I've found a bug in the jpackage tool early access which I obtained from https://jdk.java.net/jpackage/. The issue is that the icon path is incorrect in the generated desktop entry file.
Steps to reproduce the bug in Ubuntu are as follows:
I have a project in which I create an app image with the command:
jpackage create-app-image --output build/jpackage --name modsman-gui --module-path build/jlinkbase/jlinkjars --module modsman.gui/modsman.gui.MainKt --runtime-image build/image --icon icons/linux.png
This creates an app image and puts the icon inside the resources directory.
I then create a deb installer for that package with the command:
jpackage create-installer --installer-type deb --output build/jpackage --name modsman-gui --app-image build/jpackage/modsman-gui --app-version 0.0.0 --linux-bundle-name modsman-gui
When I install the resulting deb package, there's a desktop entry installed called modsman-gui.desktop. That file has the following content:
[Desktop Entry]
Name=modsman-gui
Comment=modsman-gui
Exec=/opt/modsman-gui/modsman-gui
Icon=/opt/modsman-gui/modsman-gui.png
Terminal=false
Type=Application
Categories=Unknown
That Icon property points to a path containing the default gray application icon, not the icon I specified when creating the app image. In order to set the icon properly, the Icon property must be set to:
Icon=/opt/modsman-gui/resources/modsman-gui.png
This is because the create-app-image command put the icon inside the resources subdirectory. So, this is either a bug in create-installer pointing the desktop entry to the wrong icon, or a bug in create-app-image placing the icon in the wrong place.
You can reproduce it yourself by cloning my git repo from https://gitlab.com/sargunv-mc-mods/modsman-new and running ./gradlew :modsman-gui:jpackage. Inspect the deb package created in modsman-gui/build/jpackage, and you'll find the issue described above.
Reproduced on jpackage version 13-internal downloaded on May 13.