JWebAssembly is a Java to WebAssembly compiler. It uses Java class files as input. That it can compile any language that compile to Java bytecode. As output it generates the binary format (.wasm file) or the text format (.wat file).

Status of the project

Finished Components

Java byte code parser
test framework
Public API of the Compiler

Partially Finished

Binary format file writer (145 of 201 byte code instructions)
Text format file writer (145 of 201 byte code instructions)

Open Features

Exception handling - required the next version of WebAssembly
Multiple threads - required the next version of WebAssembly
Memory Management - required the next version of WebAssembly
Gradle plugin to easy integrate it in the build process
Eclipse build command to see compiler errors in in the IDE.

Required Java Version

JWebAssembly 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 need add the annotation org.webassembly.annotation.Export

import org.webassembly.annotation.Export;

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

Java Limits

In version 1 of WebAssembly you can only compile:

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

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.