Setting Manager sessionIdLength attribute in context.xml doesn't seem to work for JBoss 5.1.0

Question

I edited JBOSS_HOME/server/all/deploy/jbossweb.sar/context.xml to be the following, but session id cookie values are still only 24 characters in length:

<Context cookies="true" crossContext="true">

<Manager pathname="" sessionIdLength="40"/>

<InstanceListener>org.jboss.web.tomcat.security.RunAsListener</InstanceListener>

</Context>