I’ve recently started a little side project to create a java implementation of a CommonJS compiler: commonjs-java which is available on github under the Apache 2 license. It’s output is targeted at browsers, bundling together all the dependencies into one javascript file.

There are plenty of utilities that can do that for you on the command line or as part of a build process (e.g. browserify) and in most cases you should stick with them, but I had a couple of requirements that drove me to want a Java implementation:

1. Support no-build-step-required hot-deploy of changes to scripts
2. Have full control over how dependencies are located and loaded

Hot deploying changes can be achieved with a build system very smoothly if you set up a way to watch for changes and automatically recompile but it tends to require some messing about to set up right. 

I also wanted to be able to deploy the resulting web apps as a standalone jar, so wanted to be able to load all the web resources from the class path. When running from an IDE during development they’d be read straight off the filesystem but when deployed they’d come out of the single jar file.

Finally, it seemed like something interesting to build…

In the end I’m really quite happy with how it all worked out. We use spark as our framework so it only takes a little glue-code to route requests to through spark and have commonjs-java compile the javascript on the fly. The script compilation step doesn’t add any noticeable delay during development and I have a toggle that enables the script to be cached to avoid constant recompiling once deployed.

The CommonJS compilation turned out to be quite straight forward. The biggest part is identifying all the require calls but rhino can provide an AST which makes it straight-forward. There are some interesting details about how to inject the require, module and exports variables at runtime as well as providing fully isolated contexts for the modules but certainly nothing ground breaking.

Adding source map support was more interesting. The spec certainly doesn’t give a lot of detail or guidance so there was quite a lot of experimentation and back and forth.

The final piece to add is a minification step. My first attempt at that used Uglify2 running in java 8’s javascript engine but it was far too slow – I suspect mostly because of java’s javascript engine. Next attempt will be to try using YUI which is written in java.