* Taken from More Servlets and JavaServer Pages * from Prentice Hall and Sun Microsystems Press, * http://www.moreservlets.com/. * © 2002 Marty Hall; may be freely used or adapted. */ public class BannedAccessFilter implements Filter { private HashSet bannedSiteTable; /** Deny access if the request comes from a banned site * or is referred here by a banned site. */ public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws ServletException, IOException { HttpServletRequest req = (HttpServletRequest)request; String requestingHost = req.getRemoteHost(); String referringHost = getReferringHost(req.getHeader("Referer")); String bannedSite = null; boolean isBanned = false; if (bannedSiteTable.contains(requestingHost)) { bannedSite = requestingHost; isBanned = true; } else if (bannedSiteTable.contains(referringHost)) { bannedSite = referringHost; isBanned = true; } if (isBanned) { showWarning(response, bannedSite); } else { chain.doFilter(request,response); } } /** Create a table of banned sites based on initialization * parameters . Remember that version 2.3 of the servlet * API mandates the use of the Java 2 Platform. Thus, * it is safe to use HashSet (which determines whether * a given key exists) rather than the clumsier * Hashtable (which has a value for each key). */ public void init(FilterConfig config) throws ServletException { bannedSiteTable = new HashSet(); String bannedSites = config.getInitParameter("bannedSites"); // Default token set: white space. StringTokenizer tok = new StringTokenizer(bannedSites); while(tok.hasMoreTokens()) { String bannedSite = tok.nextToken(); bannedSiteTable.add(bannedSite); System.out.println("Banned " + bannedSite); } } public void destroy() {} private String getReferringHost(String refererringURLString) { try { URL referringURL = new URL(refererringURLString); return(referringURL.getHost()); } catch(MalformedURLException mue) { // Malformed or null return(null); } } // Replacement response that is returned to users // who are from or referred here by a banned site. private void showWarning(ServletResponse response, String bannedSite) throws ServletException, IOException { response.setContentType("text/html"); PrintWriter out = response.getWriter(); String docType = "\n"; out.println (docType + "\n" + "