Web Image is an experimental backend for Native Image that produces a WebAssembly module from Java code.

Being part of Native Image, see the Quick Start Guide for all the prerequisites for building Native Image.

In addition, Web Image also uses wasm-as from binaryen as its assembler. Version 119 of wasm-as has to be available on your PATH.

brew install binaryen

To build Web Image, run the mx build command in the web-image suite:

cd /path/to/graal
cd web-image
mx build

Use this as you would use the regular native-image tool, but with an additional --tool:svm-wasm flag to enable the WebAssembly backend:

mx native-image --tool:svm-wasm -cp ... HelloWorld

This produces helloworld.js and helloworld.js.wasm in your working directory. The --tool:svm-wasm flag should be the first argument if possible. If any experimental options specific to the Wasm backend are used, they can only be added after the --tool:svm-wasm flag.

The JavaScript file is a wrapper that loads and runs the WebAssembly code and can be run with Node.js 22 or later:

# --experimental-wasm-exnref is only required for Node versions before 25
$ node --experimental-wasm-exnref helloworld.js
Hello World

The WebAssembly code generated by Web Image makes use of various WebAssembly features from WebAssembly 3.0:

• Garbage Collection
• Exception Handling
• Typed Function References

Support for WebAssembly 2.0 is generally assumed.

• Aleksandar Prokopec
• Christoph Schobesberger
• David Leopoldseder
• Fengyun Liu
• Patrick Ziegler