如何将java库移植到Web上？

How do you take a java library that is for the desktop or an applet and make it so you can use its functions for a web page? I know not all things are meant for the web, but can you call a "regular" java class in an apache/tomcat setup server and it "work"? Do you have to so something to it to get it work with a web page?How do you take a java library that is for the