This is OpenJDK 21's HotSpot interpreter (Zero) compiled to WebAssembly, with a bytecode-to-wasm JIT. It compiles and runs Java entirely client-side — no server, no round-trips. Everything below is a static site served from GitHub Pages.
A Swing application rendering through Java2D onto an HTML canvas — real AWT peers, painted frame-by-frame from inside the wasm JVM. Boots in ~2s.
Edit Java, compile it with the in-VM javac, and run it — console
output streams straight back. The VM stays warm, so re-runs take ~1s.
A Java program issuing OpenGL calls through JNI, translated to WebGL and rendered to a canvas — a spinning shaded triangle, same path as the Minecraft port.
Browse the JVM's in-memory filesystem — the JDK modules under /jdk,
the app classpath /app, and the /work scratch area.
Java opens a real TCP socket and fetches a URL over HTTP, tunneled through a local relay. Run the relay, then fetch live sites from inside the VM.
A real Spring Boot app on embedded Tomcat, running inside the JVM. The page calls its REST endpoint over an emulated TCP loopback — no network, no relay.
A real S3-compatible server (TinyS3, Java/Netty) running inside the tab. Upload and browse objects through the AWS SDK over an emulated TCP loopback — the server, storage, and client all in one browser page.
Inside the bytecode → WebAssembly JIT: the compile pipeline, the dispatch-loop control flow, the MacroAssembler-style emitter layer, and what it compiles.
How to embed this JVM in your own web app — load a bundle, boot it, compile source, wire a canvas for graphics/input, and add networking via the relay.
How it works. The page loads an ~8 MB wasm build of libjvm plus the JDK
runtime data (~25 MB gzipped, decompressed in the browser). A service worker
adds the cross-origin isolation headers that SharedArrayBuffer (wasm
threads) needs, since GitHub Pages can't set them itself. First load fetches the
runtime once, then it's cached.