过滤器Filter(拦截jsp页面的跳转)案例：

创建一个 Filter , class类: 其继承于 接口 Filte（接口导包：import javax.servlet.Filter;）

在 web.xml 文件中配置并映射该 Filter. 其中 url-pattern 指定该 Filter 可以拦截哪些资源, 即可以通过哪些 url 访问到该 Filter，并进行拦截；

案例：username=Tom,password=1234,设计Filter类，及jsp页面实现，输入username和password是否等于Tom和1234，不等拦截index.jsp页面跳转到hello.jsp页面，等的或，在在hello，显示；

1.建立UserNameFilter类：继承于接口Filter（接口导包：import javax.servlet.Filter;）

package com.lanqiao.javatest;

import java.io.IOException;

import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;

import org.apache.catalina.connector.Request;

public class UserNameFilter implements Filter{

    //构造方法
    public UserNameFilter(){

    }
    public void destroy() {

    }

    @Override
    public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain)
            throws IOException, ServletException {
        String initUser=filterConfig.getInitParameter("username");
        String username=request.getParameter("username");

        if(!initUser.equals(username)){
            request.setAttribute("message", "用户名不正确！！！");
            request.getRequestDispatcher("/login.jsp").forward(request, response);
            return ;
        }
        chain.doFilter(request, response);

    }

    private FilterConfig filterConfig;
    @Override
    public void init(FilterConfig filterConfig) throws ServletException {
        this.filterConfig=filterConfig;

    }

}

2.建立PasswordFilter类：继承于接口Filter（接口导包：import javax.servlet.Filter;）

package com.lanqiao.javatest;

import java.io.IOException;

import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;

public class PasswordFilter implements Filter {

    public PasswordFilter() {
        // TODO Auto-generated constructor stub
    }
    @Override
    public void destroy() {

    }

    @Override
    public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain)
            throws IOException, ServletException {
        String initPassword=filterConfig.getServletContext().getInitParameter("password");
        String password=request.getParameter("password");
        if(!initPassword.equals(password)){
            request.setAttribute("message", "密码不正确！！！");
            request.getRequestDispatcher("/login.jsp").forward(request, response);
            return ;
        }
        chain.doFilter(request, response);

    }

    private FilterConfig filterConfig;
    @Override
    public void init(FilterConfig filterConfig) throws ServletException {
        this.filterConfig=filterConfig;

    }

}

3.在WEB-FIN下的web.xml文件下的配置和映射：

<?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>day-12</display-name>

  <filter>
      <filter-name>UserNameFilter</filter-name>
      <filter-class>com.lanqiao.javatest.UserNameFilter</filter-class>
      <init-param>
          <param-name>username</param-name>
          <param-value>Tom</param-value>
      </init-param>
  </filter>

  <filter-mapping>
      <filter-name>UserNameFilter</filter-name>
      <url-pattern>/hello.jsp</url-pattern>
  </filter-mapping>

  <filter>
      <filter-name>password</filter-name>
      <filter-class>com.lanqiao.javatest.PasswordFilter</filter-class>
  </filter>
  <filter-mapping>
      <filter-name>password</filter-name>
      <url-pattern>/hello.jsp</url-pattern>
  </filter-mapping>
  <context-param>
      <param-name>password</param-name>
      <param-value>1234</param-value>
  </context-param>

</web-app>

4.index.jsp页面：输入username和password;

<%@ page language="java" contentType="text/html; charset=UTF-8"
    pageEncoding="UTF-8"%>
<!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=UTF-8">
<title>Insert title here</title>
</head>
<body>
    <font color="red">${message }</font>
    <br><br>
    <form action="hello.jsp" method="post">
        username:<input type="text" name="username" /><br>
        password:<input type="password" name="password"/><br>
        <input type="submit" value="Submit"/>
    </form>

</body>
</html>

5.hello.jsp页面：

<%@ page language="java" contentType="text/html; charset=UTF-8"
    pageEncoding="UTF-8"%>
<!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=UTF-8">
<title>Insert title here</title>
</head>
<body>
    Hello:${param.username }
</body>
</html>