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Codename One in the Browser

Codename One in the Browser

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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

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.


  • bryan says:

    Have you got any demos that can be downloaded ?

  • Shai Almog says:

    Good point. We will add some demos to the website.

  • glegris says:

    Great work !

    Did you also consider Mozilla’s J2ME VM in Javascript: […]( ?

    As it provides a J2ME layer, It could avoid to write a specific port for IO and graphic primitives.

    Nevertheless, not sure the project is as mature as TeaVM

  • Shai Almog says:

    I wasn’t familiar with that project. Interesting.
    Either way I think the TeaVM approach is probably superior in terms of compatibility to newer Java language features rather than CLDC limitations, good to know about it regardless…

  • Thomas Yuen says:

    It is good to see js support but if it is only for the enterprise (the “rich”) customer, it is not so good (bad for biz)

  • Shai Almog says:

    Enterprise subscribers paid for this port so it wouldn’t exist without them.
    More enterprise subscribers == more developers == more features and capabilities for everyone.
    So its great this is an enterprise only feature as it will make some developers upgrade and make us a stronger/larger company. Even if you can’t afford this particular feature I’m sure you can benefit from that.

  • Thomas Yuen says:

    hope u hv a lot more enterprise cust than professional cust for i am abt to sub as prof. enterprise is bit too much for now.

  • KL says:

    Hi, Because of “same origin policy”, if my js apps need to do http request to origin server, should I hardcode the server domain/ip inside the apps? If not, how do I open the http request?
    ConnectionRequest r = new ConnectionRequest();
    r.setUrl(“”); // Possible not to hard code the domain name?
    Kindly advice.
    * Can I ask here? Or where should I?

  • Shai Almog says:

    It should work exactly like it does in the simulator with the full URL’s. Keep in mind that our goal here is WORA so we want the code that works for you on the mobile native app to be the exact same code that works for the JavaScript version. You can ask here (if its relevant to the post) or in the discussion forum or in stack overflow (with the codenameone tag).
    You can also use pro support if you signup for pro.

  • KL says:

    Can I use the Display.getInstance().getProperty(“[](”, “DEFAULT”) to get the host/domain name? I tried, but it just return “DEFAULT” ..

    Still not implemented?

  • Shai Almog says:

    Why do you you expect that to be in Display?
    You are aware that Codename One isn’t a JavaScript/HTML5 API right?
    Its a native platform that allows you to target JavaScript, not the other way around.

  • KL says:

    Thanks, I am clearer now. Have to go through the developer guide again.

  • Gareth Murfin says:

    Just read this again, I can say that the Javascript port works unbelievably well, it’s astounding work.

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