Three main thread libraries are in

Three main thread libraries are in use today: POSIX Pthreads, Windows, and Java. Pthreads, the threads extension of the POSIX standard,
may be provided as either a user-level or a kernel-level library. The Windows thread library is a kernel-level library available on Windows systems.
The Java thread API allows threads to be created and managed directly in Java programs. However, because in most instances the JVM is running on top of a host operating system,
the Java thread API is generally implemented using a thread library available on the host system.
This means that on Windows systems, Java threads are typically implemented using the Windows API; UNIX and Linux systems often use Pthreads.