Does the JVM or the underlying OS handle thread state changes

Question

When I create a multi-threaded program and I use methods such as Wait or Signal to control threads among other things, does JVM control all the thread state changes or does the underlying OS have anything to do with it.

Answer 1

It depends on the implementation of the JVM. Most modern JVM's (Suns HotSpot, Oracles JRockit, IBM VMs) will use the Operating system threading model as this will give the best performance.

Early implementations used green threads - The VM was modelling the threads using itself. This was typically used when the platform or operating system it was running on didn't support threading. For example, in Java 1.1, Green Threads were used on Solaris. At the time, the common pattern to use multiple cores/CPU's in Solaris was to use multiple processes - only later were threads added to the Operating System.

The Java Language Specification does not specify how Threads must be implemented but in general, if the OS has threading support, modern JVM's will use the OS implementation. When there is no support in the OS, for example on low end mobile phones or in a Java Card implementation for example, then Green Threads will be used by the runtime.

Answer 2

In the early days of linux 2.4, at least the IBM JVM used separate processes to implement java threads. This resulted in a long time to switch between threads, as the system needed to activate a completely different process each time.

Answer 3

In general, Java threads will map to OS threads and Java will make use of OS synchronisation primitives to implement synchronized/wait/signal/..., but the mapping is not as straightforward as you might think. In fact, the JVM uses some clever tricks to improve performance and implements as much of the synchronisation code itself (at least the uncontended case).

If you are really interested in the details, have a look at the JVM source code or at cmeerw.org/notes/java_sync.html which provides some overview of how Java's synchronisation primitives are implemented on Linux and Solaris.