With sandboxing, macOS is now forcing devs to use their file selection dialog because it will grant access to the selected files - a custom dialog box has no such blessing.
It's nice, actually; it's finally making the file selection process feel uniform. Too many Java apps are still using random Swing dialogs though...
Sandboxed Java apps don’t. Here I’m talking about apps that you can’t get on the App Store, many of which are older Java programs which haven’t been updated to use the newer APIs.
The push to force sandboxed apps to use the system dialog had the nice side effect of forcing many GUI APIs to actually use the native file pickers, which meant that a lot of non-app-store apps started to use the native file picker. But Java apps don’t automatically get that upgrade.
It's nice, actually; it's finally making the file selection process feel uniform. Too many Java apps are still using random Swing dialogs though...