How Java multi-threaded program is able to use multiple CPU cores?

Question

Could someone please provide explanation how Java multi-threaded program (e.g. Tomcat servlet container) is able to use all cores of CPU when JVM is only single process on linux? Is there any good in-depth article that describes the subject in details?

EDIT #1: I'm not looking for advice how to implement multi-threaded program in Java. I'm looking for explanation of how JVM internally manages to use multiple cores on linux/windows while still being single process on the OS.

EDIT #2: The best explanation I managed to find is that Hotspot (Sun/Oracle JVM) implements threads as native threads on Linux using NPTL. So more less each thread in Java is lightweight process (native thread) on Linux. It is clearly visible using ps -eLf command that print outs not only process id (PPID) but also native thread id (LWP).

More details can be also found here:

• http://www.velocityreviews.com/forums/t499841-java-5-threads-in-linux.html
• Distinguishing between Java threads and OS threads?

EDIT #3: Wikipedia has short but nice entry on NPTL with some further references http://en.wikipedia.org/wiki/Native_POSIX_Thread_Library

Answer 1

The Linux kernel supports threads as first-class citizens. In fact to the kernel a thread isn't much different to a process, except that it shares a address space with another thread/process.

Some old versions of ps even showed a separate process for each thread by default and newer versions can enable this behavior using the -m flag.

Answer 2

The JVM is a single process with many threads. Each thread can be scheduled on a different CPU core. A single process can have many threads.

When Java software running inside the JVM asks for another thread the JVM starts another thread.

That is how the JVM manages to use multiple cores.