8-过滤器Filter和监听器Listener

一。web监听器：监听特殊事件的发生
1.监听实现步骤
　　a。写一个java类，实现特定的接口，重写相关方法
　　b。在web.xml中，牌配置
　　　　<listener>
　　　　　　<listener-class>com.listener.MyContextLisenter</listener-class>
　　　　</listener>
　　c.注解方式：在java类上写一个@WebLisenter
2.可以在web.xml中可以配置servetContext初始化的参数
　　<context-param>
　　　　<param-name>basePath</param-name>
　　　　<param-value>http://localhost:8888/StudentManager/</param-value>
　　</context-param>
　　在程序中获取方式：sce.getServletContext().getInitParameter("basePath");

3.三种监听：请求监听（ServletRequestListener），会话监听（HttpSessionListener），上下文监听（ServletContextListener）

二。过滤器
1、特点：在服务器中第一个被执行，可以拦截任意指定的路径请求
2.用于项目中的非法访问，字符集设置等等
3.实现步骤：
　　a.写一个java程序，实现Filter接口，重写方法
　　b.在web.xml中配置
　　<filter>
　　　　<filter-name>charfilter</filter-name>
　　　　<filter-class>com.filter.CharSetFilter</filter-class>
　　</filter>
　　<filter-mapping>
　　　　<filter-name>charfilter</filter-name>
　　　　<!-- 拦截根目录下的所有请求 -->
　　　　<url-pattern>/*</url-pattern>
　　</filter-mapping>
　　c.dofilter方法里面的最后一句，应该将请求继续放行china.doFilter(req,resp);

　　web.xml配置文件

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://xmlns.jcp.org/xml/ns/javaee" xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_3_1.xsd" id="WebApp_ID" version="3.1">
  <display-name>javaweb06</display-name>
<!--   <listener> -->
<!--       <listener-class>com.listener.MyRequestListener</listener-class> -->
<!--   </listener> -->
    <context-param>
        <param-name>basePath</param-name>
        <param-value>http://localhost:8888/StudentManager/</param-value>
    </context-param>
  <filter>
      <filter-name>charset</filter-name>
      <filter-class>com.filter.CharSetFilter</filter-class>
  </filter>
  <filter-mapping>
      <filter-name>charset</filter-name>
      <url-pattern>/*</url-pattern>
  </filter-mapping>
    
  <servlet>
      <servlet-name>loginservlet</servlet-name>
      <servlet-class>com.control.LoginServlet</servlet-class>
  </servlet>
  <servlet-mapping>
      <servlet-name>loginservlet</servlet-name>
      <url-pattern>/login</url-pattern>
  </servlet-mapping>
  <servlet>
      <servlet-name>loginservlet2</servlet-name>
      <servlet-class>com.control.LoginServlet2</servlet-class>
  </servlet>
  <servlet-mapping>
      <servlet-name>loginservlet2</servlet-name>
      <url-pattern>/login2</url-pattern>
  </servlet-mapping>
  
  <welcome-file-list>
    <welcome-file>index.html</welcome-file>
    <welcome-file>index.htm</welcome-file>
    <welcome-file>index.jsp</welcome-file>
    <welcome-file>default.html</welcome-file>
    <welcome-file>default.htm</welcome-file>
    <welcome-file>default.jsp</welcome-file>
  </welcome-file-list>
</web-app>

实例1：

package com.listener;

import javax.servlet.ServletRequestEvent;
import javax.servlet.ServletRequestListener;

public class MyRequestListener implements ServletRequestListener{
    @Override
    public void requestInitialized(ServletRequestEvent sre) {
        sre.getServletContext();
        sre.getServletRequest();
        System.out.println("请求开始");
    }
    
    @Override
    public void requestDestroyed(ServletRequestEvent sre) {
        System.out.println("请求结束");
    }
}

实例2：

package com.listener;

import javax.servlet.ServletContext;
import javax.servlet.annotation.WebListener;
import javax.servlet.http.HttpSessionEvent;
import javax.servlet.http.HttpSessionListener;
@WebListener
public class MySessionListener implements HttpSessionListener{
    
    @Override
    public void sessionCreated(HttpSessionEvent se) {
        String path = se.getSession().getServletContext().getInitParameter("basePath");
        System.out.println("初始化参数为："+path);
        ServletContext app = se.getSession().getServletContext();
        Integer online = (Integer)app.getAttribute("online");
        if(online==null){
            online = 0;
        }
        System.out.println("当前在线人数是："+ ++online);
        app.setAttribute("online", online);
    }
    
    @Override
    public void sessionDestroyed(HttpSessionEvent se) {
        ServletContext app = se.getSession().getServletContext();
        Integer online = (Integer)app.getAttribute("online");
        System.out.println("当前在线人数为："+ --online);
        app.setAttribute("online", online);
    }
}

实例3：

package com.filter;

import java.io.IOException;

import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.annotation.WebFilter;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

@WebFilter("/student/*")
public class IsLoginFilter implements Filter{

    @Override
    public void doFilter(ServletRequest req, ServletResponse resp, FilterChain arg2)
            throws IOException, ServletException {
        //1.从session中获取用户信息
        HttpServletRequest request = (HttpServletRequest) req;
        HttpServletResponse response = (HttpServletResponse) resp;
        String username = (String) request.getSession().getAttribute("username");
        
        System.out.println(request.getRequestURI());
        if(request.getRequestURI().endsWith("/login.jsp")||request.getRequestURI().endsWith("/login3")){
            arg2.doFilter(req, resp);
            return;
        }
        
        if(username==null){//非法登录
            response.sendRedirect("login.jsp");
        }else{
            arg2.doFilter(req, resp);
        }
    }

}

实例4：

package com.filter;

import java.io.IOException;

import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.annotation.WebFilter;

/**
 * 将所有请求的字符集统一修改
 * @author allen
 */
@WebFilter("/*")
public class CharSetFilter implements Filter{
    @Override
    public void doFilter(ServletRequest req, ServletResponse resp, FilterChain arg2)
            throws IOException, ServletException {
        System.out.println("请求字符集修改为utf-8");
        req.setCharacterEncoding("utf-8");
        arg2.doFilter(req,resp);//将请求继续往下放行
    }
}