为什么请求调度程序参数仅对接收 servlet 可用

Question

我的 index.jsp

 <%
    String token = "abc"
    request.getRequestDispatcher("/LoginServlet?Token="+token).forward(request,response);
    %>

实际上在去LoginServlet控件之前会去ServletFilter url映射到 /*(in web.xml) ，所以每个请求都会经过这个ServletFilter

在 ServletFilter 中 request.getparameter("Token") 返回 null，但在 LoginServlet 中令牌返回 "abc"

请建议我如何在 ServletFilter 中获取令牌

Answer 1

我认为您对 sendRedirect 和 forward 的工作方式感到困惑。

使用 forward 和 sendRedirect 的过滤器流程如下所示，

前进

1. Filter (Your request came here first, before JSP called, and 
   you are trying to get "Token" parameter which is no where set yet)

2. JSP (Your JSP setting parameter, as it is forward, control will not 
   go to Filter again)

3. Servlet (you are accessing parameter, which is set in JSP 
   and you got the "Token" parameter)

发送重定向

1. Filter(Your request came here first, before JSP called, and you are trying
   to get "Token" parameter which is no where set yet)

2. JSP(Your JSP setting parameter, as it is redirect, control will go to browser 
   back and spwan new request,which intercept by web.xml and calls Filter again) 

3. Filter (So you got data here)

4. Servlet (So you got data here) 

试试这个看看输出

  <%

    String token = "abc";
    // Below 2 lines, try commenting each and see flow
    //request.getRequestDispatcher("/LoginServlet?Token="+token).forward(request,response);   
    response.sendRedirect("LoginServlet?Token="+token);

   %>