With the introduction of the HTML5 <canvas> element, could Swing theoretically be implemented in Google Web Toolkit (GWT) by using the <canvas> tag for drawing?
I'm aware of efforts to port source code from using Swing calls to GWT calls, but what I'm after is a pure behind the scenes port where a Swing application would compile under GWT without any source code modifications.
Is that theoretically possible? Why? Why not?
It is theoretically possible, though it will be non-trivial amount of work and questionable utility. What you need to do is implement custom Look and Feel that defines UI delegates generating GWT widgets and on the GWT side, capture the mouse and keyboard events on the client and feed them to the Swing event queue.
The problem is that the typical Swing application there are too many input events and widget state updates that will have to go back and forth between your browser and backend. This would kill your application responsiveness.
There are some challenges.
Local data storage Swing applications can use the disk to store data. Assuming local data store available in HTML 5 works, it'd still be difficult to auto-magically map file/disk access to local data store calls.
Multithreading & Synchronization Swing applications typically create threads, and that's not possible with GWT. Tricky to get it right.
Network Access Swing apps can connect to arbitrary network locations, which doesn't work with GWT.
Using java language features not available in browser Anything outside core-java is inaccessible, so the automatic port will likely fail.
Memory & Performance optimizations Garbage Collection patterns are entirely different. How do you optimize for optimal download sizes? How do you map multiple swing screens to use something like code-splitting for performance gains?
Look and Feel All said and done, you have to get your hands dirty to write some CSS code to get the right look and feel. An automatic port cannot do that.
Given all this, I think it is not-possible for anything but trivial applications. And for trivial applications you might as well re-write the code.
I would say it's possible, given this technology: http://www.creamtec.com/products/ajaxswing/demos.html
What AjaxSwing may not have yet is Java2D/HTML5 Canvas (frankly, I haven't checked). But how far away is that, with IE9 jumping on board?