I'm not comforatble with the container-specific code, so I think we need to abandon that here and switch to using javax.servlet.ServletRegistration. All we need to do is add support for that into ServletRegistrationBean. Probably to reach feature parity with this patch we need to scoop up a bean of type MultipartConifgElement and register it with the "free" servlets (i.e. the ones that do not already have a ServletRegistrationBean) - that would be in EmbeddedWebApplicationContext.getServletContextInitializerBeans() if you want to revise and re-submit.

I'm not sure about how to use ServletRegistrationBean by itself, hence I don't know how to modify it in this context to add extra support.

The ServletRegistrationBean just provides simple getters/setters to configure a servlet. It is similar to the servlet element in web.xml. The magic happens in the configure method, it simply delegates to servlet 3.0 ServletRegistration.Dynamic. We would need to call setMultiPartConfig here, probably from a local property.

The EmbeddedWebApplicationContext has some additional logic that allows Servlet beans to be picked up if there are no ServletRegistrationBeans. We would need some additional logic here as well.

Begging your pardon, but how do I wire up a minimal embedded setup using that for test purposes? So far, I have the following code, but I don't know how to bridge that to either EmbeddedWebApplicationContext or ServletRegstirationBean. What's missing so I can test my changes?

    @Test
    public void containerWithAutomatedMultipartTomcatConfiguration() {
        this.context = new AnnotationConfigEmbeddedWebApplicationContext(
                ContainerWithEverythingTomcat.class,
                MultipartAutoConfiguration.class);
        try {
            assertNotNull(this.context.getBean(MultipartConfigElement.class));
            assertNotNull(this.context.getBean(StandardServletMultipartResolver.class));
        } finally {
            this.context.close();
        }
    }

    @Configuration
    public static class ContainerWithEverythingTomcat {
        @Bean
        MultipartConfigElement multipartConfigElement() {
            return new MultipartConfigElement("");
        }

        @Bean
        TomcatEmbeddedServletContainerFactory containerFactory() {
            return new TomcatEmbeddedServletContainerFactory();
        }
    }

Breakthrough! Since I updated my test code with this:

        this.context = new AnnotationConfigEmbeddedWebApplicationContext(
                ContainerWithEverythingTomcat.class,
                WebMvcAutoConfiguration.class,
                MultipartAutoConfiguration.class);

...it seems the servlet is getting created and I can see my tweaks working. Let me work on this some more, and I might have an updated patch to inspect on Monday.