From an API perpective, you can always use reflection to cheat past the option to create virtual threads in pre-21 (without previews) java bytecode, but you nee

From an API perpective, you can always use reflection to cheat past the option to create virtual threads in pre-21 (without previews) java bytecode, but you need to do more to your code than just flip the switch to support virtual threads. A virtual thread thread pool by definition is unbound. If you're binding data to a thread (eg. Thread locals, you now have a seemingly unbound list of threads that is now effectively a memory leak). I bumped into that one a few months ago with Netty that has a per thread cache for some things (thankfully you can turn off that cache). It was creating a significantly large waste of RAM that slowed down the application alone. The other big one is as I mentioned the synchronized limitation. If you assume naively that anything can run in a virtual thread without worries, you're opening yourself up to deadlocks or at least significantly low performance code if you're relying on libraries/code that are synchronized using java monitors. There may be more examples of gotchas, but these two are the most notable examples I have right now.