I have written a JVM in Rust

https://andreabergia.com/blog/2023/07/i-have-written-a-jvm-in-rust/

by lukastyrychtr • 3 years ago

718 181 3 years ago

andreabergia.com

haspok• 3 years ago

Nice project, congrats!

One thing struck me as a bit odd:

In particular, it does not support: generics

What kind of support is there for generics in the JVM? Maybe I'm too naive to assume that due to type erasure on bytecode level everything is just an Object, ie. a reference type? Or do you mean the class definition parser - but then, you don't really have any checks in place to see if the class file is valid (other than the basic syntax)?

xxs10• 3 years ago

pretty much this - generics have (rare) implications to the reflection (but it's unsupported as well) but overall they are replaced with the nearest class/interface when compiled.

OTOH lack of string interning is super strange [it's trivial to implement], and w/o it JVM is not a thing. String being equal by reference is important, and part of JLS.

Lack of thread makes the entire endeavor a toy project.

senorrib• 3 years ago

“I want to stress that this is a toy JVM, built for learning purposes and not a serious implementation.”

3cats-in-a-coat2• 3 years ago

Java strings are compared by reference, if they do not match, they're compared by value. There's no guarantee every single string has a single instance. That would hurt performance.

whizzter• 3 years ago

Not entirely correct, last I checked string interning was ONLY guaranteed for those strings defined in source and read in during class loading, strings created via the String constructor (f.ex. via StringBuilder) CAN duplicate those strings that you hardcoded in your sources, to get the "canonical" string in those cases you have to invoke String.intern() if memory serves me correct.

https://docs.oracle.com/en/java/javase/11/docs/api/java.base...()

Also interning strings to optimize equality checks to be able to use pointer comparison is dangerous for external inputs since iirc at some point interned strings could permanently be stored (unless implemented by a WeakSet) and attackers could fill up your heap (or cause other GC issues since the entire interning functionality is a cache) by filling up your interning lists with crap.

znpy• 3 years ago

Lack of thread makes the entire endeavor a toy project.

yeah, as stated by the author in the line that says "I want to stress that this is a toy JVM, built for learning purposes and not a serious implementation."

newmana• 3 years ago

They might be talking about the checkcast operation: https://docs.oracle.com/javase/specs/jvms/se8/html/jvms-6.ht...

This is generated when you do something like: final Main value = list.get(0);

http://henrikeichenhardt.blogspot.com/2013/05/how-are-java-g...

xxs10• 3 years ago

The cast is added by javac, so it just needs to verify the object on the stack to be compatible w/ the provided class. That part is very simple.

andreabergia5• 3 years ago

Thanks!

About the generics - some people have pointed out the same on reddit, and yeah, you are correct. The only thing that should be done is to read the Signature attribute that encodes the generic information about classes, methods, and fields (https://docs.oracle.com/javase/specs/jvms/se7/html/jvms-4.ht...)

As a matter of fact, I just did a test and the following code works! :-)

    public class Generic {
        public static void main(String[] args) {
            List<String> strings = new ArrayList<String>(10);
            strings.add("hey");
            strings.add("hackernews");

            for (String s : strings) {
                tempPrint(s);
            }
        }

        private static native void tempPrint(String value);
    }

maverwa3• 3 years ago

Great project, congrats! Mad respect.

Started working on something very similar a few years back and gave up pretty soon for some stupid reason. Maybe I should try again, am getting better at getting stuff done.

zote• 3 years ago

Please do

cmrdporcupine6• 3 years ago

You should. I've been doing this kind of stuff on the side for 25+ years, and nothing ever gets "done." But the educational value is much higher than you think. And the skills learned can eventually propagate out into interesting paid work.

I think of them as retirement projects before I retire. When I actually retire I'll maybe finish them.

(That said, I have in the past tried taking jobs that were adjacent to my "research" interests, and found the joy of building these things from scratch is much better than fiddling with the levers on the side of someone else's thing they built from scratch years ago. I like working on and improving production systems, but if they intersect too closely to my personal interests, it can be demoralizing.)

andreabergia5• 3 years ago

Thanks!

