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

[Andrew Dinn]

On 20/11/2024 13:02, Glavo wrote:
> I agree with what Andrew said:
> 
>     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.

regards,


Andrew Dinn
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