为什么重定向路径需要加上req.getContextPath()，转发则不加呢?

为什么重定向路径需要加上req.getContextPath()+"/main.jsp"，转发则不加req.getContextPath()或去跟路径呢?

================================================

public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {

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

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------doFilter--FirstFilter");

}