How to set value to environment variable in xml file in the Tomcat

[Shirode, Sangita \(Sangita\)]

Hi,
 
I need to set one of the parameter in the xml configuration file to the
environment variable + some text.
 
eg.
<?xml version='1.0' encoding='utf-8'?>

<Context crossContext="true" docBase=$WEB_DIR"rest_of_path"
path="alias_path" reloadable="true">

</Context>

 

where $WEB_DIR is an environment variable.
How can I acheive this?
 
Thanks,
Sangita