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I have a java app rendering some sprites using LWJGL and OpenGL. It works fine until I move it to remote virtual machine with no phisical graphic card, Mesa 3d with related stuff is used to emulate it there. When I connect via ssh and start job manually it works fine. But running as a cron job it throws an exception

Caused by: java.lang.RuntimeException: org.lwjgl.LWJGLException: Could not open X display connection
        at org.lwjgl.opengl.Display.<clinit>(Display.java:141) ~[lwjgl.jar:na]
        ... 7 common frames omitted
Caused by: org.lwjgl.LWJGLException: Could not open X display connection
        at org.lwjgl.opengl.LinuxDisplay.openDisplay(Native Method) ~[lwjgl.jar:na]

Obviously something is wrong with X11.

Another update:

I found out it fails at the check for either Xrandr or XF86VidMode are supported. I got them installed but they kind of disabled. I tried to explicitly add RANDR but that didn't help.

xvfb-run -a '--server-args=+extension RANDR -screen 0 1024x768x16' /home/username/start.sh

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  • Obviously? Don't think so. X11 needs a DISPLAY environment variable. It's likely that is missing.
    – EEAA
    Apr 29, 2014 at 2:12
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    'Obviously' was there in context of my admition that I'm a Linux noob. Now the context is gone by all-mighty moderator releasing his power. I've updated question
    – Viktor Stolbin
    Apr 29, 2014 at 2:17
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    BTW check over on unix.stackexchange.com. unix.stackexchange.com/questions/10121/… unix.stackexchange.com/questions/25684/… You can also search for 'x11 cron application' there are many Google hits, some might apply to your situation. I don't really know the specifics here, I have never needed to do this.
    – Zoredache
    Apr 29, 2014 at 2:59
  • BTW could moderators please comment why is this off-topic? If there's another place for questions like this I'd be happy to move it there.
    – Viktor Stolbin
    Apr 30, 2014 at 18:26

2 Answers 2

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X11 servers don't simply get started on demand. There has to be a running X11 server, your application has to know about the existence of the X11 server it should be using via the DISPLAY variable, and the account must be authorized to connect to the X11 server that the DISPLAY variable is pointing to (xauth).

X11 forwarding built into SSH is great because it deals with all these details automatically in a way you don't have to know anything. But this won't help you from a cron.

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Your problem is, that the program expects a feature, which may be present on a "real" X server, but which is absent from Xvfb.

Adding features to Xvfb is probably more work than you are willing to put into this. So you'll be looking for alternatives, of which there are some:

  • Get the application to stop depending on the display mode extension.
  • Run the application on a "real" X server instead.
  • Run the application on another X server.

If it is your own application, you should be able to figure out at what point it tries to use the display mode extension. Might the application be unhappy with the settings you chose for Xvfb? If you are lucky, it may be as simple as the application requiring 32 bit color depth, and specifying x32 on the command line rather than x16 would help.

A real X server might still be an option, even on a virtual machine. It depends on the capabilities on graphics emulation on that virtual machine.

Alternatively it may be worth trying with Xvnc, which I believe has more features than Xvfb.

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  • Thank you for this, I managed to proceed, please check the question updated.
    – Viktor Stolbin
    Apr 30, 2014 at 1:28

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