Learn CHEERPJ with Real Code Examples

CheerpJ lets you run Java applications in the browser without needing JVM installation.

It converts Java bytecode (.class, .jar) into a mix of JS and Wasm.

Provides a browser-based JVM emulation to run complex Java apps.

Supports Java AWT/Swing applications via HTML5 canvas rendering.

Allows enterprise legacy Java applications to be ported to the web with minimal changes.

Ahead-of-time Java-to-Wasm/Javascript compiler

CheerpJ Runtime Environment (CJRE)

Browser-based JVM with classloader/Web APIs

Swing/AWT rendering stack mapped to HTML5

Secure sandbox execution

Bytecode Translation - convert Java to WebAssembly/JS

CJRE - browser-hosted Java Runtime Environment

Swing/AWT Emulation - map Java UI to HTML5

ClassLoader - dynamic class loading via web

Filesystem Emulation - Java file APIs mapped to virtual FS

index.html - main launcher

cheerpj.js - runtime loader

cheerpj-*-wasm.js - Wasm modules

app.jar - your Java program

assets/ - Additional Java resources

Prepare Java .jar or .class files

Configure CheerpJ loader

Initialize CJRE in browser

Run main class using cheerpjRunMain

Debug and optimize loading performance

Beginner: run a simple Java console app

Intermediate: run Swing UI with buttons

Advanced: migrate legacy multi-jar applications

Expert: integrate CheerpJ with custom JS UI

Auditor: optimize Wasm payload + caching

CheerpJ vs TeaVM: CheerpJ runs *unmodified* bytecode; TeaVM requires source-level transformation

CheerpJ vs GWT: CheerpJ is JVM-compatible; GWT is Java->JS transpiler

CheerpJ vs Go-WASM: CheerpJ targets legacy Java preservation; Go-WASM targets new apps

CheerpJ vs WebAssembly Java Runtimes: CheerpJ has strongest AWT/Swing support

CheerpJ vs Emscripten: Different languages; Emscripten better for C/C++ performance

2017 – CheerpJ initial release

2019 – Strong Swing/AWT support

2021 – CJRE performance improvements

2023 – CheerpJ 2.0, Wasm-focused runtime

2024–2025 – Enhanced enterprise tooling & DX

CJRE - CheerpJ Runtime Environment

Bytecode - compiled Java code

Wasm - WebAssembly binary format

Swing/AWT - Java UI frameworks

Classloader - dynamic Java class loader