The 3.3 documentation mentioned that ant-style variable substitution was available in the server.xml configuration. The documentation for 5.5 (and 6.0) doesn't seem to mention that anywhere. It should be included. Something like: Configuration attribute values may use variable substitution (aka dynamic variable values or system property variable values). The syntax for doing so is ${varname} Valid variable names come from any system properties (JVM Option) that are available at the time. Values are only resolved once and not updated until the server is restarted.
Thanks for the report. I have added some text to the docs for trunk 6.0.x and 5.5.x which will be included in 7.0.0 onwards, 6.0.21 onwards and 5.5.29 onwards.