I can create a PR for this if this seems like a good idea to @lihaoyi

Go for it, though note we'll need some compatibility story for Java <21

Is this something that #74 unblocks?

I would recommend against this until JAVA-23 is released, du to the "pinning issue", effectively making Virtual Threads useless if your code/libs uses synchronized a lot.
https://www.infoworld.com/article/3713220/java-virtual-threads-hit-with-pinning-issue.html

Doing it opt-in should be fine. We can run benchmarks etc. without synchronized to see the benefit. That way we can be ready on day one when Java 23 lands with a fix to the thread pinning issue.

I have done this at $work for all my services, with Java 21, it works fine, no need to delay to Java 23.

If there are issues because of undertow, you may consider switching to Helidon, which is the new web server from Oracle, built for working with virtual threads.

https://helidon.io

https://github.com/helidon-io/helidon/tree/main/webserver

https://central.sonatype.com/artifact/io.helidon.webserver/helidon-webserver/overview

https://javadoc.io/doc/io.helidon.webserver/helidon-webserver/

FYI for anyone reading, this bounty is still open in case anyone else wants to take a crack at this

@lihaoyi Since I started this issue, I am going to finish it too. :)

while building cask, twirl throws java.lang.reflect.InvocationTargetException

while building cask, twirl throws java.lang.reflect.InvocationTargetException

I get that too when building on JDK >= 21. Maybe the newer 2.0.x works better.