How can i make a servlet (class) temporarily unavailable, except for ADMIN

Question

Hello All!
I am rather new to the programming field and have already completed a web-project in Java only with Servlets. (no jsp). I have a login procedure, means I have a user management for users and admins. If an admin is logged in, he/she should be able to deactivate the servlet with a mouse-click as long he activates it again with the same. if the servlet is deactivated, other users/admins trying to access the servlet should get a "servlet is currently unavailable" message. but the admin, who is logged in, still should be able to work with the servlet! how can i realise that?
in other words, by clicking that particular button, the servlet should be made "non-multi-thread" and other way round by activating it.
any ideas??? how can the standard "servlet currently unavailable" page be displayed? I have tomcat 4 on a linux machine! i'd be really grateful if someone oculd help me.
another question i have is, how to define a "pseudo" link address to the servlet. now, the servlet can only be accessed by typing like this: http://servername.xy.com:8080/ProjectName/servlet/ServletName (because I have activated the servlet mapping in the server.xml/web.xml with /servlet/*)
but I want a link like this: http://servername.xy.com:8080/shortname
How can I do this in an easy way?? I have treid to create a web.xml in der WEB-INF folder of the servlet with following content, but it is not working:
<?xml version="1.0" encoding="ISO-8859-1"?>
<!DOCTYPE web-app PUBLIC
"-//Sun Microsystems, Inc.//DTD Web Application 2.3//EN"
"http://java.sun.com/dtd/web-app_2_3.dtd">
<web-app>
<servlet>
<servlet-name>
shortname
</servlet-name>
<servlet-class>
ClassName
</servlet-class>
</servlet>
</web-app>
Thanks a lot in advance for your kind help,
lisa

Answer 1

Ok,
You'll need to find a tutorial on servlet filters. Its not that hard a concept.
http://java.sun.com/j2ee/1.4/docs/tutorial/doc/Servlets8.html#wp64572
Filters need to be configured in the web.xml.
Basically they are a buffer between a request and your servlet.
Any requests for the servlet, go through the filter first. It lets you do some processing before/after the servlet gets called. Its a good way of putting in some generic code that need to be run for many servlets - security checks are often implemented in this fashion.
This should give you an idea of the sort of thing you need. I haven't really written one before, so I copied this out of the tutorial and did some basic framework for it....
public final class TestFilter implements Filter {
   private FilterConfig filterConfig = null;
   public void init(FilterConfig filterConfig) throws ServletException {
      this.filterConfig = filterConfig;
   public void destroy() {
      this.filterConfig = null;
   public void doFilter(ServletRequest request,  ServletResponse response, FilterChain chain) throws IOException, ServletException {
        // if you want to get the session will need to cast request to an HttpServletRequest
        Session session = request.getSession();
        String requestURL = request.getRequestURL();
        // check if this URL is for a servlet that is disabled for this user
       // somehow you have to keep track of this ... maybe in the servletContext ?
    User user = (User) session.getAttribute("user")     
      boolean disabled = checkDisabled(requestURL, user);
      // if its not disabled, go ahead
      if (!disabled)
        chain.doFilter(request, response);
    else{
        // send the servlet off somewhere else - requestDispatcher maybe?
}