Sorcery with WebAssembly, GraalVM, and Java!

Alternative title: How I like to write my JavaScripts: in Java via WebAssembly!

TL;DR: the code is here if you want to try it! But, I suggest you read on.

Second: I don't have the ability to test or document any of this on Windows. I'm sorry. Though I've committed the final built WebAssembly artifact so one imagines that, given node on Windows, you could run the application...

OK, read on!

One of my favorite doctors, Dr. Niephaus, is at it again!

He's doing a presentation at WASM.io this very day looking at how to use a new WebAssembly backend for GraalVM! Yes, you heard me, you can write Java code and target WebAssembly!

I've waited a long time for this possibility! They first teased it in the innocuous waining paragraphs of a blog looking at the then nascent support for RISC-V! I've been waiting for it ever since!

The full length talk from WASM.io should be online soon, and I can't wait to watch it and greatly expand upon the pittance of a scrap of a rotten morself of knowledge that is this post on which we must contentedly dine, for the moment...

Now, there are - as you might imagine - several limitations. First, it's nowhere near done! Second, it's work that's being done on at the very least GraalVM for Java 25. Third, it's targeting Oracle GraalVM (as opposed to GraalVM community edition). Fourth, there's no networking support.

The third one isn't necessarily a dealbreaker for a lot of folks, but keep it in mind. Not sure whether this stuff ever lands in the community edition. I'm also not sure it matters, since the licensing for Oracle GraalVM is so permissive with respect to cost (free) and usecases. I'm not a lawyer and my knowledge of the situation could be out of date.. blah blah. Anyway, I'm locked in.

So, next question. How do i get this early access JDK?

I'm using the excellent SDKMAN.io, which as of 28 March, 2025, doesn't yet have a distribution for this early access build of GraalVM for Java 25, so I just needed to do so manully.

Here's a script that - depending on whether you're on macOS or Linux and x86 or ARM - downlaods the right version of the JDK by consulting this releases file.

#!/usr/bin/env bash
source "$HOME/.sdkman/bin/sdkman-init.sh"

function setup(){
  if [[ "$OSTYPE" == "darwin"* ]]; then
      platform="darwin"
  elif [[ "$OSTYPE" == "linux"* ]]; then
      platform="linux"
  fi
  arch=$(uname -m)
  if [[ "$arch" == "x86_64" ]]; then
      arch="x64"
  elif [[ "$arch" == "arm64" ]] || [[ "$arch" == "aarch64" ]]; then
      arch="aarch64"
  fi
  json_url="https://raw.githubusercontent.com/graalvm/oracle-graalvm-ea-builds/refs/heads/main/versions/25-ea.json"
  temp_file=$(mktemp)
  curl -s "$json_url" > "$temp_file"
  if [ $? -ne 0 ]; then
      echo "Failed to download version information"
      rm "$temp_file"
      exit 1
  fi
  latest_version=$(jq -r '[.[] | select(.latest == true)] | .[0].version' "$temp_file")
  base_url=$(jq -r --arg version "$latest_version" '[.[] | select(.version == $version)] | .[0].download_base_url' "$temp_file")
  filename=$(jq -r --arg version "$latest_version" --arg arch "$arch" --arg platform "$platform" \
    '[.[] | select(.version == $version)] | .[0].files[] | select(.arch == $arch and .platform == $platform) | .filename' "$temp_file")
  echo $filename
  download_url="${base_url}${filename}"
  curl -o graalvm.tgz -L -O "$download_url"
  OD=temp_extract
  mkdir -p $OD
  tar -xzf graalvm.tgz -C $OD
  cd $OD
  folder=$(  find . -maxdepth 1 -type d -mindepth 1   | head -n1 )
  echo "the folder is $folder "
  mkdir -p $HOME/bin
  install_dir=$HOME/bin/${folder}
  mv $OD/${folder} $install_dir
  sdk install java 25.ea.15-graal $install_dir/Contents/Home
  
}

sdk list java | grep 25.ea.15-graal || setup

That's a little much, perhaps, but it works for me. I now have Oracle GraalVM for Java 25 installed and I can now make it the default for my system:

sdk default java  25.ea.15-graal 

If everything's worked, you can do:

java --version 

Now let's try a simple example. Create a new directory, say sample. In there, create a new class called Main.java.

public class Main {

	public static void main(String [] args) {
		System.out.println("Hello, WebAssembly!");
	}
}

Now you'll test that it's worked by compiling and running it on the JRE:

javac Main.java && java Main 

Now we're ready to make this work in WebAssembly. But first, you'll need a set of tools in a package called binaryen. I'm on macOS, so I use Homebrew.sh: brew install binaryen. Easy!

You can get a typical GraalVM native image thusly:

javac Main.java && native-image  Main  

Run ./main in the same directory. Zoom!

If that's worked, then we can finally do the thing and get a WebAssembly artifact:

javac Main.java && native-image   --tool:svm-wasm Main  

You should see three files:

...
main.js
main.js.wasm
main.js.wat
...

You can run main.js with node (tho, note you'll need Node 22 or higher!) thusly:

node main.js

Do you see Hello, WebAssembly!? If so, congrats! You just wrote your first real Java.. Script!

The possibilities here are endless! Did you see they even hava javac running in the browser?