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Tutorial 1 — Hello, Fiji

Goal: run a Java program inside a WebAssembly component.

This is the smallest end-to-end demonstration of what Fiji is — standard javac output running under a Wasm component runtime, producing the output you'd expect.

What you need​

  • wasmtime on your PATH (a supported major version).
  • The Fiji release tarball extracted somewhere accessible.
  • A WASI-targeted JDK image. The same one used by fiji verify-release works; export its path as FIJI_JDK_IMAGE.

The workload​

HelloFiji.java:

public class HelloFiji {
    public static void main(String[] args) {
        System.out.println("Hello from Fiji.");
    }
}

No special imports. No annotations. No Fiji-aware API surface. The .class is produced by standard javac --release 21 and ships alongside the source in the release tarball at tutorials/01-hello-fiji/ so you don't need a host JDK to run the tutorial.

Running it​

From the tutorials/01-hello-fiji/ directory inside the release tarball:

wasmtime run \
  --dir=. \
  ../../components/fiji-jvm-composed.component.wasm \
  HelloFiji
# → Hello from Fiji.

That's the whole demonstration. The same JAR-of-classes that would run on java -cp . HelloFiji on a desktop JDK runs inside a WebAssembly component on any host that speaks the component model.

What just happened​

  1. wasmtime loaded the Fiji JVM component (fiji-jvm-composed.component.wasm).
  2. The host (wasmtime here, via WASI) handed Fiji the workload class name (HelloFiji).
  3. Fiji's runtime — a real JVM, packaged as a Wasm component — resolved the class, executed the bytecode, and the println call wrote bytes through standard WASI stdio.

No JNI. No platform-specific binary. No transpilation. The same .class file would run identically on any JDK.

Going further​

The release tarball ships a second tutorial, 05-same-artifact-several-runtimes/, showing the same workload + the same component artifact running under multiple host categories — standalone Wasmtime, Node via jco, with pointers to browser and embedded-library hosts.