Intro: Spring Boot Version: 3.4.4 Java Version: 23 Tomcat Servlet Container has been the default & preferred server in my application from the beginning.

Problem: Some auto-configuration classes are defined as static inner classes inside top-level configs (e.g., EmbeddedWebServerFactoryCustomizerAutoConfiguration$UndertowWebServerFactoryCustomizerConfiguration). These are conditionally loaded based on classpath presence (e.g. Undertow.class).

Here's the excerpt for convenience: 


import io.undertow.Undertow;


@Configuration(proxyBeanMethods = false)
@ConditionalOnClass({ Undertow.class, SslClientAuthMode.class })
public static class UndertowWebServerFactoryCustomizerConfiguration {

    @Bean
    public UndertowWebServerFactoryCustomizer undertowWebServerFactoryCustomizer(Environment environment,
            ServerProperties serverProperties) {
        return new UndertowWebServerFactoryCustomizer(environment, serverProperties);
    }

    @Bean
    @ConditionalOnThreading(Threading.VIRTUAL)
    UndertowDeploymentInfoCustomizer virtualThreadsUndertowDeploymentInfoCustomizer() {
        return (deploymentInfo) -> deploymentInfo.setExecutor(new VirtualThreadTaskExecutor("undertow-"));
    }

}

Here is the maven dependency that includes io.undertow.Undertow. 

<dependency>
    <groupId>io.undertow</groupId>
    <artifactId>undertow-core</artifactId>
    <version>2.3.18.Final</version>
</dependency>

However, since these inner config classes are not registered in AutoConfiguration.imports, they cannot be excluded via spring.autoconfigure.exclude.

This creates a situation where:

  • I need to use Undertow manually in an isolated use case (non-servlet container)
  • I do not want Spring Boot to auto-configure anything related to Undertow, especially not an additional Servlet Container
  • But Spring Boot sees Undertow.class and tries to auto-configure the factory anyway
  • I cannot exclude just the inner config class, and excluding the top-level class disables unrelated embedded server support like Tomcat.

Expected: Support spring.autoconfigure.exclude for inner config classes or provide a mechanism to opt-out of specific servlet container support (like server.exclude=undertow).

I am aware that there is/was a similar ask with issue number #5427 which was declined as it was deemed "not reasonable" and to quote @philwebb :

We can't easily disable inner-configurations. I'd rather take each need on a case-by-case basis and work out why a specific inner-configuration couldn't be used.

I kindly request this requirement to be reconsidered as it has been 9 years over the other issue and I can't really come up with an elegant solution.