So, the agent intercepts the classes being loaded and inserts stubs and bits so that classes can be hotswapped.

But once the JVM has a class to run, how can it find and access added fields to other classes. Hotswap doesn't allow it, but even with "modified" classes, how can this work? What's the catch. How does it affect performance?

That is beautiful. Let's create a language and start hacking it. Why not add multiple inheritance on fly, for example?

People use Java specifically because it is NOT dynamic. Those who like dynamic ones go to scripting languages. For "increased productivity" you will pay with maintenance nightmare.

JavaRebel doesn't make language dynamic, it doesn't impose any dependencies on the application, it just reloads compiled classes. And your are only supposed to use it in development, so there is for sure no maintenance involved.

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cheers
Aman

Sorry - simple misunderstanding, and I like the part about using it in development. I was partially misled by leo's message.

which was probably because I didn't explain how and why I use scripting, and would like to replace it. I use it to reduce the development cycle of edit compile run observe stop edit, etc.
In my opinion this seriously hampers productivity in a huge way. If you can run the program, and fix it up, develop it while it's actually running, you can interactively develop.
The closest thing to getting that experience is (was) by using scripting to wire together objects (which themselves are probably Java objects, but could be scripted too). I put a lot of the business logic in these scripts, while the (java) objects provide low level functions like "withdraw from account", "deposit into account", "start transaction", "end transaction", etc.

But with Java becoming dynamic through the use of hotswapping you can have all the code in Java. Java codes well, it's clean, precise, predictable, debuggable, typed, etc, all the good stuff.
Javascript IMO kind of sucks. I used to like it, but once you get complex with it it just sucks - it's just NOT the cute little language at all in anything but trivial settings.
I don't like a lot of the other scripting languages either, like Python and Ruby. Groovy, maybe. Java also doesn't suffer from the easy-come-easy-go fashionable phases - it's just clean and keeps on going and going.
If only we could keep out highly questionable constructs like the recent property proposals that would be great...

Anyway - making Java dynamically programmable is absolutely huge. We need to have this feature built into the JVM. Hotswap got us only part of the way.