OSGi: Register a VaadinServlet using Http Whiteboard specification

[denis-anisimov]

At the moment developer should care himself about VaadinServlet registration for his OSGi bundle.

There are several ways to do it:

• Using @WebServlet annotation within a WAB file which has a Web-ContextPath manifest header
• Register a Servlet via HttpService .See https://github.com/vaadin/flow/blob/master/flow-tests/test-root-context/src/main/java/com/vaadin/flow/osgi/Activator.java
• Register a Servlet (or servlet service factory) using Http Whiteboard specification (see https://osgi.org/specification/osgi.cmpn/7.0.0/service.http.whiteboard.html) and https://github.com/vaadin/flow/issues/4367.

The first way has been used in original OSGi tests for jetty-osgi-boot as a web server container within Apache Felix. See https://github.com/vaadin/flow/blob/7314427ae8c40104e627f6a2a4e41cb4ff121f74/flow-tests/test-root-context/bnd.bnd#L8

But once I changed the web container from jetty-osgi-boot to felix-jetty this way doesn't work anymore: the container recognizes a bundle as a WAB it says that the context root is occupied ( by the way felix-jetty doesn't allow to use "/" as a context root for a custom WAB) but web servlets are not instantiated and nothing works. I don't know whether this is something specific for felix-jetty or may be I missed something additionally or additional config is required. It's possible to register a servlet programmatically though either via HttpService or via Http Whiteboard specification.

This is done as a modern way of OSGi tests in Flow.

But

• there is an additional issue with that (will be created separately)
• we deploys a servlet in plain servlet container out of the box. So we should do the same in OSGi env
• the servlet is register using Http Whiteboard specification in FW8: https://github.com/vaadin/framework/blob/master/osgi-integration/src/main/java/com/vaadin/osgi/servlet/VaadinServletRegistration.java
• in case of non-root mapping we should register a frontend servlet in the same way as we do in plain servlet container.

[denis-anisimov]

See also #4797

[denis-anisimov]

We need servlet auto-registration for https://github.com/vaadin/flow/issues/5125. This should be a custom OSGi specific servlet rather than plain VaadinServlet.

We should not auto-register the servlet in case the application contains a custom servlet.