在servletfilter中正確的使用重定向

在filter中使用重定向時要注意，在重定向后不需要在調用 filterChain.doFilter()。因為過濾已經結束了再調用的話會產生getWriter（）方法非法使用的錯誤

package com.zsh.project.exam.config;

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;

/**
 * @author shihai.zhang
 * @date 2019/3/19 0019
 */
@WebFilter(urlPatterns = {"/student/*" ,"/teacher/*"})
public class SecurityFilter implements Filter {
    @Override
    public void init(FilterConfig filterConfig) throws ServletException {

    }

    /**
     * 在filter中進行重定向時需要用if else 不然會導致 getWriter（）被非法調用的錯誤
     * @param servletRequest
     * @param servletResponse
     * @param filterChain
     * @throws IOException
     * @throws ServletException
     */
    @Override
    public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {
        HttpServletRequest httpServletRequest=(HttpServletRequest)servletRequest;
        HttpServletResponse httpServletResponse = (HttpServletResponse) servletResponse;
        HttpSession httpSession = httpServletRequest.getSession();
        if(httpSession.getAttribute("userName")==null){
            ((HttpServletResponse) servletResponse).sendRedirect("login");
        }else{
            filterChain.doFilter(servletRequest, servletResponse);
        }
    }

    @Override
    public void destroy() {

    }
}