java web(6) Filter和监听器

Filter

       Filter:过滤器，用来过滤网站的数据；

              处理中文乱码

              登录验证

 

1. 导包
2. 编写过滤器 实现Filter接口，重写对应的方法即可
3. import javax.servlet.*;
   import java.io.IOException;
   
   public class CharaterEncodingFilter implements Filter {
       public void init(FilterConfig filterConfig) throws ServletException {
   
       }
    
       /*
           filterChain：链
           1.过滤所有的代码，在过滤特定的请求的时候都会执行
           2.必须要让过滤器继续同行
   
        */
       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....");
           filterChain.doFilter(servletRequest,servletResponse); //让我们的请求继续走，如果不走，程序到这里就被拦截了
           System.out.println("CharacterEncodingFilter执行后");
   
       }
   
       public void destroy() {
   
       }

3.在web.xml中配置filter映射

        <filter>

    <filter-name>filter</filter-name>

    <filter-class>com.luo.filter.CharaterEncodingFilter</filter-class>

</filter>



<filter-mapping>

    <filter-name>filter</filter-name>

    <url-pattern>/*</url-pattern>

</filter-mapping>

监听器

       实现一个监听器有n种

1. 编写一个监听器（实现一个监听器的接口）
2. 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，看你的一举一动
       //一旦创建一个session就会触发一个这个事件
       public void sessionCreated(HttpSessionEvent httpSessionEvent) {
   
           ServletContext ctx = httpSessionEvent.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);
       }
   
       //销毁session监听
       //一旦销毁一个session就会触发一个这个事件
       public void sessionDestroyed(HttpSessionEvent httpSessionEvent) {
   
           ServletContext ctx = httpSessionEvent.getSession().getServletContext();
           Integer onlineCount = (Integer) ctx.getAttribute("OnlineCount");
   
           if (onlineCount==null){
               onlineCount=new Integer(0);
           }else {
               int count=onlineCount.intValue();
               onlineCount=new Integer(count-1);
           }
   
           ctx.setAttribute("OnlineCount",onlineCount);
       }
   }

 

1. web.xml中注册监听器
2. <!-- 注册监听器-->
   <listener>
       <listener-class>com.luo.listener.OnlineCountListener</listener-class>
   </listener>

 

1. 看情况是否使用

 

1. 过滤器和监听器的常见应用

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

req.setCharacterEncoding("utf-8");
resp.setCharacterEncoding("utf-8");
resp.setContentType("text/html;charset=utf-8");

 HttpServletRequest request = (HttpServletRequest) req;
 HttpServletResponse response = (HttpServletResponse) resp;

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

 filterChain.doFilter(req,resp);

 

public class LoginoutServlet extends HttpServlet {

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

        Object user_session = req.getSession().getAttribute(LoginSessionId.loginSessionId);
        if (user_session!=null){
            req.getSession().removeAttribute(LoginSessionId.loginSessionId);
            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 LoginoutServlet extends HttpServlet {

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

        Object user_session = req.getSession().getAttribute(LoginSessionId.loginSessionId);
        if (user_session!=null){
            req.getSession().removeAttribute(LoginSessionId.loginSessionId);
            resp.sendRedirect("/Login.jsp");
        }else{
            resp.sendRedirect("/Login.jsp");
        }
    }

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