instanciate a servlet when tomcat starts

[SAMEUNIE]

Hi,

I've a servlet that loads properties from a file to load them in the System
properties.
I'd like this servlet to be instanciated when Tomcat starts.
With Jserv, i used the zone.properties file.
I think it must be simple, but i really don't now tomcat and all the
configuration file!

Thanks 

Sandrine