jdservicevnc

/etc/systemd/system $ cat jdownloaderVNC.service
[Unit]
Description=Remote desktop service (VNC)
After=syslog.target network.target

[Service]
Type=forking
# Clean any existing files in /tmp/.X11-unix environment
ExecStartPre=/bin/sh -c '/usr/bin/vncserver -kill %i > /dev/null 2>&1 || :'
ExecStart=/sbin/runuser -l jdownload -c "/usr/bin/vncserver %i -geometry 1024x600"
PIDFile=/home/jdownload/.vnc/%H%i.pid
ExecStop=/bin/sh -c '/usr/bin/vncserver -kill %i > /dev/null 2>&1 || :'

[Install]
WantedBy=multi-user.target

--
pi@baerchen:/etc/systemd/system $ sudo service jdownloaderVNC start
Job for jdownloaderVNC.service failed. See 'systemctl status jdownloaderVNC.service' and 'journalctl -xn' for details.
pi@baerchen:/etc/systemd/system $ sudo service jdownloaderVNC status
● jdownloaderVNC.service - Remote desktop service (VNC)
   Loaded: loaded (/etc/systemd/system/jdownloaderVNC.service; disabled)
   Active: failed (Result: resources) since So 2016-09-04 16:19:40 CEST; 1s ago
  Process: 27905 ExecStart=/sbin/runuser -l jdownload -c /usr/bin/vncserver %i -geometry 1024x600 (code=exited, status=0/SUCCESS)
  Process: 27900 ExecStartPre=/bin/sh -c /usr/bin/vncserver -kill %i > /dev/null 2>&1 || : (code=exited, status=0/SUCCESS)

Sep 04 16:19:40 baerchen systemd[1]: PID file /home/jdownload/.vnc/baerchen.pid not readable (yet?) after start.
Sep 04 16:19:40 baerchen systemd[1]: Failed to start Remote desktop service (VNC).
Sep 04 16:19:40 baerchen systemd[1]: Unit jdownloaderVNC.service entered failed state.

--
pi@baerchen:/home/jdownload/.vnc $ cat xstartup
#!/bin/sh

# Uncomment the following two lines for normal desktop:
# unset SESSION_MANAGER
# exec /etc/X11/xinit/xinitrc

[ -x /etc/vnc/xstartup ] && exec /etc/vnc/xstartup
[ -r $HOME/.Xresources ] && xrdb $HOME/.Xresources
xsetroot -solid grey
vncconfig -iconic &
#x-terminal-emulator -geometry 80x24+10+10 -ls -title "$VNCDESKTOP Desktop" &
x-window-manager &
java -jar /home/jdownload/bin/JDownloader.jar &