I know the feeling - this project, like most of my other side projects, got abandoned a couple of times. But I was really curious about implementing a GC and, for once, I managed to finish something. I'm glad I did! :-)

techn00• 3 years ago

See also https://jacobin.org/ for JVM 17 written in Go.

xmcqdpt22• 3 years ago

Also https://github.com/lihaoyi/Metascala for a JVM implemented in Scala running on the JVM.

dimgl• 3 years ago

Seems… redundant, no?

leshow• 3 years ago

That is a very interesting name for a programming project lol. The Jacobins were a revolutionary political club during the French Revolution in the 1790's. It's also the name of a magazine at https://jacobin.com

snordgren• 3 years ago

It's starts with the letters "ja", that's all that matters for a Java-related project.

cmrdporcupine6• 3 years ago

Great learning project, I'm glad the author is having fun. Implementing a VM from scratch is a blast, and I have learned so much in the past doing that kind of thing.

If they're interested in bolting on a GC, it couldn't hurt to look at MMtk. (https://www.mmtk.io/) Some high quality collection algorithms, written to be pluggable to various VMs, and written in Rust.

sbt567• 3 years ago

Uh, first time hearing mmtk. Thanks for the link!

cmrdporcupine6• 3 years ago

I only became aware of it because a former employer (RelationalAI) was heavily interested in replacing Julia's GC with it (for some workloads): https://pretalx.com/juliacon2023/talk/BMBEGY/

celeritascelery6• 3 years ago

Note that MMTK is x86 only. I was going to use it for a toy project but I have a Mac.

cmrdporcupine6• 3 years ago

That's a bummer -- I guess I never noticed that, when I played with it before it was on an M1 Mac, but compiled into an x86 Julia executable & running through Rosetta (which surprisingly did not suck).

Haven't read, but I bet it's likely related to expectations around the x86_64 memory model & atomics. In the long run I see no reason why it couldn't be made portable, but I imagine the authors efforts are elsewhere for now.

hajmo97• 3 years ago

How often will this post be reposted on HN ?

Recent posts:

https://news.ycombinator.com/item?id=36735344 - 6 days ago

https://news.ycombinator.com/item?id=36717967 - 7 days ago

https://news.ycombinator.com/item?id=36710803 - 7 days ago (OP)

Btw. nice project!

capableweb• 3 years ago

Doesn't really count as a repost if none of the previous submissions didn't get any traction.

dgb232• 3 years ago

I agree, plenty of technically interesting projects don’t get discussion on hn. There’s always a bit of luck and context involved.

freedomben• 3 years ago

Getting the attention needed for front page is a huge chance. I've seen great stuff get posted 5 or more times before it makes it out of obscurity. HN is highly non-deterministic in these things.

bkkz5046• 3 years ago

but more importantly does edbrowse work here?

tenaf0• 3 years ago

Shameless plug of my similar project: https://github.com/tenaf0/rust-jvm3

FrustratedMonky• 3 years ago

Just for kicks, has anybody tried to Rube-Goldberg it, and see how many VM's can stack on top of each other. Like have Java App running on JVM written in RUST, Running on WASM, Running on JVM, etc.. etc...

_joel• 3 years ago

It's turtles, all the way down..

PTOB• 3 years ago

And Logo drives the turtles.

post-it• 3 years ago

See also: The Birth & Death of JavaScript, A talk by Gary Bernhardt from PyCon 2014 (https://www.destroyallsoftware.com/talks/the-birth-and-death...)

Zambyte• 3 years ago

This talk isn't about VMs, but it is about an infinite tower of interpreters. You may find it interesting: https://youtu.be/SrKj4hYic5A

sylware• 3 years ago

Can I compile it with the rust-written rust compiler?

encody• 3 years ago

Missed opportunity to call it Just

orlp• 3 years ago

Just is already 'taken' in the Rust community as it is a command runner utility like make: https://github.com/casey/just

entropicdrifter• 3 years ago

Ooh, I like that. `just build` feels like a good plea to make to the Rust compiler lol

1letterunixname2• 3 years ago

Haha. Shout out to homeboy Xoogler Rodarmor.

brunoborges• 3 years ago

JustVM

celeritascelery6• 3 years ago

I am curious if your ran into limitations due to the lifetimes on this signature

fn execute_instruction( &mut self, vm: &mut Vm<'a>, call_stack: &mut CallStack<'a>, instruction: Instruction, ) -> Result<InstructionCompleted<'a>, MethodCallFailed<'a>>

When I try to add a lifetime to the `Err` variant of a `Result` and that lifetime is invariant (which it is due to `vm` and `call_stack`) it usually means that I can't use the question mark operator or have early returns in the code[1]. This makes error handling more verbose and less readable. Is that your experience as well?

