Servlets :: How Is RequestDispatcher Implemented

How to implement a RequestDispatcher, so I checked tomcat's source code, the ApplicationDispatcher class, but there are many places I can't understand.

I found in the ApplicationDispatcher class that the method used to forward request and response to the designated resource is the invoke(ServletRequest,ServletResponse) method, and this part is responsible for executing forward:

support.fireInstanceEvent(InstanceEvent.BEFORE_DISPATCH_EVENT,
servlet, request, response);
// for includes/forwards
if ((servlet != null) && (filterChain != null)) {
filterChain.doFilter(request, response);
}
// Servlet Service Method is called by the FilterChain
support.fireInstanceEvent(InstanceEvent.AFTER_DISPATCH_EVENT,
servlet, request, response);

I can't understand this part of code, does this mean the RequestDispatcher.forward() and RequestDispatcher.include() methods are all executed in a filterchain? What is the mechanism of it?