filter实现用户登录后才能发文章的功能

首先，BBS论坛是游客和用户都可以访问的，但是只有用户登录后才可以发帖

如图中，当前没人在线，当我在下方发帖处输入信息，并发帖

<form method="post" action="WriteArticleServlet.do">
      标题：<input type="text" placeholder="请输入标题" name="title"><br/>
      内容:<textarea name="content"></textarea><br/>
      <button type="submit">发帖</button>
  </form>

此处是发帖处的代码

@WebServlet(name = "WriteArticleServlet",urlPatterns = {"/WriteArticleServlet.do"})
public class WriteArticleServlet extends HttpServlet {
.....

此处是writerArticleServlet的代码
于是，创建了一个filter

package filter;

import entity.User;

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

@WebFilter(filterName = "WriteFilter",urlPatterns = {"*.do"})//此处，对.do结尾的进行过滤，一定要写对这个，不然Tomcat就不断报错
public class WriteFilter implements Filter {
    public void destroy() {
    }

    public void doFilter(ServletRequest req, ServletResponse resp, FilterChain chain) throws ServletException, IOException {
        HttpServletRequest httpServletRequest = (HttpServletRequest)req;
        HttpServletResponse httpServletResponse = (HttpServletResponse)resp;
        HttpSession session = httpServletRequest.getSession();
        User user = (User)session.getAttribute("user");//获得当前的用户，如果不存在返回null
        if(user!=null) {
            System.out.println("通过filter");
            chain.doFilter(req, resp);////关于chain.doFilter(request,response)
            //他的作用是将请求转发给过滤器链上下一个对象。这里的下一个指的是下一个filter，如果没有filter那就是你请求的资源。 一般filter都是一个链,web.xml 里面配置了几个就有几个。一个一个的连在一起
            //request -> filter1 -> filter2 ->filter3 -> .... -> request resource.
        }else{
            System.out.println("没有通过filter");
            httpServletResponse.sendRedirect("login.html");
        }
    }

    public void init(FilterConfig config) throws ServletException {

    }

}