Hi Zac,
I think working actively on the now a bit dead GCJ project
https://gcc.gnu.org/java/ would be more valuable than trying to pack a
jvm + your application into an MCU, because I definitely think that the
nature of the JVM platform doesn't make it runnable inside 512kb RAM.
Additionally I don't see why one would want Java and it's garbage
collector thread in a RTOS, because you cannot do Real Time with Java,
or really hardly otherwise the garbage collector thread will do it's
work in the middle of your execution and you won't have a deterministic
execution time.
You can naturally write the Java code in a really hard way: ensuring you
preallocate all objects and never deallocating, that is never creating
temporaries inside your main loop. Additionally JITting with a vm
requires a cache where your store the compiled code.
The only way you would really get a good java support on an RTOS would
be to change the runtime platform nature of java, that is :
- Compiling java as binaries, with an ABI similar to C++ one.
- Replacing the garbage collector concept with a reference counted
automatic java reference management (std::shared_ptr like concept)
Then I think you could get it to work and find real use case with RTOS.
But you'll have some more issues, because Java object system uses
function call dispatching mainly via a VTABLE as in C++ with the virtual
keyword (i.e. Basically java has all functions being virtual, and by the
way overridable), and this typically has a serious impact on
performance, because the compiler doesn't know which code path will be
taken before execution and cannot arrange the binary well for this scenario.
So I think java on the targets that riot is thought for might not be
really interesting.
But you have C++ which is a really pretty language and offers all
paradigms and expressiveness of Java and even more. If you know how to
code in java, you can code as well so in C++, and by learning it you'll
discover that you can employ other paradigms to improve readability and
quality of your code (i.e. With tag & SFINAE dispatching, functional
programming, compile time checking).
In case you still want to work on this, I would advice you to look at
the LLVM platform, there exists already some projects capable of
compiling java bytecode into LLVM bitcode, that you can in the end
transform to a native executable well optimized. Actually the OpenJDK
uses the LLVM platform to just-in-time compile java.
https://llvm.org/svn/llvm-project/java/trunk/docs/java-frontend.txt
Cheers,
--
Damien Buhl
alias daminetreg
On 29/01/2016 20:16, Zac Harvey wrote:
> Hi all,
>
> I was wondering if the following would be *possible* to do (but not
> necessarily easy or the most elegant solution):
>
> 1. Fork the open source OpenJDK HotSpot Java Virtual Machine code
> (written in C). Refactor its code to automatically look at a particular
> memory address to load an executable JAR.
> 2. Compile that forked HotSpot VM code along with RIOT-OS and generate
> an executable for a given platform (say 32-bit ARM)
> 3. Flash the executable to the ARM chip
> 4. Flash a JAR to some ROM that is memory addressable from that chip
> 5. ...
> 6. Profit.
>
> The intention here is that when the chip powers up, the RIOT-OS app
> (again a forked HotSpot JVM) loads an executable JAR from a specific
> place in memory. Boom: you're running Java on a RTOS.
>
> I understand there are various projects out there attempting to run a
> JVM on an embedded tech (Java Card, etc.). But to my knowledge, none of
> these projects leverage the above proposed technique. And the reason for
> that is either:
>
> 1. I'm overlooking something obvious that makes this approach a
> non-starter; or
> 2. No one's ever tried this before
>
> I'm willing to bet that its the former reason and not the latter. But in
> that case, I'd be interested in what the things are that I'm overlooking!
>
> Thanks for any-and-all-help!
> _______________________________________________
> devel mailing list
> [email protected]
> https://lists.riot-os.org/mailman/listinfo/devel
>
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