[brabel]

It's not recommended though.

See https://openjdk.org/jeps/429

If you keep ThreadLocal variables, they get inherited by child Threads. If you make many thousands of them, the memory footprint becomes completely unacceptable. If the memory used by ThreadLocal variables is large, it also makes it more expensive to create new Threads (virtual or not), so you lose most advantages of Virtual Threads by doing that.

[bberrry]

I don't think that's correct. ThreadLocals should behave just like on regular OS threads, the difference is that you can suddenly create millions of them.

You used to be able to depend on OS threads getting reused because you were pooling them. You can do the same with virtual threads if you wish and you will get the same behavior. The difference is we ought to spawn new threads per task now.

Side note, you have to specifically use InheritableThreadLocal to get the inheritance behavior you speak of.