java重定向jsp 404,使用eclipse编写error，为什么跳转不到编写的error.jsp页面，仍然是404...

index.jsp

FirstFilter

com.imooc.filter.FirstFilter

FirstFilter

/index.jsp

404

/error.jsp

package com.imooc.filter;

import java.io.IOException;

import javax.servlet.Filter;

import javax.servlet.FilterChain;

import javax.servlet.FilterConfig;

import javax.servlet.ServletException;

import javax.servlet.ServletRequest;

import javax.servlet.ServletResponse;

import javax.servlet.http.HttpServletRequest;

import javax.servlet.http.HttpServletResponse;

public class FirstFilter implements Filter {

@Override

public void destroy() {

System.out.println("destory-----FirstFilter");

}

@Override

public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain)

throws IOException, ServletException {

System.out.println("start---FirstFilter--doFilter");

//        chain.doFilter(request, response);

HttpServletRequest req = (HttpServletRequest) request;

HttpServletResponse response2 = (HttpServletResponse) response;

//重定向

response2.sendRedirect(req.getContextPath()+"/main.jsp");

//转发

//        req.getRequestDispatcher("main.jsp").forward(request, response);

//        req.getRequestDispatcher("main.jsp").include(request, response);

System.out.println("end---FirstFilter--doFilter");

}

@Override

public void init(FilterConfig filterConfig) throws ServletException {

System.out.println("init-----FirstFilter");

}

}

以上分别是web.xml和FirstFilter的代码，请大神告知下