Non-HTTP(S) Servlets

Question

I'd like to use servlets to provide services over a non-HTTP protocol (e.g. FTP, NNTP, SMTP, etc.). The existence of the Servlet -> GenericServlet -> HTTPServlet hierarchy suggests this is supposed to be possible but I'm not sure how to do it. The main problem I see is getting my servlet container to know what protocol (IP port) I want to use.
So, are non-HTTP servlets supposed provide their own logic to set up a socket listener in the Servlet.init() method, manage their own thread pool, and dispatch requests to servlet instances as they are received? Or, is there some service provider API I can use to plug a non-HTTP service into my servlet container?
BTW, I'd like to do this in a J2EE v1.3 server.
Thanks, Dave

Answer 1

I'd like to use servlets to provide services over a
non-HTTP protocol (e.g. FTP, NNTP, SMTP, etc.)My first reaction would be "don't".
In an HTTP request, there is a bunch of headers, which the servlet container uses to decide which servlet it will invoke. There are no such headers in e.g. FTP. Even if you manage to subclass Servlet to FTPServlet, the web server doesn't know how that FTP requests should be routed to that servlet.
The piece of code that listens to port 80 etc really wants to see "GET", "POST" etc plus an URL on the first line of incoming data. That code won't respond well to seeing SMTP's "HELO".
That being said, you can put such protocols in a web server or app server. Write a thread that creates a server socket for the appropriate port and then sits in accept(). When a connection comes in, it starts a new per-client thread which talks SMTP or whatever over the incoming socket. Pretty basic client/server socket programming.
Also, to me, the servlet model appears inappropriate for FTP et al: a servlet services one request, then exits. FTP, NNTP and SMTP are all more like a discussion than request-reply.
In theory, it would be possible to fashion e.g. an SMTP server as a sort of a servlet. Instead of doGet() and such perhaps you'd have doHELO(), doMAIL_FROM(), doDATA() etc. But that seems overly complicated... Seems easier to me to do simply:
    while (true)
        read socket input a line at a time;
        if (in data mode) {
            check for end-of-message;
            append to string buffer;
        } else {
            tokenize it;
            obey the incoming command;
    }Maybe if you really want to use the servlet model, write a Servlet-like class that has a doCommand() method and make the "obey the incoming command" bit above call that. I'm not sure how much that buys, though. There are enough small differences in things like command tokenizing to make code reuse in the various protocol "servlets" difficult.
Btw, FTP in particular is an incredibly messy protocol. Implement that one only if you absolutely must, and even then crib some existing public domain implementation as a base. Active/passive, separate command/data channels, a wide range of buggy FTP clients, security difficult to get right, ... ouch...