JavaWeb学习笔记--filter开发

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("==============
耗时"+totalTime+"毫秒
==============");        
        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("==============
耗时"+totalTime+"毫秒
==============");        
        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
==============