCreate() - creates a bean
}

For a full-blown example see the SpringInstantiator class (present in vaadin-spring.jar, simply clone the skeleton-starter-flow-spring project.

Alternative Hacky Way

More hacky way is to introduce your own custom VaadinService and override either VaadinService.loadInstantiators() or VaadinService.createInstantiator(). Please see Vaadin: Custom VaadinServlet and VaadinServletService on how to do that.