CheerpJ 2.1 is now available. Check it out! Release notes here.

CheerpJ is

the Java compiler for the Web

converts any Java client application into standard HTML5/WebAssembly/JavaScript

works from Java bytecode, without requiring access to the source code

compatible with 100% of Java (reflection and dynamic classes included)

turn-key solution, works on Java Swing clients, Java applets, and Java libraries

actively developed and supported by Leaning Technologies experts


The solution for converting Java clients to HTML5

100% of Java SE

Reflections, proxy classes, multi-threading are all supported thanks to unique technical solutions.

Performance oriented

Based on both WebAssembly and highly-optimised JavaScript, CheerpJ can achieve excellent runtime performance and a small download footprint.

Full runtime environment

Based on a full, unmodified OpenJDK runtime environment: supports Swing, Filesystem, Audio, Printing.

First class HTML5 interoperability

Access the DOM, WebAPIs (e.g. WebGL, WebAudio) and browser resources from Java. Make Java calls from JavaScript.

Browsers as compilation target

CheerpJ converts your Java client into a standard HTML5, automatically. No longer ask your end users to maintain a local Java installation.

Convert or migrate

Convert full applications or individual components (such as libraries) and expose JavaScript APIs to use them from HTML5.


Three Key Components


The CheerpJ compiler processes your Java bytecode ahead of time, generating optimised JavaScript that can run on any browser.

CheerpJ Runtime

In parallel to your converted application, deploy a higly-optimised runtime in WebAssembly and JavaScript, keeping 100% of the execution client-side. Based on a full unmodified build of OpenJDK .

CheerpJ Java compiler

Whenever new classes are generated (e.g. through proxy classes), CheerpJ will convert them on the fly for you.

Thanks to these three components, CheerpJ provides a seamless experience of executing a converted Java application on any browser, fully client-side. No plugins and no installation are required from the user.




  • Command line CheerpJ utilities
  • Cloud-hosted CheerpJ Runtime
  • Documentation and tutorials
  • Community support on Gitter
  • Available for technical evaluations

Read the licence agreement

FREE, for non-commercial purposes


  • Command line CheerpJ utilities
  • Cloud-hosted CheerpJ Runtime
  • Documentation and tutorials
  • Community support on Gitter
  • Technical support from our support team
  • Available to individual developers and small businesses

from £750 per year


  • Command line CheerpJ utilities
  • Cloud or Self-hosted CheerpJ Runtime
  • Documentation and tutorials
  • Community support on Gitter
  • Premium technical support from our support team
  • Access to our development team for compilation log reviews, video calls, consulting
  • Available to organisations of any size

Contact us for a quote


Download CheerpJ

By clicking on the ‘Download’ above, you are accepting the CheerpJ non commercial licence agreement.

Why not starting with a tutorial:
How to get started with CheerpJ?


We are experts at converting and migrating legacy Java


Examples of CheerpJ in action

Browser based Java playground

Code editor and runtime all in one. Feel free to compile some Java.

CheerpJ Applet Runner

Our own free solution to help anyone run legacy Java Applets on Chrome without having to install Java.

Demos page

CheerpJ’s demo collection, complete with source code to reproduce and modify the results.


Developers area