javaweb学习23：Filter实现权限拦截

监听器：GUI编程中经常使用；
过滤器：重点理解过滤器；

需求：
- 用户登录后才能进入主页！
- 用户注销后就不能进入主页了！
- 核心：Session的用法；

思路：
- 1，用户登录之后，向Session中放入用户的数据；
- 2，进入主页的时候，要判断用户是否已经登录；要求：过滤器实现；

代码案例：login.jsp

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

<form action="/servlet/login" method="post">
    用户名：<input type="text" name="username">
    <br>
    <input type="submit">
</form>

</body>
</html>

代码案例：success.jsp

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>成功页面</title>
</head>
<body>
<%--
<%
    Object user_session = request.getSession().getAttribute("USER_SESSION");
    if(user_session==null){
        response.sendRedirect("/login.jsp");
    }
%>--%>


<h1>主页</h1>

<p>
    <a href="/servlet/logout">注销</a>
</p>

</body>
</html>

代码案例：error.sjp

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

<h1>错误</h1>

<p>
    <a href="/login.jsp">返回首页</a>
</p>

</body>
</html>

代码案例：登录Servlet

public class LoginServlet extends HttpServlet {

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        //获取前端请求的参数:
        String username = req.getParameter("username");
        if(username.equals("admin")){//登录成功
            req.getSession().setAttribute("USER_SESSION",req.getSession().getId());
            //登录成功之后，重定向到成功页面
            resp.sendRedirect("/sys/success.jsp");
        }else{//登录失败

            resp.sendRedirect("/error.jsp");

        }


    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doGet(req, resp);
    }
}

代码案例：退出Servlet


public class LogoutServlet  extends HttpServlet {

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {

        //移除登录的那个session即可实现推出操作：
        Object user_session = req.getSession().getAttribute("USER_SESSION");
        if(user_session !=null ){
            req.getSession().removeAttribute("USER_SESSION");
            resp.sendRedirect("/login.jsp");
        }else{
            resp.sendRedirect("/login.jsp");
        }

    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doGet(req, resp);
    }
}

代码案例：过滤器


/**
 * 过滤器
 */
public class SysFilter implements Filter {


    public void init(FilterConfig filterConfig) throws ServletException {

    }

    public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {
        System.out.println("开始处理...");
        HttpServletRequest req=(HttpServletRequest)request;
        HttpServletResponse resp=(HttpServletResponse)response;
        Object username = req.getSession().getAttribute("USER_SESSION");

        if(username == null){
            resp.sendRedirect("/error.jsp");
        }

        chain.doFilter(request,response);//放行
        System.out.println("结束处理...");
    }

    public void destroy() {

    }
}

代码案例：web.xml

<?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">
    

    <!--注册一个Servlet-->
    <servlet>
        <servlet-name>loginServlet</servlet-name>
        <servlet-class>com.servlet.LoginServlet</servlet-class>
    </servlet>

    <servlet-mapping>
        <servlet-name>loginServlet</servlet-name>
        <url-pattern>/servlet/login</url-pattern>
    </servlet-mapping>


    <!--注册一个Servlet-->
    <servlet>
        <servlet-name>logoutServlet</servlet-name>
        <servlet-class>com.servlet.LogoutServlet</servlet-class>
    </servlet>

    <servlet-mapping>
        <servlet-name>logoutServlet</servlet-name>
        <url-pattern>/servlet/logout</url-pattern>
    </servlet-mapping>


    <!--配置Filter过滤器-->
    <filter>
        <filter-name>SysFilter</filter-name>
        <filter-class>com.lesson.demo01.SysFilter</filter-class>
    </filter>

    <filter-mapping>
        <filter-name>SysFilter</filter-name>
        <!--只要是/servlet/的任何请求，都会经过这个过滤器 -->
        <url-pattern>/sys/*</url-pattern>
    </filter-mapping>


</web-app>