Development of simple Servlet2.3 filter: Run a name: Introduction of new Java Servlet 2.3 specification, many of the most exciting features, one of which is filter (filter). Filter can change a request or modify response (Response). Filter is not a servlet, it is just a servlet to accept the pre-processes before the request. That is to say, when the user sends a request to servlet, this Filter is executed before the servlet is processed, and then the execution of the servlet. 2: The role FILTER can be used in the following aspect 1: Access to a specific resource (web page, JSP page, servlet) 2: Accessing resources record 3: Converting the resource 3: Write a Filter a filter must implement Javax. The servlet.filter interface, that is, the three methods below: 1: DOFILTER (ServletRequest, servletResponse, filterchain): This is a method of completing the filtering behavior. This is also the method of upstream filter call. The introduced Filterchain object provides information to be called by the subsequent filters. 2: INIT (FilterConfig): This is an initialization method called by a container. It guarantees that the container is called before the first DOFILTER () call is called. You can get the initialization parameters specified in the web.xml file. 3: DESTROY (): The container is called before the instance is terminated before the filter instance is destroyed.

4: Sample function: Restrict client IP access code as follows: package com.servlet.filter; import java.io. *; import javax.servlet. *; Import javax.servlet.http. *; Import javax.servlet.filter ; public class myfilter implements Filter {private FilterConfig filterConfig = null; public void init (FilterConfig filterConfig) {this.filterConfig = filterConfig;} public void doFilter (ServletRequest req, ServletResponse res, FilterChain chain) throws ServletException, IOException {try {res. SetContentType ("text / html; charset = GBK"); PrintWriter Out = Res.GetWriter (); string remoteaddr = (httpservletRequest) REQ) .GETREMOTEADDR (); if ((RemoteAddr.indexof ("127.0.0.1")! = -1) || ("LocalHost")! = -1)) {// Here to determine out.println (" "); OUT. Println ("

Haha, IP 127.0.0.1 and IP localhost can not access this website ~~ "); Out.Println (" "); out.close (); Return;} else {out.println (" "); out.println ("

Congratulations, you have successfully visited this website ~~ " ); Out.Println (" "); out.close ();}} catch (EXC EPTION E) {E.PrintStackTrace ();}} public void design: {}} Implementation: GetRemoteAddr () method through the servletRequest class gets access to the client IP of this app, then do some processing for this IP, if not If you meet the conditions, you can limit access. This can protect the application well.