JWebAssembly

JWebAssembly is a Java bytecode to WebAssembly compiler. It uses Java class files as input. That it can compile any language that compile to Java bytecode like Clojure, Groovy, JRuby, Jython, Kotlin and Scala. As output it generates the binary format (.wasm file) or the text format (.wat file). The target is to run Java natively in the browser with WebAssembly.

The difference to similar projects is that not a complete VM with GC and memory management should be ported. It's more like a 1: 1 conversion. The generated WebAssembly code is similar in size to the original Java class files.

Status of the project

The project is currently not production ready but you can run already some tests.

• Java byte code parser
• test framework
• Public API of the Compiler see class JWebAssembly
• Gradle Plugin
• Binary format file writer and Text format file writer (195 of 201 Java byte code instructions)
• Support for native methods #2
• Exception handling - required the next version of WebAssembly
• Multiple threads - required the next version of WebAssembly
• Memory Management - currently with a polyfill on JavaScript side
• invoke static method calls
• invoke instance method calls
• invoke interface method calls
• invoke dynamic method calls
• String support
• Optimizer - Optimize the WASM output of a single method after transpiling before writing to output
• Library for accessing the DOM
• Hello World sample

Status of Required WebAssembly Features

The following table shows the status of future WebAssembly features required by JWebAssembly in nightly builds in various implementations. These features are already used by the trunk version of JWebAssembly.

Feature	V8	SpiderMonkey	WABT
float-to-int	yes	yes	yes
Sign-extension	yes	yes	yes
Multi-value	yes	-	yes
Reference Types	yes	yes	yes
Garbage collection	-	partly	-
Exceptions	partly	-	partly

• For V8 it based on the V8 - node.js integrations builds.
• For SpiderMonkey it based on the nightly build of jsshell.
• For WABT it based on libwabt.js via node module wabt@nightly.

To use it also some flags and switches are needed.

Required Java Version

The JWebAssembly compiler requires Java SE 8 or higher. It is tested with Java SE 8 on travis-ci.org.

Usage

Exporting functions

To export a Java function to make it accessible from JavaScript, you must add the annotation de.inetsoftware.jwebassembly.api.annotation.Export.

import de.inetsoftware.jwebassembly.api.annotation.Export;

@Export
public static int add( int a, int b ) {
    return a + b;
}

importing functions

To import a JavaScript function to make it accessible from Java, you must add the annotation de.inetsoftware.jwebassembly.api.annotation.Import. The method can be declared native or can have a Java implementation which will be ignored on compiling.

import de.inetsoftware.jwebassembly.api.annotation.Import;

@Import( module = "global.Math", name = "max" )
static int max( int a, int b) {
    return Math.max( a, b );
}

Java Limits

In version 1 of WebAssembly you can only compile:

• static methods
• use the data types int, long float and double

This is state of JWebAssembly 0.1.

Alternatives

• TeaVM

For Tool Developer

If you want to develop some tools like plugins for a build system or an IDE, then you need

• to include the full contents of the packages de.inetsoftware.jwebassembly and de.inetsoftware.classparser and its subpackages.
• Create an instance of de.inetsoftware.jwebassembly.JWebAssembly class and use its API.