How Java Virtual Machine can work on system without virtualization support?

Question

If hardware support is a must for virtualization, how can Java Virtual Machines run on machines without support for virtualization ? Or is JVM not a virtual machine ?

Answer 1

If hardware support is a must for virtualization, ...

Let me stop you right there :-)

There is a difference in concept between the JVM (software virtualization) and (for example) a VMWare VM (hardware-assisted virtualization).

The JVM (and other software-based VMMs such as the ones that allow to to emulate x86 on Solaris hardware - I think Bochs and possibly DosBox fall into this category) runs like any other application, using the operating system to gain access to the hardware, or emulating its own hardware purely in software.

VMWare, and the other VMMs optimised for speed, rely on hardware support. In other words, they run on the hardware as if they have full access to the hardware and, only when they try to do something they're not supposed to does the OS captures that attempt and fake it.

That's why VMWare runs so much faster than the software-only emulators. It's because, for the vast majority of the time, it's actually running on the real hardware.