1.Filter拦截请求
    处理请求 直接给予响应 转发 重定向 放行
    浏览器发出的请求都会经过Filter 前提是配置为/*
    从服务器内部发送出来的请求转发forward，默认是不经过Filter的
    从服务器内部发送出来的请求重定向sendRedirect，默认是会经过Filter的
    image.png
    如果是同一次请求，默认不经过Filter，新请求则会经过。我们可以自行设置

    1. <filter>
    2.         <filter-name>one</filter-name>
    3.         <filter-class>Filter.FilterOne</filter-class>
    4.     </filter>
    5.     <filter-mapping>
    6.         <filter-name>one</filter-name>
    7.         <url-pattern>/*</url-pattern>
    9. <!--        默认，写不写都可以-->
    10.         <dispatcher>REQUEST</dispatcher>
    11. <!--        设置转发也可以经过Filter；如果想要转发和重定向都经过Filter，两个都要写上-->
    12.         <dispatcher>FORWARD</dispatcher>
    14.     </filter-mapping>

    2.Filter处理请求
    好多真实资源之前的一个操作 用户认证
    通常拦截pattern / .xxx
    多个Filter都匹配同一个pattern 执行顺序？

    多个filter都会执行
    看起来是按照我们的web.xml的配置顺序来执行

    <?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns="http://xmlns.jcp.org/xml/ns/javaee"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_4_0.xsd"
         version="4.0">

    <filter>
        <filter-name>one</filter-name>
        <filter-class>Filter.FilterOne</filter-class>
    </filter>
    <filter-mapping>
        <filter-name>one</filter-name>
        <url-pattern>/*</url-pattern>
    </filter-mapping>

    <filter>
        <filter-name>three</filter-name>
        <filter-class>Filter.FilterThree</filter-class>
    </filter>
    <filter-mapping>
        <filter-name>three</filter-name>
        <url-pattern>/*</url-pattern>
    </filter-mapping>

    <filter>
        <filter-name>two</filter-name>
        <filter-class>Filter.FilterTwo</filter-class>
    </filter>
    <filter-mapping>
        <filter-name>two</filter-name>
        <url-pattern>/*</url-pattern>
    </filter-mapping>

    <servlet>
        <servlet-name>login</servlet-name>
        <servlet-class>controller.LoginController</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>login</servlet-name>
        <url-pattern>/login</url-pattern>
    </servlet-mapping>

</web-app>

    package controller;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;

public class LoginController extends HttpServlet {
    @Override
    protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        req.setCharacterEncoding("UTF-8");
        System.out.println("我是真实的目标，取值，调用业务，转发");
        req.getRequestDispatcher("welcome.jsp").forward(req,resp);
    }
}

    package Filter;

import javax.servlet.FilterChain;
import javax.servlet.ServletException;
import javax.servlet.http.HttpFilter;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;

public class FilterOne extends HttpFilter {
    @Override
    protected void doFilter(HttpServletRequest request, HttpServletResponse response, FilterChain chain) throws IOException, ServletException {
        System.out.println("FilterOne执行了，放行前");
        chain.doFilter(request,response);
        System.out.println("FilterOne放行后");
    }

    @Override
    public void destroy() {
    }
}

    FilterTwo、FilterThree做测试用，除了名字其余都和FilterOne一样

    index.jsp**

    <%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
  <head>
  </head>
  <body>
    <form action="login" method="post">
      账号：<input type="text" name="username" value=""><br>
      密码：<input type="password" name="password" value=""><br>
      <input type="submit" value="登录">
    </form>
  </body>
</html>

    welcome.jsp

    <%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
    <head>
    </head>
    <body>
        welcome!
    </body>
</html>

    运行结果如下
    image.png


    image.png
    **