Sbt run with javaagent

[fokot]

I want to run our app with opentelemetry java agent like java -javaagent:path/to/opentelemetry-javaagent.jar -jar myapp.jar. When I build the app and run it after dist it works. But I was not able to make it run with sbt run. The things I tried
These doesn't work:

• sbt run in play app with addSbtPlugin("com.github.sbt" % "sbt-javaagent" % "0.1.8") + settings (I know they work as they work in non play app)
• sbt run in play app with

run / fork := true,
run / forkOptions := ForkOptions().withRunJVMOptions(
   Vector(
     "-javaagent:opentelemetry-javaagent.jar"
))

These works:

• sbt run in non play app with addSbtPlugin("com.github.sbt" % "sbt-javaagent" % "0.1.8")
• sbt runMain SomeHelloWorldMainClass in play app with addSbtPlugin("com.github.sbt" % "sbt-javaagent" % "0.1.8")

What should I do?

[ihostage]

Hi, @fokot!
I'm not sure that it's possible for DEV-mode right now. For example Kamon wrote a special Sbt plugin to add their javaagent (Kanela) into Play DEV-mode.

[mkurz]

I think run / fork := true does not even have an effect here. Play implements it's own run task and I think fork is not considered anyway.

So the app will be run in the same JVM process like sbt. So would you can try is:

1. Create a file called .sbtopts in the root of your project (the folder where you start sbt)
2. To that file add:

-J-javaagent:opentelemetry-javaagent.jar

And restart sbt. Let me know if it works then.
Of course this is just for dev mode (when running sbt). For production you need to use another approach.

BTW: Instead of .sbtopts you can also use a .jvmopts file but without the -J:

-javaagent:opentelemetry-javaagent.jar

More details here: https://www.scala-sbt.org/1.x/docs/Command-Line-Reference.html#Sbt+runner+arguments

[ihostage]

Usually, tracing agents like as OTel agent instrument classes that loaded by PlatformClassloader in preMain function. That why I think it won't work with Play DEV-mode that loads classes by custom ClassLoaders and that why Kamon copy&paste our run task and add a few logic to instrument Play application in DEV-mode.

https://github.com/kamon-io/sbt-kanela-runner/blob/4af613be5dce2736cfefb90946bc684d1c3c8f25/sbt-kanela-runner-play-2.8/src/main/scala/kamon/instrumentation/sbt/play/KanelaReloader.scala#L279

But, of course, we can add an abstract solution for any Java agents (not only Kanela) in Play-Run-Support module in the future 🤔

[fokot]

I will try it out and let you know. Running in prod is no problem (it works) but I want to test things in dev mode. I do not need dynamic recompile. I just do not want to run dist all the time. Stopping and running run after code change would work for me. And it works with non play app.

[fokot]

With .sbtopts agent is starting, I see it logs, but I do not see any metrics, so probably it is not attached to run, even tough forking is off.

[fokot]

Is there a way to start play app with runMain and some class? I don't care about hot reload. I would restart sbt after every change

[ihostage]

Try to use play.core.server.ProdServerStart as a main class with runMain task.

[fokot]

I had some problems, but probably with our configuration

[fokot]

runProd is working with sbt-javaagent plugin.