On Windows NT as well. On my first job around 2000 I did a little bit of Java programming. As far as I recall, JVM scheduled all their threads on top of a single OS thread.
Windows NT natively supported threads when Java was released, and Java on Windows NT (and Windows 95) in 1996 used OS threads. There were some problems when running on machines with more than one CPU, but I never figured out if this was the the fault of the OS or the JVM.
On Windows NT as well. On my first job around 2000 I did a little bit of Java programming. As far as I recall, JVM scheduled all their threads on top of a single OS thread.