关于 Cannot forward after response has been committed的错误

这个错误顾名思义：就是在给客户端做出提交响应完毕后，就不能再次使用request的转发。

代码原先是这样的：

package com.bdqn.jsp.study.web.filter;

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

@WebFilter(filterName = "checkLoginFilter", urlPatterns = "/*")
public class CheckLoginFilter implements Filter {
    @Override
    public void init(FilterConfig filterConfig) throws ServletException {

    }

    @Override
    public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {
        HttpServletRequest request = (HttpServletRequest) servletRequest;
        HttpServletResponse response = (HttpServletResponse) servletResponse;
        if (request.getRequestURI().contains("/user")) {
            Object value = request.getSession().getAttribute("user");
            if (value == null) {
                response.sendRedirect(request.getContextPath() + "/login.jsp");
                //return;
            }
        }
        filterChain.doFilter(request, response);
    }

    @Override
    public void destroy() {

    }
}

注意这里是个filter主要作用：用于判断用户是否登录后才能访问相关页面，地址栏里有“/user”的地址信息，就认为改地址是登录后才能进行访问

然而在某一个servlet里有如下相关代码：

  if (returnValue != null) {
      request.getRequestDispatcher("/" + returnValue.toString()).forward(request, response);
  }

如果这么写就出问题了，因为在filter里response做出重定向后，还要执行doFilter方法，因此当执行servlet中的转发操作时就会报错了，因为在response.sendRedirect()会认为已经给response的commited属性设置成true了，因此不能再进行转发