Rusty Java - Transition the Java Runtime from C/C++ to Rust.

[Jesper Wilhelmsson]

Hi Eric,

As you probably already figured out, this is a no-go for so many reasons. I'll just add another one on the pile for good luck.

Even if we could just snap our fingers and have the perfect translation of the JVM in rust, just ready to go, all tests passing, drop in replacement in the src directory, we could still not do it.

The JVM is developed and maintained by engineers who in many cases have worked with C++ development for decades. They know the C++ language, they know the JVM source code. Re-writing things in a different language like rust would unavoidably change how things are implemented, like synchronization/locking or gory details in the GC implementation. There are things in the JVM internals that are built the way they are because C++ works the way it does. Moving to a new programming language would change how these things are implemented - not only the syntax of the language, but the algorithms themselves as well.

This means that not only would you have to teach all JVM developers world wide a new programming language and new tools for development and debugging that at least some of them have never seen before and have no interest in learning, which in itself is a major and truly expensive task, you would also loose all institutional knowledge these developers have about how things in the JVM works after having worked in the code base for decades.

Change for the sake of change is almost never a good idea. We routinely say no to changes that cleans up code or make use of new language features if they don't bring an actual benefit as well. Cleaning up code that no one is working in is not really useful for instance. Moving all code to a new language would change code that has been running without bugs or changes for 20+ years. I started with the assumption that we could do the translation perfectly without bugs, but we all know that's not true. There will be bugs introduced all over the place. New interesting bugs that will take years to weed out. So unless your intention here is to destabilize the Java platform for many years to come and force a large group of C++ developers to start looking for a new job because they actually are C++ developers by choice ...

As all developers who suggest changes to the JDK should have done already, I suggest you also read https://openjdk.org/guide/#why-is-my-change-rejected

/Jesper

On 20 Nov 2024, at 18:52, Kennke, Roman <rkennke at amazon.de> wrote:



  If we're translating the Java runtime into a better language, that
  language would be Java.


Although rewriting HotSpot in Java would be equally hard work, it would
be much more rewarding and interesting.
Having worked on two Java-in-Java VMs (JikesRVM and SubstrateVM -- oh
and also a Lisp-in-Lisp OS i.e. Interlisp-D) I can agree to the
interesting but I'm much less sanguine about the 'rewarding'.

There are many aspects of how pure Java operates that make implementing
Java in Java a significant challenge. You end up having to allow some
smaller or larger subset of the language to be 'impure', i.e. get
substituted/compiled specially so it is not actually operating like Java
code in one way or another. That's necessary in in order to be able to
perform operations that are not constrained by the the Java's highly
opinionated and deliberately constrained memory model, threading and
synchonization model, etc. You also still need to pirate on native
libraries to do things like i/o (unless you relish writing things like
device drivers, file system management, etc in Java -- can be done, just
as it was with Interlisp-D, but pheweee!).

Once you concede that latter point you have to ponder where exactly it
is best to draw the lines between Java code, code masquerading as Java
code but compiled/translated as if it were something else and explicitly
invoked foreign code. I actually think Hotspot makes a very good
trade-off, implementing a lot of stuff in the JDK, providing various
flavours of intrinsics and JVM callouts to sidestep some of the language
limitations and using standardized foreign libraries to do a lot of the
heavy lifting.

Also, now that we have a JPMS we can maintain those boundaries very
flexibly, redrawing the line to accommodate to new hardware, new
libraries and new language features (e.g. as was done with method
handles). It does not really do to be absolutist here.


I agree with all that you said.

Plus, we should not forget the cost-benefit equation: HotSpot is a *huge* code base with dozens or more engineers having poured more than quarter century of work into it. Rewriting this in Rust or any other language would be a prohibitively expensive undertaking, with comparably little benefit, IMO.

I might like the idea to write *new* components in Rust, or replace certain subsystems with new implementations when it makes sense, and if we can figure out how to interface between the C++ and Rust parts. It has actually already been done before. IIRC, the research group around Steve Blackburn wrote a new (research-y) HotSpot GC entirely in Rust.

Roman




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