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Codename One in the Browser

We are very excited to announce the alpha release of the Code...
Codename One in the Browser

Codename One in the Browser

We are very excited to announce the alpha release of the Codename One Javascript port. This brings us one step closer to the coveted write once run anywhere ideal. Starting with Codename One version 3.0, you will be able to deploy your projects as Javascript applications that run directly in the browser. The process is simple:

  1. Select the “JavaScript” build target from the right click menu.

  2. Log into the Codename One build server.

  3. Download the application as a .zip archive that will run on any web server - or try the app instantly in your browser using the “Preview” link.


I mention above that this is an alpha release because not all features have yet been implemented. The port is still under active development, and most/all of the Codename One platform features will be implemented over the next few months. If you build an app and it doesn’t work, it is likely due to a feature not being implemented yet.

I have created a Project Status page for the project to record the status of each feature.

Browser Support

The goal is for this port to work in all modern browsers, but development has primarily occurred on Chrome, Safari, and Firefox (on the desktop), with mobile testing on Android’s Chrome browser and iOS’s Safari browser. Once all of the features are complete, I will focus on ensuring support for the other major browsers (i.e. IE).


When I first started working on the port, I didn’t know whether it would even be viable, performance-wise. I feared that we might finish the port and find that the interface was sluggish, unresponsive, and clunky. The single-threaded architecture of Javascript was viewed as a limitation, and it was thought that the EDT might cause the Javascript main thread to lock and block the UI, causing the dreaded “This page is unresponsive” alert in the browser.

I am happy to say that our fears were unfounded. Performance, on the desktop, seems comparable to the current JavaSE port - and in some cases it is even better. The true test of performance, however, is on mobile devices. On newer devices (e.g. iPhone 6, iPad Air 2) the port is quite responsive; Animations and transitions are smooth - though not quite as smooth as in the iOS port. On the previous generation of devices (e.g. Nexus 5, Nexus 7), I would describe the port as “usable”, but animations and transitions are a little jerky. As we move into even older devices, the apps remain usable, but the slow responsiveness, animations, and transitions become more noticeable.

So… at present, your apps should work very well in desktop browsers and quite well in new mobile devices.

The Secret Ingredient: TeaVM

As recently as 4 months ago most of us believed that a Javascript port of Codename One was impossible due to Codename One’s extensive use of threads and concurrency, for which Javascript has no equivalent. Developers had been working for years on solutions to try to add threading to the browser, but no full solution existed. And Codename One needed a full solution. It needed all of the bells and whistles of Java’s threading features: wait/notify/synchronized, etc..

The turning point was when Alexey Andreev told me that he had a way to support threads in TeaVM using a sophisticated transformation of the Java code into continuations. This break-through was nothing short of amazing, and it paved the way for Codename One in the browser. One of the most surprising things is how performant TeaVM is able to be despite these transformations. Alexey has taken great care, each step of the way, to ensure that the generated Javascript code is both concise (for a small file size), and performant. Benchmarks have shown the code to be as fast or faster than GWT on the same code - though we can’t really test that on Codename One since GWT doesn’t support threads - and thus can’t run Codename One.

The Future

Shai always reminds me to just take this one step at a time, and not to get ahead of myself. But let’s cast that aside for a moment and ruminate on the future potential of Codename One now that it can run inside the browser. I believe that Codename One is currently the closest thing to an heir-apparent to Swing that exists; and indeed the API is quite similar to Swing. It was carefully crafted to be easily portable to other platforms. Many suggested features or APIs have been rejected by Codename One because they wouldn’t be portable, or wouldn’t fit into the WORA paradigm.

Being able to run inside the browser opens up a whole array of possibilities for us Codename One developers. Enterprise applications that must target the “web first”, can now be written entirely in Java. No more messing around with HTML and Javascript for the client side. Write the application once, deploy to the web, and also provide native application bundles for users that require the enhanced experience of a native app.

This port could also provide a gateway to some of the more niche platforms like Firefox OS and Chrome OS where the small user-base makes it hard to justify a full native port.

Ultimately, the future direction will depend on you, the developers who are using Codename One to realize your creative solutions. How do you see web deployment fitting into your development cycle? How can the port be improved to meet your needs? We want to hear from you.

Same Origin

One of the limitations of JavaScript is the same origin policy which means you can’t connect to a server other than the one the app was delivered from. Since resources (images, res files etc.) are stored in the server this makes the JavaScript impractical for local execution by just opening a file. So you can’t download the build result zip, extract it and run it in your browser, you need to place it in a webserver for it to work.

As part of that work we also included a special servlet that will allow you to open a connection request to any destination. All requests will be proxied thru that servlet and thus allow you to connect to any domain you choose seamlessly.

However, we wanted developers to be able to preview their build results easily without requiring a specific server setup. To that end we generate a monolithic HTML file build result that we provide as a "preview" that you can open using the QR code or preview link as a result of the build. Its not as optimized as the actual result but you can see it instantly without setting up a server.


You will be able to send a JavaScript build with the next update of the plugin coming really soon!

During the technology preview phase we will allow everyone to build to the JavaScript target and experience the technology. Once the port enters beta status it will become an enterprise only feature since it was the funding from enterprise customers that allowed this work to happen.

Continued support from enterprise accounts is crucial for the growth of Codename One, it will allow us to build more offerings such as this and expand/bolster our existing offerings.

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Posted by Steve Hannah

Steve writes software for Codename One. He is an open source enthusiast who loves to tinker with new technologies.