You develop an application in Java which uses AWT on Windows platform. Deploy the build in Unix box and when you hit the URL of the web application only to find that the charts, barcodes etc., are not displayed on the page.
Check the logs and you will see some Exception related to X11 display variable/HeadlessException. This happens when there is no X11 service running.
When you get errors like: java.awt.HeadlessException: No X11 DISPLAY variable was set, but this program performed an operation which requires it
Or, any other error related to X11 or graphics display.
Steps to set the DISPLAY variable:
1) Login with root privileges. This is to ensure that all the DISPLAY settings are common for all the privileges/profiles within root.
2) Execute this script: startx &
This will run the startx command in the background. The startx command is an easy way to start an X session if you’re working on a single computer or boot Linux to runlevel 3 mode (X11, multiuser and networking enabled). There are other ways to start X. The startx command is a handy shell script that will pass on command-line options to the X server for your X sessions. The startx command is typically used to pass starting color-depth information to the X server, as well as to find client commands or options to run for the session (usually your .xinitrc file in your home directory).
3) Execute this script to set the DISPLAY variable: export DISPLAY=localhost:0.0
This sets and Environment variables called DISPLAY. EVs store useful information for programs defaults or system state. If you look at your EV you will see a variable called “DISPLAY” this variable stores the address for X clients to connect to. These addresses are in the form: hostname:displaynumber.screennumber
4) Execute this command: xhost +
The xhost program is used to add and delete host (computer) names or user names to the list of machines and users that are allowed to make connections to the X server. This provides a rudimentary form of privacy control and security.
* xhost + hostname: Adds hostname to X server access control list.
* xhost – hostname: Removes hostname from X server access control list.
* xhost + : Turns off acccess control (all remote hosts will have access to X server)
* xhost – : Turns access control back
Once this is done, you can see all graphic related charts and images come up in your application.