Java Filter(过滤器)和Listener(监听器)

# Java Filter(过滤器)和Listener(监听器)

1.Filter(过滤器)

Filter：过滤器，用来过滤网站数据；

CharacterEncodingFilter

public class CharacterEncodingFilter implements Filter {
    //初始化：web服务器启动，就已经初始化，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.过滤器中所有代码，在过滤器特定的请求时候都会执行
    2.必须要让过滤器想继续执行
    */
    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...");
        //让请求继续执行，如果不写，程序到这里就被拦截停止！
        filterChain.doFilter(servletRequest, servletResponse);
        System.out.println("CharacterEncodingFilter执行后...");
    }

    //销毁：web服务器关闭的时候，过滤器会被销毁
    public void destroy() {
        System.gc();
        System.out.println("CharacterEncodingFilter销毁");
    }
}

ShowServlet

public class ShowServlet  extends HttpServlet {

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

        //resp.setCharacterEncoding("utf-8");
        resp.getWriter().write("你好啊！世界！");
    }

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

web.xml

<servlet>
    <servlet-name>ShowServlet</servlet-name>
    <servlet-class>com.peng.servlet.ShowServlet</servlet-class>
</servlet>
<servlet-mapping>
    <servlet-name>ShowServlet</servlet-name>
    <url-pattern>/s</url-pattern>
</servlet-mapping>

<filter>
    <filter-name>CharacterEncodingFilter</filter-name>
    <filter-class>com.peng.filter.CharacterEncodingFilter</filter-class>
</filter>
<filter-mapping>
    <filter-name>CharacterEncodingFilter</filter-name>
    <!--servlet下的所有请求-->
    <url-pattern>/servlet/*</url-pattern>
    <!--整个网站的所有请求-->
    <!--<url-pattern>/*</url-pattern>-->
</filter-mapping>

2.监听器

OnlineCountListener

package com.peng.listener;


import javax.servlet.ServletContext;
import javax.servlet.http.HttpSessionEvent;
import javax.servlet.http.HttpSessionListener;

//统计网站在线人数：统计session
public class OnlineCountListener implements HttpSessionListener {
    //创建session监听：
    //一旦创建session就会触发一次这个事件
    public void sessionCreated(HttpSessionEvent se) {
        ServletContext ctx = se.getSession().getServletContext();

        System.out.println(se.getSession().getId());

        Integer onlineCount = (Integer) ctx.getAttribute("OnlineCount");
        if (onlineCount == null) {
            onlineCount = new Integer(1);
        } else {
            int count = onlineCount.intValue();
            onlineCount = new Integer(count + 1);
        }

        ctx.setAttribute("OnlineCount",onlineCount);
    }

    public void sessionDestroyed(HttpSessionEvent se) {
        ServletContext ctx = se.getSession().getServletContext();
        Integer onlineCount = (Integer) ctx.getAttribute("OnlineCount");
        if (onlineCount == null) {
            onlineCount = new Integer(1);
        } else {
            int count = onlineCount.intValue();
            onlineCount = new Integer(count - 1);
        }

        ctx.setAttribute("OnlineCount",onlineCount);
    }

    /*
    Session销毁：
    1.手动销毁 se.getSession().invalidate();
    2.自动销毁 web.xml配置
    <session-config>
        <session-timeout>1</session-timeout>
    </session-config>
    */
}

web.xml

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

3.过滤器、监听器常见应用

3.1GUI编程中经常使用

package com.peng.listener;

import java.awt.*;
import java.awt.event.WindowEvent;
import java.awt.event.WindowListener;

public class TestPanel {
    public static void main(String[] args) {
        Frame frame = new Frame("新年快乐");
        Panel panel = new Panel(null);
        frame.setLayout(null);

        frame.setBounds(300,300,500,500);
        frame.setBackground(new Color(0,0,255));

        panel.setBounds(50,50,300,300);
        panel.setBackground(new Color(0,255,0));

        frame.add(panel);
        frame.setVisible(true);

        //监听事件，监听关闭事件
        frame.addWindowListener(new WindowListener() {
            public void windowOpened(WindowEvent e) {
                System.out.println("打开");
            }

            public void windowClosing(WindowEvent e) {
                System.out.println("关闭ing");
                System.exit(0); //1 不正常 0 正常
            }

            public void windowClosed(WindowEvent e) {
                System.out.println("关闭ed");
            }

            public void windowIconified(WindowEvent e) {
                System.out.println("");
            }

            public void windowDeiconified(WindowEvent e) {
                System.out.println("");
            }

            public void windowActivated(WindowEvent e) {
                System.out.println("激活");
            }

            public void windowDeactivated(WindowEvent e) {
                System.out.println("未激活");
            }
        });
    }
}

3.2 用户登录之后才能进入主页

SysFilter

package com.peng.filter;

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

public class SysFilter implements Filter {

    public void init(FilterConfig filterConfig) throws ServletException {

    }

    public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {

        HttpServletRequest request = (HttpServletRequest)servletRequest;
        HttpServletResponse response = (HttpServletResponse)servletResponse;

        if (request.getSession().getAttribute("username")==null){
          response.sendRedirect("/error.jsp");
        }

        filterChain.doFilter(servletRequest, servletResponse);
    }

    public void destroy() {

    }
}

LoginServlet

package com.peng.servlet;

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

    }

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

LogoutServlet

package com.peng.servlet;

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 LogoutServlet  extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
       Object session =  req.getSession().getAttribute("username");
       if (session!= null){
           req.getSession().removeAttribute("username");
           resp.sendRedirect("/login.jsp");
       }
    }

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

web.xml

<filter>
    <filter-name>SysFilter</filter-name>
    <filter-class>com.peng.filter.SysFilter</filter-class>
</filter>
<filter-mapping>
    <filter-name>SysFilter</filter-name>
    <url-pattern>/sys/*</url-pattern>
</filter-mapping>
<servlet>
    <servlet-name>login</servlet-name>
    <servlet-class>com.peng.servlet.LoginServlet</servlet-class>
</servlet>
<servlet-mapping>
    <servlet-name>login</servlet-name>
    <url-pattern>/login</url-pattern>
</servlet-mapping>

<servlet>
    <servlet-name>logout</servlet-name>
    <servlet-class>com.peng.servlet.LogoutServlet</servlet-class>
</servlet>
<servlet-mapping>
    <servlet-name>logout</servlet-name>
    <url-pattern>/logout</url-pattern>
</servlet-mapping>