Experimental feature in GraalVM

GraalVM Implementation of WebAssembly

GraalVM can run programs compiled to WebAssembly. It can interpret and compile WebAssembly code in the binary format or embed it into other programs. The support for WebAssembly is in the early stages of its development.

Getting Started #

As of GraalVM for JDK 21, the GraalVM WebAssembly runtime (known as Wasm) is available as a standalone distribution. You can download a standalone based on Oracle GraalVM or GraalVM Community Edition.

  1. Download the Wasm 23.1 standalone for your operating system:
  2. Unzip the archive:

    Note: If you are using macOS Catalina and later you may need to remove the quarantine attribute:

     sudo xattr -r -d com.apple.quarantine <archive>.tar.gz


     tar -xzf <archive>.tar.gz
  3. A standalone comes with a JVM in addition to its native launcher. Check the version to see GraalVM WebAssembly runtime is active:
     ./path/to/bin/wasm --version

Running WebAssembly Programs #

You can run a program written in the language that compiles to WebAssembly on GraalVM. For example, put the following C program in a file named floyd.c:

#include <stdio.h>

int main() {
  int number = 1;
  int rows = 10;
  for (int i = 1; i <= rows; i++) {
    for (int j = 1; j <= i; j++) {
      printf("%d ", number);
  return 0;

Compile it using the most recent Emscripten compiler frontend version. It should produce a standalone floyd.wasm file in the current working directory:

emcc -o floyd.wasm floyd.c

Then you can run the compiled WebAssembly binary on GraalVM as follows:

wasm --Builtins=wasi_snapshot_preview1 floyd.wasm

In this example, the flag --Builtins specifies builtin modules that the Emscripten toolchain requires.

Embedding WebAssembly Programs #

The compiled WebAssembly binary code can be accessed programmatically with GraalVM Polyglot API, which allows embedding GraalVM WebAssembly into user programs. Here is a simple example of how to call WebAssembly code from a Java application:

import org.graalvm.polyglot.*;
import org.graalvm.polyglot.io.ByteSequence;
//Load the WASM contents into a byte array
byte[] binary = readBytes("example.wasm");
Context.Builder contextBuilder = Context.newBuilder("wasm");
Source.Builder sourceBuilder = Source.newBuilder("wasm", ByteSequence.create(binary), "example");
Source source = sourceBuilder.build();
Context context = contextBuilder.build();


Value mainFunction = context.getBindings("wasm").getMember("main").getMember("_start");

For more polyglot examples, visit the Embedding Languages guide.

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