Question

What exactly the main purpose of using RequestDispatcher, for example when it’s executed in Filter like the following example:

public void doFilter(ServletRequest request, ServletResponse response,
    FilterChain chain) throws ServletException, IOException {
     HttpServletRequest aHttpServletRequest = (HttpServletRequest) request;
     aHttpServletRequest.getRequestDispatcher("/init.jsp").include(request, response);
     chain.doFilter(request, response);
}

Answer 1

In your case, the output of JSP is prepended to every page that filter is attached. This is a technique to easily add a common header to all your pages.

It’s a little bit confusing when RequestDispatcher is used int his context. Normally, when you want to dispatch your request to another servlet or JSP for process, you use RequestDispatcher to forward to another resource. In this case, your request is not dispatched anywhere else, instead you include output generated by another resource to your current response.