Write a script that starts JDownloader in headless mode
java -Djava.awt.headless=true -jar JDownloader.jar >/dev/null 2>/dev/null
you should run that script as different user
eg
su -c "/.../script.sh &" user
You can either place it in /etc/rc.local or write a systemd file for it
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JD-Dev & Server-Admin
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