[1] https://users.rust-lang.org/t/nll-and-early-return-not-allow...

celeritascelery6• 3 years ago

EDIT: Looks like this is not an issue because the invariant lifetime 'a is not used for the mutable reference of vm or call_stack. So it's not the invariance that is the problem, but rather how Rust reasons about the lifetime of mutable references, which this avoids.

In that case I don't understand what the point of 'a is on VM and CallStack. You can create[1][2] those with any unbounded lifetime (including 'static[3]), which means it is not constraining anything. What is the lifetime 'a doing here? Why not remove it?

[1] https://github.com/andreabergia/rjvm/blob/be9c54066c64a82879...

[2] https://github.com/andreabergia/rjvm/blob/be9c54066c64a82879...

[3] https://github.com/andreabergia/rjvm/blob/be9c54066c64a82879...

andreabergia5• 3 years ago

I wanted to express the fact that everything that gets allocated (call stack, frames, classes, and objects) is alive and valid until the "root" VM is, thus I used 'a more or less everywhere.

I also struggled with a got a ton of errors from the borrow checker initially, and I fixed many of those with a lot of explicit lifetimes, but it's not impossible that in some places they are unnecessary.

nunobrito• 3 years ago

Good work!

celeritascelery6• 3 years ago

I have a few questions about the garbage collection. One of the hard parts of implementing a garbage collector is making sure everything is properly rooted (especially with a moving collector). you have the `do_garbage_collection` method marked unsafe[1], but don't explain what the calling code needs to do to ensure it is safe to call. How do you ensure all references to the heap are rooted? This is not a trivial problem[2][3][4].

Also note that I cloned the repo and tried to run `cargo test` every test fails with 'should be able to add entries to the classpath: InvalidEntry(".../vm/rt.jar")' vm/tests/integration/real_code_tests.rs:15:10

[1] https://github.com/andreabergia/rjvm/blob/be9c54066c64a82879...

[2] https://manishearth.github.io/blog/2021/04/05/a-tour-of-safe...

[3] https://without.boats/blog/shifgrethor-iii/

[4] https://coredumped.dev/2022/04/11/implementing-a-safe-garbag...

munificent2• 3 years ago

It's pretty straightforward. Their VM maintains its own notion of a callstack instead of using the native callstack. That lets them iterate over it and find all of the parameters and locals on the VM's callstack and use them as roots.

There is a performance cost for a VM having its own virtual callstacks like this, but it makes GC tracing much simpler. (It also makes implementing interesting concurrency and control flow primitives like coroutines or continuations much easier too.)

celeritascelery6• 3 years ago

Seems like that would take care of roots for the bytecode's themselves, but not for "native" functions[1]. Allocating a new object could call gc[2], and native functions are using the native callstack. It seems like it would be easy to allocate in a native function and any unrooted references would be invalidated. In fact I see a case like that here[3]. That method creates a reference with `expect_concrete_object_at` and then calls gc with `new_java_lang_class_object`. It avoids UB by not using `arg` after the call that gc's, but there is nothing stopping you from using `arg` again (and having an invalid reference).

[1] https://github.com/andreabergia/rjvm/blob/main/vm/src/native...

[2] https://github.com/andreabergia/rjvm/blob/be9c54066c64a82879...

[3] https://github.com/andreabergia/rjvm/blob/be9c54066c64a82879...

amelius• 3 years ago

GCs are pretty easy, and just a matter of good accounting. That is, until you start doing concurrent GC then it becomes hellishly difficult.

Agingcoder• 3 years ago

I'm doing a (free) operating system (just a hobby, won't be big and professional like gnu) for 386 (486) AT clones.

:-)

1letterunixname2• 3 years ago

There's a no-std tutorial on how to write a demo kernel in Rust. https://os.phil-opp.com

osdev.org, sandpile.org, RBIL, and freevga. The biggest PITA is hardware support. There are many good vintage hardcopy books with recipes for things like reliable port IO and undocumented hardware tricks.

- Intel® 64 and IA-32 Architectures Software Developer’s Manual Combined Volumes: 1, 2A, 2B, 2C, 2D, 3A, 3B, 3C, 3D, and 4

- Microsoft MS-DOS Programmer's Reference (also includes real-mode BIOS calls)

- PC Interrupts

- Undocumented PC

- PC Intern

- Programmer's Guide To The EGA, VGA, And Super VGA Cards

- Graphics Programming Black Book Special Edition

Also, it's worth toying with advances in OS dev past the era of monolithic, microkernel, and hybrid.

