OpenTracing AutoConfiguration crashes with a non-brave OT implementation

OpenTracingAutoConfiguration creates a Brave Tracer if OpenTracing classes are on the classpath.

https://github.com/spring-cloud/spring-cloud-sleuth/blob/master/spring-cloud-sleuth-core/src/main/java/org/springframework/cloud/sleuth/instrument/opentracing/OpentracingAutoConfiguration.java#L40-L41

This crashes the app if a non-brave OT implementation is used. To Repro:

1. Launch SpringBoot2 with a Datadog JavaAgent with no Brave classes present: https://docs.datadoghq.com/tracing/setup/java/#installation-and-getting-started
2. App will crash on startup: java.lang.ClassNotFoundException: brave.opentracing.BraveTracer

As a workaround, the OT configuration can be disabled.

https://github.com/spring-cloud/spring-cloud-sleuth/blob/master/spring-cloud-sleuth-core/src/main/java/org/springframework/cloud/sleuth/instrument/opentracing/OpentracingAutoConfiguration.java#L39

Seems like a simple fix would be to add a check for a brave implementation to the OT autoconfiguration.

@ConditionalOnClass(Tracer.class, BraveTracer.clss)

Happy to open a PR if that helps.

[realark]

@adriancole Thanks for the explanation. That makes sense. Usually better to crash early than behave strangely later.

In terms of putting OT on the classpath, that’s an early design decision of the dd tracer. We’re working to decouple the dd tracer from OT so we don’t have to inject OT classes for javaagent users.

[devinsba]

This won’t actually work the way you are expecting it to. If you disable the BraveTracer and create an instance of some OtherTracer then you will end up with 2 different tracing systems; sleuth (built on brave) and your OT tracer. The OT support in Sleuth is explicitly to allow the use of OT instrumentation not to swap the tracer since sleuth is a tracer itself historically.