Reactive auto-configuration not working with Spring Boot 2.2.0 M3

[andifalk]

Using Spring HATEOAS 1.0.0 M2 together with Spring Boot 2.2.0 M3 in reactive web mode is not working.
Steps to reproduce this issue:

1. Configure a new spring project with:
   • Select Java, Spring Boot 2.2.0 M3
   • Select HATEOAS dependency
   • Select WebFlux dependency
2. Add annotation '@EnableHypermediaSupport(type = EnableHypermediaSupport.HypermediaType.HAL, stacks = WebStack.WEBFLUX)'

Then the spring application always starts using Tomcat in Servlet mode.
According to debug mode output the autoconfiguration does not detect a reactive web application:
WebFluxAutoConfiguration:
Did not match:
- not a reactive web application (OnWebApplicationCondition)
Matched:
- @ConditionalOnClass found required class 'org.springframework.web.reactive.config.WebFluxConfigurer' (OnClassCondition)

I tried adding '@EnableWebFlux' annotation, but then another error pops up:
The bean 'requestMappingHandlerAdapter', defined in class path resource [org/springframework/boot/autoconfigure/web/servlet/WebMvcAutoConfiguration
$EnableWebMvcConfiguration.class], could not be registered. A bean with that name has already been defined in class path resource [org/springframework/web/reactive/config/DelegatingWebFluxConfiguration.class] and overriding is disabled.

It seems that the Spring HATEOAS dependency always forces a servlet based auto configuration via '@import(HypermediaHttpMessageConverterConfiguration.class)' in class org.springframework.boot.autoconfigure.hateoas.HypermediaAutoConfiguration

If I remove the Spring HATEOAS dependency in gradle/maven build file then application starts in reactive mode using netty again as expected.

[odrotbohm]

Can you clarify whether you have seen this working before? Given the arrangement in Spring Boot's auto-configuration, it looks like nothing in particular has changed for this release. I.e. I am suspecting that some parts of the auto-configuration have always triggered inclusion of configuration that somehow triggers Servlet stuff.

Here's a few things I found out during first level debugging:

• the HATEOAS starter pulls in the web starter transitively, which probably causes the bootstrap of the servlet infrastructure. This would have to be removed, I guess.

• if you manually exclude this, Boot will still issue some auto-configuration that depends on some WebMVC stuff:

  Caused by: java.lang.NoClassDefFoundError: org/springframework/web/servlet/mvc/method/annotation/RequestMappingHandlerAdapter
    at org.springframework.boot.autoconfigure.hateoas.HypermediaHttpMessageConverterConfiguration$HalMessageConverterSupportedMediaTypesCustomizer.configureHttpMessageConverters (HypermediaHttpMessageConverterConfiguration.java:70)
  

It looks like this will need a bit more refactoring on the Boot side. I'll ping the team and consult them for how to proceed.

[andifalk]

Can you clarify whether you have seen this working before? Given the arrangement in Spring Boot's auto-configuration, it looks like nothing in particular has changed for this release. I.e. I am suspecting that some parts of the auto-configuration have always triggered inclusion of configuration that somehow triggers Servlet stuff.

I actually tried this the first time with current Spring Boot 2.2.0 milestone 3, so I cannot tell if it was broken before

Here's a few things I found out during first level debugging:

• the HATEOAS starter pulls in the web starter transitively, which probably causes the bootstrap of the servlet infrastructure. This would have to be removed, I guess.
• if you manually exclude this, Boot will still issue some auto-configuration that depends on some WebMVC stuff:

  Caused by: java.lang.NoClassDefFoundError: org/springframework/web/servlet/mvc/method/annotation/RequestMappingHandlerAdapter
    at org.springframework.boot.autoconfigure.hateoas.HypermediaHttpMessageConverterConfiguration$HalMessageConverterSupportedMediaTypesCustomizer.configureHttpMessageConverters (HypermediaHttpMessageConverterConfiguration.java:70)
  

That's what I also tried to exclude the servlet web starter dependency then I also got the same error.

It looks like this will need a bit more refactoring on the Boot side. I'll ping the team and consult them for how to proceed.

Thanks very much :-)

[odrotbohm]

It looks like it hasn't ever worked before. I could get a sample project to bootstrap by explicitly disabling the hypermedia auto-configuration and activating Spring HATEOAS manually:

@EnableAutoConfiguration(exclude = HypermediaAutoConfiguration.class)
@EnableHypermediaSupport(type = HypermediaType.HAL)

There already is a ticket at spring-projects/spring-boot#16020, so I'll close this one in favor of the other.