install xscreensaver in the usual way using the Synaptic Package Manager.
configure xscreensaver from the terminal using xscreensaver-demo (without privileges - no sudo su).

then how to autostart xscreensaver in debian jessie.

create a file in /home/yourname/.config/autostart called Xscreensaver.desktop thus:-

#!/usr/lib/env/ xdg-open

[Desktop Entry]
Version=1.0
Type=Application
Terminal=false
Icon[en-US]=gnome-panel-launcher
Icon=gnome-panel-launcher
Exec=xscreensaver
Name[en-US]=Xscreensaver.desktop
Name=Xscreensaver

for yourname read your account name. apart from the line Exec=xscreensaver this probably doesn't do much.
and that's it. i share this after months of trawling the internet and not finding out how to do it.
xscreensaver will create a file called .xscreensaver in /home/yourname/.
this worked for me.