When would javax.servlet.Filter.doFilter(ServletRequest req, ServletResponse res) be called with anything other than HttpServletRequest/Response?

Question

I've been writing servlets and filters for years and it is something that has always irked me a little - the checking and casting of ServletRequest and ServletResponse parameters to HttpServletRequest and HttpServletResponse , particularly with filters as there is no standard HttpFilter base class to do this for you - unlike servlets.

The question is - when would you ever see anything other that HttpServletRequest / HttpServletResponse being passed to your servlet/filter and what should you do if you do?

Answer 1

在一些尚未实现的世界中，除了HttpServlet之外还有其他servlet类型。

Answer 2

From what I understood going through API, servlet specifications is that the servlet framework has been created for HTTP/web use specifically.

The protocols mentioned under reference section of the servlet specification are all related to HTTP only viz.
RFC 1945 Hypertext Transfer Protocol (HTTP/1.0)
RFC 2616 Hypertext Transfer Protocol (HTTP/1.1)
RFC 2617 HTTP Authentication: Basic and Digest Authentication
RFC 2324 Hypertext Coffee Pot Control Protocol (HTCPCP/1.0) -- this is basd on HTTP

All these are HTTP protocols.

Also the spec mandates this:

All servlet containers must support HTTP as a protocol for requests and responses, but may also support other request / response based protocols such as HTTPS (HTTP over SSL).

And the reason for having GenericServlet -> HttpServlet is to have the Generalization -> towards Specialization kind of hierarchy in the API structure in the API.