SpringBoot Actuator fails to start when deploying a WebFlux application as a war file to Tomcat

I have an existing WebFlux application that runs still on a standalone tomcat9 application server.

Problem: I'm trying to add -actuator dependency, but then the application won't start anymore.

@SpringBootApplication
public class MainApp extends SpringBootServletInitializer {
    public static void main(String[] args) {
        SpringApplication.run(MainApp.class, args);
    }

    @Override
    protected SpringApplicationBuilder configure(SpringApplicationBuilder application) {
        return application.sources(MainApp.class);
    }
}

pom.xml:

<parent>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-parent</artifactId>
    <version>2.6.4</version>
</parent>

    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-webflux</artifactId>
    </dependency>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-actuator</artifactId>
    </dependency>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-tomcat</artifactId>
    </dependency>

Result: ```

o.s.b.w.s.c.AnnotationConfigServletWebServerApplicationContext: Exception encountered during context initialization - cancelling refresh attempt: org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'webHandler' defined in class path resource [org/springframework/boot/autoconfigure/web/reactive/WebFluxAutoConfiguration$EnableWebFluxConfiguration.class]: Initialization of bean failed; nested exception is org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'healthEndpointWebFluxHandlerMapping' defined in class path resource [org/springframework/boot/actuate/autoconfigure/health/HealthEndpointReactiveWebExtensionConfiguration$WebFluxAdditionalHealthEndpointPathsConfiguration.class]: Bean instantiation via factory method failed; nested exception is org.springframework.beans.BeanInstantiationException: Failed to instantiate [org.springframework.boot.actuate.endpoint.web.reactive.AdditionalHealthEndpointPathsWebFluxHandlerMapping]: Factory method 'healthEndpointWebFluxHandlerMapping' threw exception; nested exception is java.lang.IllegalStateException: Found multiple extensions for the endpoint bean healthEndpoint (healthEndpointWebExtension, reactiveHealthEndpointWebExtension)

Caused by: java.lang.IllegalStateException: Found multiple extensions for the endpoint bean healthEndpoint (healthEndpointWebExtension, reactiveHealthEndpointWebExtension) at org.springframework.boot.actuate.endpoint.annotation.EndpointDiscoverer.convertToEndpoint(EndpointDiscoverer.java:196) ~[spring-boot-actuator-2.6.4.jar:2.6.4] at org.springframework.boot.actuate.endpoint.annotation.EndpointDiscoverer.convertToEndpoints(EndpointDiscoverer.java:182) ~[spring-boot-actuator-2.6.4.jar:2.6.4] at org.springframework.boot.actuate.endpoint.annotation.EndpointDiscoverer.discoverEndpoints(EndpointDiscoverer.java:125) ~[spring-boot-actuator-2.6.4.jar:2.6.4] at org.springframework.boot.actuate.endpoint.annotation.EndpointDiscoverer.getEndpoints(EndpointDiscoverer.java:117) ~[spring-boot-actuator-2.6.4.jar:2.6.4] at org.springframework.boot.actuate.autoconfigure.health.HealthEndpointReactiveWebExtensionConfiguration$WebFluxAdditionalHealthEndpointPathsConfiguration.healthEndpointWebFluxHandlerMapping(HealthEndpointReactiveWebExtensionConfiguration.java:67) ~[spring-boot-actuator-autoconfigure-2.6.4.jar:2.6.4] at jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[?:?] at jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[?:?] at jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[?:?] at java.lang.reflect.Method.invoke(Method.java:566) ~[?:?] at org.springframework.beans.factory.support.SimpleInstantiationStrategy.instantiate(SimpleInstantiationStrategy.java:154) ~[spring-beans-5.3.16.jar:5.3.16] at org.springframework.beans.factory.support.ConstructorResolver.instantiate(ConstructorResolver.java:653) ~[spring-beans-5.3.16.jar:5.3.16] at org.springframework.beans.factory.support.ConstructorResolver.instantiateUsingFactoryMethod(ConstructorResolver.java:638) ~[spring-beans-5.3.16.jar:5.3.16] at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.instantiateUsingFactoryMethod(AbstractAutowireCapableBeanFactory.java:1352) ~[spring-beans-5.3.16.jar:5.3.16] at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBeanInstance(AbstractAutowireCapableBeanFactory.java:1195) ~[spring-beans-5.3.16.jar:5.3.16] at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:582) ~[spring-beans-5.3.16.jar:5.3.16] at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:542) ~[spring-beans-5.3.16.jar:5.3.16] at org.springframework.beans.factory.support.AbstractBeanFactory.lambda$doGetBean$0(AbstractBeanFactory.java:335) ~[spring-beans-5.3.16.jar:5.3.16] at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:234) ~[spring-beans-5.3.16.jar:5.3.16] at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:333) ~[spring-beans-5.3.16.jar:5.3.16] at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:208) ~[spring-beans-5.3.16.jar:5.3.16] at org.springframework.beans.factory.support.DefaultListableBeanFactory.getBeansOfType(DefaultListableBeanFactory.java:671) ~[spring-beans-5.3.16.jar:5.3.16] at org.springframework.context.support.AbstractApplicationContext.getBeansOfType(AbstractApplicationContext.java:1308) ~[spring-context-5.3.16.jar:5.3.16] at org.springframework.beans.factory.BeanFactoryUtils.beansOfTypeIncludingAncestors(BeanFactoryUtils.java:378) ~[spring-beans-5.3.16.jar:5.3.16] at org.springframework.web.reactive.DispatcherHandler.initStrategies(DispatcherHandler.java:120) ~[spring-webflux-5.3.16.jar:5.3.16] at org.springframework.web.reactive.DispatcherHandler.setApplicationContext(DispatcherHandler.java:115) ~[spring-webflux-5.3.16.jar:5.3.16] at org.springframework.context.support.ApplicationContextAwareProcessor.invokeAwareInterfaces(ApplicationContextAwareProcessor.java:128) ~[spring-context-5.3.16.jar:5.3.16] at org.springframework.context.support.ApplicationContextAwareProcessor.postProcessBeforeInitialization(ApplicationContextAwareProcessor.java:102) ~[spring-context-5.3.16.jar:5.3.16] at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.applyBeanPostProcessorsBeforeInitialization(AbstractAutowireCapableBeanFactory.java:440) ~[spring-beans-5.3.16.jar:5.3.16] at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.initializeBean(AbstractAutowireCapableBeanFactory.java:1796) ~[spring-beans-5.3.16.jar:5.3.16] at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:620) ~[spring-beans-5.3.16.jar:5.3.16] ... 68 more ```

Sidenote: the application works, also with actuator, when run locally as embedded tomcat webserver. It's just the standalone tomcat won't start as soon as actuator is present.

Comment From: bclozel

I think this setup is invalid, as Spring Boot doesn't support WebFlux application deployed as WAR in Servlet containers.

Comment From: membersound

As per this answer: https://stackoverflow.com/a/71422629/1194415 It should be possible though!

And in fact my webflux application already works on a tomcat standalone. So this is for sure not invalid! It's just that the actuator dependency does not work in this setup. Everything else does! Please reopen.

Comment From: bclozel

There are other answers on StackOverflow from team members saying the opposite: this, this and this.

You've chosen to take a path that's not supported by the Spring Boot team. It might work for some features, but as you've found out there are many things that aren't supported. We don't plan to invest in this area.