在servletfilter中正确的使用重定向

在filter中使用重定向时要注意，在重定向后不需要在调用 filterChain.doFilter()。因为过滤已经结束了再调用的话会产生getWriter（）方法非法使用的错误

 1 package com.zsh.project.exam.config;
 2 
 3 import javax.servlet.*;
 4 import javax.servlet.annotation.WebFilter;
 5 import javax.servlet.http.HttpServletRequest;
 6 import javax.servlet.http.HttpServletResponse;
 7 import javax.servlet.http.HttpSession;
 8 import java.io.IOException;
 9 
10 /**
11  * @author shihai.zhang
12  * @date 2019/3/19 0019
13  */
14 @WebFilter(urlPatterns = {"/student/*" ,"/teacher/*"})
15 public class SecurityFilter implements Filter {
16     @Override
17     public void init(FilterConfig filterConfig) throws ServletException {
18 
19     }
20 
21     /**
22      * 在filter中进行重定向时需要用if else 不然会导致 getWriter（）被非法调用的错误
23      * @param servletRequest
24      * @param servletResponse
25      * @param filterChain
26      * @throws IOException
27      * @throws ServletException
28      */
29     @Override
30     public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {
31         HttpServletRequest httpServletRequest=(HttpServletRequest)servletRequest;
32         HttpServletResponse httpServletResponse = (HttpServletResponse) servletResponse;
33         HttpSession httpSession = httpServletRequest.getSession();
34         if(httpSession.getAttribute("userName")==null){
35             ((HttpServletResponse) servletResponse).sendRedirect("login");
36         }else{
37             filterChain.doFilter(servletRequest, servletResponse);
38         }
39     }
40 
41     @Override
42     public void destroy() {
43 
44     }
45 }

注：在jsp中地址最好用<:ulr>来产生URL，否则当地址栏地址变了之后上下文也会变，会导致错误