1. Capability-based like seL4. It has a number of inherent performance and security advantages including capabilities and excellent IPC.

2. POSIX compatibility layer. Even embedded OSes without the concept of threads or processes can implement POSIX.

3. Hypervisor. They're much easier to add with intel's VT-[xd]. Failing that, fall back to emulation. Translational emulation is very performant.

4. Get good at generalizing interrupt handlers, making them fast, avoiding race conditions, and using lock-free patterns.

Also:

5. Rewriting or trapping unsupported instructions including x87 and MMX.

6. The failure of pure microkernel was the added complexity and management of sequencing multiple resources in a transactional manner. There are great theoretical security and operational advantages in microkernel architectures but they never caught on widely in a pure form.

vacuity• 3 years ago

2. POSIX compatibility layer. Even embedded OSes without the concept of threads or processes can implement POSIX.

Do you have resources about this? I can't quite fathom how it would work, but then again I have no expertise.

3. Hypervisor. They're much easier to add with intel's VT-[xd]. Failing that, fall back to emulation. Translational emulation is very performant.

Speaking of which, QubesOS was on HN recently. The essence is having many VMs to minimize cross-app attack surface and privilege escalation.

6. The failure of pure microkernel was the added complexity and management of sequencing multiple resources in a transactional manner. There are great theoretical security and operational advantages in microkernel architectures but they never caught on widely in a pure form.

I recently saw an interesting brief article[0] about how memory-safe languages can supercede the compartmentalization that microkernels provide. I'm reminded of Theseus[1], written in rUsT, which happens to reflect the sentiment. I've actually been putting off rereading the Theseus USENIX paper. Again, I'm not nearly qualified to answer whether the security of this is comparable to the best of that of microkernels, barring formal verification. Still, I think it should be explored more.

  [0] https://catern.com/microkernels.html (Write modules, not microkernels)
  [1] https://www.usenix.org/conference/osdi20/presentation/boos

lionkor• 3 years ago

Sorry, but I followed most of this tutorial, just to be greeted with "and luckily enough we dont have to write this because theres a crate for it" every time a new concept is introduced. Interrupts? x86_64 crate does all that. Keyboard handling, etc? Theres a crate for that.

Of course theres a crate for all that, but thats not the point of making an OS.

appleaday1• 3 years ago

Oracle oracle oracle.

nine_k• 3 years ago

This project is like the ground floor of the JVM, not the whole tower. I like though how the project's page is direct and clear about that.

A lot of the foundation and ground-floor mechanics are pretty interesting though.

ChuckMcM• 3 years ago

That is pretty awesome! When I joined the Java effort in '92 (called Oak at the time) the group I was with was looking at writing a full OS in Java. The idea being that you could get to just the minimal set of things needed as "machine code" (aka native methods) you could reduce the attack surface of an embedded OS. (originally Java was targeted to run in things like TV's and other appliances). We were, of course, working in C rather than Rust for the native methods. The JVM in Rust though adds a solid level of memory safety to the entire process.

mshockwave• 3 years ago

writing a full OS in Java

IMAO, Android kind of achieve that...kind of. They write lots of OS logics in Java (or Kotlin) but mixing lots of system services written in native code at the same time, interconnected by the famous (or infamous?) Bind IPC.

tgtweak4• 3 years ago

Embedded JVM is actually huge/pervasive and runs things as benign as the chip on your credit card.

ActorNightly4• 3 years ago

Android is mostly things that run java, not java itself. You can look at the source code, there is a relatively small amount of java in there.

soperj• 3 years ago

It's a modified version of the linux kernel no? That'd be mostly C.

grishka• 3 years ago

Android isn't conventional Java. For starters, its runtime uses its own bytecode (dex) that's based on registers instead of a stack. But then, also, many things that aren't related to GUI are C++ with a thin Java wrapper on top.

When I think about a "Java OS", I imagine a JVM running in kernel mode, providing minimal OS functionality (scheduler, access to hardware I/O ports) and there not being any kind of userspace.

spullara• 3 years ago

There was one for a while though wasn't really targeted at users:

https://en.wikipedia.org/wiki/JavaOS

tgtweak4• 3 years ago

Still exists and thriving today as javacard.

pjmlp6• 3 years ago

Besides the sibling comments,

- SavageJE

- microEJ

- PTC and Aonix bare metal Java runtimes

- SunSPOT mit SquawkVM

bpye• 3 years ago

This idea has definitely been tried a few times [0, 1].

