Question: **I imagine that no, they aren't, because every process has its own memory space, of course.**

But how does the whole JVM thing actually work? Is there a separate JVM in a separate process for every Java program that I launch? Do Java programs running in a system share anything at all? Are there differences between OSs and JVM implementations? Can I *make* programs share variables (i. e. directly through the JVM rather than the usual IPC mechanisms)? Are there more exotic one-process JVMs for special purposes?

Generally, what are recommendable reads about the guts of JVMs? The [spec](http://java.sun.com/docs/books/jls/third_edition/html/j3TOC.html)? The source code of some implementation? Websites? Books?

Answer: 
> 
> In Java, are static class members
>  shared among programs?
> 
> 
> 

A class is defined by its full name and the class loader that loaded it. If the same class is within the same JVM process and the two programs loaded the class through the same class loader then the static members are shared. Classloading rules are of extreme importance.

> 
> I imagine that no, they aren't,
>  because every process has its own
>  memory space, of course.
> 
> 
> 

If you are using two separate JVMs to launch two apps, you are correct. But take the case of application/servlet containers such as tomcat: they load several apps through the same process (the tomcat host process).

> 
> But how does the whole JVM thing
>  actually work? Is there a separate JVM
>  in a separate process for every Java
>  program that I launch? Do Java
>  programs running in a system share
>  anything at all?
> 
> 
> 

Every time you type `>java -cp...` at the command line, you are creating a new process. Bear in mind that when you run eclipse or call an ant java task with `fork=true` you are also creating new processes.

> 
> Are there differences between OSs and
>  JVM implementations? Can I make
>  programs share variables (i. e.
>  directly through the JVM rather than
>  the usual IPC mechanisms)? Are there
>  more exotic one-process JVMs for
>  special purposes?
> 
> 
> 

Like a poster said, there are projects like Terracota that facilitate this for you. A general approach for this kind of sharing is a distributed cache.
Just to follow on to what Kevin mentioned, there are no JVM implementations that I am aware of that allow you to share static variables across JVM instances. As previous answers have stated (correctly) - each JVM instance is it's own process.

Terracotta (which is a clustering technology, not a JVM) allows for arbitrary sharing of object instances across JVM process boundaries - including sharing static variables. Thus the moniker "network attached memory". For all intents and purposes, when using Terracotta technology in your JVM process, all of the threads in all of the VMs behave as if they were all looking at a single, large, shared heap. (With caveats of course, since that's not physically possible, Terracotta manages it through sophisticated sharing and replication algorithms which involve network io to move data - so therefore there are times when latency and throughput will not compare to native memory access)

There are abundant examples in the Cookbook section of the Terracotta site - I recommend you start with the Hello [Clustered Instance Recipe](http://www.terracotta.org/web/display/orgsite/Recipe?recipe=helloclusteredinstance) to get a feel for how it works.