刚开始接触技术，对这片领域充满了好奇，第一次写博客心里有点小激动~，开通此博客的主要目的是为了将自己学到的一些东西记录下来，这是一个复习与总结的过程，有利于巩固自己对知识的理解。俗话说的好，好记性不如烂笔头，学习——思考——总结——记录，技术的学习周期一般是这样的，希望我能坚持下去，能在技术这条路上走的更远！Say good luck to myself！
好了，废话不多说直接进入主题：本文主要记录web开发中过滤器的应用。
通常，我们在登录一些网站后比如说登录某论坛，如果想发表评论，系统往往会提示：“您还没有登录，请登录后在。。。。”这里系统检测用户是否登录的过程就用到了过滤器。过滤器主要是实现Filter接口，比如：
package com.chris.filter; import java.io.IOException; import java.util.Arrays; import java.util.List; 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; import javax.servlet.http.HttpServletResponse; import org.apache.catalina.connector.Request; import com.sun.xml.internal.bind.v2.runtime.unmarshaller.XsiNilLoader.Array; public class LoginFilter implements Filter { private String sessionKey; private String rediretUrl; private String uncheckedUrls; @Override public void init(FilterConfig filterConfig) throws ServletException { ServletContext servletContext=filterConfig.getServletContext(); sessionKey=servletContext.getInitParameter("userSessonKey"); rediretUrl=servletContext.getInitParameter("rediretUrl"); uncheckedUrls=servletContext.getInitParameter("uncheckedUrls"); } @Override public void destroy() { } @Override public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException { HttpServletRequest httpServletRequest=(HttpServletRequest) servletRequest; HttpServletResponse httpServletResponse=(HttpServletResponse) servletResponse; //1、得到用户请求路径 String servletPath=httpServletRequest.getServletPath(); //2、放行不需要过滤的页面，如登录界面之类的 List<String> urls=Arrays.asList(uncheckedUrls.split(",")); if(urls.contains(servletPath)){ filterChain.doFilter(httpServletRequest, httpServletResponse); return; } //3、从session中获取用户并判断用户是否登录过，如果没有登录过则重定向到登录界面 Object user= httpServletRequest.getSession().getAttribute(sessionKey);if(user==null){ httpServletResponse.sendRedirect(httpServletRequest.getContextPath()+rediretUrl); return; } //4、否则放行 filterChain.doFilter(httpServletRequest, httpServletResponse); } }
文中sessionKey，rediretUrl，uncheckedUrls是初始化在web.xml中的，在LoginFilter 中通过FilterConfig对象获取这些值，这里FilterConfig类似于ServletConfig：
<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://java.sun.com/xml/ns/javaee" xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd" id="WebApp_ID" version="2.5">
<display-name>LoginFilter</display-name>
<welcome-file-list>
<welcome-file>login/index.jsp</welcome-file>
</welcome-file-list>
<context-param>
<param-name>userSessonKey</param-name>
<param-value>USERSESSONKEY</param-value>
</context-param>
<context-param>
<param-name>rediretUrl</param-name>
<param-value>/login/login.jsp</param-value>
</context-param>
<context-param>
<param-name>uncheckedUrls</param-name>
<param-value>/login/a.jsp,/login/login.jsp,/login/index.jsp,/login/dologin.jsp</param-value>
</context-param>
<filter>
<filter-name>loginFilter</filter-name>
<filter-class>com.chris.filter.LoginFilter</filter-class>
</filter>
<filter-mapping>
<filter-name>loginFilter</filter-name>
<url-pattern>/login/*</url-pattern>
</filter-mapping>
</web-app>
本例子中为了方便，LoginFilter 类中的form表单的action是跳转到另一个jsp中的——dologin.jsp.
<%@ page language="java" contentType="text/html; charset=ISO-8859-1" pageEncoding="ISO-8859-1"%> <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> <html> <head> <meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1"> <title>Insert title here</title> </head> <body> <% String username = request.getParameter("username"); if (username != null && !username.trim().equals("")) {
//如果username不为空则将username放入到session中，以便LoginFilter 类中进行用户是否登录的判断 session.setAttribute(application.getInitParameter("userSessonKey"), username); response.sendRedirect("index.jsp"); }else{ response.sendRedirect("login.jsp"); } %> </body> </html>
到这里，一个简单的检测用户是否登录的过滤器就完成了。本人理解比较浅显，请各位大神指正！