Filter(过滤器)与Listener(监听器)详解

11、Filter（重点）

Filter：过滤器，用来过滤网站的数据；

• 处理中文乱码
• 登陆验证...

Filter开发步骤：

1. 导包

2. 编写过滤器

   1. 导包不要错

      

   2. 实现Filter接口，重写对应的方法即可

      public class CharacterEncodingFilter implements Filter {    //初始化：web服务器启动，就已经初始化了，随时等待过滤对象出现！    public void init(FilterConfig filterConfig) throws ServletException {        System.out.println("CharacterEncodingFilter初始化");    }    //Chain : 链    /*    1. 过滤器中的所有代码，在过滤特定请求的时候都会执行    2. 必须要让过滤器继续通行        filterChain.doFilter(servletRequest,servletResponse);     */    public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {        servletRequest.setCharacterEncoding("utf-8");        servletResponse.setCharacterEncoding("utf-8");        servletResponse.setContentType("text/html;charset=UTF-8");        System.out.println("CharacterEncodingFilter执行前...");        filterChain.doFilter(servletRequest,servletResponse); //让我们的请求继续走，如果不写，程序到这里就被拦截停止！        System.out.println("CharacterEncodingFilter执行后...");    }    //销毁：web服务器关闭的时候，过滤器会销毁    public void destroy() {        System.out.println("CharacterEncodingFilter销毁");    }}

   3. 在web.xml中配置 Filter

      <servlet>    <servlet-name>ShowServlet</servlet-name>    <servlet-class>com.kuang.servlet.ShowServlet</servlet-class></servlet><servlet-mapping>    <servlet-name>ShowServlet</servlet-name>    <url-pattern>/servlet/show</url-pattern></servlet-mapping><servlet-mapping>    <servlet-name>ShowServlet</servlet-name>    <url-pattern>/show</url-pattern></servlet-mapping><filter>    <filter-name>CharacterEncodingFilter</filter-name>    <filter-class>com.kuang.filter.CharacterEncodingFilter</filter-class></filter><filter-mapping>    <filter-name>CharacterEncodingFilter</filter-name>    <!--只要是 /servlet的任何请求，都会经过这个过滤器-->    <url-pattern>/servlet/*</url-pattern>    <!--<url-pattern>/*</url-pattern>--></filter-mapping>

12、监听器

实现一个监听器的接口； （有N中）

1. 编写一个监听器

   实现监听器的接口...

   //统计网站在线人数 ： 统计sessionpublic class OnlineCountListener implements HttpSessionListener {    //创建session监听：看你的一举一动    //一旦创建一个session就会触发一次这个事件！    public void sessionCreated(HttpSessionEvent httpSessionEvent) {        ServletContext context = httpSessionEvent.getSession().getServletContext();        System.out.println(httpSessionEvent.getSession().getId());        Integer onlineCount = (Integer) context.getAttribute("OnlineCount");        if (onlineCount==null){            onlineCount = new Integer(1);        }else {            int count = onlineCount.intValue();            onlineCount = new Integer(count+1);        }        context.setAttribute("OnlineCount",onlineCount);    }    //销毁session监听    //一旦销毁session就会触发一次这个事件！    public void sessionDestroyed(HttpSessionEvent httpSessionEvent) {        ServletContext context = httpSessionEvent.getSession().getServletContext();        Integer onlineCount = (Integer) context.getAttribute("OnlineCount");        if (onlineCount==null){            onlineCount = new Integer(0);        }else {            int count = onlineCount.intValue();            onlineCount = new Integer(count-1);        }        context.setAttribute("OnlineCount",onlineCount);    }    /*    Session销毁：    1. 手动销毁 getSession.invalidate()    2. 自动销毁 在web.xml中设置session过期时间     */}

2. web.xml中注册监听器

   <!--注册监听器--><listener>    <listener-class>com.kuang.listener.OnlineCountListener</listener-class></listener>

