>Is it possible to add a jar residing on a remote webserver to the
>classpath of an application that I run locally?
no. The things you put on the classpath are files, not URLS. To pull
that off you need to use a custom classloader. There is one already
written just for that.

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Canadian Mind Products, Roedy Green.
http://mindprod.com Java custom programming, consulting and coaching.
There is a concept called URL classloading. You can see JBoss source to
get started.
google:"URL Classloading"