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
Gradle Plugin

Partially Finished

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

Open Features

Support for native methods #2
Exception handling - required the next version of WebAssembly
Multiple threads - required the next version of WebAssembly
Memory Management - required the next version of WebAssembly with GC

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
floar-to-int	yes	yes	yes
Sign-extension	yes	yes	yes
Multi-value	yes	-	yes
Reference Types	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

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 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 need 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

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.