Tomcat: fix auto-configuration of VaadinServlet

[mvysny]

Vaadin's ServletDeployer is apparently not being called, and therefore the default VaadinServlet isn't being configured automatically if the app doesn't provide one.

The problem apparently is that Tomcat doesn't perform a full classpath scanning. Vaadin-Boot forces Tomcat to perform classpath scanning, but only in project jar file, see TomcatWebServer.enableClasspathScanning().

Perhaps setting metadata-complete to false can persuade Tomcat to perform full classpath scanning: https://stackoverflow.com/questions/9820379/what-to-do-with-annotations-after-setting-metadata-complete-true-which-resolv

However, that's definitely not easy: it can't be programmatically configured for Tomcat Embedded: https://stackoverflow.com/questions/15988578/specify-a-custom-web-xml-to-an-embedded-tomcat

Oddly enough Vaadin Lookup is initialized correctly, but that's because LookupServletContainerInitializer is ServletContainerInitializer mentioned in META-INF/services, which causes Tomcat to load it.

We could use the same technique to load and run ServletDeployer, something like:

public class VaadinInitializer implements ServletContainerInitializer {
    @NotNull
    private final ServletDeployer servletDeployer = new ServletDeployer();
    @NotNull
    private final JSR356WebsocketInitializer jsr356WebsocketInitializer = new JSR356WebsocketInitializer();
    @Override
    public void onStartup(Set<Class<?>> c, ServletContext ctx) throws ServletException {
        servletDeployer.contextInitialized(new ServletContextEvent(ctx));
        jsr356WebsocketInitializer.contextInitialized(new ServletContextEvent(ctx));
    }
}

but that's tricky: ServletDeployer fails if Lookup wasn't initialized, and therefore this would depend on the ordering of ServletContainerInitializers, which is fragile.

[mvysny]

If anyone has any clue how to enable classpath scanning for Embedded Tomcat, please let me know.

[mvysny]

((StandardContext) ctx).setDefaultWebXml(TomcatWebServer.class.getClassLoader().getResource("boot/web.xml").toExternalForm()); does nothing - the file is ignored and not loaded.

[mvysny]

server.getHost().setAutoDeploy(true); does nothing.