[Jetty-support] How can I have at most X current instances of a given servlet, and let the X+1 ... X+n wait until there is a vacant slot?

[<nospam0000@c.dk>]

I have a small Cocoon application running with the enclosed Jetty 
2.4.19, where I submit an XSL-FO file, and get a rendered version back, 
where I now - due to the unpredictable resource usage by FOP and our 
prints being of different sizes - would like to ensure that I have at 
most X instances running at one time, and then making the rest wait 
until one of the currently active is finished. 

I have looked at the Jetty docs, but I do not think that such a facility 
is named something that I might look for.  Is there a kind person who 
can let me know where I need to look now?

Best regards,
-- 

  Thorbjoern Ravn Andersen      "...plus...Tubular Bells!"