All decent modern web servers support some sort of "plugin" architecture. Apache can dynamically load "modules" which are specified in the httpd.conf file.
The best way to understand this is to examine your httpd.conf, and also check out the installation instructions for any of the add-on servlet/JSP engines. In brief, though, you compile the "module" (if necessary), place it somewhere where Apache can see it, and add two sections to the httpd.conf. The first addition tells Apache how to load the module; the second addition configures it and tells Apache when to invoke it.
Most servlet containers use a small module which can be configured to respond to either a URL-path (all URLs starting with "/servlets/" for example) or specific file
patterns ("*.jsp" for example). When invoked, the module communicates with the servlet container, typically over a local socket connection, passes it the request details and gathers up the response generated which it then gives to Apache to send to the client.
This may sound a bit complicated, but in practice it is very efficient, and allows Apache to do what it is best at (serving static files and directories) while the Servlet container just handles servlets, JSPs and other
Java stuff.
