Set the property java.awt.headless=true
Easiest to do in your servlet container. Without this you may get complaints when rendering with JAI on linux.

To do this with tomcat set JAVA_OPTS BEFORE calling startup.sh or before calling catalina.sh. (or you can edit those files, but it isn't advised)

export JAVA_OPTS='-Djava.awt.headless=true'

With Jetty you can start up with java and just pass the -Djava.awt.headless=true in directly:

java org.mortbay.jetty.Server jetty.xml -Djava.awt.headless=true

If you are using GeoServer's embedded Jetty you can modify the build.xml file, just add it to the jetty.jvmarg property:

<property name="jetty.jvmarg"  value="-Djetty.home=$\{basedir\}/server -Djava.awt.headless=true" />

Or, modify the last line of startup.sh like so:

exec "$_RUNJAVA" -Djava.awt.headless=true -jar bin/start.jar

(This is now the default in the startup.sh script, please give feedback if it messes up anything on linux by always setting this option to true - I'm inclined to think it won't, since its a server application)

I have not tested Resin on linux, but I think it should be done in the resin.conf file, by adding a line like:

<caucho.com>
  <system-property java.awt.headless=true/>
</caucho.com>

If some could confirm this then just edit this page.