servlet的两种转发

request	response
request.sendRedirect()	response.getRequrstDispatcher().forword(request,response)
共发两次请求	转发第一次的请求，可带参数
地址栏会变	地址栏不变

eg:
login.jsp

<form action="../LoginServlet" method="get">
    用户名：<input type="text" name="username"><br>
    密码：<input type="password" name="password"><br>
    <input type="reset" value="重置">
    <input type="submit" value="提交">
</form>

LoginServlet.java(doGet()):

HttpSession loginsession = request.getSession();
        //String name = (String)loginsession.getAttribute("username1");
        String username = request.getParameter("username");
        String password = request.getParameter("password");
        if(username.length() >= 6) {
            while(password.equals("1234")) {
//              response.sendRedirect("9(important)/success.jsp");
                request.getRequestDispatcher("9(important)/success.jsp").forward(request, response);
            }
//          response.sendRedirect("9(important)/login.jsp");
            request.getRequestDispatcher("9(important)/login.jsp").forward(request, response);
        }
        response.sendRedirect("9(important)/login.jsp");
//      request.getRequestDispatcher("9(important)/login.jsp").forward(request, response);
        
        loginsession.invalidate();

使用第1、2个注释的代码段的时候：
报错：

image.png