Since I used this post :
https://board.jdownloader.org/showthread.php?t=75534
to solve my problem, I need to use the command
java -Djava.awt.headless=true -jar JDownloader.jar
to start JDownloader each time.
So to autostart I opened /etc/rc.local and put :
sudo -c "java -Djava.awt.headless=true -jar JDownloader.jar" jd2
(jd2 is the user)
It does not work : It does not start automatically. Any idea please?:outch: