介绍
自定义的Filter类必须实现Filter接口，并且实现Filter接口定义的init() doFilter() destory()方法。其中init为初始化，destory为销毁 doFilter为实现过滤。
doFilter(ServletRequest request, ServletResponse response, FilterChain chain)
该方法的三个参数：request、response用来传递给下一个Filter或者JSP和Servlet，
chain通过调用其doFilter方法来调用下一个Filter，获得调用原始JSP或Servlet等其他内容。
该方法完成实际的过滤操作，当客户端请求方法与过滤器设置匹配的URL时，Servlet容器将先调用过滤器的doFilter方法。FilterChain用户访问后续过滤器。
这里的ServletRequest和ServletResponse一般需要转换成具体的Servlet实现对于的对象，如：HttpServletRequest和HttpServletResponse。
有些过滤器比较消耗资源，所以需要防止重复过滤，可以在doFilter()方法中判断。
定义过滤器
public class MyFilter implements Filter {
public void init(FilterConfig fc) {
//过滤器初始化代码
}
public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain){
//在这里可以对客户端请求进行检查
//沿过滤器链将请求传递到下一个过滤器。
chain.doFilter(request, response);
//在这里可以对响应进行处理
}
public void destroy( ) {
//过滤器被销毁时执行的代码
}
}
过滤器映射
<filter>
<filter-name>MyFilter</filter-name>
<filter-class>
com.servlet.TimeTrackFilter
</filter-class>
<init-param>
<param-name>developer</param-name>
<param-value>Developername</param-value>
</init-param>
</filter>
<!--针对一个Servlet做过滤-->
<filter-mapping>
<filter-name>MyFilter</filter-name>
<servlet-name>MyServlet</servlet-name>
</filter-mapping>
<!--针对URL Pattern做过滤-->
<filter-mapping>
<filter-name>MyFilter</filter-name>
<url-pattern>/book/*</url-pattern>
</filter-mapping>
注意：/* 表示拦截所有资源及不存在的资源
*.jsp 表示拦截所有jsp文件
===========================================================================
例子：登录验证过滤器
dl.html 登录界面
<!DOCTYPE HTML PUBLIC "=//w3c//dtd html 4.0 transitional/cn">
<html lang='zh'>
<head>
<title>登录</title>
</head> <body bgcolor="#FFFFFF">
<h1 align="center"><b>欢迎登陆系统</b></h1>
<form name="login" method = "post" action="servlet/Main">
<p></p>
<table width = "52%"border="2" align ="center">
<tr bgcolor ="#FFFFCC">
<td align = "center" width="43%"><div align="center">用户名：</div></td>
<td width="57%">
<div align="left">
<input type ="text" name = "userID" value="">
</div>
</td>
</tr> <tr bgcolor = "#ccff99">
<td align = "center" width = "43%">
<div align= "center">密码:</div>
</td>
<td width = "57%">
<div align = left>
<input type="password" name= "password" value="">
</div>
</td>
</tr>
</table> <p align="center">
<input type="reset" name= "reset" value = "重置">
<input type="submit" name= "tj" value = "提交">
</form>
</body>
</html>
Main.java 进行页面重定向，判断用户名密码是否正确
public class TimeTrackFilter implements Filter {
private FilterConfig filterConfig=null;
public void destroy() {
this.filterConfig=null;
}
public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain)
throws IOException, ServletException {
Date startTime , endTime;
double totalTime;
StringWriter sw = new StringWriter();
System.out.println("我在Filter中1");
try{
Thread.sleep(2000);
}catch(InterruptedException e){
System.out.println(e.toString());
}
startTime = new Date();
String login=(String) request.getParameter("userID");
if(login==null||"".equals(login)){ //如果没有输入用户名，重定位
HttpServletResponse resp =( HttpServletResponse )response;
resp.sendRedirect("../dl.html");
}
else
chain.doFilter(request, response);
//对响应进行处理
endTime = new Date();
totalTime = endTime.getTime()- startTime.getTime();
System.out.println("我在Filter中2");
PrintWriter writer = new PrintWriter(sw);
writer.println("==============\n耗时"+totalTime+"毫秒\n==============");
filterConfig.getServletContext().log(sw.getBuffer().toString());
}
public void init(FilterConfig fConfig) throws ServletException {
this.filterConfig=fConfig;
}
}
TimeTrackFilter.java Main的过滤器，如果没有输入用户名，则重定位到dl.html，否则就继续
public class TimeTrackFilter implements Filter {
private FilterConfig filterConfig=null;
public void destroy() {
this.filterConfig=null;
}
public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain)
throws IOException, ServletException {
Date startTime , endTime;
double totalTime;
StringWriter sw = new StringWriter();
System.out.println("我在Filter中");
try{
Thread.sleep(2000);
}catch(InterruptedException e){
System.out.println(e.toString());
}
startTime = new Date();
String login=(String) request.getParameter("userID");
if(login==null||"".equals(login)){ //如果没有输入用户名，重定位
HttpServletResponse resp =( HttpServletResponse )response;
resp.sendRedirect("../dl.html");
}
else
chain.doFilter(request, response);
//对响应进行处理
endTime = new Date();
totalTime = endTime.getTime()- startTime.getTime();
System.out.println("我在Filter中");
PrintWriter writer = new PrintWriter(sw);
writer.println("==============\n耗时"+totalTime+"毫秒\n==============");
filterConfig.getServletContext().log(sw.getBuffer().toString());
}
public void init(FilterConfig fConfig) throws ServletException {
this.filterConfig=fConfig;
}
}
LoginSuccess.jsp登录成功页面：
<%@ page language="java" import="java.util.*" pageEncoding="gb2312"%>
<html>
<head>
<title>登录成功</title>
</head>
<body>
<%String name = request.getParameter("userID"); %>
<h1>欢迎！"<%out.print(name);%> "您已成功登录系统...</h1>
</body>
</html>
LoginFail.java 登录失败页面
<%@ page language="java" import="java.util.*" pageEncoding="gb2312"%>
<html>
<head>
<title>登录失败</title>
</head>
<body>
<h1>登录失败，请重新登录...</h1>
</body>
</html>
控制台输出效果：
我在Filter中1
我在Servlet Main中
重定向
我在Filter中2
七月 08, 2015 10:22:21 上午 org.apache.catalina.core.ApplicationContext log
信息: ==============
耗时2015.0毫秒
==============