用JavaWeb实现一个过滤器

1，首先在src下创建一个class，里边实现Filter的方法，但是要注意导入的包是下面这个包

import javax.servlet.*;
import java.io.IOException;

public class Filter1 implements Filter {
    @Override
    public void init(FilterConfig filterConfig) throws ServletException {
        System.out.println("初始化");
    }
    @Override
    public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {
        System.out.println("过滤器开始执行");
        filterChain.doFilter(servletRequest,servletResponse);//读取下一个servlet或filter
        System.out.println("过滤器结束执行");
    }
    @Override
    public void destroy() {
        System.out.println("销毁Filter");
    }
}

2 然后配置web.xml文件，为所有的类实现过滤方法

<filter>
        <filter-name>f1</filter-name>
        <filter-class>
            com.ly.web.Filter1
        </filter-class>

    </filter>
    <filter-mapping>
        <filter-name>f1</filter-name>
        <url-pattern>/*</url-pattern>
    </filter-mapping>

启动服务器就可以看到过滤器可以使用了

3，过滤器的应用
3.1.如何防止用户未登录就执行后续操作
String name=(String)session.getAttribute(“key”);
if(name==null){
//跳转到登录页面
}
3.2.设置编码方式–统一设置编码
3.3.加密解密(密码的加密和解密)
3.4.非法文字筛选
3.5.下载资源的限制
过滤器的特点:在servlet之前和之后都会被执行

案例
防止用户未登录就执行后续操作,登录后退出删除掉cookie然后就重新输入，实现了未登录不得访问的功能

功能所需的全部代码如下

package com.yhp.web;

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

@WebServlet(value = "/testcookie")
public class CookieServlet extends HttpServlet {
    @Override
    protected void service(HttpServletRequest req, HttpServletResponse response) throws ServletException, IOException {
        System.out.println("CookieServlet");
        //1.创建cookie
        Cookie cookie = new Cookie("c1","cookieTest");
        //2.响应给客户端
        response.addCookie(cookie);
        //3.跳转页面
        response.sendRedirect("show.jsp");
    }
}

package com.yhp.web;

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


@WebServlet(value="/loginout")
public class LoginoutServlet extends HttpServlet {
    @Override
    protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
         //让session失效
        HttpSession session = req.getSession();
      //  session.removeAttribute("loginuname");
        session.invalidate();//让所有的session的相关值都清除
        resp.sendRedirect("index.jsp");
    }
}

package com.yhp.web;

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


@WebServlet(value = "/login")
public class LoginServlet  extends HttpServlet {
    @Override
    protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        System.out.println("loginservlet");

        String uname = req.getParameter("uname");
        if("admin".equals(uname)){
            req.getSession().setAttribute("loginuname",uname);
            resp.sendRedirect("/success.jsp");//丢失request的值
        }else{
            Cookie cookie = new Cookie("username",uname);
            resp.addCookie(cookie);
            resp.sendRedirect("/index.jsp");
        }
    }
}

package com.yhp.util;


import javax.servlet.*;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;



public class Filter2 implements Filter {
    @Override
    public void init(FilterConfig filterConfig) throws ServletException {
        System.out.println("初始化filter2");
    }
    @Override
    public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {
        System.out.println("过滤器开始执行2");
        HttpServletRequest request=(HttpServletRequest)servletRequest;
        HttpServletResponse response=(HttpServletResponse)servletResponse;
        request.setCharacterEncoding("utf-8");//处理所有的servlet乱码

        String loginuname = (String)request.getSession().getAttribute("loginuname");
        String requestURI = request.getRequestURI();//获得请求地址
        System.out.println("requestURI="+requestURI);
        //对某些资源放行（用户未登录的情况下可以访问的资源,或session有值的情况下可以访问）
        if(requestURI.endsWith("/")||requestURI.endsWith("message.jsp")||requestURI.endsWith("/index.jsp")||
        requestURI.endsWith("/login")||loginuname!=null){
            filterChain.doFilter(servletRequest, servletResponse);//调取下一个servlet或filter
        }else{
            response.sendRedirect("/message.jsp");
        }
        System.out.println("过滤器结束执行2");
    }

    @Override
    public void destroy() {
        System.out.println("销毁filter2");
    }
}

index.jsp

<%--
  Created by IntelliJ IDEA.
  User: Administrator
  To change this template use File | Settings | File Templates.
--%>
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
  <head>
    <title>$Title$</title>
  </head>
  <body>
   <h1>index.jsp</h1>
  <a href="/testcookie">testcookie</a>
  <p>

   <%
      String val ="";
       Cookie[] cookies = request.getCookies();
       if(cookies!=null){
           for (Cookie cookie : cookies) {
               if(cookie.getName().equals("username")){
                   val = cookie.getValue();
                   break;
               }
           }
       }
      // request.setAttribute("val",val);
      %>

    <form method="post" action="/login">
     用户名:<input type="text" name="uname" value="<%=val%>"><input type="submit" value="登录">
   </form></p>
  </body>
</html>

message.jsp

<%--
  Created by IntelliJ IDEA.
  User: Administrator
  To change this template use File | Settings | File Templates.
--%>
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>Title</title>
</head>
<body>
   <h1>请先登录，再访问，登录页面<a href="index.jsp">index.jsp</a> </h1>
</body>
</html>

show.jsp

<%--
  Created by IntelliJ IDEA.
  User: Administrator
  To change this template use File | Settings | File Templates.
--%>
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>Title</title>
</head>
<body>
  <h1>show.jsp</h1>
  <h1>欢迎您:${loginuname} 回来  <a href="/loginout">退出</a>  </h1>
<%

    Cookie[] cookies = request.getCookies();
    if(cookies!=null){
        for (int i = 0; i < cookies.length; i++) {
            out.print("cookieName="+cookies[i].getName()+",cookieValue="+cookies[i].getValue());
        }
    }

%>
</body>
</html>

success.jsp

<%--
  Created by IntelliJ IDEA.
  User: Administrator
  To change this template use File | Settings | File Templates.
--%>
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>Title</title>
</head>
<body>
 <h1>登录成功,欢迎您:${loginuname} 回来    <a href="/loginout">退出</a> </h1>
</body>
</html>

web.xml

<?xml version="1.0" encoding="UTF-8"?>

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


-<filter>

<filter-name>f2</filter-name>

<filter-class>com.yhp.util.Filter2 </filter-class>

</filter>


-<filter-mapping>

<filter-name>f2</filter-name>

<url-pattern>/*</url-pattern>

</filter-mapping>

</web-app>