Make DefaultInstantiator extendable

Currently, if you extend DefaultInstantiator and register your new custom instantiator through SPI, the custom Instantiator must be instantiated through a no-arg constructor. This constructor will not have access to a VaadinService instance and thus the private VaadinService service member variable in the parent DefaultInstantiator can never be set. This, in turn, means the init method will return false and the custom Instantiator won’t be picked up. If the init method is overridden to return true, the methods requiring the VaadinService instance will fail.

[mvysny]

Note that in Vaadin 14.6, it will be possible to override an InstantiatorFactory, removing the need to have a custom VaadinServlet.

For Vaadin 14.5.x and lower, use something like the following:

@WebServlet(urlPatterns = "/*", asyncSupported = true)
public class MyServlet extends VaadinServlet {

    @Override
    protected VaadinServletService createServletService(DeploymentConfiguration deploymentConfiguration) throws ServiceException {
        VaadinServletService service = new VaadinServletService(this,
                deploymentConfiguration) {
            @Override
            protected Instantiator createInstantiator() throws ServiceException {
                return new MyInstantiator(this);
            }
        };
        service.init();
        return service;
    }

    public static class MyInstantiator extends DefaultInstantiator {
        public MyInstantiator(VaadinService service) {
            super(service);
        }
        // ... etc - override other methods as necessary.
    }
}

[mvysny]

Yeah I can see InstantiatorFactory but it’s not clear how to pass in a custom implementation to Vaadin! See https://github.com/vaadin/docs/issues/739