【JavaWeb】JavaWeb从入门到实战(12)

前言：

本文内容：过滤器Filter、监听器

推荐免费JavaWeb入门到实战视频：【狂神说Java】JavaWeb入门到实战_哔哩哔哩_bilibili

过滤器Filter

Filter：过滤器，用来过滤网站的数据

• 中文乱码
• 登录验证
• …

Filter基本使用

• 导包

• 编写过滤器(注意Filter包为Servlet下的)

• 实现Filter接口，重写方法(这里测试解决乱码问题)

  package com.jokerdig.filter;
  
  import javax.servlet.*;
  import java.io.IOException;
  
  /**
   * @author Joker大雄
   * @data 2022/4/9 - 10:03
   **/
  public class CharacterEncodingFilter implements Filter {
  
  
      // 初始化 在web服务器启动的时候就已经初始化
      public void init(FilterConfig filterConfig) throws ServletException {
          System.out.println("编码过滤器初始化");
      }
  
      public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {
          // 解决乱码
          request.setCharacterEncoding("utf-8");
          response.setCharacterEncoding("utf-8");
          response.setContentType("text/html;charset=UTF-8");
          System.out.println("执行前");
          // chain  链
          // 用来放行
          chain.doFilter(request,response);
          System.out.println("执行后");
      }
      // 销毁 在web服务器关闭时执行销毁
      public void destroy() {
          System.out.println("编码过滤器已销毁");
      }
  }

  用来测试乱码的Servlet

  package com.jokerdig.servlet;
  
  import javax.servlet.ServletException;
  import javax.servlet.http.HttpServlet;
  import javax.servlet.http.HttpServletRequest;
  import javax.servlet.http.HttpServletResponse;
  import java.io.IOException;
  
  /**
   * @author Joker大雄
   * @data 2022/4/9 - 10:07
   **/
  public class ResponseServlet extends HttpServlet {
  
      @Override
      protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
          resp.getWriter().write("测试过滤器解决中文乱码");
      }
  
      @Override
      protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
          doGet(req,resp);
      }
  }

• 配置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">
  
  <!--    配置-->
      <servlet>
          <servlet-name>response</servlet-name>
          <servlet-class>com.jokerdig.servlet.ResponseServlet</servlet-class>
      </servlet>
      <servlet-mapping>
          <servlet-name>response</servlet-name>
          <url-pattern>/servlet/resp</url-pattern>
      </servlet-mapping>
      <servlet-mapping>
          <servlet-name>response</servlet-name>
          <url-pattern>/resp</url-pattern>
      </servlet-mapping>
  <!--    配置过滤器-->
      <filter>
          <filter-name>filterEncoding</filter-name>
          <filter-class>com.jokerdig.filter.CharacterEncodingFilter</filter-class>
      </filter>
      <filter-mapping>
          <filter-name>filterEncoding</filter-name>
  <!--        过滤/servlet下的所有请求-->
          <url-pattern>/servlet/*</url-pattern>
      </filter-mapping>
  </web-app>

监听器

实现一个监听器的接口：

• 编写一个监听器，实现监器接口，重写方法(这里使用session监听器测试网站在线人数)

  package com.jokerdig.listener;
  
  import javax.servlet.ServletContext;
  import javax.servlet.http.HttpSessionEvent;
  import javax.servlet.http.HttpSessionListener;
  
  /**
   * @author Joker大雄
   * @data 2022/4/9 - 10:32
   **/
  
  // 统计网站在线人数
  public class OnlyCountListener implements HttpSessionListener {
  
      // 创建session监听  session创建就触发
      public void sessionCreated(HttpSessionEvent se) {
          ServletContext context = se.getSession().getServletContext();
          Integer onlyCount =(Integer) context.getAttribute("onlyCount");
  
          // 判断是否有人在线
          if(onlyCount==null){
              onlyCount= new Integer(1);
          } else {
              int count = onlyCount.intValue();
              onlyCount= new Integer(count+1);
          }
          context.setAttribute("onlyCount",onlyCount);
  
      }
      //销毁session监听  session销毁就触发
      public void sessionDestroyed(HttpSessionEvent se) {
          ServletContext context = se.getSession().getServletContext();
          Integer onlyCount =(Integer) context.getAttribute("onlyCount");
          // 判断是否有人在线
          if(onlyCount==null){
              onlyCount= new Integer(0);
          } else {
              int count = onlyCount.intValue();
              onlyCount= new Integer(count-1);
          }
          context.setAttribute("onlyCount",onlyCount);
  
          /*
              Session销毁：
                  手动销毁 se.getSession().invalidate();
                  自动销毁 在web.xml配置session-timeout
           */
      }
  }

• 使用并不多，仅作了解