Role of the JVM

Question

Would the JVM (and probably also the CLI) be considered a virtual machine (the equivalent of the x86 in a "normal" program stack) or a virtual OS (the equivalent of Windows)?

Answer 1

Strictly speaking, it is a virtual machine, ie: it executes a special low-level language (similar to x86 ASM. CLI uses MSIL, JVM uses "byte codes") and translates them into the target machine's op-codes (x86, x86_64, ARM .. etc.) for execution on the host CPU.

It also manages marshaling (ie: correct handling and passing through of variables to native memory stack/heap) to allow function calls from inside the managed world to the outside OS on which the VM runs.

Practically though, neither the JVM nor the CLI alone are very helpful except for automated garbage collection and CPU-architecture-independence, but they are complemented by a large base library (the Java classes, or the .NET BCL) which allows you to do many platform-y things without having to call platform specific APIs and use marshaling manually for everything.

That's why there is a different Java Runtime Environment for each OS. Each one's JVM translates to a specific CPU arch, and uses different platform specific-APIs to accomplish what the unified base library exposes to you as a friendly API inside the managed world.

Hope that helps you.

Answer 2

The jvm is considered a real computer, only not realized in hardware. The machine has it's own storage capacity, it's own memory model, it's own specific behaviour of it's central processing unit and it's own internal machine code. This machine is extendable with new possibilities and modules that are represented with classes, API's, etc...

It has it's own stack based architecture, like most virtual machines.