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

A JVM is not virtual in the same sense as a VirtualBox or VMWare virtual machine. It is a 'machine' that implements the Java bytecode, not a virtualized version of actual hardware.

The term-of-art 'virtual machine' was coined a very long time ago for the following scenario:

1. make up a computer, like Knuth's MIX.
2. write a computer program that implements the made-up computer.
3. run programs

When this virtual machine runs, it's a completely ordinary program, running completely in user mode. It needs no special help from the hardware or operating system to work reasonably well. This is especially true of the JVM, since the Java byte code does not deal with low-level hardware I/O or other things which are hard to simulate.

Later, historically, (to pick a particular instance), IBM invented VM/370. VM/370 uses the other sense of the term 'virtual machine'. In this later sense, the hardware and operating system cooperate to allow a single physical machine to host multiple virtual instances of (more or less) the same architecture, in which multiple copies of the whole operating system are written as if they are running on more or less bare hardware. Later, the X86 was designed with features to facilitate this.

So, yes, any virtual machine is making use of some physical hardware, unless you implement it with pieces of paper passed around a table (pace John Searle). But when the virtual machine bears no resemblance to the machine it is running on, then there's no need for special help from the operating system and hardware, and no need for anything as complex as VM/370, or VMware.