day107-过滤器与监听器

过滤器

filter：过滤器用来过滤网站的数据

• 处理中文乱码

• 登陆验证

• ......

导入maven包

 <dependencies>
     <dependency>
         <groupId>javax.servlet</groupId>
         <artifactId>javax.servlet-api</artifactId>
         <version>3.1.0</version>
     </dependency>
 ​
     <dependency>
         <groupId>javax.servlet.jsp</groupId>
         <artifactId>jsp-api</artifactId>
         <version>2.3.3</version>
     </dependency>
 ​
     <dependency>
         <groupId>javax.servlet.jsp.jstl</groupId>
         <artifactId>jstl-api</artifactId>
         <version>1.2</version>
     </dependency>
 ​
     <dependency>
         <groupId>taglibs</groupId>
         <artifactId>standard</artifactId>
         <version>1.1.2</version>
     </dependency>
 ​
     <dependency>
         <groupId>mysql</groupId>
         <artifactId>mysql-connector-java</artifactId>
         <version>8.0.31</version>
     </dependency>
 </dependencies>

编写过滤器

 public class CharacterEncodingFilter implements Filter {
     @Override
     public void init(FilterConfig filterConfig) throws ServletException {
         //初始化:启动web服务器的时候初始化
         System.out.println("CharacterEncodingFilter初始化");
     }
 ​
     @Override
     public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {
         /*
          1. 过滤器中的所有代码，再过滤特定的请求时都会执行
          2. 必须要让过滤器持续执行
          */
         servletRequest.setCharacterEncoding("utf-8");
         servletResponse.setCharacterEncoding("utf-8");
         servletResponse.setContentType("text/html;charset=UTF-8");
 ​
         System.out.println("before CharacterEncodingFilter");
         filterChain.doFilter(servletRequest,servletResponse);
         System.out.println("after CharacterEncodingFilter");
     }
 ​
     @Override
     public void destroy() {
         //销毁：关闭web服务器的时候，过滤会销毁
         System.out.println("CharacterEncodingFilter销毁");
     }
 }

编写servlet

 public class showServlet extends HttpServlet {
     @Override
     protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
         resp.getWriter().write("你好世界");
     }
 ​
     @Override
     protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
         super.doPost(req, resp);
     }
 }

配置servlet和过滤器

 <servlet>
     <servlet-name>ShowServlet</servlet-name>
     <servlet-class>com.gu.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.gu.filter.CharacterEncodingFilter</filter-class>
 </filter>
 <filter-mapping>
     <filter-name>CharacterEncodingFilter</filter-name>
     <url-pattern>/servlet/*</url-pattern>
 </filter-mapping>

在配置时，对servlet设置两个路径/servlet/show和/show

而过滤器只过滤/servlet/*下的内容

结果

在/servlet/show可以成功转义成中文，而/show还是乱码

监听器

//统计网站在线人数，统计session
 public class OnlineListener implements HttpSessionListener {
 ​
     @Override
     public void sessionCreated(HttpSessionEvent httpSessionEvent) {
         //创建session监听
         //一旦创建一个session，会触发一次这个事件
         ServletContext servletContext = httpSessionEvent.getSession().getServletContext();
 ​
         System.out.println(httpSessionEvent.getSession().getId());
 ​
         Integer onlineCount = (Integer) servletContext.getAttribute("OnlineCount");
 ​
         if (onlineCount == null){
             onlineCount = new Integer(1);
         }else {
             int count = onlineCount.intValue();
 ​
             onlineCount = new Integer(count+1);
         }
         servletContext.setAttribute("OnlineCount",onlineCount);
     }
 ​
     @Override
     public void sessionDestroyed(HttpSessionEvent httpSessionEvent) {
         //销毁session监听
         ServletContext servletContext = httpSessionEvent.getSession().getServletContext();
         Integer onlineCount = (Integer) servletContext.getAttribute("OnlineCount");
 ​
         if (onlineCount == null){
             onlineCount = new Integer(1);
         }else {
             int count = onlineCount.intValue();
 ​
             onlineCount = new Integer(count-1);
         }
         servletContext.setAttribute("OnlineCount",onlineCount);
 ​
     }
 }

展示：

 <body>
 <h1>当前有 <span style="color: cornflowerblue"><%=request.getSession().getAttribute("onlineCount")%></span>在线</h1>
 </body>

over