Spring Boot Auto Configuration not usable in WebFlux project

[jrehwaldt]

Currently, when including org.zalando:tracer-spring-boot-starter into Spring 5 reactive applications Spring cannot start (NoClassDefFoundError: javax/servlet/Filter) because of TracerAutoConfiguration's hard dependency on javax.servlet.Filter.

Caused by: java.lang.IllegalStateException: Failed to introspect Class [org.zalando.tracer.spring.TracerAutoConfiguration] from ClassLoader [sun.misc.Launcher$AppClassLoader@18b4aac2]
	at org.springframework.util.ReflectionUtils.getDeclaredMethods(ReflectionUtils.java:659)
	at org.springframework.util.ReflectionUtils.doWithMethods(ReflectionUtils.java:556)
	at org.springframework.util.ReflectionUtils.doWithMethods(ReflectionUtils.java:541)
	at org.springframework.util.ReflectionUtils.getUniqueDeclaredMethods(ReflectionUtils.java:599)
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.getTypeForFactoryMethod(AbstractAutowireCapableBeanFactory.java:726)
        ....
Caused by: java.lang.NoClassDefFoundError: javax/servlet/Filter

Expected Behavior

The application should start, but disable servlet filtering. All other configurations should be preserved.

Actual Behavior

It fails.

Possible Fix

Extract TracerFilterAutoConfiguration into separate class and add @ConditionalOnWebApplication there.

[whiskeysierra]

Thanks for reporting! Your proposal would fix it, but we wouldn't have proper support for Spring WebFlux afterwards. We have a similar discussion in zalando/logbook#331.

[jrehwaldt]

I know. But at least all other configuration properties such as MDCListener, AspectJ, ... etc would work as expected. All else might or might not be added in the future.

[whiskeysierra]

Yes, makes sense. Would you be willing to open a pull request by any chance?

[jrehwaldt]

I'll have a look next week.