servlet-filter的一个例子

Filter的意思是可以在Servlet实例进行实质性的处理之前，起到“过滤”的作用.

一个Filter可以过滤一个或多个Servlet，Filter必须实现javax.Servlet.Filter接口，并且必须定义以下三个方法:init(),destory(),doFilter().

下面的例子演示记录客户端主机信息的Filter：

程序代码：

/** * 该实例演示记录客户端主机信息的Filter * www.javatx.cn */ package cn.javatx; import java.io.IOException; import javax.servlet.Filter; import javax.servlet.FilterChain; import javax.servlet.FilterConfig; import javax.servlet.ServletContext; import javax.servlet.ServletException; import javax.servlet.ServletRequest; import javax.servlet.ServletResponse; import javax.servlet.http.HttpServletRequest; /** * @author ljfan * */ public class logFilter implements Filter { private ServletContext context; /* (non-Javadoc) * @see javax.servlet.Filter#init(javax.servlet.FilterConfig) */ public void init(FilterConfig arg0) throws ServletException { // TODO Auto-generated method stub context = arg0.getServletContext(); } /* (non-Javadoc) * @see javax.servlet.Filter#doFilter(javax.servlet.ServletRequest, javax.servlet.ServletResponse, javax.servlet.FilterChain) */ public void doFilter(ServletRequest arg0, ServletResponse arg1, FilterChain arg2) throws IOException, ServletException { // TODO Auto-generated method stub context.log("[" + arg0.getRemoteHost() + "] request: " +((HttpServletRequest)arg0).getRequestURI()); arg2.doFilter(arg0, arg1); context.log("[" + arg0.getRemoteHost() + "] done"); } /* (non-Javadoc) * @see javax.servlet.Filter#destroy() */ public void destroy() { // TODO Auto-generated method stub context = null; } }

当前web应用的web.xml配置为:

程序代码：

<?xml version="1.0" encoding="UTF-8"?> <web-app version="2.4" xmlns="http://java.sun.com/xml/ns/j2ee" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee http://java.sun.com/xml/ns/j2ee/web-app_2_4.xsd"> <filter> <filter-name>logFilter</filter-name> <filter-class>cn.javatx.logFilter</filter-class> </filter> <filter-mapping> <filter-name>logFilter</filter-name> <url-pattern>/*</url-pattern> </filter-mapping> </web-app>

最后用myeclipse插件自动部署到tomcat路径下，启动tomcat，在浏览器中输入
http://localhost:8080/Filter/ (Filter为此web应用名称)

可以在tomcat 目录的log目录下的localhost.2006-01-02.log看到以下消息:

2006-1-2 16:25:51 org.apache.catalina.core.ApplicationContext log
信息: [127.0.0.1] request: /Filter/
2006-1-2 16:25:51 org.apache.catalina.core.ApplicationContext log
信息: [127.0.0.1] done

这表明logFilter已经被server调用成功.
当然在实际应用中filter还有其他用处,象登陆验证，编码转化等.