[0] https://en.wikipedia.org/wiki/Singularity_(operating_system)

[1] https://en.wikipedia.org/wiki/Midori_(operating_system)

bingemaker3• 3 years ago

When I see such cool projects, I feel very overwhelmed. How do you get started with Rust and master basics to even attempt doing such a thing? Can OP explain?

nop_slide2• 3 years ago

Likewise. Not to go onto too much of tangent, but on a more personal note I've been generally struggling with this feeling a lot lately.

I've been a professional software developer for almost 10 years, and I _know_ I'm competent (and not an impostor) as demonstrated by my current position and ability to ship things.

However, lately after viewing developer blogs I become overwhelmed that I actually don't know enough and am not a "real" developer. I seem to have formed a notion of an ideal developer in my head and I compare myself against this imagined construct which leads to these feelings. I admire how these people have so much deep knowledge and can express themselves so clearly and concisely, then wonder why I am not like that.

I barely have the energy after work after taking care of my family to do anything further, and I know programming isn't everything but I do have a desire to learn more and improve myself.

I recognize this isn't healthy nor is it rational, but it's just a feeling I can't shake lately.

theLiminator• 3 years ago

Well, you're probably comparing yourself against the top 1% of developers. It's okay to not be the very best, being in the top 30% of this field already is very rewarding.

dist1ll• 3 years ago

What you're describing is very common amongst developers. So common in fact, that I've written a post about this https://alic.dev/blog/comparisons

In short: recognizing your insecurities is the first step. The next step is figuring out what's important to you, shedding impossible to achieve and irrational ambitions, prioritizing your goals in life, and articulating concrete steps to further them.

p91paul• 3 years ago

I happen to personally know the author, and I'm not really surprised he pulled this off. Using him as a baseline of who is a real developer his extremely unhealthy. Please don't :)

bingemaker3• 3 years ago

Thanks for sharing

naltun• 3 years ago

Not OP nor am I a Rust expert. I can speak regarding another technology: sockets.

I've been deep-diving into sockets recently. 2 weeks ago I had only a high-level understanding of sockets (learned from casually reading manpages, docs, blog posts, etc.). I decided to read as much as possible because I wanted to understand networking fundamentals, and after a week I learned enough to write some sockets code in Python and C. I know Python quite well, so reviewing the ``sockets'' library made more sense after my deep dive.

If you want to get better at technology A using language X, I suggest either reading/watching as much as you can about tech A, and build stuff with it in language Y. Then you can circle back to learning language X and you've already mastered much of the concepts around technology A.

e: spelling

aardvark1792• 3 years ago

Break things down. A simple language VM is going to have a way to represent objects in memory, a byte code interpreter, a simple garbage collector, and a way to load things.

A byte code interpret is a stack, some way to represent functions on that stack, and then a loop to interpret beach byte code and move the program counter.

andreabergia5• 3 years ago

Well, _I_ feel impostor syndrome half the times I open HN honestly!

I did have a bit of experience with VMs before, I wrote many years ago a short series of posts about it on my blog, and at my previous job I dabbled a bit in JVM byte code to solve one very unusual problem we had for a customer. I also read the _amazing_ https://craftinginterpreters.com/ years ago and that gave me some ideas.

But this project was definitely big and complex. It took me a lot of time, and it got abandoned a couple of times, like many of my side projects. But I'm happy I finished it. :-)

sn9• 3 years ago

How much do you code in your free time? Like average hours per week?

If it's zero (and no judgement from me if it is; plenty of other things to focus on), then it shouldn't be surprising that someone for whom that number is (speculatively) 10-20 hours per week on average for years has impressive side projects.

15155• 3 years ago

Do some embedded work, implement a bare metal program on an ARM microcontroller in C or Rust. Make a LED blink. Then, make the same LED blink in pure ASM. The RP2040 is easy to bring up.

The magic will fade away quickly.

aardvark1792• 3 years ago

Very well done. Building VMs is always fun, and I’m sure it was an interesting learning experience when combined with Rust’s type system.

If you’re looking for a job then ping me on Twitter, Mastodon or my work email, I’m sure you can figure them out from my user id here.

fmiras• 3 years ago

You're awesome, thanks for sharing!

WhyNotHugo• 3 years ago

built for learning purposes and not a serious implementation

This is a typical way to present a project that ends up replacing the established/existing implementation.

I have written a JVM in Rust

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