I am trying to package Flow and Spring MVC into a Spring Boot runnable jar (i.e. I intend to start my app with java -jar).
I am aware of the [example in the docs]
(https://vaadin.com/docs/v10/flow/spring/tutorial-spring-basic-mvc.html) which shows how to setup Vaadin Flow with Spring MVC. However, this doesn’t seem to work when packaging the application into a jar with Spring Boot’s embedded Tomcat. The WebApplicationInitializer is the problem here. I cannot use it with Spring Boot in an embedded Tomcat. However, something needs to run to register Vaadin’s SpringServlet as the dispatcher. This is [expected behaviour]
(https://github.com/spring-projects/spring-boot/issues/522) when using Spring Boot in this way. One has to use the ServletContextInitializer instead of WebApplicationInitializer.
So, I modified the example to
public class AppContextInitializer implements ServletContextInitializer {
@Override
public void onStartup(ServletContext servletContext) throws ServletException {
AnnotationConfigWebApplicationContext context = new AnnotationConfigWebApplicationContext();
registerConfiguration(context);
servletContext.addListener(new ContextLoaderListener(context));
ServletRegistration.Dynamic registration = servletContext
.addServlet("dispatcher", new SpringServlet(context));
registration.setLoadOnStartup(1);
registration.addMapping("/*");
}
private void registerConfiguration(
AnnotationConfigWebApplicationContext context) {
// register your configuration classes here
}
}
But now, if I run with java -jar the embedded Tomcat is unable to start up:
java.lang.IllegalStateException: Cannot initialize context because there is already a root application context present - check whether you have multiple ContextLoader* definitions in your web.xml!
It seems Spring is trying to add additional ContextLoader definitions. However, I need to add mine, as the SpringServlet is required as the dispatcher.
Is there a way to achieve a fat jar with Spring Boot, combining Spring MVC and Vaadin Flow?
Do you have an example putting Vaadin Flow and Spring MVC into a Spring Boot fat jar?
If not, could you give confirmation that it isn’t currently possible (or rather easily possible) so I can stop trying and chose another way? 
Thanks.