How to create custom JVM ?

Question

I was reading item #6.10 on http://www.cafeaulait.org/javafaq.html and I began wondering how the big players go about creating their own implementation of a JVM. Would an experimental something or another be possible (and feasible) for one guy?

Answer 1

For one thing, you may want to have a look at Apache Harmony They have come a long way, so their project history may actually give you a good idea on the effort required. I myself would not bet on it being feasible for one guy

Answer 2

technically, all the information people need to create a new JVM is in the public specifications for the language and the targetted platform. A JVM would need to behave differently depending on whether it is meant to run on a desktop computer or a mobile phone, even if the bytecode interpretation would be largely identical.

A few places to start looking for information:

http://java.sun.com/javame/licensees/index.jsp
Reading The "Java Virtual Machine Specification" by Tim Lindholm
http://www.jcp.org/en/jsr/detail?id=30

From what I have seen of JVM implementations by Sun, IBM or smaller companies like Esmertec, writing a simple JVM is a several man-months project but adding JSR after JSR to support more functionality can take years afterwards.

Now, if all you need is a simple bytecode interpreter, it's not that bad, but it's still quite a bit of code to write.

Answer 3

I understand that, currently, the big players license the Java library from Sun. They then add their own refinements. The main difference between implementations is the bytecode->machine code compiler.

Answer 4

A handmade JVM would be a great way to learn about virtual machines in general, the issues of program language design (through the JVM spec) and the nitty gritty of parsing and so forth.

If you choose to take it in that direction, you could also explore optimizations, which is where it can get interesting, and you can take research papers and implement their algorithms.

That being said, if you're less interested in the long and arduous task of creating a VM from scratch, you might want to modify an existing open source VM like Kaffe. It will show you what a virtual machine does, but not necessarily how Java code works in Sun's JVM:

Kaffe is a clean room implementation of the Java virtual machine, plus the associated class libraries needed to provide a Java runtime environment.

This way, you could study the details, but dive in to implementing more interesting features.