Not only is Qt open source, it is also free software according to FSF. Core Qt and most modules are released under the LGPL, the rest under the GPL.
Qt even has a binding poison-pill promise that will make the code BSD-licensed should Qt (the company) stop releasing it. See https://www.kde.org/community/whatiskde/Software_License_Agr... and https://www.kde.org/community/whatiskde/kdefreeqtfoundation....
I've worked with a team who develops a windows app which still uses QT4. They cannot migrate to QT5 since it requires a commercial license.
Still, LGPL libraries can be used for commercial software, this is a rather common use case.
https://docs.oracle.com/javase/8/docs/technotes/guides/deplo...
(Disclaimer: I work for Oracle but I don't speak for them)
(I can't comment on whether it is supported well since I've never used that part of NetBeans.)
Eclipse was apparently working on support for it at one point, but it looks to me like the work has stalled (just my impression from their bug database, I don't really know):
Since Java 8, the JDK offers tooling to package applications, making it standard the way many third party bundlers used to work.
In Java 9 there is a linker, which will allow for an application personalised JRE, coupled with the startup code.
The biggest problem was that using Swing properly required a mix of programming and designer skills, which most didn't bother to learn.
At least on enterprise space of our customer base, we are still on Java and .NET for native applications, no Electron on sight.
What about JavaFX 1/2? Do you have any information about its popularity and usefulness?
It is the Java version of XAML, and it is quite nice to use, but the uptake has been on the slow side. There were quite a few sessions at JavaONE 2016:
https://oracle.rainfocus.com/scripts/catalog/oow16.jsp?event...
Probably the place where it has seen more uptake has been in car infotainment systems, like by Volkswagen.
https://static.rainfocus.com/oracle/oow16/sess/1462531766187...
However our Java desktop native projects are still mostly Swing based, or make use of either Eclipse or Netbeans as desktop framework.
And of course there is the Android Java fork, which doesn't offer any JavaFX compatibility (some guys on the community are porting it though).
The filesize may be a little bigger than native apps, and redundant if you install more than one of such apps, but in this day and age of high bandwidth and storage that's an acceptable tradeoff. I spent 2015 working on a Win/Mac/Linux app where this would have been the preferred solution, it was annoying that in 2016 it still doesn't exist and as a result we're getting janky Node.js-based platforms like Electron (http://electron.atom.io/) instead.
Qt and GTK are the only mature enough x-platform non-web-based options it seems, but Qt is not open source and GTK's look is dated.