Java2Script provides an Eclipse Java to JavaScript transpiler, with a nearly complete implementation of the Java Virtual Machine with AWT and Swing in JavaScript, with simple, automated parallel creation of both class files and js files. To date, over 100 applets have been converted.
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README.md

README.md

Note

These notes are for Java developers who want to convert their Java applets or Java applications to JavaScript, allowing continued, simultaneous one-source development of both Java and JavaScript.

Developers of java2script/SwingJS itself should read README-developers.md

Bob Hanson (hansonr@stolaf.edu)

About java2script/SwingJS

java2script/SwingJS is not a one-time "port" to JavaScript. It is a system that allows real-time Java-to-JavaScript conversion within Eclipse, with immediate JavaScript testing and debugging in a browser based on code development in Java. Using the java2script/SwingJS Eclipse plug-in, both Java .class files and their equivalent .js files are created simultaneously.

java2script/SwingJS includes an Eclipse plug-in (technically a "drop-in"), net.sf.j2s.core.zip (see https://github.com/BobHanson/java2script/blob/master/sources/net.sf.j2s.core/dist/dropins), along with a JavaScript version of the Java Virtual Machine (SwingJS, https://github.com/BobHanson/java2script/blob/master/sources/net.sf.j2s.java.core/SwingJS-site.zip) to allow the rapid and automated production of browser-ready JavaScript versions of Java applications and applets.

QuickStart

See https://github.com/BobHanson/java2script/tree/master/dist and the README file in that directory.

History - 2017-

https://github.com/BobHanson/java2script (development master)

Current development "Version 3 development master" involve a completely rewritten transpiler (2017) “CompilationParticipant” that follows the Eclipse Java compiler. The implementation nearly perfectly emulates the Java Virtual Machine. It includes fully qualified methods, compile-time method binding, generic methods and classes, Java 8 lambda functions and streams, Java reflection and dynamic class loading for efficient modular just-in-time performance, Java Swing components, modal and nonmodel dialogs, audio, jpdf, the AWT event thread, and many other added packages. Java applications and applets can both be run in JavaScript in any browser.

Version 3 also implements real-time creation of HTML test files for immediate JavaScript testing of any changes made to the Java code. Basically, when the source file is saved in Eclipse (Photon), the JavaScript updates automatically, and a reload of the page in the browser shows the changes immediately. This makes for very easy side-by-side Java and JavaScript debugging.

Unlike Version 2, Version 3 requires minimal reworking of Java classes - primarily just for performance and threading* reasons, maximizing JavaScript performance without compromising any Java performance and making Java-to-JavaScript conversion almost trivial. (Of course, we are still finding occasional bugs in the transpiler and run-time "JavaScript-JVM.")

*Note that java2script/SwingJS cannot not support Thread.wait(), Thread.notify(), or Thread.sleep(). Nonetheless, in all cases we have found simple state-based alternatives that essentially reproduce this behavior in JavaScript using javax.Swing.Timer. These methods work equally well in Java and JavaScript, requiring just a bit of redesign of Java methods.

Examples include:

Physlets (https://github.com/BobHanson/Physlets-SwingJS) conversion to JavaScript by Wolfgang Christian (wochristian@davidson.edu) and Bob Hanson (in progress).

Jmol-SwingJS https://github.com/BobHanson/Jmol-SwingJS (code only)

Varna https://github.com/BobHanson/VARNA and https://chemapps.stolaf.edu/swingjs/varna

History - 2010-2016

https://github.com/java2script/java2script (stable master)

Ported from SourceForge by Zhou Renjian, developed further by Zhou Renjian and Udo Borkowski (primarily) as "Version 2.0".

Extensively used by Bob Hanson in 2012-2016 for the conversion of Jmol/java to JSmol/javascript (http://jmol.sourceforge.net/), leading to extensive distribution and widespread use on the web. As of 3/2018, still the only known widely-available java2sript application.

Developed further in 2016 by zhou Renjian, Udo Borkowski, Bob Hanson, Andrew Raduege, Nadia El Mouldi, and Paul Falstad (http://www.falstad.com) for the first round of post-JSmol application of java2script, adding javax.swing classes and dubbed "SwingJS". Used by Bob Hanson during 11/2016 for conversion of the PhET gas applications applets.

Version 2 allowed extensive development of JavaScript versions of Java applets, including JSmol, Falstad applets, and PhET applets. However, this version did not produce "fully qualified" method signatures, resulting in error-prone and performance-reducing run-time binding of methods. Java class loading procedures were not consistent with the sequence of events used by the Java class loader, and parameter and array typing was minimal. Thus, while functional, Version 2 was not fully generalizable and required extensive reworking of Java code to be "java2script compliant."

Synchronized with https://github.com/BobHanson/java2script as the stable master version in 2018.

Examples can be found at https://chemapps.stolaf.edu/swingjs/phet/site/swingjs/examples and https://chemapps.stolaf.edu/jmol/jsmol

History - 2005-2010

https://sourceforge.net/projects/j2s (last updated 6/2010)

Initially developed by Zhou Renjian (aka "Josson Smith", zhourenjian@gmail.com, 2005-2017), Soheil Hassas Yeganeh (soheil.h.y@gmail.com, 2006), Sal Ferro (svferro@gmail.com, 2006), Sebastian Gurin (sgurin@softpoint.org, 2008), and others. The project was a "proof-in-concept" that saw limited actual application.