3. 看情况是否使用！

13、过滤器、监听器常见应用

监听器：GUI编程中经常使用；

public class TestPanel1 {    public static void main(String[] args) {        Frame frame = new Frame("中秋节快乐"); //新建一个窗体        Panel panel = new Panel(null); //面板        frame.setLayout(null); //设置窗体的布局        frame.setBounds(50,50,300,300);        frame.setBackground(Color.blue); //设置背景颜色        panel.setBounds(70,70,70,70); //设置背景颜色        panel.setBackground(Color.GREEN);        frame.add(panel);        frame.setVisible(true);        //监听事件，监听关闭事件，使用适配器        frame.addWindowListener(new WindowAdapter() {            @Override            public void windowClosing(WindowEvent e) {                System.exit(0);            }        });    }}

实现：用户登录之后才能进入主页！用户注销后就不能进入主页了！

1. 用户登陆之后，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：在过滤器中实现！

   //ServletRequest    HttpServletRequestHttpServletRequest httpServletRequest = (HttpServletRequest) servletRequest;HttpServletResponse httpServletResponse = (HttpServletResponse) servletResponse;if (httpServletRequest.getSession().getAttribute(Constant.USER_SESSION)==null){    httpServletResponse.sendRedirect("/error.jsp");}filterChain.doFilter(servletRequest, servletResponse);

项目全部代码：

public class Constant {    public final static String USER_SESSION = "USER_SESSION";}=============================================================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")){ //登陆成功            req.getSession().setAttribute(Constant.USER_SESSION,req.getSession().getId());            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);    }}==========================================================================public class LogoutServlet extends HttpServlet {    @Override    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");        }    }    @Override    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {        doGet(req, resp);    }}===============================================================================public class SysFilter implements Filter {    public void init(FilterConfig filterConfig) throws ServletException {    }    public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {        /*        实现会员分级：            if (req.getSession().setAttribute(Constant.USER_SESSION).level==VIP1){                response.sendRedirect("/vip1/index.jsp");            }            if (req.getSession().setAttribute(Constant.USER_SESSION).level==VIP2){                response.sendRedirect("/vip2/index.jsp");            }            if (req.getSession().setAttribute(Constant.USER_SESSION).level==VIP3){                response.sendRedirect("/vip3/index.jsp");            }         */        //ServletRequest    HttpServletRequest        HttpServletRequest httpServletRequest = (HttpServletRequest) servletRequest;        HttpServletResponse httpServletResponse = (HttpServletResponse) servletResponse;        if (httpServletRequest.getSession().getAttribute(Constant.USER_SESSION)==null){            httpServletResponse.sendRedirect("/error.jsp");        }        filterChain.doFilter(servletRequest, servletResponse);    }    public void destroy() {    }}

<%@ page contentType="text/html;charset=UTF-8" language="java" %><html><head>    <title>Title</title></head><body><h1>错误</h1><h3>没有权限，用户名错误</h3><p><a href="/Login.jsp">返回登录页面</a></p></body></html>====================================================================<%@ page contentType="text/html;charset=UTF-8" language="java" %><html><head>    <title>Title</title></head><body><h1>主页</h1><p><a href="/servlet/logout">注销</a></p></body></html>=======================================================================<%@ page contentType="text/html;charset=UTF-8" language="java" %><html><head>    <title>Title</title></head><body><h1>登录</h1><form action="/servlet/login" method="post">    用户名：<input type="text" name="username"><br>    <input type="submit"></form></body></html>========================================================================<?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">    <servlet>        <servlet-name>LoginServlet</servlet-name>        <servlet-class>com.kuang.servlet.LoginServlet</servlet-class>    </servlet>    <servlet-mapping>        <servlet-name>LoginServlet</servlet-name>        <url-pattern>/servlet/login</url-pattern>    </servlet-mapping>    <servlet>        <servlet-name>LogoutServlet</servlet-name>        <servlet-class>com.kuang.servlet.LogoutServlet</servlet-class>    </servlet>    <servlet-mapping>        <servlet-name>LogoutServlet</servlet-name>        <url-pattern>/servlet/logout</url-pattern>    </servlet-mapping>    <filter>        <filter-name>SysFilter</filter-name>        <filter-class>com.kuang.servlet.SysFilter</filter-class>    </filter>    <filter-mapping>        <filter-name>SysFilter</filter-name>        <url-pattern>/sys/*</url-pattern>    </filter-mapping>    </web-app>