过滤器的简单例子

设置过滤器；必须登录成功才能访问sys目录下的success.jsp

login.jsp

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>登录</title>
</head>
<body>
<form action="/login" method="get">
    用户名：<input type="text" name="username">
    <input type="submit" value="登录">
</form>
</body>
</html>

error.jsp

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>Title</title>
</head>
<body>
<h1>登录失败，用户名错误或没有权限访问</h1>

<a href="login.jsp">返回登录界面</a>
</body>
</html>

success.jsp

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>主页</title>
</head>
<body>
<h1>主页</h1>
<h1>登录成功</h1>
<a href="/logout">注销</a>
</body>
</html>

loginServlet

protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        String name = req.getParameter("username");
        if("admin".equals(name)){
            req.getSession().setAttribute(constant.USER_SESSION,req.getSession().getId());
            resp.sendRedirect("/sys/success.jsp");
        }else{
            resp.sendRedirect("/error.jsp");
        }

logoutServlet

protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        Object user_session = req.getSession().getAttribute(constant.USER_SESSION);
        if(user_session!=null){
            req.getSession().removeAttribute(constant.USER_SESSION);
            resp.sendRedirect("/login.jsp");
        }else{
            resp.sendRedirect("/login.jsp");
        }

工具类utils包里存放session的名字，方便以后修改

public class constant {
    public final static String USER_SESSION="USER_SESSION";
}

web.xml

<filter>
        <filter-name>SysFilter</filter-name>
        <filter-class>com.bxb.filter.SysFilter</filter-class>
    </filter>
    <filter-mapping>
        <filter-name>SysFilter</filter-name>
        <url-pattern>/sys/*</url-pattern>
    </filter-mapping>

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原文地址：https://www.cnblogs.com/bxbo/p/13453389.html