ADDITIONAL SYSTEM INFORMATION :
Fedora 35
Flatpak
Java Version: Java 8 (1.8.0_312)

A DESCRIPTION OF THE PROBLEM :
The class FileDialog which opens up a native file dialog does not work properly under modern Linux sandbox solutions such as Flatpak.

Take the following java application https://flathub.org/apps/details/com.atlauncher.ATLauncher .

This java application has been packaged under a sand-boxed application distribution format called "flatpak".

"flatpak" isolates applications from accessing the file system outside of the sandbox. Because the sandbox is in effect, while FileDialog will create a respective QT / GTK dialog, it will fail to display any files outside of the sandbox.

To resolve this limitation with flatpak, the flatpak developers created "portals" (https://flatpak.github.io/xdg-desktop-portal/). These portals allow an application to request features / services from outside of the sandbox. The important one for this issue is opening files.

The following is the specification for FileChooser:
https://flatpak.github.io/xdg-desktop-portal/#gdbus-org.freedesktop.portal.FileChooser

In order to properly resolve this issue, FileDialog will have to check DBUS for the freedesktop portals & then utilize them to create the respective dialog instead.

All links:
ATLauncher on FLATHUB: https://flathub.org/apps/details/com.atlauncher.ATLauncher
ATLauncher on GitHub: https://github.com/ATLauncher/ATLauncher
Relevant issue: https://github.com/ATLauncher/ATLauncher/issues/564
xdg-desktop-portal on GitHub: https://github.com/flatpak/xdg-desktop-portal
xdg-desktop-portal documentation: https://flatpak.github.io/xdg-desktop-portal/#common-conventions