java2js

Java to JavaScript (java2js)

java2js can translate simple Java programs to JavaScript and runs them using a JavaScript based JDK in the browser without the use of a VM on a server.

Try it out! The demo files are located in the docs folder of this repository, its a barebones implementation that shows what is possible with java2js.

Java classes included in the java2js JDK

java.io
File	InputStream	PrintStream

java.lang
Boolean	Byte	Character
Double	Exception	Float
Integer	Long	Short
String	StringBuilder	System
Thread

java.security
MessageDigest

java.time
Instant

java.util
AbstractCollection	AbstractList	AbstractMap
AbstractSet	ArrayList	Arrays
Collections	Formatter	HashMap
HashSet	IllegalFormatException	Itr
LinkedList	Random	Scanner
Set	Stack

Features

The java2js transpiler supports:

• imports
• primitive type casting
• static classes
• static methods
• array literals
• arrays with initial size
• two dimensional arrays
• lambda arrow functions
• triple quote strings
• try/catch blocks

API

await jdk.init(root)

This function imports the standard java.lang classes into the global scope. You must use it before translating or running files.

• root: (optional) path to the JS JDK folder, by default it is ./jdk (the java2js JS JDK), for online use on codepen or similar code sharing sites you can use this link as the root path: 'https://quinton-ashley.github.io/java2js/jdk'

await jdk.transpile(javaFile)

Translates a Java class and loads it.

• javaFile: a String with a Java class in it

returns a String with the JavaScript transpilation of the Java class

jdk.run(jvmArgs)

Runs the main method

• jvmArgs: (optional) String array of JVM arguments.

jdk.workerPath

java2js might cause the main thread to stall when transpiling a large Java file. This can cause your website to appear unresponsive/frozen. You can (optionally) utilize the java2js worker script to transpile Java asynchronously in a seperate JS thread. However due to CORS security 'jav2js_worker.js' must be hosted on your own domain, define 'jdk.workerPath' to be the location of that file.

By default workerPath is undefined unless you are using java2js on a local server, in that case workerPath is the path to 'jav2js_worker.js' in the parent folder of jdk.root

Known limitations

• not very good error reporting

• no support for method overloading, though a workaround might be possible by making a function with the og name route to each of the variations of the overloaded function

• no support for private/public methods yet, though this could be done since they are included in modern JavaScript classes

• no three dimensional arrays

• no third level static classes

• no method can be named .length() because it will be replaced with .length for getting the length of Strings in JS

Why did you make this?

I created this package so intro level computer science students could write Java code but still use my QuintOS retro game engine library, which is web based.

Credits

This project builds upon the java-to-javascript transpiler by @wyattades. I also did a modern JS implementation of the fundamental classes in the JDK (Java Development Kit), which I got the inspiration to do from java2javascript by @BobHanson, @zhourenjian, @abego, and others.

Contribute

I've only done a partial implementation of the Java 17 JDK in JavaScript, so if you're interested in adding more please go for it and submit a pull request!

Commercial Use

This project is licensed under the copyleft GNU GENERAL PUBLIC LICENSE 3.0 (only) license. If you'd like to request legal permission to use this code in a commerical product or closed source project please email me ([e-mail]) with a proposal of how you would like to use java2js.