In your view advancement is that in "green threads" they overloaded Thread.sleep(..), so it doesn't call real Thread.sleep() but doing something like Futures +

In your view advancement is that in "green threads" they overloaded Thread.sleep(..), so it doesn't call real Thread.sleep() but doing something like Futures + ExecutionService underneath instead. Java already had tons of non-blocking io/http and many other frameworks without "green threads" but using futures and executionservice. Green threads look like syntactic sugar.